OA16696A - Current regulator. - Google Patents

Abstract

The present application describes a current regulator for providing a regulated current from an input voltage. The current regulator comprises a voltage regulator circuit, operable to provide a regulated voltage, which comprises a plurality of Zener diodes connected in parallel.

Description

The présent invention relates to a current régulation device.

In particular, the présent invention relates to a current regulator suitable for supplying a drive current to devices such as light emitting diodes (LEDs), and other devices which are sensitive to fluctuations in current.

The reduced cost and continued improvement in the performance I0 of LEDs has led to their increased application in recent years.

They are widely employed, for example, as illumination éléments in backlighting applications, such as within the backlight of liquid crystal displays (LCDs) . Backlights of this type are used to provide uniform and constant illumination of an array of LCD 15 éléments which make up the display. LEDs are also commonly employed in other applications such as within lighting assemblies, status indicators and displays on a variety of equipment and installations. Within ail of these applications, LEDs are typically arranged in sériés connected strings and are 20 provided with a substantially constant current, via a constant current driver circuit. Such driver circuits therefore include a current régulation means.

It is well known that variations in the drive current supplied 25 to an LED, or a chain of LEDs, forming part of a lighting system can adversely affect the performance of the system. For example, in large lighting or signage applications, uncertainty in the drive current can lead to corresponding uncertainty in power consumption. Such uncertainties are generally unwelcome in the 30 context of a lighting technology marketed on the basis of energy conservation. As well as this, variations in current can, in certain applications requiring, for instance, Red-Green-Blue (RGB) colour mixing, resuit in variations in the chromatic properties of an illuminated platform, such as a sign.

Furthermore, the useful lifetime of an LED, or series-connected chain of LEDs is related to the junction température of the/each LED, which is in turn partly related to the current flowing 5 through the/each LED. Therefore, précisé control of LED current can resuit in improvements in the predictability of LED lifetime. It is further known that variations in the current supplied by an LED driver can occur as a resuit of variations in component properties due to either manufacturing variations, or as a 10 resuit of variations in température. Other performance requirements for LED drivers for lighting Systems, relate to the reliability of a driver. Typically, this is expressed through the use of a metric referred to as Mean Time Between Failures (MTBF). For a given electronic assembly, using I5 well-established components, this metric can readily be calculated, provided that the electrical and thermal stresses placed on each component during operation are known. Due to the mlx of components typically used in conventional so-called switch-mode LED drivers, which includes switching Métal Oxide 20 Semiconductor Field Effect Transistors (MOSFETs) and

Electrolytic Capacitors, both of which are known to hâve limitations in terms of long-term reliability, corresponding limitations are placed on the MTBF of such drivers. Conversely, drivers using linear means of current régulation, in place of 25 switch-mode means, typically suf fer f rom variations in current, referred to previously.

Ir is therefore highly désirable that an LED or a chain of LEDs is supplied with a substantially constant drive current. It is 30 particularly désirable that a substantially constant drive current is produced through the use of high MTBF electronic assemblies, which use high-reliability components such as bipolar transistors and which avoid or at least limit the need for Electrolytic Capacitors. In the case of switch-mode LED drivers, wherein the current régulation function is provided by a switching voltage waveform that successively charges and discharges a circuit element such as an inductor, with such 5 discharge taking place through an LED chain, a substantially constant current can be produced within the LED chain. The current delivered to the LED chain by such a switch-mode driver is dépendent on a number of factors, including the proportion of time that the switching voltage is in the '0N^f state, during 10 which it is delivering charge to the LED chain (this proportion being referred to as the Duty Cycle of the switching waveform) . This switching process, however, leads to the génération of Electro-Magnetic Interférence (EMI) waveforms which necessitate the use of EMI filtering structures, which in turn 15 use ElectroLytic Capacitors. From the perspective of seeking to maximize the MTBF of a driver, therefore, it can be advantageous to construct a constant current LED driver, based upon a current régulation circuit that does not use any switch-mode éléments, so long as current accuracy can be 20 maintained, including the constancy of current over température .

The présent invention is concerned with the general aim of providing a regulated current from an input voltage in order to provide a stable or substantially constant drive current for supply to illumination devices such as LEDs, or other devices 25 which are adversely affected by, or sensitive to, current fluctuations. Preferred embodiments of the présent invention seek to achieve this aim preferably without the use of switch-mode circuitry within the current regulator, thereby tending to increase the lor.g-term reliability of the regulator, 30 as well as reducing or elïminating the need for Electrolytic Capacitors in an LED driver based upon the regulator, thereby increasing further, the long-term reliability of the LED driver.

Current regulator devices or circuits which seek to provide a current to an LED or LED chain that is regulated, or substantially constant, with respect to supply voltage are 5 known. So-called constant current regulators can be realised in either two-terminal or three-terminal topologies. Figure la illustrâtes the case of a two-terminal regulator, whilst figure lb shows a three-terminal current regulator.

However, even with the use of a current regulator device, variations in the drive current supplied to an LED chain can still arise for a number of reasons. Manufacturang spreads i.e. variations in the manufacturing tolérance of current determining circuit éléments - is one of the main causes of

I5 variations arising in the LED drive/supply current.

Variations also arise due to the température coefficient of the current regulator circuit - in other words the dependence of the regulator performance with respect to ambient or junction température.

As will become apparent from the following discussion relating to previously considered constant current regulators, there are a number of drawbacks associated with the prior art.

Figure 2 shows a schematic for a typical three-terminal current regulator used for the purpose of driving a chain of LEDs (also cited in US2010/0277091 - Brieda et al). The minimum 'drop voltage' across a current regulator according to the design shown in figure 2 is around 1.3V - this being equal to two

Base-Emitter voltage (vbe) drops (across transistors Q1 and Q2) .

