Switch mode power converter
View Patent ↗A switch mode power converter is provided which includes a switching cell with a supply input, an output and a control input. A summing comparator has first and second differential input pairs and an output. The output is connected to the control input of the switching cell. An oscillator provides a periodic waveform that is applied to a first one of the inputs of the first differential input pair of the summing comparator. An adjustable reference voltage source provides an adjustable reference voltage a predetermined fraction of which is applied to a second one of the inputs of the first differential input pair of the summing comparator. An error amplifier has differential outputs coupled to the second pair of differential inputs of the summing comparator and a differential input pair. A first input of the differential input pair is coupled to the output of the switching cell, and the adjustable reference voltage from the adjustable reference voltage source is applied to a second input of the differential input pair. Thus, the desired DC voltage is scaled to produce a DC reference for the comparator which generates the desired duty cycle for the pulse signal that drives the switching cell, and thus the desired regulated output voltage, with only minor corrections required across the error terminals to correct small parasitic terms.
1. A switch mode power converter comprising:
a switching cell with a supply input, an output and a control input,
a summing comparator having first and second differential input pairs and an output, the output being connected to the control input of the switching cell,
an oscillator providing a periodic waveform that is applied to a first one of the inputs of the first differential input pair of the summing comparator,
an adjustable reference voltage source providing an adjustable reference voltage a predetermined fraction of which is applied to a second one of the inputs of the first differential input pair of the summing comparator, and
an error amplifier having differential outputs coupled to the second pair of differential inputs of the summing comparator and a differential input pair, a first input of said differential input pair being coupled to the output of the switching cell, the adjustable reference voltage from the adjustable reference voltage source being applied to a second input of said differential input pair.
2. The power converter according to claim 1 , wherein a fixed fraction of a demanded output voltage developed at the output of the switching cell is applied to the first differential input of the error amplifier.
3. The power converter according to claim 1 , wherein the adjustable reference voltage source comprises a fixed reference source, an adjustable gain amplifier having an input and an output, a fixed reference voltage from the fixed reference voltage source being applied to said input and said output providing the adjustable reference voltage.
4. The power converter according to claim 3 , and further comprising a resistive voltage divider connected to the output of the adjustable gain amplifier and providing said predetermined fraction of the adjustable reference voltage.
5. The power converter according to claim 3 , wherein the adjustable gain amplifier has a feedback loop with an adjustable resistive voltage divider.
6. The power converter according to claim 5 and being implemented as an integrated CMOS circuit, the resistive voltage divider being external to the integrated circuit.
7. The power converter according to claim 1 , wherein the periodic waveform provided by the oscillator is a saw-tooth waveform.
8. The power converter according to claim 7 , wherein the saw-tooth waveform has a ramp height which is a fixed fraction of an input voltage at the supply input.
9. The power converter according to claim 1 , being a DC-DC buck converter and comprising an inductor and a capacitor in the switching cell.