LED driver and circuit for controlling a power switch to provide a driving voltage to at least one LED

This application claims the priority benefit of Taiwan application serial no. 97109122, filed on Mar. 14, 2008. The entirety the above-mentioned patent application is hereby incorporated by reference herein and made a part of specification.

1. Field of the Invention

The present invention relates to a light emitting diode (LED) driver circuit. More particularly, the present invention relates to a circuit for controlling the power switch of an LED driver circuit by pulse width modulation (PWM).

2. Description of the Related Art

FIG. 1 is a schematic diagram showing a conventional LED driver circuit 100 . The LED driver 100 consists of a boost DC/DC converter 130 and its load 120 . The load 120 includes several strings of LEDs 104 and a current mirror 103 coupled to the LED strings. The boost converter 130 includes an inductor L 1 , a switch Q 1 , a PWM controller 101 , a diode D 1 , resistors R 11 and R 12 , and a capacitor C 1 . The inductor L 1 has one end coupled to receive the input voltage Vin 1 and the other end coupled to one end of the switch Q 1 . The anode of the diode D 1 is coupled to the same end of the switch Q 1 . The capacitor C 1 has one end coupled to the cathode of the diode D 1 and the other end coupled to receive a ground voltage. The PWM controller 101 controls the switch Q 1 through the PWM signal PS 1 . When the switch Q 1 is turned on, the current of the inductor L 1 increases. When the switch Q 1 is turned off, the energy accumulated in the inductor L 1 is transferred to the capacitor C 1 to maintain a driving voltage Vout 1 for the LEDs 104 . The resistors R 11 and R 12 constitute a voltage divider and provide a feedback voltage FB 1 to the PWM controller 101 . The PWM controller 101 determines the duty cycle of the PWM signal PS 1 according to the feedback voltage FB 1 .

The driving current determines the color of the light emitted from an LED. The brightness of an LED has to be controlled by the duty cycle of its driving current. For example, when the duty cycle is 50%, the brightness is 50% of the maximum level. In the driver circuit 100 , the current mirror 103 provides driving current through the LEDs 104 when the dimming control signal DCS 1 is asserted and cuts off the driving current when the dimming control signal DCS 1 is de-asserted. Therefore the duty cycle of the dimming control signal DCS 1 is the duty cycle of the driving current of the LEDs 104 .

The switch Q 1 is a power metal oxide semiconductor field effect transistor (power MOSFET). Power MOSFET is a large external component. Every time when a power MOSFET is turned on or turned off, it consumes considerable power due to gate charging or discharging. The PWM controller 101 outputs the PWM signal PS 1 constantly. The power switch Q 1 keeps switching even when the dimming control signal DCS 1 is de-asserted, resulting in power waste and low efficiency. This problem is especially severe when the duty cycle of the dimming control signal DCS 1 is low. Moreover, this unnecessary switching tends to overcharge the capacitor C 1 and enlarge the ripple of the driving voltage Vout 1 .

Accordingly, the present invention is directed to an LED driver circuit and a circuit for controlling the power switch of an LED driver circuit. These circuits turn off the power switch to prevent unnecessary switching when the driving current of the LED is cut off. Consequently, these circuits greatly reduce power consumption and achieve higher efficiency. These circuits also features smaller ripple of the driving voltage because there is no capacitor overcharge.

According to an embodiment of the present invention, an LED driver is provided. The LED driver includes a switch, a PWM controller, a current source and a signal controller. The switch has a first end outputting a driving voltage to the LED. The PWM controller provides a PWM signal. The current source provides a driving current flowing through the LED when a dimming control signal is asserted. The signal controller turns off the switch when the dimming control signal is de-asserted and relays the PWM signal from the PWM controller to the switch so that the switch is controlled by the PWM signal when the dimming control signal is asserted.

In an embodiment of the present invention, the LED driver further includes a buffer coupled between the signal controller and the switch.

In an embodiment of the present invention, the LED driver further includes a voltage detector generating a feedback signal indicating the condition of the driving voltage. Moreover, the PWM controller determines the duty cycle of the PWM signal according to the feedback signal.

In an embodiment of the present invention, the PWM controller asserts a fault signal when the feedback signal indicates a fault condition of the driving voltage. The fault condition occurs when the driving voltage is lower than a predetermined threshold. The signal controller relays the PWM signal from the PWM controller to the switch when the fault signal is asserted even if the dimming control signal is de-asserted.

In an embodiment of the present invention, the signal controller includes an OR gate and an AND gate. The OR gate receives the dimming control signal and the fault signal. The AND gate receives the PWM signal and the output signal of the OR gate. The switch is controlled by the output signal of the AND gate.

According to another embodiment of the present invention, a circuit is provided for controlling a power switch to provide a driving voltage to at least one LED. A driving current flowing through the LED is provided when a dimming control signal is asserted. The circuit includes a PWM controller and a signal controller. The PWM controller provides a PWM signal. The signal controller turns off the power switch when the dimming control signal is de-asserted and relays the PWM signal from the PWM controller to the power switch so that the power switch is controlled by the PWM signal when the dimming control signal is asserted.

The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

FIG. 1 is a schematic diagram showing a conventional LED driver circuit.

FIG. 2 and FIG. 3 are schematic diagrams showing LED driver circuits according to different embodiments of the present invention.

FIG. 4 is a schematic diagram showing the circuit of the signal controller in an embodiment of the present invention.

FIG. 5 is a schematic diagram showing signal waveforms in an embodiment of the present invention.

Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.

FIG. 2 is a schematic diagram showing an LED driver circuit 200 according to an embodiment of the present invention. The LED driver 200 is an improvement over the conventional LED driver 100 . Briefly speaking, the LED driver circuit 200 turns off the switch SW 2 when its switching is unnecessary.