One of these 'vbe drops' - namely the one across the base-emitter junction of Q1 - occurs across RI, resulting in a current through RI of vbel/Rl. Assuming that Q2 is drawing negligible base t

current, the current through the LEDs is also equal to vbel/Rl, where vbel is the base-emitter voltage of transistor Ql. Consequently, due to the inhérent température dependence of vbe, the temperature-related variation of the LED current, expressed as a fraction of nominal LED current, is given by:

TC = (δΙίΕο/δΤ) /Iled = (δν±>β!/δΤ) /vhel_noa équation 1

Wherein, vbelnom is the nominal value of vbel at a standard température (300K) . In the design of figure 2, vbel_nom is around 0.6 V and ôvbel/δΤ is, to a very good engineering approximation, -2mV/K. Consequently, the lowest achievable value of the température coefficient, TC, for this design is - 0.0033 K’¹ (- 0.33 % per Kelvin, or -3,300 ppm per Kelvin). The currents shown for this 'standard solution'’ in Table 1 of Brieda et al indicate a variation of - 0.35 % per Kelvin. This value of TC would resuit in the current provided to the LED string varying by -/+ 9.25¾ over a température range of +/- 55 Kelvin.

The solution proposed by Brieda et al suffers from a température coefficient TC of - 0.0650 % per Kelvin (- 650 ppm/K). This results in a variation in LED current of -/+ 3.6% over +/- 55 Kelvin. This variation renders the Brieda solution unsuitable for many applications where fluctuations in ambient température are expected and where the opticai output, in ternis of Luminous Flux and/or chromatic indices, of an assembly of LEDs is/are required to remain substantially constant.

In summary, therefore, although the Brieda design offers some advantages in terme of cost-efficiency, this design is capable of delivering minimum values of température coefficient, TC, of around 650 ppm/K in magnitude. This magnitude of TC is still significant and leads to variations of around -/ + 4% in LED r

current over the specified température range of -30C to + 80C.

Also known in the art is a généralisée! two-terminal circuit topology capable of providing a substantially constant current, limited by the current and voltage handling capabilities of a Silicon bipolar transistor. This generalised topology ïs shown in Figure 3.

Within this topology, a Voltage Regulating Device (VRD) is used to regulate the voltage across a sériés combination of a base-emitter voltage, vbe, and a current programming resistor, R. If the regulated voltage across the VRD is Vreg, then the current through the resistor R is given by:

ïn = (Vreg - vbe)/R equat ion 2

By allowing two such currents to mutually bias the base-emitter junctions of the two bipolar transistors shown in figure 3, the total regulated current through the regulator is given by:

Ιγ = 2.1 r = 2.(Vreg-vbe)/R équation 3

The température coefficient of this current, defined (as before) as the fractional change in I_? with température, is given by :

TC = (SI_T/ST)/I_r = (SVregtâT- âvbe/âT)/(Vreg- vbe) équation 4

It is known in the art that for a Silicon bipolar transistor, the value of 5vbe/5T is around -2mV/K and that vbe, being the voltage across a forward-biased Silicon pn junction is around 0.7V.

The thermal behaviour of the regulated current therefore dépends upon the nature and thermal behaviour of the VRD. In light of this, a particular design, based on this generalised topology has been disclosed in which the VRD comprises a sériés combination of a forward biased PN junction diode and a 'bandgap reference' diode. This design is shown in figure 4. For this design, the régulation voltage, Vreg is given by:

Vreg - Vdinde + 1¾ équation 5

It is a property of a bandgap reference diode, that the voltage across it, Vbg (typically 1.23V) is substantially invariant with température, whereas, the voltage across a forward-biased PN junction diode, Vdïode, will vary with température in the same way as a base-emitter junction (it also being a forward-biased PN junction, carrying substantially the same current as the diode) . Therefore, the thermal behavior of Vreg will be identical to that of vbe, thereby producing a zéro température coefficient, TC, for the regulator current.

There are, however, limitations placed on the performance and cost of regulators of this design. In particular, a Silicon bandgap reference diode, maintaining a température stabilised voltage across it of 1.23V, opérâtes up to a typical maximum current of 20mA. This places an upper limit on the total regulator current, I_T, of 40mA.

Furthermore, the very low dif ferential impédance of the bandgap diode (typically less than 1Ω) makes it difficult to ensure that devices of this type can be connected in parallel, whilst sharing current between them. Figure 5 illustrâtes the problem. It depicts the I/V characteristics of two bandgap diodes, lying (for worst-case illustration) at each end of the manufacturing ?

spread in Vbg - for a typical Silicon bandgap diode, this spread (Vbg₂ - VbgJ is around 8mV. It can readily be seen, that if two such diodes are placed in parallel, the diode with the lowest value of Vbg (VbgO will take a certain amount of current (shown as Ibgi) before the other diode begins to take any current. Consequently, there will be a range of VRD current, over which no current-sharing takes place and over which therefore, the current handling capabilities of the VRD and therefore of the current regulator as a whole, remains limited by the current-handling capabilities of a single bandgap reference diode.

By inspecting the I/V characteristic of a bandgap diode with a maximum current handling capability of 20mA (such as the LT1004-1.2) it can be seen, that the voltage across Bandgap Diode 1 in figure 5, has a value which is substantially 8mV higher than its nominal (low current) value, thereby ensuring that Bandgap Diode 2 is turned-on, when the current through Bandgap Diode 1 has reached a value of around 14mA. This means that Bandgap Diode 1 and Bandgap Diode 2 do not share current, until the current through Bandgap Diode 1 has reached a value that is only a few milliamps short of its maximum rated value.

