WO2014116100A1

WO2014116100A1 - Constant current led driver for ssl device using hv-dmos

Info

Publication number: WO2014116100A1
Application number: PCT/MY2014/000010
Authority: WO
Inventors: Arjuna Marzuki; Mohd Hakimi OTHMAN; Siti Rohaya ABDUL AZIZ
Original assignee: Usains Infotech Sdn. Bhd.
Priority date: 2013-01-25
Filing date: 2014-01-24
Publication date: 2014-07-31

Abstract

An on-chip controller for SSL driver (101) that includes control circuitry comprising Bipolar, CMOS and DMOS power devices on a single monolithic substrate is provided. The SSL driver control circuitry (101) includes a sensing stage and a control stage. In the solid state lighting integrated circuit driver (101), the control stage triggers the transistor (102) to an "on" state and an "off" state and the sensing stage sends feedback signal to the control stage thereby providing a constant output current and maintains the brightness of LEDs.

Description

CONSTANT CURRENT LED DRIVER FOR SSL DEVICE USING HV-DMOS FIELD OF INVENTION: The present disclosure relates generally to Solid State Lighting (SSL). More particularly, the present disclosure relates to an Integrated Circuit (IC) on-chip controller which provides constant current to an LED (Light Emitting Diode) driver circuit. BACKGROUND OF THE INVENTION:

Solid State Lighting (SSL) refers to a type of lighting that uses semiconductor Light-Emitting Diodes (LEDs), Organic Light-Emitting Diodes (OLED), or Polymer Light-Emitting Diodes (PLED) as sources of illumination rather than electrical filaments, plasma (used in arc lamps such as fluorescent lamps), or gas. An LED driver circuit is used to power these light-emitting diodes and the brightness of these light-emitting diodes, particularly a white LED, is proportional to a conduction current flowing through the LEDs. This conduction current needs to be regulated to obtain constant luminous intensity and to avoid it from exceeding the LED's maximum current rating. Therefore, there is a need for a control circuit / controller that will control the constant output current to maintain the brightness of LEDs.

Many circuitries utilized in maintaining constant output current are known in prior art. For example, U.S. Pat. No. 6,452,369 to Lang describes a buck converter of straightforward construction which is conceived to charge a storage battery by supplying a current adapted to a particular state of charge of the storage battery. U.S. Pat. Nos. 7,245,089 to Yang and 7,750,616 to Liu describe LED driver circuitry that ensures maintenance of a constant level of output current. However, these patents do not disclose short circuit protection to be incorporated with the control circuitry as short circuit protection is a major concern which controls the actuation of a transistor included in the control circuit that drives the LED driver circuitry. Besides the above prior art, certain prior art applications such as U.S. patent application Nos. 20100225249 to Liao; Chia-Wei ; et al, 201 10193491 to CHOUTOV; Dmitri A. ; et al. and 201 10084607 to Hopper; Peter J. ; et al. relate to the field of solid state LED drivers. 20100225249 discloses an integrated LED driver control circuitry with an error amplifier that detects an excess amount of current flowing through a sensing resistor, which results in a high voltage across the resistor and feedback of said high voltage level to an actuating transistor through the opto-coupler. It leads to the usage of more external components. 20110193491 and 20110084607 disclose an integrated solid state LED driver which includes an integrated LED driver control circuitry but it does not disclose feedback from the output end of the LED driver circuitry. It faces the disadvantage of unreliable LED Driver due to many employed external components and inefficiency in LED Driver. To overcome the aforementioned disadvantages of conventional systems there is felt a need for a control circuitry which provides constant output current for LED driver circuit and further reduces loss and increases the efficiency of LED driver circuit. SUMMARY OF THE INVENTION

The present invention envisages a Solid State Lighting (SSL) Driver IC for providing constant current output to a LED driver circuitry, the SSL Driver IC is coupled to the LED driver circuitry and a sense resistor, the SSL Driver IC comprises a control circuitry for the LED driver circuit, wherein the control circuitry includes a combination of Bipolar, CMOS and DMOS power devices on a single monolithic substrate.

