# US 8,569,975 B2 · Control circuit for switching power supply

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- Number: US 8,569,975 B2
- Document ID: US8569975B2
- Status: Granted
- Kind: B2
- Application number: 13/089,694
- Filed: 2011-04-19
- Published: 2011-11-10
- Granted: 2013-10-29
- Earliest claimed: 2010-04-23
- Assignees: Rohm Co., Ltd.
- Applicants: Daisuke Uchimoto; Naoki Inoue; Yoju Imamura
- Inventors: Daisuke Uchimoto; Naoki Inoue; Yoju Imamura
- Web page: https://patentgrep.com/patent/US8569975B2

## CPC

- H02M 3/156: with automatic control of output voltage or current, e.g. switching regulators
- G09G 3/3406: {Control of illumination source (illumination devices structurally associated with liquid crystal cells G02F1/1336)}
- H05B 45/38: using boost topology
- H05B 45/44: with an active control inside an LED matrix
- G09G 2320/064: by time modulation of the brightness of the illumination source
- Y02B 20/30: Semiconductor lamps, e.g. solid state lamps [SSL] light emitting diodes [LED] or organic LED [OLED]
- 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)
- G09G: ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION (arrangements for transferring data between digital computers and displays G06F3/14; static indicating arrangements comprising an association of a number of separate sources or light control cells G09F9/00; static indicating arrangements comprising integral associations of a number of light sources H01J, H01K, H10H, H05B33/12; scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission, details thereof H04N1/00)
- H05B: ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
- Y02B: CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS

## Abstract

A control IC controls a switching power supply configured to supply a driving voltage Vout to one terminal of an LED string which is driven in an intermittent manner. A sample-and-hold circuit performs sampling of a detection voltage Vs that corresponds to a driving voltage Vout in the on period, and holds the sampled detection voltage Vs in the off period. In the off period, a pulse modulator generates a pulse signal having a duty ratio adjusted such that a hold detection voltage VsH output from the sample-and-hold circuit matches the detection voltage Vs. A driver drives a switching transistor according to the pulse signal.

## Claims

### Claim 1

A control circuit for a switching power supply configured to supply a driving voltage to one terminal of a light emitting element configured to be intermittently driven, the control circuit comprising:
a sample-and-hold circuit configured to (i) sample a detection voltage that corresponds to the driving voltage generated in an on period of the light emitting element, and (ii) to hold the detection voltage thus sampled and to output a hold detection voltage that corresponds to the detection voltage in an off period of the light emitting element, the hold detection voltage being higher than the detection voltage;
a pulse modulator configured to generate a pulse signal having a duty ratio adjusted in an off period of the light emitting element such that the detection voltage matches a hold detection voltage output from the sample-and-hold circuit; and
a driver configured to drive a switching element of the switching power supply according to the pulse signal.

### Claim 2 (depends on 1)

A control circuit according to claim 1, wherein the sample-and-hold circuit comprises:
a capacitor arranged such that one terminal thereof is set to a fixed electric potential in the on period;
a buffer configured to generate a voltage that corresponds to the detection voltage; and
a switch arranged between the other terminal of the capacitor and an output terminal of the buffer, and configured to switch on in the on period of the light emitting element, and to switch off in the off period thereof,
and wherein the voltage that occurs at the capacitor is output as the hold detection voltage.

### Claim 3 (depends on 1)

A control circuit according to claim 1, wherein the pulse modulator is configured to adjust the duty ratio of the pulse signal in the on period of the light emitting element such that a voltage that occurs at the other terminal of the light emitting element matches a predetermined reference voltage.

### Claim 4 (depends on 2)

A control circuit according to claim 2, wherein the pulse modulator is configured to adjust the duty ratio of the pulse signal in the on period of the light emitting element such that a voltage that occurs at the other terminal of the light emitting element matches a predetermined reference voltage.

