TWI434614B - Control circuit and method for led drivers - Google Patents

Description

Control circuit and method applied to LED driver

The present invention relates to a light emitting diode (LED) driver, and more particularly to a control circuit and method for use in an LED driver.

LED drivers are one of the applications of switched power converters. In battery-powered systems, such as LED flashlights, the input current is detected and controlled so that the lower the battery voltage, the lower the input current, so that even when the battery is almost exhausted, the LED can be illuminated, thus maximizing Battery usage time. However, this method has two disadvantages: First, the brightness of the LED is proportional to the output current, not the input current, so the control input current cannot accurately control the brightness of the LED. Second, in the boost system, the input current is usually greater than the output current. Therefore, the efficiency is poor, and considerable power is wasted on the sensing resistor.

For example, referring to the boosting structure LED driver 10 of FIG. 1, the transistor Q is switched as a power switch by the controller 12, and the sensing resistor R is connected in series with the transistor Q to detect the input current Iin, and the voltage drop VR of the sensing resistor R is fed back to The controller 12, once the voltage VR is greater than the reference voltage, is turned off for a fixed period of time to release the energy stored in the inductor L. In this method, the peak value of the input current Iin is controlled. When the battery voltage Vin drops, the reference voltage drops at a fixed slope, so the peak value of the input current Iin decreases. Therefore, the lower the battery voltage Vin, the lower the input current Iin. Although this method can maximize the battery life, but the non-output current Io is controlled, its size is difficult to determine, and will vary greatly with external components, resulting in mass production troubles. Also, since the power consumption of the sensing resistor R is large, it is difficult to improve the efficiency.

One of the objects of the present invention is to provide a control circuit and method for use in an LED driver.

One of the objects of the present invention is to provide a control circuit and method for improving the efficiency of an LED driver.

One of the objects of the present invention is to provide a control circuit and method for a battery powered system having a long battery life.

In accordance with the present invention, the LED driver senses the input voltage and the output current is controlled. The output current is detected to generate a feedback voltage, and the feedback voltage or the reference voltage is adjusted according to the input voltage, and an error signal is generated from a difference between the feedback voltage and the reference voltage to control the output current.

Referring to the boosting structure LED driver 14 of FIG. 2, the feedback circuit 16 is connected to the LED to feedback the current Io of the LED to the control circuit 18, and the control circuit 18 detects the input voltage Vin and generates an error according to the feedback voltage Vfb. The signal Sc, the pulse width modulation (PWM) comparator 20 compares the error signal Sc and the sawtooth wave signal Sr to generate a pulse width modulation signal Spwm, and the flip-flop 22 generates a driving signal Sd according to the signal Spwm and the pulse CLK to control the power as the power switch. The crystal M is used to adjust the output current Io supplied to the LED. In this embodiment, the feedback circuit 16 includes a sense resistor R series LED to sense the output current Io, and the feedback voltage Vfb is the voltage drop of the sense resistor R. In the control circuit 18, the voltage source Vref supplies a reference voltage Vref to the reference voltage regulator 24, and the reference voltage regulator 24 detects the input voltage Vin and adjusts the reference voltage to Vrefo=f(Vin), which is in accordance with the input voltage Vin. Falling down, the error amplifier 26 generates an error signal Sc based on the difference between the feedback voltage Vfb and the reference voltage Vrefo. Since the negative feedback loop forces the feedback voltage Vfb to be equal to the reference voltage Vrefo, the output current Io will also decrease as the reference voltage Vrefo decreases. In a battery-powered system, when the battery voltage Vin drops, the output current Io also drops, thus extending battery life.

3 is an embodiment of the reference voltage regulator 24 of FIG. 2, wherein the voltage source Vini provides a reference voltage Vini to the negative input of the operational amplifier 28, and the resistor Rin is coupled to the power input terminal Vin and the positive input terminal of the operational amplifier 28. Because of the virtual short circuit, the operational amplifier 28 reflects the reference voltage Vini to its positive input terminal, so the current of the resistor Rin Ivin=(Vin-Vini)/Rin, and the current Ivin is sent to the arithmetic circuit 30 and the reference current Iref. The operation, for example, adding, subtracting, multiplying, dividing, etc., generates a current Im=f(-Ivin), the resistor Rs is connected between the voltage source Vref and the output terminal Iout of the arithmetic circuit 30, and the current Im generates the adjustment voltage VRs through the resistor Rs. The reference voltage Vref is subtracted from the adjustment voltage VRs to obtain a reference voltage Vrefo=Vref-Im×Rs. When the input voltage Vin falls, the current Im rises and the adjustment voltage VRs becomes large, so the reference voltage Vrefo falls.

