JP2012146812A - Led driving device and method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a novel method and device for driving a light-emitting diode (LED) having a light control function.SOLUTION: Pulse width modulation (PWM) of a LED driving system can be achieved by disconnecting and grounding the gate of a transistor M1. Since a transistor M0 always exists and the feedback loop of an operational amplifier OPA1 is never interrupted, output Vg of the OPA1 is kept constant. Consequently, more accurate current drive for an LED can be created, and the problem of overshoot of the output LED current can be solved in the high frequency dimming process.

Description

  The present invention relates to a light emitting diode (LED) driving circuit having a dimming function, and more particularly to an LED driving apparatus and method. The main drive is provided by a power NMOS whose gate terminal is driven by a controller having a feedback mechanism.

  LEDs are used in various applications such as background lighting and decorative lighting in addition to general lighting. In order to produce a lighting effect, an LED drive circuit is required. The LED drive circuit needs not only a function of supplying current but also a device for controlling an illumination drive signal applied to the gate terminal of the power NMOS.

  Using conventional driving techniques for LED 1 as shown in FIG. 1, the LED is driven by a simple power NMOS 3 and an operational amplifier 2 with a feedback system. The current passing through the LED can be set by the resistor 4 and the reference voltage device 5. Brightness adjustment and blinking adjustment are performed by turning on / off the switch 7 by using PWM switching. There are several problems when using traditional techniques.

  One of those problems is that current accuracy cannot be maintained. The LED current is affected by the temperature and the offset of the operational amplifier 2. The current applied to the LED is changed by changing the resistance value of the resistor 4. However, this is inconvenient for the user and increases the cost of the component parts.

  Another problem is that the dimming effect due to high frequency and the blinking effect of decorative lighting cannot be effectively realized using this conventional technique. The gate terminal 6 is switched by a switch 7 driven by PWM operation. To turn on the LED, the switch 7 is opened, and to turn off the LED, the switch 7 is closed. In particular, when switching by high frequency, the gate terminal 6 of the power NMOS 3 is grounded via the resistor 4 when the switch 7 is closed, and is connected to the output of the operational amplifier 2 when the switch 7 is opened. When the switch 7 changes from an open state to a closed state, the operational amplifier 2 quickly charges the gate of the NMOS 3 that lights the LED. Although the gate of the NMOS 3 is repeatedly charged by these two states, a large overshoot occurs in the LED current if the charging speed is not properly controlled. Such an LED current overshoot occurs during the PWM operation.

  The present invention aims to solve the above-mentioned problems and achieve better control and lighting effects of LEDs.

  It is an object of the present invention to provide a novel method and apparatus for driving LEDs with better current accuracy.

According to the present invention, a method for driving an LED comprises:
Generating a stable and temperature independent reference voltage from which a stable and temperature independent output LED current is derived;
A step of providing a driving function for supplying a sufficient current for lighting the LED, and a voltage level applied to the gate terminal of the current driving unit of the driving function so as to reduce an overshoot of the output LED current. Maintaining the voltage.

  According to the present invention, the step of generating a stable and temperature independent reference voltage further comprises adjusting the output LED current by a trimming means to generate an accurate output LED current.

  In accordance with the present invention, the method further includes the step of canceling a characteristic offset voltage that can affect the accuracy of the output LED current.

According to the present invention, an apparatus for driving an LED is
A voltage generating means for generating a stable and temperature-independent reference voltage from which a stable and temperature-independent output LED current is derived;
A current drive unit connected to the LED and driving the LED;
A voltage tracking / gate capacity driving unit for applying a voltage level to the gate terminal of the driving unit; and a high-speed PWM controller unit for maintaining the voltage level applied to the gate terminal of the current driving unit at a predetermined voltage level. .

According to the present invention, the voltage generating means includes
A reference voltage generator for initiating generation of the stable and temperature independent reference voltage process;
An LED current accuracy adjustment circuit connected to the reference voltage generator and enabling adjustment of the reference voltage to a desired voltage level; and an LED output current connected to the LED current accuracy adjustment circuit according to a user's need DAC to enable digital logic setting
Is further provided.

