Light emitting element driving apparatus and driving method thereof

Patent metadata

Field	Value
Publication number	US-2018317292-A1
Application number	US-201815956911-A
Country	US
Kind code	A1
Filing date	Apr 19, 2018
Priority date	Apr 26, 2017
Publication date	Nov 1, 2018
Grant date	—

Abstract

A light emitting element driving apparatus includes the following: current source; error amplifier generating error signal based on comparison of a first reference voltage with the highest voltage among total forward drop voltages of element arrays or feedback voltage corresponding to a drive voltage; first and second switches for ON/OFF control to apply the feedback voltage to first and second input terminals of the error amplifier respectively; slope voltage generating circuit; PWM comparator generating a pulse modulation signal; drive amplifier controlled by the pulse modulation signal and drives a switching element supplying the drive voltage to the element arrays; PWM width detector that outputs a pulse width detection signal having different levels between when the pulse width is less than predetermined threshold value and when equal to or greater than the threshold value, and switches ON/OFF of the first and second switches based on the pulse width detection signal.

First claim

What is claimed is:

1 . A light emitting element driving apparatus, comprising:
a current source configured to supply a current to each of a plurality of element arrays in which a plurality of light emitting elements are connected in series; an error amplifier configured to generate an error signal based on a comparison of a first reference voltage with one of the highest voltage among total forward drop voltages of the plurality of element arrays, and a feedback voltage corresponding to a drive voltage supplied to the plurality of element arrays; a first switch configured to perform an ON/OFF control to apply the feedback voltage to a first input terminal of the error amplifier or not to apply the feedback voltage to the first input terminal; a second switch configured to perform an ON/OFF control to apply the highest voltage among the total forward drop voltages to a second input terminal of the error amplifier or not to apply the highest voltage to the second input terminal; a slope voltage generating circuit configured to generate a slope voltage; a PWM comparator configured to generate a pulse modulation signal having a duty ratio based on the slope voltage and the error signal; a drive amplifier that is controlled by the pulse modulation signal output from the PWM comparator and drives a switching element configured to supply the drive voltage to the plurality of element arrays; and a PWM width detector configured to detect a pulse width of the pulse modulation signal, wherein the PWM width detector outputs a pulse width detection signal having different levels between when the pulse width of the pulse modulation signal is less than a predetermined threshold value and when the pulse width is equal to or greater than the threshold value, and switches ON/OFF of the first switch and the second switch based on the pulse width detection signal.

2 . The apparatus of claim 1 , wherein when the pulse width of the pulse modulation signal is less than the threshold value, the first switch and the second switch are turned on and turned off, respectively, and when the pulse width of the pulse modulation signal is equal to or greater than the threshold value, the first switch and the second switch are turned off and turn on, respectively.

3 . The apparatus of claim 1 , wherein a DC potential of the slope voltage generated by the slope voltage generating circuit is switched depending on whether the pulse width of the pulse modulation signal is less than the threshold value or the pulse width is equal to or greater than the threshold value.

4 . The apparatus of claim 3 , wherein the slope voltage generating circuit is controlled by a slope offset adjuster that is turned on or off by the pulse width detection signal, and
wherein the DC potential of the slope voltage is adjusted depending on whether the slope offset adjuster is turned on or turned off

5 . The apparatus of claim 4 , wherein when the pulse width of the pulse modulation signal is less than the threshold value, the DC potential of the slope voltage is controlled to be higher than the DC potential when the pulse width of the pulse modulation signal is equal to or greater than the threshold value.

6 . The apparatus of claim 4 , wherein the slope offset adjuster comprises an operational amplifier, a transistor, a first resistor, and a current mirror circuit,
wherein a voltage source is connected to a non-inverting input terminal of the operational amplifier, wherein a control electrode of the transistor is connected to an output terminal of the operational amplifier, wherein an inverting input terminal of the operational amplifier is commonly connected to a first main electrode of the transistor, wherein the first resistor is connected between the first main electrode of the transistor and a ground potential, wherein an input stage of the current mirror circuit is connected to a second main electrode of the transistor, wherein an output stage of the current mirror circuit is connected to one end of a second resistor, wherein the other end of the second resistor is coupled to the slope voltage, and wherein the DC potential of the slope voltage is dependent on a magnitude of a current flowing between the first main electrode and the second main electrode of the transistor.

7 . The apparatus of claim 4 , wherein the error amplifier, the PWM comparator, and the slope offset adjuster are incorporated in a single semiconductor integrated circuit (IC), and
wherein an output of the error amplifier is connected to one input terminal of the PWM comparator and led out to a first external terminal of the semiconductor integrated circuit.

8 . The apparatus of claim 7 , wherein a series connection body of a resistor and a capacitor, which are formed of separate parts, is connected between the first external terminal and a ground potential.

9 . The apparatus of claim 3 , further comprising:
a first comparator configured to output, as a first comparison output signal, a comparison of the highest voltage among the total forward drop voltages of the plurality of element arrays with a second reference voltage; a second comparator configured to output, as a second comparison output signal, a comparison of the highest voltage with a third reference voltage; and a logic part configured to respectively generate a first control signal and a second control signal for controlling ON/OFF of the switching element via the drive amplifier based on the first comparison output signal and the second comparison output signal.

10 . The apparatus of claim 9 , wherein the first control signal is enabled when the highest voltage among the total forward drop voltages of the plurality of element arrays exceeds the second reference voltage for the first time after the pulse width of the pulse modulation signal detected by the PWM width detector exceeds the threshold,
wherein the second control signal is enabled each time the highest voltage exceeds the third reference voltage, and wherein the switching of the switching element being allowed when both the first control signal and the second control signal are enabled.

11 . The apparatus of claim 10 , wherein a relationship of VREF 1 <VREF 2 <VREF 3 is established among the first reference voltage, the second reference voltage, and the third reference voltage, wherein the first reference voltage is VREF 1 , the second reference voltage is VREF 2 and the third reference voltage is VREF 3 .

12 . The apparatus of claim 1 , wherein the current source is of a sink current type provided between cathode sides of the element arrays and a ground potential.

13 . The apparatus of claim 1 , wherein the current source is of a source current type that supplies a current from the drive voltage side to anode sides of the element arrays.

14 . The apparatus of claim 6 , wherein the current mirror circuit is in an ON state when the pulse width of the pulse modulation signal is less than the threshold value, and is in an OFF state when the pulse width of the pulse modulation signal is equal to or greater than the threshold value.

15 . A method of driving the light emitting element driving apparatus of claim 9 , wherein when the pulse width of the pulse modulation signal is less than the threshold value, a first feedback control mode in which the feedback voltage and the first reference voltage are compared is set and the drive voltage is controlled to be a predetermined magnitude, and
wherein when the pulse width of the pulse modulation signal is equal to or greater than the threshold value, a second feedback control mode i

Assignees

• Rohm Co Ltd

Inventors

• Katsura Koji
• Kanemitsu Ryosuke
• Miura Keisuke

Classifications

• H05B45/38

  using boost topology · CPC title

• H05B45/46

  having LEDs disposed in parallel lines · CPC title

• H05B33/0821 Primary

  Electricity · mapped topic

• H05B33/0818

  Electricity · mapped topic

• H05B45/397

  Current mirror circuits · CPC title