Furthermore, due to the nonlinear nature of the I/V characteristic of a bandgap diode, where the differential impédance (rate of change of voltage with current) is significantly higher at low current than at high current, as the current through Bandgap Diode 1 increases by 6mA, up to its rated maximum of 20mA, the current through Bandgap Diode 2 will increase by significantly less than this (around 3mA).

Consequently, replacing the bandgap diode in each VRD of a circuit according to figure 4, with a parallel combination of two such bandgap diodes, allowing for manufacturing variations tf in Vbg, can be reliably expected to increase the current handling capability of each VRD by only 9mA, compared with the desired 20mA. Therefore, the reliably expected increase in the current handling capability of the current regulator as a whole 5 would be only around 18mA, as opposed to the desired 40mA. This is effectively a process of diminishing returns in terms of current handling per unit cost. The importance of this is significant, in view of the fact that bandgap reference diodes are not simple diode structures, but fairly complex integrated circuits, containing several circuit éléments. A typical 1.23 Volt bandgap reference diode contains around 13 bipolar transistors and 8 resistors, making it a significant contributor to the overall cost of the current regulator.

An alternative approach, in the case of a circuit according to the design of figure 4, would be to form parallel combinations of the entire low current VRD (where each such low current VRD is, as shown, a sériés combination of forward-biased PN junction diode and bandgap reference diode) to form a high current VRD.

This, however, would mean replicating both the bandgap diode and the PN junction diode, thereby again, increasing signifîcantly, the cost of the regulator.

As such, the réalisation of the general topology shown in Figure

4 does not offer a cost-effective solution to the challenge of providing a low température coefficient current regulator which is programmable over a wide range of constant currents.

Embodiments of the présent invention seek to alleviate the problems and drawbacks associated with the previously considered current regulator devices. Considération of the LED driver requirements of a range of different applications, leads to the observation that there exists a need for a current <

ΙΟ regulator device having improved thermal performance together with accurate current setting capabilities and which is preferably opérable over a wide range of programmable current values. Furthermore, in view of the price sensitivîty of many of these applications, LED drivers aimed at addressing these needs should ideally be cost-effective. In circuit design terms, this means realising solutions that use simple current topologies and simple components. For example, a cost-effective solution would be one that keeps transistor count low. This would hâve the added benefit of maximizing the MTBF of the current regulator and therefore of an LED driver incorporating it. It is also désirable to provide a current regulator device which exhibits a lower sensitivîty to the manufacturing tolérance of current determining circuit éléments than previously considered solutions.

According to a first aspect of the présent invention there is provided a current regulator for providing a regulated current from an input voltage, the current regulator comprising:

a driver circuit comprising a resistor and a transistor; and a voltage regulator circuit opérable to provide a regulated voltage to said driver circuit, wherein said voltage regulator circuit comprises a plurality of Zener diodes connected in parallel.

Preferably, the driver circuit and the voltage regulator circuit form a first current regulator circuit. Preferably, the first current regulator circuit is cross-coupled to a second current regulator circuit. Preferably, in this case, the second current regulator circuit may comprise:

a second driver circuit comprising a resistor and a transistor; and

U a second voltage regulator circuit opérable to provide a reguiated voltage to said second driver circuit, wherein said voltage regulator circuit comprises a plurality of Zener diodes connected in parallel.

Alternatively, the driver circuit and the voltage regulator circuit form a first current regulator circuit which is connected to a résistive summing circuit.

According to a second aspect of the présent invention there is provided a current regulator for providing a reguiated current from an input voltage, the current regulator comprising:

a first current regulator circuit and a second current regulator circuit, wherein the output of the first current regulator circuit is cross-coupled to said second current regulator circuit, each of the first and second current regulator circuits comprising:

a driver circuit comprising a resîstor and a transistor; and a voltage regulator circuit opérable to provide a reguiated voltage to the respective driver circuit, wherein said voltage regulator circuit comprises a plurality of Zener diodes connected in parallel.

According to embodiments of the second aspect of the présent invention the output of the first current regulator circuit is cross-coupled to said second current regulator circuit such that the collector of the transistor of the first current regulator circuit is connected to the positive terminal of the voltage regulator circuit of the second current regulator circuit.

Preferably, the Zener diodes of the/each voltage regulator circuit comprise silicon Zener diodes. The transistors used in a cross-coupled current regulator circuit of this type preferably form a complimentary pair wherein one transistor is a Silicon bipolar transistor of the PNP type and the other is a Silicon bipolar transistor of the NPN type.

According to a third aspect of the présent invention there is provided a voltage regulator circuit for use in a current regulator circuit comprising a plurality of Zener diodes connected in parallel.

Embodiments of the présent invention advantageously exploit the well-defined breakdown voltage of Zener diodes as a means to regulate the voltage applied to the driver circuit of a current regulator device in order to generate a stabilised current for supply to a given load.

The provision of a plurality of Zener diodes which are connected in parallel to form the voltage regulator circuit according to embodiments of the présent invention is advantageous in that it readily facilitâtes the génération of a wide range of regulated current values (It)· Specifically, the current programming range of a current regulator embodying the présent invention can advantageously be selected according to the number of Zener diodes used in each voltage regulator circuit, or voltage regulator device (VRD). As such, according to embodiments of the présent invention, it is not necessary to parallelise, or replicate, the whole circuit in order to achieve a range of constant current values. Thus, the parts that are replicated according to the présent invention (i.e. the Zener diodes) are simple, relatively inexpensive circuit éléments. This advantageously provides a very cost-ef fective solution to the problem of providing a range of regulated current values, thereby allowing embodiments of the présent invention to be useful for stabilizing the drive current for a diverse range of applications.