  According to the present invention, the LED current accuracy adjusting circuit is realized by trimming means for adjusting the reference voltage generated by the reference voltage generator.

  According to the present invention, the trimming means is realized by a combination of a fuse and a resistor for providing a voltage dividing effect.

  According to the present invention, the device for driving the LED further comprises an offset cancellation unit for removing the offset voltage that may affect the accuracy of the LED output current.

  The above-described invention can be used to overcome the problems of the prior art such as the absolute current accuracy of the LED, the temperature dependence of the current, the current overshoot due to PWM operation, etc.

  The absolute current accuracy is improved according to the present invention mainly by a trimming technique to improve the accuracy of the reference voltage generated by the resistor ladder circuit. The accuracy is further improved by using an op amp offset cancellation technique. The temperature dependence of the LED current is mitigated by generating a voltage independent of the temperature used to generate the temperature independent LED current by the voltage generating means.

  Current overshoot due to PWM operation is overcome by a high speed PWM controller for maintaining the voltage level applied to the gate terminal of the current drive unit at a predetermined voltage level.

It is a circuit diagram used in the conventional LED drive technology. It is a block diagram of a 1st embodiment of the present invention. It is a block diagram of the 2nd Embodiment of this invention. FIG. 6 is a circuit diagram of an exemplary implementation of a reference voltage generator based on a second embodiment of the present invention. FIG. 6 is a circuit diagram of a typical implementation example of an LED current accuracy adjustment circuit and a DAC based on the second embodiment of the present invention. It is a circuit diagram which shows the technique of offset cancellation. It is a circuit diagram which shows the technique of offset cancellation. FIG. 6 is a circuit diagram of an exemplary implementation of an offset cancellation technique. FIG. 5 is a circuit diagram showing a typical implementation example of a voltage tracking / gate capacitance drive unit, an offset cancellation unit, a high-speed PWM controller, a current setting unit, and a current drive unit. The waveform of the LED output current I _LED is shown for a signal that is PWM controlled based on the conventional method. For the signal that is PWM controlled according to the present invention, the waveform of the LED output current I _LED is shown.

  FIG. 2 shows a first preferred embodiment according to the present invention.

The LED driving circuit includes a voltage generator 100 connected to an LED driving main system 200 that drives the LED 300. The voltage generator 100 is for outputting the LED current I _{LED in} which the output of the LED driving main system 200 is stable and not influenced by temperature. The LED driving main system 200 performs sufficient current driving to finally turn on the LED 300.

A second preferred embodiment according to the present invention comprises a block as shown in FIG. The reference voltage generator 101 constitutes the voltage generator 100 together with the LED current accuracy adjusting circuit 102 and the DAC (digital-analog converter) 103, and supplies the voltage generator 100 to the LED driving main system 200. The main purpose of the voltage generator 100 is to finally generate an LED current I _LED that is stable and temperature independent of the output of the LED driving main system 200. The DAC 103 is also incorporated in the voltage generator 100. The DAC 103 can control the LED current I _LED at the output of the LED driving main system 200 by digital logic setting. The digital step of the current step and the current logic digital setting can be adjusted by the user via software. Therefore, the required LED brightness can be easily set. The user does not need to change the current setting resistor unit 204 to change the LED current. Further, fine adjustment of the LED current can be performed by the current accuracy unit 102. This ensures that the generated LED current can meet certain accuracy requirements.

