# US 2014/0197747 A1 · Current driver for led diodes

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- Number: US 2014/0197747 A1
- Document ID: US20140197747A1
- Status: Published application
- Kind: A1
- Application number: 14/154,367
- Filed: 2014-01-14
- Published: 2014-07-17
- Earliest claimed: 2013-01-17
- Assignees: STMicroelectronics S.r.l.
- Applicants: STMicroelectronics S.r.l.
- Inventors: Marco Martini; Salvatore Pantano
- Web page: https://patentgrep.com/patent/US20140197747A1

## CPC

- H05B 45/14: using electrical feedback from LEDs or from LED modules
- H02M 1/36: Means for starting or stopping converters
- H05B 45/395: Linear regulators
- 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
- H02M: APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF (transformers H01F; dynamo-electric converters H02K47/00; controlling transformers, reactors or choke coils, control or regulation of electric motors, generators or dynamo-electric converters H02P)
- Y02B: CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS

## Abstract

A current driver for a string of LEDs includes a first series connection of a first transistor and a first resistance and a second series connection of a second transistor and a second resistance. The first and second series connections are coupled in parallel between the string of LEDs and a voltage reference. An operational amplifier selectively drives the first and second transistors in response to a clock signal. A switch device driven by the clock signal alternately applies a reference voltage and a respective one of the voltages across the first and second resistances to inverting and non-inverting inputs of the operational amplifier in response to the clock signal. A storage circuit is coupled to the output of the operational amplifier to store the drive signals for the first and second transistors for application to the first and second transistors in the absence of output from the operational amplifier.

## Claims

### Claim 1

A current driver for at least one LED diode, comprising:
a first series connection of a first transistor and a first resistance,
a second series connection of a second transistor and a second resistance (Rs2),
said first and second series connections being arranged in parallel to each other along an electric path between the at least one LED diode and a voltage reference node,
an operational amplifier, and
a switch device driven by a clock signal and able to apply at a non-inverting input terminal and inverting input terminal of the operational amplifier alternately a reference voltage and a voltage across the first resistance or the voltage across the second resistance according to a half-period of the clock signal,
said switch device configured to apply an output signal of the operational amplifier to the first and second transistor according to the half-period of the clock signal,
a storage element arranged to maintain turned on the first and second transistors when not driven by the output signal of the operational amplifier.

### Claim 2 (depends on 1)

The current driver according to claim 1, wherein said first and second resistances have equal value.

### Claim 3 (depends on 1)

The current driver according to claim 1, wherein said storage element comprises a first and a second capacitor coupled between respective first and second control terminals of the first and second transistors and the voltage reference node.

### Claim 4 (depends on 1)

The current driver according to claim 1, wherein the clock signal assumes a low level during a first half-period and a high level during a second half-period, the switch device configured:
to apply at the non-inverting input terminal and the inverting input terminal of the operational amplifier the reference voltage and the voltage across the first resistance, respectively, and to apply the output signal at a non-inverting output of the operational amplifier to the first transistor during the first half-period of the clock signal, and
to apply at the inverting input terminal and the non-inverting input terminal of the operational amplifier the reference voltage and the voltage across the second resistance, respectively, and to apply the output signal at an inverting output of the operational amplifier to the second transistor during the second half-period of the clock signal.

### Claim 5 (depends on 1)

The current driver according to claim 1, wherein the current driver comprises a start device configured to pre-charge the storage element at start-up.

### Claim 6 (depends on 5)

The current driver according to claim 5, wherein said start device comprises another switch device configured to apply both voltages across the first and second resistances at the inverting input terminal of the operational amplifier, the reference voltage at the non-inverting input terminal of the operational amplifier and the output signal of the operational amplifier at the storage element, said another switch device controlled by a start signal active only at start-up while the switch device is controlled by the signal at the output from a logic AND gate receiving at inputs the clock signal and the start signal.

### Claim 7 (depends on 5)

The current driver according to claim 5, wherein the storage element comprises a first and a second capacitor coupled between respective first and second control terminals of the first and second transistors and the voltage reference node, the operational amplifier comprising an inverting output terminal and a non-inverting output terminal, the output signal at the non-inverting output terminal being applied to both the first and second capacitor at start-up.

### Claim 8

A circuit, comprising:
a first series connection of a first transistor and a first resistance coupled between a supply node and a reference node;
a second series connection of a second transistor and a second resistance coupled between the supply node and the reference node;
an operational amplifier having a first output node and a second output node;
a first switch configured to selectively couple the first output node to a first control terminal of the first transistor;
a second switch configured to selectively couple the second output node to a second control terminal of the second transistor; and
a clock signal configured to alternately actuate the first and second switches.

### Claim 9 (depends on 8)

The circuit of claim 8, further comprising a load circuit coupled between the supply node and the first and second series connections.

### Claim 10 (depends on 9)

The circuit of claim 9, wherein the load circuit comprises a plurality of series connected LEDs.

### Claim 11 (depends on 8)

The circuit of claim 8, wherein the operational amplifier comprises a first input and a second input, the circuit further comprising:
a third switch configured to selectively couple a voltage across the first resistor to the first input of the operational amplifier; and
a fourth switch configured to selectively couple a voltage across the second resistor to the second input of the operational amplifier;
wherein the clock signal is further configured to alternately actuate the third and fourth switches.

### Claim 12 (depends on 11)

The circuit of claim 11, further comprising an input node configured to receive a reference voltage, the circuit further comprising:
a fifth switch configured to selectively couple the reference to the first input of the operational amplifier; and
a sixth switch configured to selectively couple the reference voltage to the second input of the operational amplifier;
wherein the clock signal is further configured to alternately actuate the fifth and sixth switches.

### Claim 13 (depends on 8)

The circuit of claim 8, wherein the operational amplifier comprises a first input and a second input, the circuit further comprising:
a first start-up switch configured to selectively couple the first input to the second input; and
a second start-up switch configured to selectively couple the first control terminal of the first transistor to the second control terminal of the second transistor.

### Claim 14 (depends on 13)

The circuit of claim 13, further comprising a circuit configured to control said first switch and second switch to couple the first and second output nodes of the operational amplifier simultaneously to both the first control terminal of the first transistor and the second control terminal of the second transistor during start-up.

### Claim 15

A current driver device for an array of LED diodes, said array comprising a plurality of channels, each channel comprising a plurality of series connected LEDs, said current driver device comprising, for each channel, a current driver circuit, each current driver circuit comprising:
a first series connection of a first transistor and a first resistance,
a second series connection of a second transistor and a second resistance (Rs2),
said first and second series connections being arranged in parallel to each other along an electric path between one of said channels and a voltage reference node,
an operational amplifier, and
a switch device driven by a clock signal and able to apply at a non-inverting input terminal and inverting input terminal of the operational amplifier alternately a reference voltage and a voltage across the first resistance or the voltage across the second resistance according to a half-period of the clock signal,
said switch device configured to apply an output signal of the operational amplifier alternately to the first and second transistor according to the half-period of the clock signal,
a storage element arranged to maintain turned on the first and second transistors when not driven by the output signal of the operational amplifier