For Silicon Zener diodes with Zener voltages, Vz, of less than around 5.5V, there exists a value of current, Iz,opt, through the Zener diode at which the rate of change of Zener voltage with température substantially equals the rate of change of base-emitter voltage, vbe, of a Silicon bipolar transistor 10 (substantially-2mV/K) . Zener diodes with these Zener voltages, however, differ according to both the value of Iz,opt at which this thermal balance condition is met, and the value of Zener impédance, Z_z at any given current. Preferred embodiments of the présent invention make use of the fact that in a 15 cross-coupled circuit, a VP.D can be constructed, using low-voltage Zener diodes, which are chosen on the basis of having a current, Iz,opt, at which the rate of change of the Zener voltage with température is substantially equal to the rate of change of the base-emitter voltage, vbe, of a Silicon 20 bipolar transistor with température.

Furthermore, according to a particularly preferred embodiment, Zener diodes are selected such that the rate of change of Zener voltage with température, ÔVz/δΤ should exhibit minimal 25 variation with current, for values of Zener current around Iz,opt, thereby facilitating a wide range of programmable currents through a regulator embodying the présent invention, over which the température dependency of each current within this programmable range is advantageously small.

Thus, according to preferred embodiments of the présent invention, the Zener diodes exhibit a low Zener voltage - i.e. less than 5.5V. Preferably, the Zener diodes exhibit a Zener voltage of between 2.0V and 3.0V. Itwillbe appréciatea by those skilled in the art that the Zener voltage of a given Zener diode is defined, in accordance with the définition of the nominal Zener voltage, as the voltage across the diode at a defined diode current. A typical Zener diode current at which the Zener voltage is measured is 5mA.

Preferred embodiments of the présent invention make use of the fact that Silicon Zener diodes with low values of Zener voltage tend to hâve higher values of différentiel Zener impédance, Z_z compared with both higher voltage Zeners and bandgap diodes. These higher values of Z_z advantageously ensure, within limits defined by the manufacturing tolérance in Zener voltage, that such Zener diodes can be connected in parallel and share, approximately evenly, the current through the parallel combination. This beneficially ensures that several regulator current ranges can be chosen, over which the température dependency of current is small and has a value of zéro within the range. Each said range relates to a given number of Zener diodes per VRD.

Preferably, embodiments of the présent invention seek to alleviate the problem that would normally occur as a resuit of manufacturing variations in the Zener voltage cf any given Zener diode, or indeed manufacturing variations in rectifying diodes, such as those used in prior art ref 2, namely corresponding variations in programmed regulator current, I_T. This is done by ensuring that the current through a regulator according to the présent invention varies in accordance with the average Zener voltage within each parallel Zener diode stack, where variations in this average value will obey a statistical distribution governed by the Central Limit Theorem of statistics, whereby the standard déviation of the mean Zener voltage within each VRD is reduced by a factor of the square root of the number of Zener diodes per VRD, compared with the standard déviation in the Zener voltage of a single Zener diode. This leads to a reduced variation in the mean Zener voltage 5 within a VRD and therefore reduced fractional variations in regulated current, in higher current variants of a current regulator circuit according to the présent invention.

As will be discussed in more detail herein, the voltage 10 régulation device (VRD) according to embodiments of the présent invention is highly advantageous in that the parallel combination of Zener diodes not only serves to provide a voltage régulation function, but in preferred embodiments it can also serve to compensate for the température dependence of the drive 15 transistor in order to achieve a thermal balancing function, over a wide range cf currents, comprising a number of sub-ranges, where each sub-range corresponds to a particular number of paralleled Zener diodes per VRD. Current regulator circuits according to the présent invention advantageously provide a 20 regulated current for which the température dependence of the regulated current is beneficially reduced to a value measured in tens of parts per million per Kelvin. Tndeed, according to particularly preferred embodiments of the présent invention, the value of the température coefficient, TC, is seen to be 25 substantially zéro at spécifie preferred currents across each sub-range.

Furthermore, it will be appreciated that since this performance may be achieved, according to embodiments of the présent 30 invention, by means of a circuit containing only bipolar transistors, Zener diodes and resistors, embodiments of the présent invention represent a particularly cost-effective current regulator. As such, embodiments of the présent

I6 invention find particular application in LED lighting, LCD backlights, including those for large public displays, as well as LED displays, architectural lighting and channel lettering applications, without recourse to additional means for correcting for thermal drift in regulator current.

In summary, preferred embodiments of the présent invention advantageously provide a cost-effective régulation circuit, with improved thermal performance (i.e. température coefficient values which are less than those assocîated with the previously considered solutions), which is opérable over a range of programmable current values, and which is accurately set.

According to a fourth aspect of the présent invention there is provided an illumination apparatus comprising one or more LEDs, the illumination apparatus comprising a current regulator according to an embodiment of the first or second aspect.

The illumination apparatus may, for example, comprise a lighting fixture, containing LEDs, together with one or more LED drivers, where each of the said LED drivers contains one or more current regulators.

For a better understanding of the présent invention, and to show how the same may be carried into effect, reference will now be made, by way of example to the accompanying drawings in which:

Figure 1 shows generalized current regulator circuit topologies according to the prior art;

Figure 2 shows the generalized topology of a three-terminal current regulator circuit according to the prior art;

Figure 3 shows the generalized topology of a two-terminal current regulator circuit according to the prior art;

Figure 4 shows a current regulator circuit design according to the prior art;

Figure 5 depicts a graphical représentation of the current/voltage (I/V) characteristics of two bandgap diodes;

Figure 6 shows a current regulator circuit according to a embodiment of the présent invention;

Figure 7 shows a current regulator circuit according to a second 15 embodiment of the présent invention; and

Figure 8 depicts a graphical représentation of the current/voltage (I/V) characteristics of two Zener diodes.

