Current mode hysteretic control with clocked ramp
Current in a switching converter is controlled using a current-mode hysteretic controller. The high-side switch (usually a PMOS) is turned off when the current in the coil exceeds a certain peak control current. The low-side switch (usually an NMOS) is turned off when the current in the coil falls below a certain valley control current. A current ramp is added to one of these control currents, either peak or valley. The current ramp is initiated by a reference clock signal, which has the effect of synchronizing the switching converter to the reference clock.
1. A current mode hysteretic control circuit, in a DC-DC switching converter, comprising:
a high-side switch, configured to turn off when a current in a coil exceeds a peak current limit.
a low-side switch, configured to turn off when the current in the coil falls below a valley current limit;
a ramp generator, configured to generate a current ramp, initiated with a reference clock signal, and reset when said valley current limit or said peak current limit is reached by said coil current, wherein said current ramp is used to lock a frequency of said switching converter; and
said switching converter configured with an offset between said peak current limit and said valley current limit, to provide hysteretic current-mode operation.
2. The circuit of claim 1 , wherein said DC-DC switching converter is configured to switch with a frequency similar to said reference clock signal.
3. The circuit of claim 1 , further comprising an operational transconductance amplifier configured to generate a control current, proportional to a difference between a target output voltage and an actual output voltage, and to supply said peak current limit and said valley current limit.
4. The circuit of claim 1 , further comprising said coil, having one end connected between said high-side switch and said low-side switch, and another end at an output of said DC-DC switching converter.
5. The circuit of claim 1 , wherein said offset is configured to be added to either or both said peak current limit and said valley current limit.
6. The circuit of claim 1 , wherein said DC-DC switching converter is multi-phase.
7. The circuit of claim 1 , wherein said current ramp is added to said valley current limit.
8. The circuit of claim 1 , wherein said current ramp is subtracted from said peak current limit.
9. The circuit of claim 7 , wherein said current ramp is configured with a slope having a similar order of magnitude to a down slope of a current in said coil, and wherein said down slope is calculated by dividing said output of said DC-DC switching converter by said coil value.
10. The circuit of claim 1 , wherein said current ramp is added to said peak current limit, and is configured with a slope having a similar order of magnitude to an up slope of a current in said coil.
11. The circuit of claim 1 , wherein a slope of said current ramp is configured to have a value between that of an up slope and a down slope of said coil current.
12. The circuit of claim 1 , wherein said current ramp is added to said valley current limit and said peak current limit, wherein said ramp has a similar order of magnitude as a down slope of said coil current.
13. The circuit of claim 1 , wherein said current ramp is added to said valley current limit, wherein said ramp has a similar order of magnitude as a slope equal to, or less than, (S1*S2)/(S2−S1), where S1 is a positive slope of the current in the coil when the high-side switch is on, and S2 is a negative slope of the current in the coil when the low-side switch is on.
14. The circuit of claim 1 , wherein said offset between said peak current limit and said valley current limit is controlled to be proportional to a calculated predicted peak-to-peak current ripple in said coil.
15. The circuit of claim 14 , wherein said offset is controlled to be about 1.5 times that of said calculated predicted peak-to-peak current ripple in said coil.
16. The circuit of claim 1 , where said high-side switch is a PMOS device and said low-side switch is an NMOS device.
17. A method for current mode hysteretic control, in a DC-DC switching converter, comprising:
turning off a high-side device, when a current in a coil exceeds a peak current limit;
turning off a low-side device, when the current in the coil falls below a valley current limit;
adding a current ramp to one of the current limits; and
initiating the current ramp with a reference clock signal, and resetting the current ramp when the coil current reaches the valley current limit, or the peak current limit, synchronizing a switching converter to the reference clock, providing hysteretic current-mode operation.
18. The method of claim 17 , wherein said DC-DC switching converter switches with a frequency similar to that of said reference clock signal.
19. The method of claim 17 , further comprising generating a control current with an operational transconductance amplifier, proportional to a difference between a target output voltage and an actual output voltage across said coil, which supplies said peak current limit and said valley current limit.
20. The method of claim 17 , further comprising adding an offset to either or both said peak current limit and said valley current limit.
21. The method of claim 17 , wherein said current ramp is added to said valley current limit.
22. The method of claim 17 , wherein said current ramp is subtracted from said peak current limit.
23. The method of claim 21 , wherein said current ramp has a slope having a similar order of magnitude to that of a down slope of a current in said coil.
24. The method of claim 19 , wherein said current ramp is added to said peak current limit, and has a slope having a similar order of magnitude to an up slope of a current in said coil.
25. The method of claim 17 , wherein a slope of said current ramp has a value between that of an up slope and a down slope of a current in said coil.
26. The method of claim 17 , wherein said current ramp is added to said valley current limit and said peak current limit, wherein said ramp has a similar order of magnitude as a down slope of said coil current.
27. The method of claim 17 , wherein said current ramp is added to said valley current limit, wherein said ramp has a similar order of magnitude as a slope equal to, or less than, (S1*S2)/(S2−S1), where S1 is a positive slope of the current in the coil when the high-side device is on, and S2 is a negative slope of the current in the coil when the low-side device is on.
28. The method of claim 17 , wherein an offset between said peak current limit and said valley current limit is proportional to a calculated predicted peak-to-peak current ripple in said coil.
29. The method of claim 28 , wherein said offset is about 1.5 times that of said calculated predicted peak-to-peak current ripple in said coil.