Slope compensation module
A slope compensation module provides slope compensation of a switched-mode power supply using current mode control. The slope control unit comprises a capacitor coupled between an input and an output of the slope control unit, a switch for discharging the capacitor and a constant current source for charging the capacitor. Slope compensation parameters may be changed during operation with a programmable constant current source. The slope compensation module may also function as an analog sawtooth waveform frequency generator, and as an analog pulse width modulation (PWM) generator. Charging the capacitor generates a linearly decreasing (negative slope) ramp voltage for modulating a feedback error voltage into a slope compensated feedback error voltage. Capacitor charging may be controlled from a pulse width modulation signal. Opening of the switch may be programmably delayed, and a minimum closed time thereof may also be programmed during operation of the slope compensation module.
1. A slope compensation module for use with current mode control in a switched-mode power supply (SMPS) controller,
said SMPS controller comprising:
an error amplifier comprising an output; and
a voltage comparator comprising a first input;
said slope compensation module comprises:
a slope compensation capacitor coupled between the output of the error amplifier and the first input of the voltage comparator;
a slope compensation switch coupled in parallel with the slope compensation capacitor; and
a programmable constant current source coupled to the an output side of the slope compensation capacitor and to the slope compensation switch;
wherein when the slope compensation switch is open the slope compensation capacitor charges through the programmable constant current source to a circuit common and thereby generates a linearly decreasing (negative slope) ramp voltage.
2. The slope compensation module according to claim 1 , wherein when the slope compensation switch is closed the slope compensation capacitor discharges.
3. The slope compensation module according to claim 1 , wherein the slope compensation switch is a single pole double throw switch and the programmable constant current source is decoupled from the output side of the slope compensation capacitor when the slope compensation switch is in a first position, and coupled to the output side of the slope compensation capacitor when the slope compensation switch is in a second position.
4. The slope compensation module according to claim 1 , wherein the linearly decreasing ramp voltage is adapted to automatically adjust a feedback loop error voltage.
5. The slope compensation module according to claim 1 , wherein the programmable constant current source is formed by a constant current digital-to-analog converter (IDAC) coupled with an IDAC register.
6. The slope compensation module according to claim 1 , wherein the slope compensation capacitor comprises a plurality of slope compensation capacitors programmably selectable for a desired capacitance value.
7. A switched-mode power supply (SMPS) controller having a slope compensation module, comprising:
a voltage reference;
an error amplifier coupled with the voltage reference and comprising an output; and
a voltage comparator comprising a first input;
a slope compensation module having an input coupled to an output of the error amplifier, and an output, wherein the slope compensation module comprises:
a slope compensation capacitor coupled between the output of the error amplifier and the first input of the voltage comparator;
a slope compensation switch coupled in parallel with the slope compensation capacitor; and
a programmable constant current source coupled to an output side of the slope compensation capacitor;
wherein a second input of the voltage comparator is adapted for coupling to a current-to-voltage circuit that measures a current signal;
a period generator; and
a reset dominant latch comprising
a set input coupled to an output of the period generator,
a reset input coupled to an output of the voltage comparator, and
an output that is coupled to and controls opening and closing the slope compensation switch,
the output of the reset dominant latch also provides a control signal;
the period generator provides a plurality of pulses at a pulse period to the set input of the reset dominant latch, wherein each time a pulse is received at the set input of the reset dominant latch the output thereof goes to a second logic level from a first logic level;
when the output of the reset dominant latch is at the second logic level the slope compensation switch opens and the slope compensation capacitor charges through the programmable constant current source to a circuit common, thereby generating a linearly decreasing ramp voltage for modulating a feedback error voltage from the error amplifier into a slope compensated feedback error voltage;
when the slope compensated feedback error voltage that is coupled to the first input of the voltage comparator is less than a voltage representing the current through the SMPS inductor at the second input of the voltage comparator, the output of the reset dominant latch returns to the first logic level and the slope compensation switch closes thereby discharging the slope compensation capacitor and returning the first input of the voltage comparator to the feedback error voltage from the error amplifier.
8. The slope compensation module according to claim 7 , further comprising a buffer amplifier coupled between the output of the error amplifier and the slope compensation capacitor.
9. The SMPS controller according to claim 7 , further comprising a buffer amplifier coupled between the output of the error amplifier and the slope compensation capacitor and switch.
10. The SMPS controller according to claim 7 , further comprising a loop compensation network coupled between the output and the second input of the error amplifier.
11. The SMPS controller according to claim 7 , wherein the voltage representing the current through the SMPS inductor is generated with a current-to-voltage sensor.
12. The SMPS controller according to claim 11 , wherein the current-to-voltage sensor comprises a current transformer in a current path of the SMPS inductor, a rectifier diode coupled to the current transformer and a load resistor coupled to the resistor.
13. The SMPS controller according to claim 11 , wherein the current-to-voltage sensor comprises a resistor in a current path of the SMPS inductor and a diode coupled to the resistor.
14. The SMPS controller according to claim 7 , wherein the period generator is programmable for selecting one of a plurality pulse period times.
15. The SMPS controller according to claim 7 , wherein the slope compensation switch is a metal oxide semiconductor field effect transistor (MOSFET).
