[SOLVED] Could someone please explain what this circuit is doing


I was looking at the power section of the basys 3 schematic(https://reference.digilentinc.com/_media/basys3:basys3_rm.pdf), which use two DC-DC buck converters (LTC 3633, LTC 3621) for the power rails of the peripherals and the FPGA:

The power on sequence of the artix-7 requires that we first enable the 1V rail, then 1.8V, and finally the 3.3V rail, which is why the enable (RUN2 of LTC3633) of the 3.3V rail (SW2 on the LTC3633) is connected to the PGOOD1v8 pin on the LTC3621.

My question is what is the purpose of this circuit:

My guess is it is amplifying the current from the PGOOD1V8 for some reason, although I could be wrong. Does anyone have any idea of what the purpose of this circuit is?

Thank You

Part of a low-voltage lockout scheme? A way to enhance the stability of the power converters when they are lightly loaded during the power-up sequence?

" VU " is the power input voltage. (NOT an indicator of signal level on an audio channel, for all you old recording and broadcast guys!) When the power input voltage exceeds approx 1.75 volts (twice the turn-on threshold of N-MOSFET Q4), Q4 applies a load of about 30 mA to the 3.3 volt rail. After the 1.8 volt rail has stabilized and asserted its “Power Good” signal, Q5 turns on and removes the extra load from the 3.3 volt rail. (I am assuming that “PGOOD1V8” is an active-HI indicator of acceptable conditions on the 1.8V rail.)

Many switching supplies become unstable when they are lightly loaded. This circuit may be there to ensure that the 3.3V regulator always sees a minimum load, even during the start-up sequence before the system is drawing load current from the 3.3V rail.

I don’t know if Linear’s models for these power converter IC’s accurately model instability under light-load conditions. If they do, it could be fun to play with this circuit in LTSpice on a rainy Friday afternoon when there isn’t much happening around the lab.



Thank you very much @dchisholm, that helps!

P.S: Yes PGOOD is active high

Q4 is turned on by the pullup (R181) to VU (Unregulated Voltage), this prevents any leakage currents from charging the output caps and keeps the 3.3V bus at or close to 0V. When PGOOD1V8 becomes active it turns on Q5 which turns off Q4 removing the additional load from the 3.3V bus. This circuit is not required for normal operation of the LTC3633 and is only there to help ensure correct power on sequence. The LTC3633 has no minimum load requirements.