The LED driver 200 drives the LEDs 204 and may be divided into a boost DC/DC converter 230 and its load 220 . The load 220 includes the LED strings 204 and a current mirror 203 coupled to the LEDs 204 . The current mirror 203 provides driving current flowing through the LEDs 204 when the dimming control signal DCS 2 is asserted and cuts off the driving current when the dimming control signal DCS 2 is de-asserted. The boost DC/DC converter 230 includes a control circuit 210 , a switch SW 2 , an inductor L 2 , a diode D 2 , a capacitor C 2 , and resistors R 21 and R 22 . The control circuit 210 includes a PWM controller 201 and a signal controller 202 . The PWM controller 201 provides a PWM signal PS 2 . The signal controller 202 turns off the switch SW 2 when the dimming control signal DCS 2 is de-asserted and relays the PWM signal PS 2 from the PWM controller 201 to the switch SW 2 so that the switch SW 2 is controlled by the PWM signal PS 2 and one end of the switch SW 2 outputs the driving voltage Vout 2 to the LEDs 204 when the dimming control signal DCS 2 is asserted. In this way the signal controller 202 prevents unnecessary switching and reduces power consumption by turning off the switch SW 2 when the LEDs 204 need not be driven. In this embodiment the switch SW 2 is a power MOSFET.

The resistor R 21 has one end coupled to receive the driving voltage Vout 2 . The resistor R 22 has one end coupled to the other end of the resistor R 21 . The other end of the resistor R 22 is coupled to receive a ground voltage. The resistors R 21 and R 22 constitute a voltage detector. This voltage detector generates a feedback signal FB 2 indicating the condition of the driving voltage Vout 2 . The PWM controller 201 determines the duty cycle of the PWM signal PS 2 according to the feedback signal FB 2 .

The other elements of the boost converter 230 are similar to their counterparts in the conventional boost converter 130 . In some applications a buffer (not shown) may be coupled between the signal controller 202 and the switch SW 2 to enhance the driving capability of the signal controller 202 .

FIG. 3 is a schematic diagram showing an LED driver circuit 300 according to another embodiment of the present invention. The components in the LED driver 300 work in the same way as their counterparts in the LED driver 200 do, except that the PWM controller 301 and the signal controller 302 have some additional functions. The PWM controller 301 asserts a fault signal FS when the feedback signal FB 3 indicates a fault condition of the driving voltage Vout 3 . The feedback signal FB 3 is provided by the voltage detector consisting of the resistors R 31 and R 32 . In this embodiment, the fault condition occurs when the driving voltage Vout 3 is lower than a predetermined threshold. For example, the threshold may be 90% of the maximum output voltage level. The signal controller 302 relays the PWM signal PS 3 from the PWM controller 301 to the switch SW 3 when the fault signal FS is asserted even if the dimming control signal DCS 3 is de-asserted. When the dimming control signal DCS 3 is de-asserted, the LEDs 304 are not driven and no driving current flows through them. However, the driving voltage Vout 3 maintained by the capacitor C 3 may decrease gradually due to leakage. In this case, the signal controller 302 resumes the operation of the switch SW 3 when the driving voltage Vout 3 falls below the predetermined threshold to maintain the driving capability of the driving voltage Vout 3 .

FIG. 4 is a schematic diagram showing the circuit of the signal controller 302 according to an embodiment of the present invention. The signal controller 302 includes an OR gate 401 and an AND gate 402 . The OR gate 401 receives the dimming control signal DCS 3 and the fault signal FS. The AND gate 402 receives the PWM signal PS 3 and the output signal of the OR gate 401 . The switch SW 3 is controlled by the output signal SDS of the AND gate 402 . It is easily seen that the signal controller 302 relays the PWM signal PS 3 to the switch SW 3 when either the fault signal FS or the dimming control signal DCS 3 is asserted, and the signal controller 302 turns off the switch SW 3 when both the fault signal FS and the dimming control signal DCS 3 are de-asserted.

FIG. 5 is a timing chart of important signals in the LED driver circuit 300 , which shows a typical operating scenario of the LED driver circuit 300 . At time T 1 , the dimming control signal DCS 3 is asserted. The signal controller 302 relays the PWM signal PS 3 to the switch SW 3 . At time T 2 , the dimming control signal DCS 3 is de-asserted. The signal controller 302 suppresses the PWM signal PS 3 and turns off the switch SW 3 . At time T 3 , the driving voltage Vout 3 drops below the threshold and the fault signal FS is asserted. Consequently the signal controller 302 relays the PWM signal PS 3 to the switch SW 3 . At time T 4 , the dimming control signal DCS 3 is asserted. The signal controller 302 keeps relaying the PWM signal PS 3 . At time T 5 , the driving voltage Vout 3 has risen above the threshold, the fault condition is gone and the fault signal FS is de-asserted. Since the dimming control signal DCS 3 is still asserted, the signal controller 302 keeps relaying the PWM signal PS 3 to the switch SW 3 . At time T 6 , the dimming control signal DCS 3 is de-asserted. The signal controller 302 suppresses the PWM signal PS 3 and turns off the switch SW 3 .

In summary, the LED driver circuit in each of the above embodiments includes a control circuit to control the operation of the power switch. The control circuit includes a PWM controller and a signal controller. The control circuit turns off the power switch when there is no need to maintain the driving voltage. Consequently the above embodiments have the advantages of lower power consumption, higher efficiency, preventing capacitor overcharging, and smaller ripple of the driving voltage. The present invention is not confined to LED driver circuits. For example, the control circuit of the present invention is applicable to any PWM converter circuit with a load which can be turned on or turned off in response to an external signal.

It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.