  The LED driving main system 200 includes a voltage tracking / gate capacitance driving circuit 201, a current driving unit 205, and a current setting unit 204. These units are used in conjunction with the offset cancellation unit 202 and the high speed PWM controller 203. The LED unit 300 is used together with the above-described block and lights up. As shown in FIG. 8, the LED current ILED is set based on ILED = Vz / RD. Here, RD is a resistance value of the resistor RD that functions as the current setting unit 204. The voltage Vz follows the voltage Vy generated by the voltage generation unit 101. The tracking of Vy and Vz is performed by the voltage tracking unit 201. The voltage tracking unit 201 keeps the voltage Vz always equal to the voltage Vy by the feedback mechanism of the operational amplifier 201. This feature always provides accurate LED output current. In addition to maintaining the voltage Vz equal to Vy, this unit also functions as a gate capacitance charging unit. The voltage tracking unit 201 is connected to the gate of a current driving unit 205 including an NMOS transistor. During the transition period following the voltage Vy, the voltage following unit 201 needs to quickly charge a larger capacity present at the gate of the current drive unit 205. By charging this gate capacitance, the voltage at the switch 203A can be increased. The voltage at the switch 203A can be increased to turn on the current setting unit 205. Only then does the LED current ILED drive the LED on. Thus, the transient gate capacity driving capability of the voltage tracking unit 201 is another important feature to ensure that the LED light can be properly turned on / off during dimming or blinking operation. .

The voltage tracking / gate capacitance drive unit 201 involves offset inaccuracy. This offset causes a decrease in the accuracy of the LED current. Here, the accuracy of the LED current means that the LED current I _LED obtained at the output of the LED driving main system 200 differs from the desired LED current based on the digital logic setting by a magnitude corresponding to the offset value. means. An offset cancellation unit 202 adjusts the offset as close to zero volts as possible. By this adjustment, the LED current is not changed by the offset, and the accuracy is improved.

  The high speed PWM controller unit 203 helps control the gate voltage of the current drive unit 205. Using this controlled voltage, the LED current can be dimmed digitally using a high frequency digital switching signal to reduce LED current overshoot. Lighting effects such as controlled blinking timing of LEDs and RGB color adjustment can also be realized using the present invention.

FIG. 4 shows a reference voltage generator 101 according to a second preferred embodiment of the present invention. Voltage Vx is determined based on current 101A through resistor R _A 101B. The operational amplifier 101C functions as a buffer and prevents the influence of the load on the reference voltage. This supplies a reference voltage to an LED circuit composed of a plurality of channels, and operates stably even when a large number of LED channels are turned on simultaneously.

  FIG. 5 shows an implementation example of the LED current accuracy adjustment circuit 102 and the DAC 103 based on the second embodiment of the present invention.

  The voltage Vx of the reference voltage generator 101 is applied to the voltage dividing resistor ladder RC of the LED current accuracy adjustment circuit 102 and the resistor ladder RB of the DAC 103 via the operational amplifier OPA2 101C.

The DAC 103 operates by turning on one of the switches 103A provided along the resistance ladder RB. As a result, Vy changes as the effective resistance of the voltage divider is changed. Therefore, the LED current I _{LED at} the output of the LED driving main system 200 changes. However, the current consumed by the resistor ladder RC-RB network remains constant regardless of the DAC setting. The size of the DAC switch may be minimal because the current flowing through the input of the voltage tracking / gate capacitance driving unit 201 is not large. The voltage tracking / gate capacitance driving unit 201 can be realized by using an operational amplifier OPA1 as shown in FIG. In this case, a large current does not flow through the non-inverting input of the operational amplifier OPA1.

The operation of the LED current accuracy adjustment circuit 102 is similar to the DAC. The function of the LED current accuracy adjustment circuit 102 is referred to as “trimming”. The effect changes the voltage division of RC-RB like DAC. However, trimming changes the current consumption as the fuse combination changes. The fuse combination is selected based on the desired Vy value. Trimming is performed by cutting the fuse 102A to change the effective total resistance of RC. The voltage Vy can be finely adjusted by trimming the network 102 in FIG. This fine adjustment further improves the accuracy of the voltage Vy after the required DAC switch 103A is selected. In FIG. 8, a voltage Vy equal to the voltage Vz is used to set the LED current ILED. here,
ILED = Vz / RD
The relationship is established. Note that RD is the resistance of the resistor unit 204. Since Vy directly controls the accuracy of the LED current ILED, it is important to maintain the accuracy of this voltage. The trimming unit 102 shown in FIG. 5 allows the voltage Vy to be adjusted to a target accuracy according to the required predetermined LED current. The trimming unit includes four fuses 102A, NMOS transistors, and resistors included in a resistor row RC. The voltage Vy has the following relationship:
Vy = {RB / (RB + RC)} × Vx
Can be adjusted based on. The trimming circuit as described above changes the voltage Vy by changing the resistance value of the resistor row RC. Increasing the resistance value of the resistor string RC can be achieved by cutting the fuses connected in parallel to the resistors connected in series in the resistor string RC. By increasing the resistance value of the resistor string RC, the voltage Vy can be adjusted to a lower value. On the other hand, the increase in the voltage Vy can be reduced by cutting the resistance value of the resistor string RC by cutting the fuse connected to the NMOS gate. By using this trimming technique, it is possible to produce an accurate voltage Vy that guarantees good precision LED current.

