Input voltage feedforward in constant amplitude ramp
In some examples, an apparatus includes a constant amplitude ramp generator having a differential output and first and second generator inputs, in which the first generator input is coupled to a power terminal. The apparatus also includes an input voltage feed-forward circuit having a feed-forward input and a feed-forward output, in which the feed-forward input is coupled to an input voltage terminal, and the feed-forward output is coupled to the second generator input.
1. An apparatus, comprising:
a constant amplitude ramp generator having a differential output and first and second generator inputs, in which the first generator input is coupled to a power terminal; and
an input voltage feed-forward circuit having a feed-forward input and a feed-forward output, in which the feed-forward input is coupled to an input voltage terminal, and the feed-forward output is coupled to the second generator input.
2. The apparatus of claim 1 , further comprising a power converter, in which the power terminal is a switch terminal of the power converter, the power converter is coupled to the input voltage terminal, and the power converter is configured to switch power from the input voltage terminal to an output voltage terminal.
3. The apparatus of claim 2 , wherein the power converter is switched based on a differential signal provided at the differential output.
4. The apparatus of claim 1 , wherein the input voltage feed-forward circuit includes:
a voltage divider having a divider output, in which the voltage divider is coupled between the input voltage terminal and a ground terminal, and the divider output is coupled to the second generator input; and
a buffer having a buffer input, a buffer output, and a buffer supply input, wherein the buffer input is coupled to the divider output, and the buffer supply input is coupled to a voltage supply terminal.
5. The apparatus of claim 4 , wherein the constant amplitude ramp generator includes:
a second buffer having a second buffer input and a second buffer output, in which the second buffer input is the first generator input coupled to the power terminal;
a first transistor having a first control terminal;
a first capacitor coupled between the second buffer output and the first transistor, in which the first transistor is coupled between the first capacitor and the buffer output;
a second transistor having a second control terminal, in which the second transistor is coupled between the first capacitor and a first output terminal, and the second control terminal is coupled to the power terminal;
a third transistor having a third control terminal;
an inverter having an inverter input and an inverter output, the inverter input coupled to the power terminal, and the inverter output coupled to the first and third control terminals;
a resistor coupled between the ground terminal and the third transistor, in which the third transistor is coupled between the first output terminal and the resistor;
a second capacitor coupled between the first output terminal and the ground terminal; and
a filter coupled between the first output terminal and a second output terminal.
6. The apparatus of claim 1 , wherein the input voltage feed-forward circuit is configured to bias the constant amplitude ramp generator based on an input voltage provided at the input voltage terminal.
7. The apparatus of claim 6 , wherein the input voltage feed-forward circuit includes:
a transistor configured in a source-follower arrangement, the transistor having a control terminal, a source, and a drain, wherein the control terminal is coupled to the divider output, and the drain is coupled to a voltage supply terminal; and
a current source coupled between the source and a ground terminal.
8. The apparatus of claim 7 , wherein the transistor injects a bias voltage to the constant amplitude ramp generator, the bias voltage including a voltage proportional to the input voltage.
9. The apparatus of claim 8 , wherein the input voltage feed-forward circuit includes a voltage scaling circuit configured to scale the input voltage to provide the voltage proportional to the input voltage.
10. The apparatus of claim 9 , wherein the voltage scaling circuit is a voltage divider.
11. An apparatus, comprising:
a voltage divider having a divider output, in which the voltage divider is coupled between an input voltage terminal and a ground terminal;
a first transistor having a first control terminal, a source, and a drain, in which the first control terminal is coupled to the divider output, and the drain is coupled to a voltage supply terminal;
a current source coupled between the source and the ground terminal;
a buffer having a buffer input and a buffer output, the buffer input coupled to a signal input terminal;
a second transistor having a second control terminal;
a first capacitor coupled between the buffer output and the second transistor, in which the second transistor is coupled between the first capacitor and the source;
a third transistor having a third control terminal, in which the third transistor is coupled between the first capacitor and a first output terminal, and the third control terminal is coupled to the signal input terminal;
a fourth transistor having a fourth control terminal;
an inverter having an inverter input and an inverter output, the inverter input coupled to the signal input terminal, and the inverter output coupled to the second and fourth control terminals;
a resistor coupled between the ground terminal and the fourth transistor, in which the fourth transistor is coupled between the first output terminal and the resistor;
a second capacitor coupled between the first output terminal and the ground terminal; and
a filter coupled between the first output terminal and a second output terminal.
12. The apparatus of claim 11 , wherein the first output terminal and the second output terminal form a differential output.
13. The apparatus of claim 11 , wherein the filter is a second order filter.
14. The apparatus of claim 11 , wherein the second control terminal is coupled to receive a drive signal of a power converter that switches a voltage provided at the input voltage terminal to provide an output voltage.
15. A system, comprising:
a power converter including a switch terminal and a control terminal, the power converter configured to switch a signal from an input voltage terminal to an output voltage terminal; and
a controller coupled to the power converter, the controller configured to control the power converter to switch the signal from the input voltage terminal to the output voltage terminal, in which the controller includes:
a ramp generator having a first and second generator inputs and a differential output;
an input voltage feed-forward circuit having a feed-forward input and a feed-forward output, in which the feed-forward input is coupled to the input voltage terminal, and the feed-forward output is coupled to the second generator input; and
control logic coupled to the ramp generator, the differential output, and the control terminal, the control logic configured to determine a value of a control signal for controlling the power converter based on a differential signal provided at the differential output.
16. The system of claim 15 , wherein the input voltage feed-forward circuit includes:
a voltage divider having a divider output, in which the voltage divider is coupled between the input voltage terminal and a ground terminal, and the divider output is coupled to the second generator input;
a first transistor having a first control terminal, a source, and a drain, in which the first control terminal is coupled to the divider output, and the drain is coupled to a voltage supply terminal; and
a current source coupled between the source and the ground terminal.
17. The system of claim 16 , wherein the ramp generator includes:
a buffer having a buffer input and a buffer output, in which the buffer input is the first generator input coupled to the control logic;
a second transistor having a second control terminal;
a first capacitor coupled between the buffer output and the second transistor, in which the second transistor is coupled between the first capacitor and the source;
a third transistor having a third control terminal, in which the third transistor is coupled between the first capacitor and a first output terminal, and the third control terminal is coupled to the control logic;
a fourth transistor having a fourth control terminal;
an inverter having an inverter input and an inverter output, the inverter input coupled to the control logic, and the inverter output coupled to the second and fourth control terminals;
a resistor coupled between the ground terminal and the fourth transistor, in which the fourth transistor is coupled between the first output terminal and the resistor;
a second capacitor coupled between the first output terminal and the ground terminal; and
a filter coupled between the first output terminal and a second output terminal.
18. The system of claim 17 , wherein the filter is a first filter, and the ramp generator includes a second filter coupled in series between the first filter and the second output terminal.
19. The system of claim 17 , wherein the control logic includes a summing comparator configured to determine the value of the control signal based on the differential signal, a feedback signal of the power converter, a reference voltage, and a bias voltage.
20. The system of claim 17 , wherein the first transistor injects a bias voltage to the ramp generator at the second transistor, the bias voltage including a supply voltage from the supply voltage terminal and a voltage proportional to the signal at the input voltage terminal.