### Claim 5 (depends on 3)

A control circuit according to claim 3, comprising:
an error amplifier arranged such that the voltage that occurs at the other terminal of the light emitting element is input to a first input terminal thereof, the detection voltage is input to a second input terminal thereof, and a third input terminal thereof selectively receives, as an input signal, the reference voltage in the on period and the hold detection voltage in the off period, and which is configured to output an error signal that corresponds to the difference between the reference voltage and the voltage that occurs at the other terminal of the light emitting element in the on period, and to output an error signal that corresponds to the difference between the detection voltage and the hold detection voltage in the off period;
a pulse width modulator configured to generate, in the on period, a pulse signal having a duty ratio that corresponds to the error signal; and
a pulse frequency modulator configured to generate, in the off period, a pulse signal having a duty ratio that corresponds to the error signal.

### Claim 6 (depends on 4)

A control circuit according to claim 4, comprising:
an error amplifier arranged such that the voltage that occurs at the other terminal of the light emitting element is input to a first input terminal thereof, the detection voltage is input to a second input terminal thereof, and a third input terminal thereof selectively receives, as an input signal, the reference voltage in the on period and the hold detection voltage in the off period, and which is configured to output an error signal that corresponds to the difference between the reference voltage and the voltage that occurs at the other terminal of the light emitting element in the on period, and to output an error signal that corresponds to the difference between the detection voltage and the hold detection voltage in the off period;
a pulse width modulator configured to generate, in the on period, a pulse signal having a duty ratio that corresponds to the error signal; and
a pulse frequency modulator configured to generate, in the off period, a pulse signal having a duty ratio that corresponds to the error signal.

### Claim 7 (depends on 5)

A control circuit according to claim 5, wherein the pulse modulator further comprises:
a feedback terminal via which a resistor and a capacitor, which are used for phase compensation, are connected;
a switch configured to receive the error signal, and to output the error signal thus received to the feedback terminal in the on period, and to output the error signal thus received to the pulse frequency modulator in the off period,
and wherein the pulse width modulator is configured to generate the pulse signal according to a voltage at the feedback terminal.

### Claim 8 (depends on 6)

A control circuit according to claim 6, wherein the pulse modulator further comprises:
a feedback terminal via which a resistor and a capacitor, which are used for phase compensation, are connected;
a switch configured to receive the error signal, and to output the error signal thus received to the feedback terminal in the on period, and to output the error signal thus received to the pulse frequency modulator in the off period,
and wherein the pulse width modulator is configured to generate the pulse signal according to a voltage at the feedback terminal.

### Claim 9

A light emitting apparatus comprising:
a light emitting element;
a switching power supply configured to supply a driving voltage to one terminal of the light emitting element; and
a current driving circuit connected to the other terminal of the light emitting element, and configured to supply an intermittent driving current that corresponds to a target luminance,
wherein the switching power supply comprises:
an output circuit comprising a switching element; and
a control circuit configured to drive the switching element, the control circuit comprising:
a sample-and-hold circuit configured to (i) sample a detection voltage that corresponds to the driving voltage generated in an on period of the light emitting element, and (ii) to hold the detection voltage thus sampled and to output a hold detection voltage that corresponds to the detection voltage in an off period of the light emitting element, the hold detection voltage being higher than the detection voltage;
a pulse modulator configured to generate a pulse signal having a duty ratio adjusted in an off period of the light emitting element such that the detection voltage matches a hold detection voltage output from the sample-and-hold circuit; and
a driver configured to drive a switching element of the switching power supply according to the pulse signal.

