Boost LED driver not using output capacitor and blocking diode
An LED driver is disclosed that boosts an input voltage to drive any number of LEDs in series. The driver includes a switch-mode current regulator that supplies regulated current pulses to the LEDs. No high voltage output capacitor is used to smooth the current pulses, so the LEDs are turned on any off at the switching frequency. Also, no blocking diode between the switching transistor and the LEDs is used. The cathode of the “bottom” LED in the string is connected to ground via a current sense resistor. In parallel with the sense resistor is connected an RC filter using a small, low voltage filter capacitor. The RC filter provides a substantially smooth feedback voltage for the current regulator to control the duty cycle of the switching transistor so that the feedback voltage matches a reference voltage.
1. A light emitting diode (LED) driver for driving a plurality of LEDs connected in series, the driver comprising:
a current regulator configured to boost an input voltage to drive the plurality of LEDs with a boosted output voltage, the current regulator comprising:
a feedback circuit comprising a first resistor and first capacitor connected in series, a first terminal of the first resistor being connected to receive a signal corresponding to a pulsed current through the plurality of LEDs, a second terminal of the first resistor being connected to a first terminal of the first capacitor, the first terminal of the capacitor providing a feedback voltage;
an error amplifier having inputs comprising the feedback voltage and a reference voltage, an output of the error amplifier determining a control voltage;
a switching transistor connected to be switched at a duty cycle corresponding to a magnitude of the control voltage, the duty cycle corresponding to a peak current generated by the current regulator;
an inductor connected to a first terminal of the switching transistor and to the input voltage for providing a varying current as the switching transistor is turned on and off; and
a first terminal of the switching transistor being connected to an anode of an LED in the plurality of LEDs connected in series, with no smoothing capacitor connected to the anode, such that pulses of current are conducted through the LEDs as the switching transistor turns on and off,
wherein a magnitude of the feedback voltage from the first capacitor is controlled by the current regulator to be approximately equal to the reference voltage by the current regulator controlling the duty cycle of the switching transistor.
2. The driver of claim 1 wherein a second terminal of the first capacitor is connected to ground.
3. The driver of claim 1 wherein the first terminal of the switching transistor is directly connected to the anode of the LED in the plurality of LEDs with no blocking diode connected in-between.
4. The driver of claim 1 further comprising a sense resistor coupled in series with the plurality of LEDs, wherein a voltage drop across the sense resistor provides the signal corresponding to the current through the plurality of LEDs.
5. The driver of claim 1 further comprising a PWM comparator having inputs being the output of the error amplifier and an output of a waveform generator, the waveform generator generating a ramping signal, an output signal of the PWM comparator controlling switching of the switching transistor.
6. The driver of claim 5 wherein the waveform generator comprises a resistive device in series with the switching transistor, wherein a voltage at the resistive device provides the ramping signal as current through the switching transistor ramps.
7. The driver of claim 5 wherein the waveform generator comprises an oscillator generating a sawtooth waveform.
8. The driver of claim 1 further comprising the plurality of LEDs connected to the first terminal of the switching transistor.
9. The driver of claim 1 wherein the first capacitor has a voltage rating that is lower than the boosted output voltage.
10. A method performed by a light emitting diode (LED) driver for driving a plurality of LEDs connected in series, the method comprising:
regulating current through the plurality of LEDs, regulating the current comprising boosting an input voltage applied to the driver to drive the plurality of LEDs with a boosted output voltage, regulating the current further comprising:
generating a pulsed current sense voltage corresponding to pulsed current through the plurality of LEDs;
filtering the current sense voltage to generate a feedback signal; and
controlling a duty cycle of a switching transistor to regulate current through the plurality of LEDs so that the feedback voltage approximately matches a reference voltage,
wherein when the switching transistor is off, a pulse of current flows through the plurality of LEDs and, when the switching transistor is on, no current flows through the LEDs.
11. The method of claim 10 wherein a first terminal of the switching transistor is connected to an anode of an LED in the plurality of LEDs, with no smoothing capacitor connected to the anode, such that pulses of current are conducted through the LEDs as the switching transistor turns on and off.
12. The method of claim 10 further comprising:
charging an inductor, connected between the input voltage and an anode of an LED in the plurality of LEDs, when the switching transistor is on, wherein no current is supplied to the plurality of LEDs when the switching transistor is on; and
discharging the inductor to supply a pulse of current through the plurality of LEDs when the switching transistor is off.
13. The method of claim 10 wherein filtering the current sense voltage to generate a feedback signal comprises applying the pulsed current sense voltage to a filter capacitor through a resistor.
14. The method of claim 10 wherein generating a pulsed current sense voltage corresponding to the current through the plurality of LEDs comprises detecting a voltage drop across a sense resistor coupled in series with the plurality of LEDs.
15. The method of claim 14 wherein filtering the current sense voltage to generate a feedback signal comprises applying the pulsed current sense voltage to a filter capacitor through a filter resistor, wherein the filter resistor is connected to the sense resistor, and the capacitor is connected to ground.
16. The method of claim 15 wherein the capacitor has a voltage rating that is lower than the boosted output voltage.
17. The method of claim 10 wherein a first terminal of the switching transistor is directly connected to an anode of an LED in the plurality of LEDs connected in series with no blocking diode connected in-between.
18. The method of claim 10 wherein controlling the duty cycle of the switching transistor comprises:
applying the feedback signal and the reference voltage to inputs of an error amplifier; and
comparing, by a PWM comparator, a control signal, set by an output of the error amplifier, to a ramping output signal of a waveform generator, an output signal of the PWM comparator controlling switching of the switching transistor to cause the feedback signal to approximately match the reference voltage.
19. The method of claim 18 wherein the waveform generator comprises a resistive device in series with the switching transistor, wherein a voltage at the resistive device provides the ramping output signal as current through the switching transistor ramps.
20. The method of claim 18 wherein the waveform generator comprises an oscillator generating a sawtooth waveform.