Since the positive input decrement of the error amplifier is equivalent to the negative input increment, the embodiment of FIG. 2 can also be changed to the embodiment of FIG. 4 to feedback the voltage regulator 32 to detect the input voltage Vin and adjust the feedback voltage accordingly. Vfbo = f (Vin), which rises as the input voltage Vin decreases. Since the negative feedback loop forces the feedback voltage Vfbo to be equal to the reference voltage Vref, when the feedback voltage Vfbo rises, the output current Io will decrease, that is, the output current Io will decrease as the input voltage Vin decreases, so the battery is In a powered system, battery life can be extended.

5 is an embodiment of the feedback voltage regulator 32 of FIG. 4, which is the same as the circuit of FIG. 3, but the resistor Rs is connected between the output terminal Iout of the arithmetic circuit 30 and the feedback circuit 16, and the voltage is fed back. The Vfb plus the adjustment voltage VRs obtains the feedback voltage Vfbo=Vfb+Im×Rs. When the current Im = f (-Ivin), when the input voltage Vin falls, the current Im rises and the adjustment voltage VRs increases, so the feedback voltage Vfbo rises.

The embodiment of FIG. 2 and FIG. 4 can also be combined. As shown in FIG. 6, the reference voltage regulator 24 and the feedback voltage regulator 32 respectively adjust the reference voltage Vref and the feedback voltage Vfb to become Vrefo=f(Vin) and Vfbo=. f (Vin), the reference voltage Vrefo decreases as the input voltage Vin decreases, the feedback voltage Vfbo rises as the input voltage Vin decreases, and the negative feedback loop forces the feedback voltage Vfbo to be equal to the reference voltage Vrefo, so when the feedback voltage Vfbo rises or When the reference voltage Vrefo drops, the output current Io will decrease.

In the embodiments of FIGS. 2, 4, and 6, the output current Io is smaller than the input current Iin, and the power consumption of the sense resistor R is low, and thus the efficiency is high. According to the LED driver of the present invention, the output current Io is detected and controlled, so that the brightness of the LED can be accurately controlled.

The above embodiments use specific power converters and circuits for convenience of description and are not limiting of the present invention. Referring to FIG. 7, the power stage 34 and the PWM circuit 36 of the power converter have various types and circuits, such as a buck architecture. And the low dropout regulator (Low DropOut; LDO), the error signal Sc provided to the PWM circuit 36 may be a current type. In this case, the error amplifier 26 may use a conduction amplifier to detect the LED current Io generation. A variety of different methods and circuits have been disclosed for the detector 38 that returns the voltage Vfb, such as from the output of the power stage 34. Those skilled in the art will appreciate that the present invention can be applied to a variety of different LED drivers from the teachings of the above embodiments, and various other embodiments can be devised using various circuits as desired.

10. . . Boost Structure LED Driver

12. . . Controller

14. . . Boost Structure LED Driver

16. . . Feedback circuit

18. . . Control circuit

20. . . PWM comparator

twenty two. . . Positive and negative

twenty four. . . Reference voltage regulator

26. . . Error amplifier

28. . . Operational Amplifier

30. . . Operation circuit

32. . . Feedback voltage regulator

34. . . Power level

36. . . PWM loop

38. . . Detector

Figure 1 is a conventional boost structure LED driver;

2 is a boost structure LED driver to which the first embodiment of the present invention is applied;

Figure 3 is an embodiment of the reference voltage regulator of Figure 2;

4 is a boost structure LED driver to which a second embodiment of the present invention is applied;

Figure 5 is an embodiment of the feedback voltage regulator of Figure 4;

Figure 6 is a boost structure LED driver to which a third embodiment of the present invention is applied;

Figure 7 is a fourth embodiment of the present invention.

14. . . Boost Structure LED Driver

16. . . Feedback circuit

18. . . Control circuit

20. . . PWM comparator

twenty two. . . Positive and negative

twenty four. . . Reference voltage regulator

26. . . Error amplifier

Claims (13)

