Floating output voltage boost-buck regulator using a buck controller with low input and low output ripple
A converter generates an output voltage differential across a floating load, such as a string of LEDs. The converter receives an input voltage Vin from a power supply, and the floating output voltage differential may be greater than or less than Vin. The converter uses a first switch and first inductor in a boost mode type configuration, and uses a second switch and second inductor in a buck mode type configuration. The inductors have a common node. The first inductor has another end coupled to ground, and the other end of the second inductor is coupled to the load. Both inductors charge and discharge together depending on the conductivities of the switches. One end of the load will be approximately zero volts, while the other end will be at a negative voltage VEE. The two inductors smooth input current/voltage ripple and output current/voltage ripple, resulting in low EMI.
1. A circuit having a converter coupled to drive a floating load comprising:
an input terminal for receiving an input voltage Vin;
a first inductor having a first end for being coupled to ground;
a first switch coupled to a second end of the first inductor for coupling the second end to the input voltage when the first switch is in an on-state to charge the first inductor;
a rectifier coupled to the second end of the first inductor for electrically coupling the second end of the first inductor to a first terminal of the load when the first switch is in an off-state, wherein the converter generates a voltage VEE at the first terminal during steady state operation;
a first capacitor coupled between the input voltage and the first terminal of the load, wherein a voltage across the first capacitor equals Vin-VEE, where VEE is a negative voltage with respect to ground at the first terminal of the load;
a second inductor having a first end coupled to the second end of the first inductor at least during a time when the first switch is in its on-state, wherein the load is coupled between a second end of the second inductor and the first capacitor; and
a controller coupled to receive a first feedback signal corresponding to an instantaneous current through the first inductor when the first switch is on, and the controller coupled to receive a second feedback signal corresponding to a load current or load voltage,
wherein the controller controls at least the first switch to regulate a peak current through the first inductor to regulate the load current or load voltage.
2. The circuit of claim 1 wherein the floating load is a light emitting diode (LED) load.
3. The circuit of claim 2 wherein the LED load further comprises a PWM dimming switch in series with a string of LEDs, such that the second capacitor is across the PWM dimming switch and the string of LEDs, wherein a duty cycle of the PWM dimming switch is used to control a perceived brightness of the LEDs.
4. The circuit of claim 1 wherein the first end of the first inductor is coupled to ground via a sense resistor for detecting the instantaneous current through the first inductor.
5. The circuit of claim 1 wherein the first end of the second inductor is directly coupled to the second end of the first inductor.
6. The circuit of claim 1 wherein the rectifier comprises a second switch controlled by the controller to have a state opposite to a state of the first switch.
7. The circuit of claim 6 wherein the first end of the second inductor is coupled to the second end of the first inductor via the first switch and a third switch, wherein the third switch is in series between the first end of the second inductor and the input voltage.
8. The circuit of claim 1 wherein the first end of the second inductor is coupled to the input voltage when the first switch is in its on-state.
9. The circuit of claim 1 wherein the load comprises a sense resistor connected in series with the load for detecting a current through the load as the second feedback signal.
10. The circuit of claim 1 wherein the converter generates a regulated current.
11. The circuit of claim 1 wherein the converter generates a regulated voltage.
12. The circuit of claim 1 wherein the controller is a packaged buck controller integrated circuit.
13. The circuit of claim 1 wherein the first switch and the second switch are N-channel MOSFETs.
14. The circuit of claim 1 further comprising a second capacitor coupled between the second end of the second inductor and VEE so as to be across the load.