LED dimming control circuit

Abstract

A LED dimming control circuit that avoids a higher output voltage than expected is provided. The LED dimming control circuit comprises an inductor, a transistor, a dimming control signal, a feedback signal, a switching control circuit, an error amplifier, and an inverted amplifier. The inductor and the transistor are coupled to a node. The switching control circuit is controlled by the dimming control signal and the feedback signal. The transistor is controlled by the switching control circuit. The output terminal of the error amplifier and the non-inverted input terminal of the error amplifier form a negative feedback path via the inverted amplifier.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a dimming control circuit. More particularly, the present invention relates to a LED dimming control circuit.

2. Description of the Related Art

FIG. 1 is a circuit diagram showing a conventional LED dimming control circuit 100. In the example shown in FIG. 1, the LED dimming control circuit 100 is implemented by a boost-type voltage converter which converts an input voltage V_in1 into an output voltage V_o1, so as to drive a LED 19. A switching control circuit 10 comprises a capacitor C₁₁, a resistor R₁₁, a comparator 13, an oscillating circuit 14, a current-to-voltage converter 15, a latch 16, a current sensing circuit 17, and a driving circuit 18. The connection point of the resistor R₁₂ and the cathode terminal of the LED 19 is used to provide a feedback signal V_fb1. The switching control circuit 10 is used to control the transistor T1 in response to a dimming control signal CN1, the feedback signal V_fb1, and the current of the inductor L1. The user can adjust the brightness of the LED 19 by changing the dimming control signal CN1. The error amplifier 12 compares the reference voltage V_r1 and the feedback signal V_fb1 so as to generate an output current I_eo1 to the switching control circuit 10. When the dimming control signal CN1 is at a high level, the LED dimming control circuit 100 operates in a normal boost-type mode. The peak current value of the inductor L1 is controlled by the output current I_eo1. When the dimming control signal CN1 is at a low level, the LED dimming control circuit 100 stops switching. The decrease of the output voltage V_o1 results in the decrease of the feedback signal V_fb1 and the increase of the output current I_eo1. When the dimming control signal CN1 changes from the low level to the high level, the inrush current of the inductor L1 will be large, resulting in a higher output voltage V_o1 than expected.

SUMMARY OF THE INVENTION

In view of the above-mentioned problems, an object of the present invention is to provide a LED dimming control circuit, capable of reducing the inrush current of the inductor and regulating an expected output voltage.

According to the present invention, the LED dimming control circuit comprises a soft start circuit, an error amplifier, an inverted amplifier, a switching control circuit, an inductor, a transistor, and a diode. The switching control circuit is used to control the transistor in response to a dimming control signal, a feedback signal, and the inductor current. When the dimming control signal is at a high level, the LED dimming control circuit operates in a normal current mode. When the dimming control signal is at a low level, the LED dimming control circuit stops switching, resulting in the decrease of the feedback signal. Since the output terminal of the error amplifier and the non-inverted input terminal of the error amplifier form a negative feedback path via the inverted amplifier, the voltage of the non-inverted input terminal of the error amplifier follows the feedback signal to be decreased accordingly. When the dimming control signal changes from the low level to the high level, the feedback signal will follow the voltage of the non-inverted input terminal of the error amplifier to be increased gradually, resulting that the output current of the error amplifier will not be too large. Therefore, the inrush current of the inductor will not be too large, resulting that a higher output voltage than expected can be avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other objects, features, and advantages of the present invention will become apparent with reference to the following descriptions and accompanying drawings, wherein:

FIG. 1 is a circuit diagram showing a conventional LED dimming control circuit;

FIG. 2 is a circuit diagram showing a LED dimming control circuit according to the present invention;

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment according to the present invention will be described in detail with reference to the drawing.

FIG. 2 is a circuit diagram showing a LED dimming control circuit 200 according to the present invention. In the example shown in FIG. 2, the LED dimming control circuit 200 is implemented by a boost-type voltage converter which converts an input voltage V_in into an output voltage V_o, so as to drive a LED 29. The LED dimming control circuit 200 comprises a soft start circuit 21, an error amplifier 22, switches S1 to S4, an inverter 30, an inverted amplifier 31, a switching control circuit 20, an inductor L, a transistor T, a diode D, a resistor R₂₂, and an output capacitor C_o. The inductor L, the transistor T, and the diode D are commonly coupled to a node N. The anode terminal of the LED 29, the diode D, and the output capacitor C_o are commonly coupled to a node O. The connection point of the resistor R₂₂ and the cathode terminal of the LED 29 is used to provide a feedback signal V_fb, where the feedback signal V_fb is coupled to the inverted input terminal of the error amplifier 22. The switching control circuit 20 is used to control the transistor T in response to a dimming control signal CN, the feedback signal V_fb, and the current of the inductor L. The user can adjust the brightness of the LED 29 by changing the dimming control signal CN. In addition, the switching control circuit 20 further comprises a capacitor C₂₁, a resistor R₂₁, a comparator 23, an oscillating circuit 24, a current-to-voltage converter 25, a latch 26, a current sensing circuit 27, and a driving circuit 28. Switches S1 and S2 are controlled by the signal CI, which is the inverted signal of the dimming control signal CN. Switches S3 and S4 are controlled by the dimming control signal CN. The switch S1 is coupled between the output terminal of the error amplifier 22 and the input terminal of the inverted amplifier 31. The switch S2 is coupled between the non-inverted input terminal of the error amplifier 22 and the output terminal of the inverted amplifier 31. The switch S3 is coupled between the output terminal of the soft start circuit 21 and the non-inverted input terminal of the error amplifier 22. The switch S4 is coupled between the output terminal of the error amplifier 22 and the inverted input terminal of the comparator 23.