Figure 6 shows a two-terminal current regulator circuit according to a first embodiment of the présent invention, the current regulator circuit having a first current regulator circuit Cl cross-coupled to a second current regulator circuit C2. The first current regulator circuit Cl comprises a driver 25 circuit having a resistor Ri and a bipolar transistor Tl. The first current regulator circuit also comprises a voltage regulator circuit VRC1 comprising a plurality of Zener diodes Zli, ZI-... Zl_n connected in parallel. The second current regulator circuit C2 comprises a driver circuit having a resistor R2 and 30 a bipolar transistor T2. The second current regulator circuit also comprises a voltage regulator circuit VRC2 comprising a plurality of Zer.er diodes Z2_:, Z22- Z2_n connected in parallel.

A voltage source drives a current IT into node W which connecte resistor Ri and the positive terminal of the voltage regulator circuit VRC1 of the first current regulator circuit Cl such that the current IT is divided between the resistor RI and VRC1. The resistor RI is connected to the emitter e of transistor Tl. The collector current of the bipolar transistor Tl, which is determined by the value of RI, the voltage produced by the VRC1 and by the base-emitter voltage Vbe of the transistor Tl, is supplied to the positive terminal of the voltage regulator circuit VRC2 of the second current regulator circuit C2 and to the base of transistor T2 at node Y. Node X connects the négative terminal of VRC1, the base of Tl and the collector of T2 . Resistor, Rn is simply a source of thermal noise, used to 'kick-start' the circuit.

Assuming negligible base current at T2, Ivrdi is equal to the collector current of T2. Furthermore, the collector current of T2 is determined by the value of R2, the voltage produced by the VRC2 and by the base-emitter voltage vbe of transistor T2. The négative terminal of VRC2 is connected to R2 forming the output node Z through which IT flows to the intended load.

By virtue of the cross-coupling of this circuit, the two transistors are advantageously provided with base-biasing currents.

According to the above embodiment, one of the resistors may be held at a constant value, whilst the other is used as a current programming resistor. Alternatively, both of the resistors may be variable in order that they both serve as current programming resistors.

According to a second embodiment of the présent invention shown in Figure 7, a current regulator circuit Cl is connected to a résistive summing circuit RSC. The person skilled in the art will appreciate that various designs for the résistive summing circuit are possible· For example, in the particular example 5 shown in Figure 7, the résistive summing circuit comprises a plurality of resistors connected in parallel.

The following describes the properties and principles of preferred embodiments of the présent invention.

Setting accuracy : The 'setting accuracy' of a current regulator according to embodiments of the présent invention is discussed herein, in terms of the variations in the current provided by such a regulator, caused by random variations in the properties 15 of circuit éléments. Whilst it should be appreciated by the reader that both random errors and deterministic errors occur in any circuit, it is the random errors that give rise to spreads in circuit performance. Deterministic errors give rise to fixed ’offsets' between designed and realised performance. The 20 setting accuracy of any constant current circuit is properly expressed as the fractional change in regulated current. Thus, for a circuit according to the general topology of figure 3:

AIfîlτ~(ΔVreg + Avhe)/(Vreg- vbe) équation 6

Wherein, ûVreg is the manufacturing spread in Vreg and ûvbe is the manufacturing spread in vbe. In the présent invention, the regulating voltage Vreg is provided by low-voltage Zener diodes 30 and therefore, AVreg = ûVz. This spread in Vreg is significantly greater (by a factor of around 10) than the spread in vbe. Therefore:

ΔΙτ/ΐ/~ ~ vbe) équation 7

Typically, for a low voltage (<5.5 Volts) Zener diode, AVz, the statistical spread in Zener voltage is around 10% - equating 5 to a spread of +/- 5% in Vz. Eqn 6 indicates that the use of low voltage Zener diodes would, in the absence of any correction means, give rise to a large variation in I_T with manufacturing tolérance in Vz - in other words, a poor current setting accuracy. It therefore becomes désirable, in accordance with embodiments 10 of the présent invention, to combine Zener diodes in such a way as to ameliorate this effect.

According to embodiments of the présent invention, use is made of a statistical theorem, known as the Central Limit Theorem.

One conséquence of this theorem is that if a variable, x, is distributed according to a normal distribution, with mean, μ and standard déviation, σ, then the mean of samples of size N, will be distributed according to a normal distribution, with the same mean, μ and a standard déviation of σ/'/Ν.

Consider the case of a Zener diode, with a nominal Zener voltage, Vz and a manufacturing tolérance, AVz. The value of AVz will be related to the standard déviation σ(νζ) of the wafer-to-wafer statistics of Vz. Typically, the value quoted for the 25 manufacturing spread in Vz will be around +/- 3. σ (Vz) - the 'six sigma spread'.

If samples of N Zener diodes are taken from this 'global' distribution, to form each parallel diode stack, then the mean 30 value of Zener voltage, <Vz> within each sample will hâve a mean value, <(<Vz>)> equal to the nominal Zener voltage, Vz and a standard déviation of σ (Vz)/'/N.

2I

According to embodiments of the présent invention which use a Voltage Régulation Circuit, VRC, the/each VRC is provided which comprises a parallel stack of Zener diodes, each with the same 5 nominal Zener voltage, Vz.

The regulator, having two diode stacks, carrying currents Isi and IS2 has a total reguiated current, It given by:

10	It = Isi + A2 Thus	équation	8
	It = K<J/Z2> - vbe) + - vbe)]/R	équation	9
15
	The variance in this total current	is given by:
	Var(fr) = (Var^J^Zz^ + Far^Kz^J/R2	équation	10
20	From the Central Limit Theorem:
	= Jiirf<Fz/>) = Var(Vz)/N	équation	11
	Therefore:
25
	Var(ïT) = 2. Var(Vz)/(N.R2)	équation	12

It can be shown that the nominal reguiated current, I_Tnara, through

	the regulator is given by:
30
	hnom - 2.(Vz - vbe)/R	équation 1	(13)
	Wherein, Vz takes its nominal	value as quoted in	the
35	manufacturer'5 datasheet.