A control circuit applied to an LED driver, the LED driver having a feedback circuit for detecting an output current thereof to generate a feedback voltage, the control circuit comprising: a reference voltage regulator connected to the power input end of the LED driver to detect an input voltage thereof The second reference voltage is generated by adjusting the first reference voltage; and the error amplifier is connected to the feedback circuit and the reference voltage regulator, and an error signal is generated according to the difference between the feedback voltage and the second reference voltage for controlling the output current. The reference voltage regulator includes: the first resistor determines a first current according to a difference between the input voltage and the third reference voltage; the operation circuit operates the first current and the reference current to generate a second current; And the second resistor is connected to the operation circuit, and the adjustment voltage is generated according to the second current, wherein the second reference voltage is equal to the first reference voltage minus the adjustment voltage. A control circuit as claimed in claim 1, wherein the error amplifier comprises a conduction amplifier. A control method applied to an LED driver, the LED driver having a feedback circuit for detecting an output current thereof to generate a feedback voltage, the control method comprising: (A) detecting an input voltage of the LED driver and adjusting the first reference voltage accordingly Generating a second reference voltage; and (B) generating an error based on a difference between the feedback voltage and the second reference voltage a signal for controlling the output current; wherein the step A includes: determining a first current according to a difference between the input voltage and the third reference voltage; calculating the first current and the reference current to generate a second current; The second current determines an adjustment voltage; and subtracting the adjustment voltage from the first reference voltage generates the second reference voltage. A control circuit for an LED driver, the LED driver having a feedback circuit for detecting an output current thereof to generate a first feedback voltage, the control circuit comprising: a feedback voltage regulator connected to the feedback circuit to detect the LED driver Inputting a voltage and adjusting the first feedback voltage to generate a second feedback voltage; and an error amplifier connected to the feedback voltage regulator, generating an error signal according to a difference between the second feedback voltage and the first reference voltage And controlling the output current; wherein the feedback voltage regulator comprises: the first resistor determines a first current according to a difference between the input voltage and the second reference voltage; and the operation circuit calculates the first current and the reference current And generating a second current; and the second resistor is connected to the operation circuit, and generating an adjustment voltage according to the second current, wherein the second feedback voltage is equal to the first feedback voltage Add this adjustment voltage. The control circuit of claim 4, wherein the error amplifier comprises a conduction amplifier. A control method applied to an LED driver, the LED driver having a feedback circuit for detecting an output current thereof to generate a first feedback voltage, the control method comprising: (A) detecting an input voltage of the LED driver and adjusting the first a feedback voltage generates a second feedback voltage; and (B) generating an error signal according to a difference between the second feedback voltage and the first reference voltage for controlling the output current; wherein the step A comprises: according to the The difference between the input voltage and the second reference voltage determines a first current; the first current and the reference current are operated to generate a second current; the adjustment voltage is determined according to the second current; and the first feedback voltage is The adjustment voltage is applied to generate the second feedback voltage. A control circuit applied to an LED driver, the LED driver having a feedback circuit for detecting an output current thereof to generate a first feedback voltage, the control circuit comprising: a reference voltage regulator connected to the power input end of the LED driver to detect an input thereof And adjusting a first reference voltage to generate a second reference voltage; a feedback voltage regulator is connected to the feedback circuit to detect the input voltage and Adjusting the first feedback voltage to generate a second feedback voltage; and the error amplifier is connected to the reference voltage regulator and the feedback voltage regulator, and generating an error signal according to a difference between the second feedback voltage and the second reference voltage For controlling the output current. The control circuit of claim 7, wherein the reference voltage regulator comprises: the first resistor determines a first current according to a difference between the input voltage and the third reference voltage; and the operation circuit calculates the first current and the reference current And generating a second current; and the second resistor is connected to the operation circuit, and generating an adjustment voltage according to the second current; wherein the second reference voltage is equal to the first reference voltage minus the adjustment voltage. The control circuit of claim 7, wherein the feedback voltage regulator comprises: the first resistor determines a first current according to a difference between the input voltage and the second reference voltage; and the operation circuit performs the first current and the reference current The operation is performed to generate a second current; and the second resistor is coupled to the operation circuit to generate an adjustment voltage according to the second current; wherein the second feedback voltage is equal to the first feedback voltage plus the adjustment voltage. The control circuit of claim 7, wherein the error amplifier comprises a conduction amplifier. A control method applied to an LED driver, the LED driver having a feedback circuit for detecting an output current thereof to generate a first feedback voltage, the control method comprising: (A) detecting an input voltage of the LED driver and adjusting the first The reference voltage generates a second reference voltage; (B) detecting the input voltage and adjusting the first feedback voltage to generate a second feedback voltage; and (C) according to the second feedback voltage and the second reference voltage The difference between the two produces an error signal for controlling the output current. The control method of claim 11, wherein the step A comprises: determining a first current according to a difference between the input voltage and the third reference voltage; and calculating the first current and the reference current to generate a second current; The second current determines an adjustment voltage; and subtracting the adjustment voltage from the first reference voltage generates the second reference voltage. The control method of claim 11, wherein the step B comprises: determining a first current according to a difference between the input voltage and the second reference voltage; and calculating the first current and the reference current to generate a second current; The second current determines an adjustment voltage; and adding the first feedback voltage to the adjustment voltage to generate the second feedback voltage.