Preferably, the DMOS power device is a high voltage switching transistor.

Typically, the control circuitry for producing constant current output comprises the following circuit: (i) an error amplifier to magnify a difference between a predetermined reference voltage level and the sensed voltage level received from the sense resistor;

(ii) a Pulse Width Modulation (PWM) comparator to generate a RESET signal using output of the error amplifier and an input ramp signal; and

(iii) a SR flip flop to generate switching pulses using the RESET signal and a set signal from a oscillator to control the operation of the DMOS power device driving the LED driver circuit.

Alternatively, the control circuitry for producing constant current output comprises the following circuit:

(i) a Pulse Width Modulation (PWM) comparator to generate a RESET signal using output of the sense resistor and an input reference voltage level; and

(ii) a SR flip flop to generate switching pulses using the RESET signal and a set signal from a oscillator to control the operation of the DMOS power device driving the LED driver circuit,

wherein the interconnection of the Pulse Width Modulation (PWM) comparator to the sense resistor and the reference voltage source is made using switches.

Further, the control circuitry for providing short circuit protection to the SSL driver IC comprises the following circuit:

(i) a comparator having one input connected to a reference voltage source and another input connected to output of the sense resistor to compare the voltage between a predetermined reference voltage level and the sensed voltage level; and

(ii) an AND gate having one input connected to output of the comparator and another input connected to the SR flip flop, an output of the AND gate being connected to turn on / off the DMOS power device in response to compared voltage levels.

Still further, the control circuitry includes a current sink / mirror to reduce loss in the sense resistor using MOSFET transistors.

In addition, the error amplifier has one input connected to a reference voltage source and another input connected to output of the sense resistor, and the error amplifier being designed without a compensation block.

In accordance with this invention there is provided a method producing constant current output in Solid State Lighting (SSL) Driver IC comprises providing an on-chip control circuitry for a LED driver circuit comprising the following steps:

(i) sensing a voltage level across a sense resistor;

(ii) comparing a predetermined reference voltage level with the sensed voltage level using a PWM comparator and generating a RESET signal;

(iii) comparing the RESET signal with a SET signal from a oscillator at a flip-flop;

(iv) producing switching pulses to control the operation of a high voltage DMOS transistor driving the LED driver circuit.

Typically, the step of sensing a voltage level across a sense resistor includes the step of providing a current sink / mirror for provision of constant current to the sensing resistor to minimize loss.

Preferably, the step of sensing a voltage level across a sense resistor includes the steps of comparing a predetermined reference voltage level with a sensed voltage level using a comparator and providing output of the comparator to an AND gate to turn on / off the high voltage DMOS transistor in response to compared voltage levels. Other aspects, embodiments, and details of the present invention will be apparent from the following description when read together with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a high level circuit diagram of an on-chip control circuitry for the SSL driver in accordance with the present invention; FIG. 2 illustrates a high level circuit diagram of the on-chip control circuitry for short circuit current protection of the SSL driver IC in accordance with the present invention; and

FIG. 3 illustrates a high level circuit diagram of the on-chip control circuitry for reducing loss in the sense resistor in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A detailed description of the preferred embodiments of the invention is disclosed herein. It should be understood, however that the preferred embodiments are merely exemplary of the invention, which may be embodied in various forms or configurations. Therefore the details disclosed herein are not to be interpreted as limiting but merely as the basis for the claims and for teaching one skilled in the art of the invention. The brightness inconsistency, unreliability and inefficiency of conventional LED driver circuits are overcome by the present invention which envisages a Solid State Lighting (SSL) Integrated Circuit (IC) for providing constant output current for an LED driver circuit. In addition to providing constant output current, the SSL IC facilitates in reducing loss or increasing the efficiency of the LED driver circuit by providing a current mirror/source. The SSL driver IC of the present invention combines DMOS, bipolar and CMOS processing steps, which are compatible with dielectric insulation, to provide a wide variety of MOS and bipolar devices which operate on different voltage levels within the same monolithic substrate.