The standard déviation in I_T is given by:

cdlr) - SqrtVarflr) = SqrtfJ/N.R^.ofFz) équation 14 *

The 'Setting Accuracy' of It is given by the spread in. Ι_τ (ΔΙτ) as a fraction of Ιτηοπι, where the spread is 6. σ (I_T) . Similarly, the manufacturing spread in Vz (ÛVz) is equal to 6. σ (Vz) .

Therefore:

Δ(Ι _r) = Sqrt(2/NR').A(Vz) équation 15

A(I_T)/I_TMm = Sqrt(2/NR^z).A(Vz)/l_T.nom

Δ(Ιτ)/Ιτ..ο™ = A(Vz)/[Sqrt(2.N).(Vz - vbe)) équation 16 équation 17

Wherein, N is the number of Zener diodes in each stack. Therefore, this fractional error in regulator current, for a regulator according to the présent invention reduces with the number of Zener diodes per stack, by a factor Sqrt(2.N).

Température coefficient and current programming range: The température coefficient of current for a regulator embodying the présent invention is given by:

TC = (δϊ'ζ/δΓ- ôvbe/âT)/(Vz - vbe) équation 18

This température coefficient is substantially zéro when the current through each Zener diode is equal to Iz,opt (the value of Zener current at which 5Vz/5T = 5vbe/ôT) . Consequently, there are values of regulator current, I_T, at which TC is substantially zéro. For optimal thermal performance, therefore, these values of I_T become 'preferred' operational currents for a regulator embodying the présent invention.

As the Zener current, Iz, départs from this optimal value, the value of TC changes. Preferred embodiments of the présent invention seek to provide a current programming range over which TC deviates from zéro by only a small amount. For illustrative purposes, we shall take this current programming range as being that over which the value of TC is bounded within the range +/75 ppm per Kelvin. Therefore, in defining upper and lower bounded values of température coefficient, TC, as TCu = 7.5xl0‘⁵ per K and TCl = -7.5xl0'⁵ per K, the upper and lower values of ôVz/δΤ corresponding to the two ends of the current programming range are:

(SVz/&T)l “ TCl.(Vzu- ^vbe) + Svbe/ST équation 19 and (ÔVz/$T)u = TCufVzi. - vbe) + ôvbe/δΓ équation 20

TC₀ is the upper bound value of TC, which corresponds to the lower bound of Zener current; TCl is the lower bound of TC, which corresponds to the upper bound of Zener current. Vz^ and Vz_L dénoté the values of Zener voltage at the upper and lower limits of Zener current respectively. These values of Vz can be accurately approximated by assuming, a-priori, that the range of current through the mean Zener diode within each stack is around 10mA to 20mA. This range of currents is centered on a value of Zener current that corresponds to the value of Iz,opt for a 2.4V Zener diode, chosen for reasons given later. Then the accurate values of I₂ (Iz,u and I_Z(L) corresponding to the lower and upper values of 6Vz/CT respectively, can usually be obtained from the Zener diode manufacturer's datasheet. The corresponding values of It are then:

/t.l' = T.N.Iz.v and Ir.L - 2-N.Iz.l équation 21

Assuming the bipolar transistors hâve high values of S(Ic/Ib) the total current through the regulator at the centre of its programming range, for a given value of N is given by:

Ιτ,αη^ 2.N.Iz,opt équation 22

From équation 9, the value of the programming resistor corresponding to this central value of current is given by:

R=2.(<Vz>~ vbe)/ÎT,_eeii équation 23

Where the <Vz> takes the vaLue of the nominal Zener voltage at Iz,opt. For values of total regulator current elsewhere within the programming range:

R =2.(<Vz> - vbe)/I_T équation 24

Current sharing: It is known in the art that difficulties arise in connecting Zener diodes in parallel. These difficulties relate to the extent to which Zener diodes share current similar to the case of bandgap reference diodes used in the prior art depicted in figure 4. If the differential Zener impédance (rate of change of Zener voltage, Vz, with current) at around the operating current per Zener diode (Iz,opt) is insuf ficiently high, or if the manufacturing spread in Vz (ûVz) is too high, then the Zener diode with the lowest Zener voltage in the stack will take ail (or at least most) of the current. To counter this problem, thereby ensuring that ail the Zeners in each stack get turned on, it is préférable to use Zener diodes with a small manufacturing spread in Zener voltage and a nominal Zener voltage for which the Zener impédance at Iz, opt is greater than a few Ohms. Thus, according to preferred embodiments, a small variation exists between the Zener voltages of the Zener diodes. The differential Zener impédance is normally regarded as a 'parasitiez' or unwanted impédance. However, inthecontext of the présent invention this usefully facilitâtes current sharing.

