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

An LED driver circuit and its control circuit for controlling its power switch are 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 at least one 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.

CROSS-REFERENCE TO RELATED APPLICATION

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.

BACKGROUND OF THE INVENTION

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. 1is a schematic diagram showing a conventional LED driver circuit100. The LED driver100consists of a boost DC/DC converter130and its load120. The load120includes several strings of LEDs104and a current mirror103coupled to the LED strings. The boost converter130includes an inductor L1, a switch Q1, a PWM controller101, a diode D1, resistors R11and R12, and a capacitor C1. The inductor L1has one end coupled to receive the input voltage Vin1and the other end coupled to one end of the switch Q1. The anode of the diode D1is coupled to the same end of the switch Q1. The capacitor C1has one end coupled to the cathode of the diode D1and the other end coupled to receive a ground voltage. The PWM controller101controls the switch Q1through the PWM signal PS1. When the switch Q1is turned on, the current of the inductor L1increases. When the switch Q1is turned off, the energy accumulated in the inductor L1is transferred to the capacitor C1to maintain a driving voltage Vout1for the LEDs104. The resistors R11and R12constitute a voltage divider and provide a feedback voltage FB1to the PWM controller101. The PWM controller101determines the duty cycle of the PWM signal PS1according to the feedback voltage FB1.

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 circuit100, the current mirror103provides driving current through the LEDs104when the dimming control signal DCS1is asserted and cuts off the driving current when the dimming control signal DCS1is de-asserted. Therefore the duty cycle of the dimming control signal DCS1is the duty cycle of the driving current of the LEDs104.

The switch Q1is 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 controller101outputs the PWM signal PS1constantly. The power switch Q1keeps switching even when the dimming control signal DCS1is de-asserted, resulting in power waste and low efficiency. This problem is especially severe when the duty cycle of the dimming control signal DCS1is low. Moreover, this unnecessary switching tends to overcharge the capacitor C1and enlarge the ripple of the driving voltage Vout1.

SUMMARY OF THE INVENTION

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.

DESCRIPTION OF THE EMBODIMENTS

FIG. 2is a schematic diagram showing an LED driver circuit200according to an embodiment of the present invention. The LED driver200is an improvement over the conventional LED driver100. Briefly speaking, the LED driver circuit200turns off the switch SW2when its switching is unnecessary.

The LED driver200drives the LEDs204and may be divided into a boost DC/DC converter230and its load220. The load220includes the LED strings204and a current mirror203coupled to the LEDs204. The current mirror203provides driving current flowing through the LEDs204when the dimming control signal DCS2is asserted and cuts off the driving current when the dimming control signal DCS2is de-asserted. The boost DC/DC converter230includes a control circuit210, a switch SW2, an inductor L2, a diode D2, a capacitor C2, and resistors R21and R22. The control circuit210includes a PWM controller201and a signal controller202. The PWM controller201provides a PWM signal PS2. The signal controller202turns off the switch SW2when the dimming control signal DCS2is de-asserted and relays the PWM signal PS2from the PWM controller201to the switch SW2so that the switch SW2is controlled by the PWM signal PS2and one end of the switch SW2outputs the driving voltage Vout2to the LEDs204when the dimming control signal DCS2is asserted. In this way the signal controller202prevents unnecessary switching and reduces power consumption by turning off the switch SW2when the LEDs204need not be driven. In this embodiment the switch SW2is a power MOSFET.

The resistor R21has one end coupled to receive the driving voltage Vout2. The resistor R22has one end coupled to the other end of the resistor R21. The other end of the resistor R22is coupled to receive a ground voltage. The resistors R21and R22constitute a voltage detector. This voltage detector generates a feedback signal FB2indicating the condition of the driving voltage Vout2. The PWM controller201determines the duty cycle of the PWM signal PS2according to the feedback signal FB2.

The other elements of the boost converter230are similar to their counterparts in the conventional boost converter130. In some applications a buffer (not shown) may be coupled between the signal controller202and the switch SW2to enhance the driving capability of the signal controller202.

FIG. 3is a schematic diagram showing an LED driver circuit300according to another embodiment of the present invention. The components in the LED driver300work in the same way as their counterparts in the LED driver200do, except that the PWM controller301and the signal controller302have some additional functions. The PWM controller301asserts a fault signal FS when the feedback signal FB3indicates a fault condition of the driving voltage Vout3. The feedback signal FB3is provided by the voltage detector consisting of the resistors R31and R32. In this embodiment, the fault condition occurs when the driving voltage Vout3is lower than a predetermined threshold. For example, the threshold may be 90% of the maximum output voltage level. The signal controller302relays the PWM signal PS3from the PWM controller301to the switch SW3when the fault signal FS is asserted even if the dimming control signal DCS3is de-asserted. When the dimming control signal DCS3is de-asserted, the LEDs304are not driven and no driving current flows through them. However, the driving voltage Vout3maintained by the capacitor C3may decrease gradually due to leakage. In this case, the signal controller302resumes the operation of the switch SW3when the driving voltage Vout3falls below the predetermined threshold to maintain the driving capability of the driving voltage Vout3.

FIG. 4is a schematic diagram showing the circuit of the signal controller302according to an embodiment of the present invention. The signal controller302includes an OR gate401and an AND gate402. The OR gate401receives the dimming control signal DCS3and the fault signal FS. The AND gate402receives the PWM signal PS3and the output signal of the OR gate401. The switch SW3is controlled by the output signal SDS of the AND gate402. It is easily seen that the signal controller302relays the PWM signal PS3to the switch SW3when either the fault signal FS or the dimming control signal DCS3is asserted, and the signal controller302turns off the switch SW3when both the fault signal FS and the dimming control signal DCS3are de-asserted.

FIG. 5is a timing chart of important signals in the LED driver circuit300, which shows a typical operating scenario of the LED driver circuit300. At time T1, the dimming control signal DCS3is asserted. The signal controller302relays the PWM signal PS3to the switch SW3. At time T2, the dimming control signal DCS3is de-asserted. The signal controller302suppresses the PWM signal PS3and turns off the switch SW3. At time T3, the driving voltage Vout3drops below the threshold and the fault signal FS is asserted. Consequently the signal controller302relays the PWM signal PS3to the switch SW3. At time T4, the dimming control signal DCS3is asserted. The signal controller302keeps relaying the PWM signal PS3. At time T5, the driving voltage Vout3has risen above the threshold, the fault condition is gone and the fault signal FS is de-asserted. Since the dimming control signal DCS3is still asserted, the signal controller302keeps relaying the PWM signal PS3to the switch SW3. At time T6, the dimming control signal DCS3is de-asserted. The signal controller302suppresses the PWM signal PS3and turns off the switch SW3.

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.