In accordance with this invention, the SSL IC includes a control circuitry for driving the LED driver circuit. The control circuitry comprises a combination of Bipolar, CMOS and DMOS power devices on a single monolithic substrate. The control circuitry includes an integrated high voltage transistor which is fabricated based on the DMOS architecture. The control circuitry provides constant output current to the LED driver circuit by controlling current flow to the HV-DMOS transistor which acts as a switch for the LED driver circuit.

Referring to the accompanying drawings, FIG. 1 illustrates schematically an on- chip control circuitry for SSL driver IC (101 ) of the present invention for producing constant current output. The SSL driver (101) is coupled to the LED driver circuit (103) and a sense resistor (104). The IC block (101 ) is capable of controlling current flow to the switching DMOS (102). The operation of the switching DMOS (102) is to convert a source of one voltage level to another. The integrated DMOS (102) is used to switch up to 350 mA constant output current over temperature and thus facilitates in maintaining the projected lifetime of LEDs by improving the thermal issue associated with the LEDs.

As shown in FIG. 1 , LED current flows through the LED driver circuit (103) to the sense resistor (104). This sensed current provides a feedback voltage (VFB). The difference between VFB and a reference voltage (VREF) from a reference voltage source (105) is magnified by an error amplifier (106). A RESET signal is generated by a PWM comparator (108) when the difference between the output of the error amplifier (106) and a ramp signal (107) is determined. The RESET signal generated by the PWM comparator (108) and a SET signal from an oscillator (109) is given to a SR flip flop (110). The switching pulses are generated by the SR flip flop (1 10) when a SET signal and a RESET signal generate a controlled duty cycle which is applied to gate terminal of the DMOS transistor ( 02). The switches pulses control the operation of DMOS transistor (102) driving the LED driver circuit (103). As used herein, the error W

amplifier block (106) is an implemented without a compensation block and the PWM comparator (108) brings the output of error amplifier (106) to a pin. Further, PWM comparator (108) has no internal compensation and frequency compensation is provided by adding poles and zeros to error amplifier (106) 5 output pin.

In accordance with this invention, two modes of SSL driver (101 ) are implemented on the same IC (101 ) namely a normal mode of SSL driver IC, in which constant current for LED driver circuit (103) is provided through a

10 combination of the error amplifier (106) and the PWM comparator (108); and a bypass error amplifier mode, in which a built-in linear regulator provides constant current for LED driver circuit (103). With continued reference to FIG. 1 , the second mode operates when connection (111) from sensing resistor (104) is connected to the negative terminal (1 12) of the PWM comparator (108) and a

15 connection at the positive terminal (1 13) of PWM comparator (108) is connected to VREF (105). The connections are made by employing switches such as transmission gates. Two mode SSL driver implemented on the same IC provides users with flexibility that supports both modes to drive the LED driver circuit (103). The two modes of operation of the SSL driver IC (101 ) facilitate in

20 reducing the costs (or external components) while maintaining the product lifetime.

FIG. 2 illustrates another configuration of the control circuitry for providing short circuit current protection for the SSL driver IC (101). The short circuit current at

25 the output of SSL driver IC (101 ) is monitored by using comparator (203). A short circuit event occurs when voltage at connection (201 ) of comparator (203) is greater than the voltage at connection (202) of the comparator (203). The control circuitry includes an AND gate (204) having one input connected to output of the comparator (203) and another input connected to the SR flip flop

30 (1 10). The output of the AND gate (204) is connected to turn on / off the DMOS transistor (102) in response to the voltage levels compared by the comparator (203). In the event of a short circuit, comparator (203) gives a logic 0 signal to the SSL driver (101 ) to shut down the DMOS transistor (102) through the AND gate (204).

FIG. 3 describes yet another configuration of the control circuitry for reducing loss in the sense resistor (104) while achieving high current for LEDs. A low loss for sense resistor (104) is achieved by using a current sink/mirror (301 ) as shown in FIG. 3, by setting low current into sense resistor (104). The basic current mirror is implemented using MOSFET transistors (303). A power MOSFET is a specific type of metal oxide semiconductor field-effect transistor (MOSFET) designed to handle significant power levels. Current sink/mirror circuit is designed to copy current through one active device by controlling the current in another active device of the circuit and by keeping the output current constant regardless of loading. Current sink/ reference (302) is used to provide the reference current to power MOSFET (303). The value of current to power MOSFET (303) can be higher than 350 mA. So the combination of current sink/mirror circuit (301 ) and power MOSFET (303) improves the efficiency of SSL driver IC (101 ).