This is shown by reference to figure 8, which depicts the I/V characteristics of two Zener diodes, ZI and Z2, with Zener voltages lying at the extremities of the manufacturing tolérance range for a given nominal Zener voltage. The Zener voltage of each Zener diode is defined, in accordance with the définition of the nominal Zener voltage, as the voltage across the diode at a defined diode current, of normally 5mA. Furthermore, in view of the design of the regulator using such Zener diodes, whereby the nominal Zener current is Iz,opt, the currents through Zener 1 and Zener 2 in figure 6, lie either side of this value. Therefore, by construction, the relationship between (Izi - Iz₂) and (Vz₂ - Vzi) is given by:

(Ιζι-/ζ2) = (^Ζ2-^7.!)/Βζ équation 24

Wherein, R_z is the Zener résistance (the real part of the Zener impedence, Z_z) at Iz,opt. For a typical 2.4 Volt Silicon Zener diode (cited for reasons given later) R_z is around 35Ω and Iz,opt is substantially 14.5mA. The manufacturing spread in Vz (Vz₂ - Vzj) is typically 0.24V. Therefore, the worst-case différence in the currents through Zener 1 and Zener 2 is around 7mA. This means that Zener 1 carries a current of Iz,opt plus 3.5mA (Izi = 18mA) whilst Zener 2 carries a current of Iz,opt minus 3.5mA (Iz₂ = 11mA) . By inspection of the thermal behaviour of a typical 2.4 Volt Silicon Zener diode, the typical rates of change of Vzi and Vz₂ with température are (at 18mA and 11mA respectively) substantially -2.1 mV/K and -1.9 mV/K. Therefore, the average rate of change of Zener voltage with température is substantially -2.0 mV/K, as desired for optimal thermal behaviour (5Vz/ST = 5vbe/5T) at a nominal current per Zener diode of Iz,opt.

Therefore, in contrast to a circuit using combinations of forward-based PN junction diodes and bandgap reference diodes, a circuit embodying the présent invention advantageously uses current-sharing voltage references (low voltage Zener diodes) within each VRD. In view of the fact that, in contrast to bandgap reference diodes, such low-voltage Zener diodes are simple PN junction structures, this provides a cost-effective means by which the current handling capabilities and therefore, the current programming range of a current regulator can be selected, according to the number of such low-voltage Zener diodes used in each VRD.

The utility of this approach is shown by reference to a sériés of different embodiments, each differing in terms of the number of Zener diodes per VRD.

The preferred sélection criteria for Zener diodes used in a preferred embodiment of the présent invention are, in light of the preceding:

1. ûVz (manufacturing spread in Vz - i.e. the variations arising between the Zener voltages of the plurality of Zener diodes comprised in a given VRD) should be low. This advantageously ensures good setting accuracy and facilitâtes current sharing.

2. The Zener impedence Z₂ should be high at operating current per diode, Iz,opt in order to advantageously enable current sharing.

%

3. Vz at Iz,opt should be low in order to achieve a low ’drop-out voltage', which is equal to 2xVz.

4. The rate of change of ôVz/δΤ with current (δνζ/δΤ.δΙζ) should be low, for currents around Iz,opt in order to advantageously achieve a wide programmable current range.

5. The value of Iz,opt should be low for cases where the setting accuracy of programmed current is particularly important, as this forces N to be high for any given value of regulator current

Inspection of thermal performance data for several commercially available Silicon Zener diodes shows that the first four of these criteria are substantially met by choosing a Zener diode with a low Zener voltage or, preferably the lowest available Zener voltage - typically, 2.4V at the Zener voltage reference current, 5mA. For applications in which setting accuracy of programmed current is particularly important, a slightly higher Zener voltage (substantially 3.0V at 5mA) can be chosen, as this is consistent with a lower value of Iz,opt, consistent with preferred sélection criterion 5.

Exemple embodiments:

For a typical 2.4V Silicon Zener diode, where the Zener voltage is measured at a Zener current of 5mA, the Zener voltage at Iz, opt (which is substantially 14.5mA) is 2.9V. Also, for a typical NPN or PNP Silicon transistor, carrying an appréciable emitter current, vbe * 0.7V.

ÛVz = 0.24 V

Iz,opt = 14.5 mA

Z_z @ Iz,opt = 35 Ω

Vz @ Iz,opt = 2.9 V

Vz @ 10mA (value taken for Vz_L) = 2.75 V

Vz @ 20mA (value taken for Vz_u) = 3.1 V (δνζ/δΤ)_σ = - 1.85 mV/K (ôVz/ÔT)_L = - 2.18 mV/K

- 9.5 mA, I_Z'U = 23.5 mA

Performance metrics can be calculated from these figures, assuming the resistors (R) hâve zéro température coefficient. In practice, ultra-low température coefficient resistors represent a significant uplift in cost. Affordable thick-film chip resistors, however, are currently available, with température coefficients of +/-25 ppm/K across the résistance range required and with résistance accuracies of +/-0.1%. The performance metrics for a range of N from 1 to 6 are given in Table 1.

N	Centre Current Ιτ,Εβη (mA.) (at which, TC = 0)	R @ It , can (Ω)	Setting Accuracy of current @ It , cen	Programmable Current Range, over which -75ppm/K < TC < +75ppm/K
	Ιτ,υ (πιΑ)
1	29	151.7	+ /- 5.5 %	19	47
2	58	75.86	+/- 3.9 %	38	94
3	87	50.57	+/- 3.2 %	57	141
4	116	37.93	+/-2.7 %	78	188
5	145	30.34	+/- 2.4 %	97	235
6	174	25.29	+/- 2.2 %	116	282

TABLE 1

This shows that embodiments of the présent invention advantageously provide a topology for a current regulator, based on which, regulators can be designed which provide a range of programmed currents from around 20 mA to around 280 mA, over which a température coefficient of current, lying between - 75 ppm per Kelvin and + 75 ppm per Kelvin (- 0.0075 % per Kelvin and + 0.0075 % per Kelvin) is maintained. Each embodiment of the présent invention comprises two bipolar Silicon transistors 5 and a number of low-voltage Silicon Zener diodes, beneficially providing a low-cost solution.

Manufacturing variations in the value of Iz,opt would be sensibly accommodated by specifying a product designed 10 according to a particular embodiment of the présent invention, over a narrower programmed current range. A realistic range of currents over which the +/- 75 ppm per Kelvin température coefficient can be specified would be around 25 mA to 220 mA.

