Peak current servo
The proposed disclosure combines peak-mode monitoring with valley-mode control, in a Buck switching converter, by means of a peak-current sampling circuit, not to turn the high side device off, but to control a slow loop, which in turn controls a variable offset incorporated into the loop control current. This helps the loop control current define the exact peak current, regardless of what other offsets, compensation ramp or peak-to-peak current ripple, are applied to the loop control current. The peak current is determined by an operational transconductance amplifier (OTA), whose maximum current is clamped to a programmed value. The loop control current is most likely implemented using a digital successive approximation register (SAR) system, but may also be implemented using a slow analog control loop.
1. A peak current servo circuit, comprising:
high side and low side devices for a switching converter;
a sampling circuit, configured to sample the peak high side device current just before the point when the high side device switches off;
a comparator, configured to compare said sample with a reference value, during a low side device on time;
a control loop, configured to successively increment or decrement a stored value in a counter, based on the output of said comparator;
wherein said peak current servo circuit is configured to use said stored value to generate an offset current to shift said peak high side device current up or down, and to match said high side device current to said reference value in cycles to come; and
a plurality of devices matched to said high side device, configured to supply a current for said reference value.
2. The peak current servo circuit of claim 1 , wherein said sampling circuit and control loop comprise PMOS pass devices and sampling capacitors.
3. The peak current servo circuit of claim 1 , wherein said sampling circuit is configured to store the final values of said peak high side device current, across an inductor, and said reference value, across said plurality of devices matched to said high side device, as inputs to said comparator.
4. The peak current servo circuit of claim 1 , wherein said comparator comprises a sense amplifier.
5. The peak current servo circuit of claim 1 , wherein said control loop is configured to modify said offset current and to remove the effect of a compensation ramp, in a switching converter.
6. The peak current servo circuit of claim 1 , further comprising an operational transconductance amplifier configured for a maximum current clamped to a programmed value, establishing a peak-current limit.
7. The peak current servo circuit of claim 1 , wherein said control loop comprises a digital successive approximation register (SAR) system, or using a slow analog control loop.
8. The peak current servo circuit of claim 1 , wherein said high side device is PMOS and said low side device is NMOS.
9. The peak current servo circuit of claim 1 , wherein said peak current servo circuit is configured to provide peak high side device current in peak-mode switching converters.
10. The peak current servo circuit of claim 1 , wherein said peak current servo circuit is configured to remove output current offset in valley-mode switching converters.
11. The peak current servo circuit of claim 1 , wherein said peak current servo circuit is configured to implement a peak-current limit in valley-mode converters.
12. A method for peak-mode monitoring with valley-mode control, in a peak current servo circuit, comprising the steps of:
providing high side and low side devices for a switching converter;
providing a sampling circuit to sample the peak high side device current just before the point when the high side device switches off;
providing a comparator, to compare said sample with a reference value, to determine if said sampled value is larger or smaller than said reference value, during the low side device on time;
providing a control loop, to successively increment or decrement a stored value based on the output of said comparator; and
providing an offset signal, to shift said peak high side device current up or down, and to match said peak high side device current to said reference value, in cycles to come.
13. The method for peak-mode monitoring with valley-mode control, in the peak current servo circuit of claim 12 , wherein said sampling circuit stores the final values of said peak high side device current across an inductor, and said reference value across a plurality of devices matched to said high side device, as inputs to said comparator.
14. The method for peak-mode monitoring with valley-mode control, in the peak current servo circuit of claim 12 , wherein said control loop modifies said offset current and removes the effect of a compensation ramp, in a switching converter.
15. The method for peak-mode monitoring with valley-mode control, in the peak current servo circuit of claim 12 , wherein an operational transconductance amplifier provides a maximum current clamped to a programmed value, establishing a peak-current limit.
16. The method for peak-mode monitoring with valley-mode control, in the peak current servo circuit of claim 12 , wherein said peak current servo circuit provides peak high-side device current in peak-mode switching converters.
17. The method for peak-mode monitoring with valley-mode control, in the peak current servo circuit of claim 12 , wherein said peak current servo circuit removes output current offset in valley-mode switching converters.
18. The method for peak-mode monitoring with valley-mode control, in the peak current servo circuit of claim 12 , wherein said peak current servo circuit provides a peak-current limit in valley-mode converters.