  When the inverting input of the operational amplifier OPA1 has a positive offset compared to the non-inverting input, the voltage Vz does not become zero volts even if the DAC setting is “0000b”. This offset cannot be corrected by trimming. This is addressed by the offset cancellation unit 202.

The concept of offset cancellation will be described with reference to FIGS. 6A and 6B. It is assumed that a unique offset Vof 202B exists between the non-inverting and inverting inputs of the operational amplifier 201 of the voltage tracking unit. This offset appears in voltage Vz 202C. This offset makes the LED current I _LED 'inaccurate. In order to cancel the offset, the current source Icx 202D is inserted, and the offset is canceled by using the voltage drop in the inserted resistor RE 202E. This additional voltage drop across resistor RE removes the inherent offset of operational amplifier OPA1 201 of the voltage tracking unit. An embodiment of the concept of offset cancellation in the present invention is as shown in FIG. The method of generating the required offset cancellation current Icx uses a trimming technique similar to that already described for the current accuracy adjustment circuit unit 202. As described above, the LED current ILED may be set as ILED = Vz / RD. Ideally, Vz = Vy, but since there is a non-ideal offset in the voltage tracking unit OPA1 201, Vz always follows the voltage Vy and is not always equal to the voltage Vy. The offset voltage Vof generates an inaccurate LED current determined by ILED = (Vy−Vof) / RD. This offset can be canceled by using an offset of the relationship Vof = Ics × RE. By using a trimming technique similar to that of the LED current accuracy adjustment circuit 102, the value of the resistor RE can be adjusted, so that a necessary offset can be canceled using a fixed Ics value. The value of the resistor RE is adjusted, and the offset voltage Vof is canceled by Ics × RE. As a result, the LED current ILED = {Vy−VofF + (Ics × RE)) / RD
Determined by. If Ics × RE is equal to Vof, an accurate LED current ILED can be obtained again based on the relationship ILED = Vz / RD = Vy / RD.

FIG. 8 shows a block diagram for implementing high-speed PWM technology in the system. Using this technique, effects such as LED dimming and LED blinking can be realized. The current driving unit 205 can be realized by using an NMOS transistor M1 as an example. Furthermore, the current setting unit 204 can be realized using a resistor _RD . In the LED driving system, PWM can be realized by cutting (and grounding) the gate of the NMOS transistor M1. In this system, since the transistor M0 203F is always present and the feedback loop of the operational amplifier OPA1 is never interrupted, the output Vg 203A of the operational amplifier OPA1 is kept constant. A logic timing waveform is generated to generate timing signals in the “θ” and “Φ” phase periods.

In the “θ” phase, the system performs normal operation and turns on the LED light. In this phase, switches 203A and 203B are closed, forming a loop with transistor 205 connected to the output of OPA1 201. In the “Φ” phase, the above loop is interrupted, the switches 203C and 203D are closed, and a new loop is formed. In this phase, the output Vg 203A connected to the gate terminal of the current drive unit 205 is grounded, and the LED is prevented from lighting. At the same time, a second loop connecting the transistor M0 203F to the output of the operational amplifier OPA1 201 is generated. If the “θ” phase arrives again, it is only necessary to charge the gate of transistor M1 to Vg in order to output the correct LED current I _LED .

