# US 9,439,259 B2 · LED backlight driving circuit for LCD panels

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- Number: US 9,439,259 B2
- Document ID: US9439259B2
- Status: Granted
- Kind: B2
- Application number: 13/354,769
- Filed: 2012-01-20
- Published: 2012-07-26
- Granted: 2016-09-06
- Earliest claimed: 2011-01-26
- Assignees: Rohm Co., Ltd.
- Applicants: Junichi Hagino; Shingo Haruta
- Inventors: Junichi Hagino; Shingo Haruta
- Web page: https://patentgrep.com/patent/US9439259B2

## CPC

- H05B 45/14: using electrical feedback from LEDs or from LED modules
- H05B 45/395: Linear regulators
- G02F 1/133612: {Electrical details}
- G05F 1/40: using discharge tubes or semiconductor devices as final control devices
- Y02B 20/30: Semiconductor lamps, e.g. solid state lamps [SSL] light emitting diodes [LED] or organic LED [OLED]
- H05B: ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
- G02F: OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G05F: SYSTEMS FOR REGULATING ELECTRIC OR MAGNETIC VARIABLES
- Y02B: CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS

## Abstract

A current drive circuit capable of reducing the influence of temperature variation or individual deviation is provided. An output transistor is a PNP bipolar transistor, and has an emitter connected to a cathode of an LED string. A current control resistor is disposed between a collector of the output transistor and a ground terminal. An output terminal of an error amplifier is connected to a base of the output transistor, a first input terminal of the error amplifier is connected to a connection point of the output transistor and the current control resistor, and a reference voltage is applied to a second input terminal of the error amplifier. The error amplifier enables a sink current sunk from the output terminal to flow to the current control resistor.

## Claims

### Claim 1

A drive circuit, for driving a light-emitting diode (LED) string, comprising:
an output transistor, being a PNP bipolar transistor, wherein an emitter of the output transistor is connected to a cathode of the LED string;
a current control resistor, disposed between a collector of the output transistor and a fixed voltage terminal that is stabilized at a specific potential;
a first error amplifier, wherein an output terminal of the first error amplifier is connected to a base of the output transistor, a first input terminal of the first error amplifier is connected to a connection point of the output transistor and the current control resistor, and a first reference voltage is applied to a second input terminal of the first error amplifier, and the first error amplifier enables a sink current sunk from the output terminal to flow to the current control resistor;
a switch power supply, providing an output voltage to an anode of the LED string and inputting a feedback voltage based on the output voltage to a feedback terminal;
a second error amplifier, amplifying an error between a base voltage of the output transistor and a second reference voltage and generating an error voltage according to the error;
a transistor, wherein a first terminal of the transistor is connected to the error voltage; and
a feedback voltage control resistor having a first end connected to a second terminal of the transistor and a second end connected to the feedback terminal, and the error voltage being output through the transistor and the feedback voltage control resistor and being input to the feedback terminal.

### Claim 2 (depends on 1)

The drive circuit according to claim 1, wherein the first error amplifier comprises:
a differential amplifier having a non-inverting input terminal and an inverting input terminal; and
a sink transistor, wherein an output of the differential amplifier is input to a control terminal of the sink transistor, and one end of the sink transistor is connected to the base of the output transistor, and the other end of the sink transistor is connected to the connection point of the output transistor and the current control resistor;
wherein the non-inverting input terminal is connected to the collector of the output transistor; and
the first reference voltage is input into the inverting input terminal.

### Claim 3 (depends on 2)

The drive circuit according to claim 2, wherein the sink transistor is an N-channel metal oxide semiconductor field effect transistor (MOSFET).

### Claim 4 (depends on 2)

The drive circuit according to claim 2, wherein the sink transistor is an NPN bipolar transistor.

### Claim 5 (depends on 1)

The drive circuit according to claim 1, further comprising a base-emitter resistor, disposed between the base and the emitter of the output transistor.

### Claim 6 (depends on 1)

The drive circuit according to claim 1, further comprising a switch power supply, for generating a drive voltage in a manner that a potential of the base of the output transistor is identical with the reference voltage and supplying the drive voltage to an anode of the LED string.

### Claim 7 (depends on 1)

A light-emitting device, comprising:
a light-emitting diode (LED) string; and
the drive circuit according to claim 1, for driving the LED string.

### Claim 8 (depends on 7)

An electronic device, comprising:
a liquid crystal panel; and
the light-emitting device according to claim 7, disposed for serving as a back light source of the liquid crystal panel.

### Claim 9

A drive circuit, for driving a light-emitting diode (LED) string, comprising:
an output transistor, being an NPN bipolar transistor, wherein a collector of the output transistor is connected to a cathode of the LED string;
a current control resistor, disposed between an emitter of the output transistor and a fixed voltage terminal that is stabilized at a specific potential;
a first error amplifier, wherein an output terminal of the first error amplifier is connected to a base of the output transistor, a first input terminal of the first error amplifier is connected to a connection point of the output transistor and the current control resistor, and a reference voltage is applied to a second input terminal of the first error amplifier, and a source current of the first error amplifier emitted from the output terminal is supplied from the cathode of the LED string;
a switch power supply, providing an output voltage to an anode of the LED string and inputting a feedback voltage based on the output voltage to a feedback terminal;
a second error amplifier, amplifying an error between a base voltage of the output transistor and a second reference voltage and generating an error voltage according to the error;
a transistor, wherein a first terminal of the transistor is connected to the error voltage; and
a feedback voltage control resistor having a first end connected to a second terminal of the transistor and a second end connected to the feedback terminal, and the error voltage being output through the transistor and the feedback voltage control resistor and being input to the feedback terminal.

### Claim 10 (depends on 9)

The drive circuit according to claim 9, wherein the first error amplifier comprises:
a differential amplifier; and
a source transistor, wherein an output of the differential amplifier is input to a control terminal of the source transistor, one end of the source transistor is connected to the base of the output transistor, and the other end of the source transistor is connected to the cathode of the LED string.

### Claim 11 (depends on 10)

The drive circuit according to claim 10, wherein the source transistor is an N-channel metal oxide semiconductor field effect transistor (MOSFET).

### Claim 12 (depends on 10)

The drive circuit according to claim 10, wherein the source transistor is an NPN bipolar transistor.

### Claim 13 (depends on 9)

The drive circuit according to claim 9, further comprising a base-emitter resistor, disposed between the base and the emitter of the output transistor.