While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.

Claims

CLAIMS:

1. A Solid State Lighting (SSL) Driver IC (101 ) for driving a LED driver circuit (103), said SSL Driver IC (101 ) coupled to the LED driver circuit (103) and a sense resistor (104), said SSL Driver IC (101) comprising a control circuitry for the LED driver circuit, characterized in that said control circuitry including a combination of Bipolar, CMOS and DMOS power devices on a single monolithic substrate.

The circuit (101 ) as claimed in claim 1 , wherein said DMOS power device is a high voltage switching transistor (102).

The circuit (101 ) as claimed in claim 1 , wherein said control circuitry for producing constant current output comprises the following circuit:

(iv) an error amplifier (106) to magnify a difference between a predetermined reference voltage level and said sensed voltage level received from the sense resistor (104);

(v) a Pulse Width Modulation (PWM) comparator (108) to generate a

RESET signal using output of said error amplifier (106) and an input ramp signal (107); and

(vi) a SR flip flop (110) to generate switching pulses using said

RESET signal and a set signal from a oscillator (109) to control the operation of said DMOS power device (102) driving the LED driver circuit (103).

(i) a Pulse Width Modulation (PWM) comparator (108) to generate a RESET signal using output of the sense resistor (104) and an input reference voltage level; and

(ii) a SR flip flop (1 10) to generate switching pulses using said RESET signal and a set signal from a oscillator (109) to control the operation of said DMOS power device (102) driving the LED driver circuit (103),

wherein the interconnection of said Pulse Width Modulation (PWM) comparator (108) to the sense resistor (104) and the reference voltage source (105) is made using switches.

The circuit (101 ) as claimed in claims 1 , 3 and 4, wherein said control circuitry for providing short circuit protection to said SSL driver IC (101) comprises the following circuit:

(i) a comparator (203) having one input connected to a reference voltage source (105) and another input connected to output of the sense resistor (104) to compare the voltage between a predetermined reference voltage level and said sensed voltage level; and

(ii) an AND gate (204) having one input connected to output of said comparator (203) and another input connected to said SR flip flop (110), an output of said AND gate (204) being connected to turn on / off the DMOS power device (102) in response to compared voltage levels.

The circuit (101 ) as claimed in claim 1 , wherein said control circuitry includes a current sink / mirror (301 ) to reduce loss in the sense resistor (104) using MOSFET transistors (303).

The circuit (101 ) as claimed in claim 3, wherein said error amplifier having one input connected to a reference voltage source (105) and another input connected to output of the sense resistor (104), and said error amplifier being designed without a compensation block.

A method producing constant current output in Solid State Lighting (SSL) Driver IC (101 ) comprises providing an on-chip control circuitry as described in Claim 1 for a LED driver circuit (103) comprising the following steps: (v) sensing a voltage level across a sense resistor (104);

(vi) comparing a predetermined reference voltage level with said sensed voltage level using a PWM comparator (108) and generating a RESET signal;

(vii) comparing said RESET signal with a SET signal from a oscillator

(109) at a flip-flop; and

(viii) producing switching pulses to control the operation of a high voltage DMOS transistor (102) driving the LED driver circuit (103).

The method as claimed in claim 8, wherein the step of sensing a voltage level across a sense resistor (104) includes the step of providing a current sink / mirror for provision of constant current to the sensing resistor (104) to minimize loss. 10. The method as claimed in claim 8, wherein the step of sensing a voltage level across a sense resistor (104) includes the steps of comparing a predetermined reference voltage level with a sensed voltage level using a comparator (203); and providing output of said comparator (203) to an AND gate (204) to turn on / off said high voltage DMOS transistor (102) in response to the compared voltage levels.