Method and apparatus for limiting inrush current during startup of a buck converter
A method of regulating an output voltage of a buck converter during a startup period in which the buck converter is first enabled includes regulating the output voltage of the buck converter to a reference voltage under current-mode control during a first part of the startup period, and regulating the output voltage of the buck converter to the reference voltage under voltage-mode control during a second, later part of the startup period. The reference voltage ramps up from an initial voltage at the beginning of the startup period to a target voltage at the end of the startup period. Buck converter embodiments are also described.
1. A method of regulating an output voltage of a buck converter during a startup period in which the buck converter is first enabled, the method comprising:
regulating the output voltage of the buck converter to a reference voltage under current-mode control during a first part of the startup period; and
regulating the output voltage of the buck converter to the reference voltage under voltage-mode control during a second, later part of the startup period,
wherein the reference voltage ramps up from an initial voltage at a beginning of the startup period to a target voltage at an end of the startup period.
2. The method of claim 1 , wherein regulating the output voltage of the buck converter to the reference voltage under current-mode control during the first part of the startup period comprises:
ramping the reference voltage at a predetermined slew rate during the startup period; and
increasing a positive current limit of the buck converter from an initial value to a final value during the startup period.
3. The method of claim 2 , wherein the positive current limit of the buck converter is increased in a stepwise linear manner during the startup period.
4. The method of claim 2 , wherein the positive current limit of the buck converter is increased periodically during the startup period.
5. The method of claim 4 , wherein the positive current limit of the buck converter is increased every nth switching cycle of the buck converter during the startup period, n being a positive integer.
6. The method of claim 5 , wherein the positive current limit of the buck converter is increased by at least one ampere every switching cycle of the buck converter during the startup period.
7. The method of claim 2 , wherein the slew rate of the reference voltage is set such that the output voltage of the buck converter catches up to the reference voltage before the reference voltage is fully ramped to the target voltage at the end of the startup period.
8. The method of claim 2 , wherein the buck converter delivers current to an output capacitance of a load coupled to the buck converter, and wherein the slew rate of the reference voltage is set such that before the reference voltage is fully ramped to the target voltage at the end of the startup period, the positive current limit no longer clamps the current delivered by the buck converter to the output capacitance.
9. The method of claim 8 , wherein the buck converter tuned to automatically switch over from current-mode control to voltage-mode control when the positive current limit no longer clamps the current delivered by the buck converter to the output capacitance.
10. A buck converter, comprising:
a power stage operable to deliver current to an output capacitance coupled to the buck converter; and
a controller operable to control switching of the power stage, including during a startup period in which the buck converter is first enabled, so as to regulate an output voltage of the buck converter,
wherein during the startup period, the controller is operable to regulate the output voltage to a reference voltage under current-mode control during a first part of the startup period and regulate the output voltage to the reference voltage under voltage-mode control during a second, later part of the startup period,
wherein the controller is further operable to ramp up the reference voltage from an initial voltage at a beginning of the startup period to a target voltage at an end of the startup period.
11. The buck converter of claim 10 , wherein the controller is operable to ramp the reference voltage at a predetermined slew rate during the startup period and increase a positive current limit of the buck converter from an initial value to a final value during the startup period.
12. The buck converter of claim 11 , wherein the controller is operable to increase the positive current limit in a stepwise linear manner during the startup period.
13. The buck converter of claim 11 , wherein the controller is operable to periodically increase the positive current limit during the startup period.
14. The buck converter of claim 13 , wherein the controller is operable to increase the positive current limit every nth switching cycle of the buck converter during the startup period, n being a positive integer.
15. The buck converter of claim 14 , wherein the controller is operable to increase the positive current limit one ampere every switching cycle during the startup period.
16. The buck converter of claim 11 , wherein the controller is operable to set the slew rate of the reference voltage such that the output voltage catches up to the reference voltage before the reference voltage is fully ramped to the target voltage at the end of the startup period.
17. The buck converter of claim 11 , wherein the controller is operable to set the slew rate of the reference voltage such that before the reference voltage is fully ramped to the target voltage at the end of the startup period, the positive current limit no longer clamps the current delivered by the buck converter to the output capacitance.
18. The buck converter of claim 17 , wherein the buck converter is tuned to automatically switch over from current-mode control to voltage-mode control when the positive current limit no longer clamps the current delivered by the buck converter to the output capacitance.
19. The buck converter of claim 11 , wherein the controller is a PID (proportional-integral-derivative) based controller operable to generate PWM (pulse width modulation) pulses for controlling the switching of the power stage, and wherein the PID-based controller is operable to adjust the width of the PWM pulses during the startup period so that the current delivered by the power stage to the output capacitance does not exceed the positive current limit.
20. A buck converter, comprising:
a first control loop operable to regulate a first output voltage of the buck converter; and
a second control loop operable to regulate a second output voltage of the buck converter independent of the first control loop,
wherein the second control loop is operable to use the first output voltage as a reference voltage during a startup period in which the buck converter is first enabled,
wherein during the startup period, the second control loop is operable to regulate the second output voltage to the reference voltage under current-mode control during a first part of the startup period and regulate the second output voltage to the reference voltage under voltage-mode control during a second, later part of the startup period,
wherein the first control loop is further operable to ramp the first output voltage from an initial voltage at a beginning of the startup period to a target voltage at an end of the startup period.