FIG. 9A shows waveforms when the circuit of FIG. 1 is operating in the conventional PWM switching method. In the phase “Φ” where the feedback loop is broken by closing the switch 7, Vz is dropped to the ground potential via the resistor 4. In this phase, Vy> Vz. When the next phase “θ” comes, the operational amplifier 2 detects a large difference of Vy> Vz, and drives the voltage Vg to be a high voltage as a result. When this reaches the “θ” phase, Vg overshoots. This is shown in the waveform diagram of 9A. Vg overshoots until it settles to a voltage level of Vg = Vz + β (where β is the gate-source voltage of the NMOS transistor 3). Because of this Vg overshoot, an overshoot occurs until the voltage Vz settles to the Vy level. This results in an overshoot of the LED current ILED. The LED current I _LED overshoots until the feedback loop pulls the voltages Vg and Vz to the desired level for outputting the correct LED current I _LED . The overshoot time is determined by the response time of the loop. The corresponding waveforms of the LED current I _LED and other voltage nodes when using the conventional method are shown in FIG. 9A.

FIG. 8 is a circuit diagram using high-speed PWM according to the present invention. The corresponding waveform is shown in FIG. 9B. In the “Φ” phase, the loop is switched to transistor M0 and Vg is kept at a level of Vz + ε slightly lower than the required voltage level of Vz + β. β is slightly lower than ε. The generation of Vz + ε is performed by the components 203F and 203E as shown in FIG. The fact that Vg is kept at a level of Vz + ε in the “Φ” phase means that every time the “θ” phase arrives again, the gate terminal of the transistor M1 is set to Vg in order to output the correct output current I _LED . This means that it is only necessary to charge an amount of β−ε. Since the value of β−ε is small, the result is an improvement to the voltage Vz. Since there is no overshoot to the voltages Vz and Vg, overshoot at the time of high-speed switching of the LED drive current is reduced. This results in uniform and accurate illumination during dimming or blinking functions when the technique of the present invention is compared to conventional techniques.

Claims (11)

An apparatus for driving an LED,
Voltage generating means for generating a reference voltage;
A current drive unit connected to the LED and driving the LED;
A voltage tracking / gate capacitance driving unit for applying a voltage level to the input terminal of the driving unit;
An apparatus comprising a high-speed PWM controller unit for maintaining a voltage level applied to an input terminal of the current drive unit at a predetermined voltage level.   The apparatus of claim 1, wherein the reference voltage results in a derivation of output LED current that is stable and temperature independent. The voltage generating means includes
A reference voltage generator for initiating generation of the reference voltage process;
An LED current accuracy adjustment circuit connected to the reference voltage generator and allowing adjustment of the reference voltage to a desired voltage level;
A DAC connected to the LED current accuracy adjusting circuit for enabling digital logic setting of the LED output current
The apparatus of claim 2 comprising:   4. The apparatus of claim 3, wherein the LED current accuracy adjustment circuit comprises trimming means for adjusting a reference voltage generated by the reference voltage generator.   The apparatus of claim 4, wherein the trimming means comprises a combination of a fuse and a resistor for providing a voltage division effect.   4. The apparatus of claim 3, further comprising an offset cancellation unit for removing offset voltage that may affect the accuracy of the LED output current. A method for driving an LED comprising:
Generating a reference voltage from which the output LED current is derived;
Providing a drive function for supplying sufficient current to light the LED;
A method comprising maintaining a voltage level applied to an input terminal of a current drive unit of the drive function at a predetermined voltage to reduce output LED current overshoot.   The method according to claim 7, wherein the driving function is a current driving unit connected to the LED and driving the LED.   8. The method of claim 7, wherein the reference voltage provides a derivation of output LED current that is stable and temperature independent.   8. The method of claim 7, wherein generating the reference voltage further comprises adjusting by trimming means to generate an accurate output LED current.   11. The method of claim 10, further comprising canceling a characteristic offset voltage that can affect the accuracy of the output LED current.

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