Efficient low current buck controller topology

Question

In SMPS there are two sources of loss: parasitics, and the controller. Usually, the load consumes more power than the controller, and the parasitic losses dominate. I have the opposite situation. I have a buck converter with a load of 50mW, input voltage of 5V and output voltage of 1V. My question is: what is the best topology that minimizes the controller current consumption? Assume that the dynamic performance is not important.

My current solution is 1 comparator + 1 opamp for a current source in order to get accurate hysteresis implementing a bang-bang controller. Is there anything better?

I should also point out that a 200mW controller will give you the same performance as a linear regulator. Since the controller needs to be powered by the input, this give an upper current consumption bound of 40mA.

Answer 1

There are chips available as dim stated. They have power reduction schemes for better light load efficiency. Such techniques have been called "Burst Mode", "Green Mode", or "Cycle Skip". You could keep working on your design and you will learn much more. When you are finished you will have something that should be cost effective but will use more PCB real estate than a special purpose chip. If you intend to go SMD the extra parts count of your proposed discrete design will not be a penalty.

If your 1 volt is not too critical and can be between two sensible limits, say 950 mV to 1050 mV, then you could set up a low current comparator with hysteresis to switch between these limits. If you then use the output of say the LP393 running on the 5V supply to inhibit your buck convertor and place lots of C across your 1V output, say 100 uF, then the whole thing goes into burst mode. You can now see that the DC output will be a sawtooth waveform always between 950 mV and 1050 mV. The average value that the DVM will read is 1V.

This BALL COCK scheme that I have outlined can work with any buck converter that does not mind being turned on and off all the time. What I have done before is just implement a switchmode buck current source and set the current up at a point above the maximum load current where things are efficient. On your job I would set the current at say 100 mA. On the scope you should see the buck convertor running at 10s of kHz being enabled and disabled at hundreds of Hz. On jobs where output ripple has to be very low I post regulate with a low drop P chan linear regulator.