16. The SMPS controller according to claim 7 , further comprising a slope compensation switch controller coupled between the output of the reset dominant latch and the slope compensation switch, wherein the slope compensation switch controller provides programmable opening delay for the slope compensation switch.
17. The SMPS controller according to claim 16 , further comprising the slope compensation switch controller providing programmable minimum closed time for the slope compensation switch.
18. The SMPS controller according to claim 7 , wherein the output voltage from the SMPS filter capacitor is provided through a resistive voltage divider.
19. The SMPS controller according to claim 7 , wherein the programmable constant current source is adapted for calibration during testing of the SMPS controller.
20. The SMPS controller according to claim 7 , wherein the slope compensation capacitor is a plurality of slope compensation capacitors programmably selectable for a desired capacitance value.
21. The SMPS controller according to claim 7 , wherein the slope compensation capacitor is a plurality of slope compensation capacitors programmably selectable for a desired capacitance value during testing of the SMPS controller.
22. The SMPS controller according to claim 7 , further comprising a digital processor coupled to the programmable constant current source and to the period generator that is programmable, wherein the digital processor provides constant current values and pulse period times, respectively, thereto.
23. The SMPS controller according to claim 7 , wherein the SMPS controller is embedded in a microcontroller.
24. The SMPS controller according to claim 7 , wherein the programmable constant current source is coupled to the output side of the slope compensation capacitor with the slope compensation switch.
25. The SMPS controller according to claim 7 , wherein the programmable constant current source is decoupled from the output side of the slope compensation capacitor when the slope compensation switch is in a first position, and coupled to the output side of the slope compensation capacitor when the slope compensation switch is in a second position.
26. An analog pulse width modulation (PWM) generator, comprising:
a capacitor having an input coupled to a first voltage reference;
a switch coupled in parallel with the input and an output of the capacitor;
a programmable constant current source coupled to the output of the capacitor;
a first voltage comparator having a first input coupled to the output of the capacitor, a second input coupled to a second voltage reference and an output that controls opening and closing of the switch;
a second voltage comparator having a first input coupled to the output of the capacitor, a second input adapted for coupling to a voltage from a switched-mode power supply (SMPS) and an output that provides a pulse width modulated (PWM) control signal for controlling a power switch of a switched-mode power supply;
wherein when a voltage at the output of the capacitor is less than a voltage from the first voltage reference the switch will open, whereby the capacitor and the programmable constant current source modulate a voltage from the first voltage reference with a linear negatively sloping voltage waveform;
wherein a voltage difference between the first and second voltage references determines a period of the PWM control signal, and a voltage difference between the voltage from the SMPS and the voltage at the second node of the capacitor determines a duty cycle of the PWM control signal.
27. The analog PWM generator according to claim 26 , wherein the programmable constant current source is firmed by a constant current digital-to-analog converter (IDAC) coupled with an IDAC register.
28. The analog PWM generator according to claim 26 , wherein the capacitor is a plurality of capacitors programmably selectable for a desired capacitance value.
29. The analog PWM generator according to claim 26 , wherein the first voltage reference is a digital-to-analog converter (DAC) and provides programmable first reference voltage values.
30. The analog PWM generator according to claim 26 , wherein the first voltage reference is fixed.
31. The analog PWM generator according to claim 26 , wherein the first voltage reference is a modulated waveform signal.
32. The analog PWM generator according to claim 26 , further comprising a buffer amplifier coupled between the output of the first voltage reference and the input of the capacitor.
33. The analog PWM generator according to claim 26 , wherein the programmable constant current source is decoupled from the output of the capacitor when the switch is in a first position, and coupled to the output of the capacitor when the switch is in a second position.
34. A method for providing slope compensation in a switched-mode power supply (SMPS) controller comprising an error amplifier with an output, a voltage comparator with a first input, a slope compensation capacitor coupled between the output of the error amplifier and the first input of the voltage comparator, a slope compensation switched coupled in parallel with the slope compensation capacitor, and a programmable constant current source coupled to an output side of the slope compensation capacitor and to the slope compensation switch, said method comprising the steps of:
providing a reference voltage;
comparing the reference voltage to output feedback voltage from the SMPS controller with the error amplifier that provides an error voltage that is the difference between the reference voltage and the voltage from the SMPS filter network;
comparing a slope compensated error voltage from the slope compensation capacitor to a voltage representation of current measured in a SMPS inductor;
providing a reset dominant latch having a set input coupled to an output of a period generator, a reset input coupled to an output of the voltage comparator, and an output that controls opening and closing of the slope compensation switch and for supplying a control signal; and
providing a plurality of pulses at a pulse period from the period generator to the set input of the reset dominant latch, wherein each time a pulse is received at the set input of the reset dominant latch the output thereof goes to a second logic level from a first logic level;
opening the slope compensation switch when the output of the reset dominant latch is at the second logic level;
generating a linearly decreasing ramp voltage for modulating the feedback error voltage from the error amplifier into a slope compensated feedback error voltage; and
returning the output of the reset dominant latch to the first logic level and closing the slope compensation switch when the slope compensated error voltage is less than the voltage representing the current through the SMPS inductor.