### Claim 10

A light emitting apparatus comprising:
a plurality of light emitting elements;
a switching power supply configured to supply an independent driving voltage to one terminal of each of the plurality of light emitting elements; and
a current driving circuit configured to supply, to each of the plurality of light emitting elements, an intermittent driving current that corresponds to a target luminance,
wherein the switching power supply comprises:
an output circuit comprising a switching element; and
a control circuit configured to drive the switching elements,
and wherein the control circuit comprises:
a sample-and-hold circuit configured to (i) sample a detection voltage that corresponds to the driving voltage generated in an on period of the light emitting element, and (ii) to hold the detection voltage thus sampled and to output a hold detection voltage that corresponds to the detection voltage in an off period of the light emitting element, the hold detection voltage being higher than the detection voltage;
a pulse modulator configured to generate a pulse signal having a duty ratio adjusted in an off period of the light emitting element such that the detection voltage matches a hold detection voltage output from the sample-and-hold circuit; and
a driver configured to drive a switching element of the switching power supply according to the pulse signal,
and wherein the output circuit comprises:
an input terminal via which an input voltage is to be applied;
a plurality of output terminals via which the driving voltages are respectively output to the respective multiple light emitting elements;
an inductor and a switching element sequentially connected in series between the input terminal and a fixed voltage terminal;
a plurality of rectifier elements respectively provided to the multiple output terminals, and arranged such that one terminal of each of the rectifier elements is connected to a corresponding output terminal, and the other terminal of each of the rectifier elements is connected to a connection node that connects the inductor and the switching element; and
a plurality of output capacitors respectively provided to the multiple output terminals, and each arranged between a corresponding output terminal and the fixed voltage terminal.

### Claim 11

An electronic device comprising:
a liquid crystal panel; and
a light emitting apparatus arranged as a backlight of the liquid crystal panel, the light emitting apparatus comprising:
a light emitting element;
a switching power supply configured to supply a driving voltage to one terminal of the light emitting element; and
a current driving circuit connected to the other terminal of the light emitting element, and configured to supply an intermittent driving current that corresponds to a target luminance,
wherein the switching power supply comprises:
an output circuit comprising a switching element; and
a control circuit configured to drive the switching element, the control circuit comprising:
a sample-and-hold circuit configured to (i) sample a detection voltage that corresponds to the driving voltage generated in an on period of the light emitting element, and (ii) to hold the detection voltage thus sampled and to output a hold detection voltage that corresponds to the detection voltage in an off period of the light emitting element, the hold detection voltage being higher than the detection voltage;
a pulse modulator configured to generate a pulse signal having a duty ratio adjusted in an off period of the light emitting element such that the detection voltage matches a hold detection voltage output from the sample-and-hold circuit; and
a driver configured to drive a switching element of the switching power supply according to the pulse signal.

### Claim 12

An electronic device comprising:
a liquid crystal panel; and
a light emitting apparatus arranged as a backlight of the liquid crystal panel, the light emitting apparatus comprising:
a plurality of light emitting elements;
a switching power supply configured to supply an independent driving voltage to one terminal of each of the plurality of light emitting elements; and
a current driving circuit configured to supply, to each of the plurality of light emitting elements, an intermittent driving current that corresponds to a target luminance,
wherein the switching power supply comprises:
an output circuit comprising a switching element; and
a control circuit configured to drive the switching elements,
and wherein the control circuit comprises:
a sample-and-hold circuit configured to (i) sample a detection voltage that corresponds to the driving voltage generated in an on period of the light emitting element, and (ii) to hold the detection voltage thus sampled and to output a hold detection voltage that corresponds to the detection voltage in an off period of the light emitting element, the hold detection voltage being higher than the detection voltage;
a pulse modulator configured to generate a pulse signal having a duty ratio adjusted in an off period of the light emitting element such that the detection voltage matches a hold detection voltage output from the sample-and-hold circuit; and
a driver configured to drive a switching element of the switching power supply according to the pulse signal, and wherein the output circuit comprises:
an input terminal via which an input voltage is to be applied;
a plurality of output terminals via which the driving voltages are respectively output to the respective multiple light emitting elements;
an inductor and a switching element sequentially connected in series between the input terminal and a fixed voltage terminal;
a plurality of rectifier elements respectively provided to the multiple output terminals, and arranged such that one terminal of each of the rectifier elements is connected to a corresponding output terminal, and the other terminal of each of the rectifier elements is connected to a connection node that connects the inductor and the switching element; and
a plurality of output capacitors respectively provided to the multiple output terminals, and each arranged between a corresponding output terminal and the fixed voltage terminal.