When the dimming control signal CN is at a high level, switches S1 and S2 are turned OFF and switches S3 and S4 are turned ON. The LED dimming control circuit 200 operates in a normal current mode. The driving circuit 28 is used to control the transistor T based on the output signal of the latch 26. The error amplifier 22 compares the voltage V_p of the non-inverted input terminal and the feedback signal V_fb so as to generate an output current to charge the capacitor C₂₁, where the resistor R₂₁ and the capacitor C₂₁ form a compensation circuit. The oscillating circuit 24 applies a pulse signal with a fixed frequency to the set terminal of the latch 26 so as to make the transistor T conductive. Once the transistor T begins to conduct, the current of the inductor L will be increased accordingly. Furthermore, the current sensing circuit 27 generates a current I_S to the current-to-voltage converter 25, where the current I_S is proportional to the current of the inductor L. The current-to-voltage converter 25 receives the current I_S to generate an output voltage V_S to the non-inverted input terminal of the comparator 23. When the output voltage VS exceeds the voltage V_n of the inverted input terminal of the comparator 23, the comparator 23 will trigger the reset terminal of the latch 26 to make the transistor T nonconductive.

When the dimming control signal CN is at a low level, the LED dimming control circuit 200 stops switching. Therefore, the output voltage V_o decreases, resulting in the decrease of the feedback signal V_fb. At this moment switches S1 and S2 are turned ON and switches S3 and S4 are turned OFF. The voltage V_p is no longer controlled by the soft start circuit 31. Since the output terminal of the error amplifier 22 and the non-inverted input terminal of the error amplifier 22 form a unit-gain negative feedback path via the inverted amplifier 31, the voltage V_p will follow the feedback signal V_fb to be decreased accordingly.

When the dimming control signal CN changes from the low level to the high level, the reference voltage V_r outputted from the soft start circuit 21 increases gradually. When the reference voltage V_r increases to a pre-determined voltage V_A, the reference voltage V_r will be kept at the pre-determined voltage V_A hereafter. Since the switch S3 is turned ON, the voltage V_p is equal to the reference voltage V_r. Under such circumstance the LED dimming control circuit 200 operates in a normal current mode. Therefore, the feedback signal V_fb will follow the voltage V_p to be increased gradually, resulting that the output current of the error amplifier 22 will not be too large. So, the inrush current of the inductor L will not be too large, resulting that a higher output voltage V_o than expected can be avoided.

While the invention has been described by a preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications.

Claims

1. A dimming control circuit used to drive a LED comprising: an inductor, wherein a first current flows through the inductor;a transistor, wherein the inductor and the transistor are coupled to a node;a dimming control signal;a feedback signal;a switching control circuit, wherein the switching control circuit is controlled by the dimming control signal and the feedback signal, and the transistor is controlled by the switching control circuit;an error amplifier comprising a first non-inverted input terminal, a first inverted input terminal, and a first output terminal; andan inverted amplifier comprising a second input terminal and a second output terminal, wherein:the first output terminal and the first non-inverted input terminal form a negative feedback path via the inverted amplifier.

2. The dimming control circuit of claim 1 further comprising a diode, wherein a terminal of the diode is coupled to the node.

3. The dimming control circuit of claim 1 further comprising: a first switch coupled between the first output terminal and the second input terminal; anda second switch coupled between the first non-inverted input terminal and the second output terminal.

4. The dimming control circuit of claim 3, wherein the first switch and the second switch are controlled by an inverted signal of the dimming control signal.

5. The dimming control circuit of claim 4 further comprising: a third switch; anda fourth switch, wherein:the third switch and the fourth switch are controlled by the dimming control signal.

6. The dimming control circuit of claim 5, wherein the switching control circuit comprising: a current sensing circuit used to provide a second current, the second current being proportional to the first current;a comparator comprising a third non-inverted input terminal, a third inverted input terminal, and a third output terminal, wherein the third inverted input terminal is coupled to a compensation circuit;a current-to-voltage converter receiving the second current so as to generate a first voltage to the third non-inverted input terminal;an oscillating circuit comprising a fourth output terminal;a latch comprising a reset terminal, a set terminal, and a first output signal, wherein the reset terminal is coupled to the third output terminal and the set terminal is coupled to the fourth output terminal; anda driving circuit used to control the transistor based on the dimming control signal and the first output signal.

7. The dimming control circuit of claim 1, wherein the feedback signal is coupled to a cathode terminal of the LED.

8. The dimming control circuit of claim 6 further comprising a soft start circuit, wherein the soft start circuit is used to generate a reference voltage based on a pre-determined voltage, and the soft start circuit comprises a fifth output terminal, wherein the voltage of the fifth output terminal is equal to the reference voltage.

9. The dimming control circuit of claim 8, wherein the third switch are coupled between the fifth output terminal and the first non-inverted input terminal, and the fourth switch are coupled between the first output terminal and the third inverted input terminal.

10. The dimming control circuit of claim 2, wherein an anode terminal of the LED is coupled to the diode.

11. The dimming control circuit of claim 1, wherein the feedback signal is coupled to the first inverted input terminal.

12. The dimming control circuit of claim 1, wherein the switching control circuit is further controlled by the first current.