In circumstances where low drop-out voltage and therefore low I5 Zener voltage, is less important than setting accuracy, it would be advantageous to use Zener diodes having a slightly higher Zener voltage, consistent with the need to maintain high Zener impédance. Such slightly higher voltage (e.g. 3.0V) Zener diodes hâve lower values of Iz, opt. This means that for any given 20 regulator current, a higher number of Zeners would be required in each stack, leading, in view of the Central Limit Theorem, to a greater setting accuracy for that regulator current.

In circumstances where the value of TC is required to be substantially zéro at a spécifie current, I_SFec, or over a small 25 range of programmed currents centred on I_spec, it is possible to choose a value of Zener voltage for which Silicon Zener diodes hâve a value of Iz,opt given by:

Iz,opt = I_spt>c/2.N équation 25

Where N is an integer and corresponds to the number of Zener diodes per VRD in such a regulator circuit.

For example, it can be shown that for Vz = 2.7 V, the corresponding value af Iz,opt for a Silicon Zener diode is typically 5 mA. Therefore, it is possible to construct a current regulator circuit, embodying the présent invention, which uses 5 two such Zener diodes per VRD and which has a température coefficient of current, TC, which is substantially equal to zéro for a total regulator current, I_T, of 20 mA.

Furthermore, in order to facilitate ease of current programming, 10 a regulator embodying the présent invention could be used, where one of the programming resistors is held constant (at the centre current value, for a given N) whilst the other is used as the programming resistor.

Claims (17)

1. AMENDED CLAIMS received by the International Bureau on 14 November 2012 (14.11.2012)

   1. A current regulator for providing a regulated current from an input voltage comprising:

   a driver circuit comprising a resistor and a transistor; and a voltage regulator circuit opérable to provide a regulated voltage to said driver circuit, wherein said voltage regulator circuit comprises a plurality of Zener diodes connected in parallel, wherein each Zener diode has the same nominal Zener voltage.
2. 2. A current regulator as claimed in claim 1, wherein said driver circuit and said voltage regulator circuit form a first current regulator circuit, and wherein said first current regulator circuit is cross-coupled to a second current regulator circuit.
3. 3. A current regulator as claimed in claim 2, wherein said second current regulator circuit comprises:

   a second driver circuit comprising a resistor and a transistor; and a second voltage regulator circuit opérable to provide a stabilised reference voltage to said second driver circuit, wherein said voltage regulator circuit comprises a plurality of Zener diodes connected in parallel, wherein each Zener diode has the same nominal Zener voltage.
4. 4 . A current regulator as claimed in claim 1, wherein said driver circuit and said voltage regulator circuit form a first current regulator circuit, and wherein said first current regulator circuit is connected to a résistive summing circuit.

   » ' 32
5. 5. A current regulator for providing a regulated current from an input voltage comprising:

   a first current regulator circuit and a second current regulator circuit, wherein the output of the first

   5 current regulator circuit is cross-coupled to said second current regulator circuit, each of the first and second current regulator circuits comprising:

   a driver circuit comprising a resistor and a transistor;

   10 a voltage regulator circuit opérable to provide a regulated voltage to the respective driver circuit, wherein said voltage regulator circuit comprises a plurality of Zener diodes connected in parallel, wherein each Zener diode has the same nominal Zener voltage.
6. 6. A current regulator as claimed in any preceding claim, wherein the current regulator device comprises a two-terminal circuit.

   20 Ί. A current regulator as claimed in any preceding claim, wherein the Zener diodes of the/each voltage regulator circuit comprise silicon Zener diodes.
7. 8. A current regulator as claimed in any preceding claim,

   25 wherein the Zener diodes of the/each voltage regulator circuit exhibit a Zener voltage of less than 5.5V.
8. 9. A current regulator as claimed in any preceding claim, wherein the Zener diodes of the/each voltage regulator circuit

   30 exhibit a Zener voltage of between 2.0V and 3.0V.
9. 10. A current regulator as claimed in any preceding claim, wherein a variation of between 0.1V and 0.3V exists between the

   Zener voltages of the Zener diodes comprised in the plurality of Zener diodes of the/each voltage régulation circuit.
10. 11. A current regulator as claimed in any preceding claim,

    5 wherein the circuit is opérable for providing a programmed regulated current of between 25mA to 220mA.
11. 12. A current regulator as claimed in any preceding claim, wherein the Zener voltage of the Zener diodes comprised in the

    10 voltage regulator circuit of the/each current regulator circuit are selected such that:

    Iz,opt = Ispec/2.N where Iz,opt is the current at which the rate of change of the Zener voltage with température substantially equals the rate

    15 of change of the base-emitter voltage vbe of the transistor of the current regulator circuit, N is an integer number of Zener diodes per voltage regulator circuit and I_gp_ec is a current regulator current at which the température coefficient is substantially zéro.
12. 13. A current regulator, as claimed in any preceding claim, wherein the transistor of the/each driver circuit comprises a silicon bipolar transistor.

    25
13. 14. A current regulator as claimed in claim 15, wherein the silicon bipolar transistor is of the NPN or PNP type.
14. 15. A current regulator as claimed in claim 14, when appended to either claim 3 or claim 5, wherein the transistor of the first 30 or second current regulator circuit is of the PNP type whilst the transistor of other current regulator circuit is of the NPN type such that the transistors form a complimentary pair.
15. 16. A current regulator as claimed in any one of claims 5 to

    13, wherein the resistor of the first and/or second diver circuit is opérable to vary in order to serve as a current programming resistor.
16. 17. A voltage regulator circuit for use in a current regulator circuit comprising a plurality of Zener diodes connected in parallel, wherein each Zener diode has the same nominal Zener voltage.
17. 18. An illumination apparatus cbmprising one or more LEDs and a current regulator as claimed in any preceding claim.