Negative LDO with TL431

[AcHmed99]

I

[amspire]

The TS431 looks pretty unstable in both sets of waveforms.

I haven't used the TL431 in regulators myself, but a few things don't look right.

Is C2 really meant to be across R3? I would think it would make the regulator overreact to transients which could cause those spikes.

Also the power mosfet driving circuit looks pretty weird to me. -15v in and -12v out - that means 3 volts across the gates of the N channel FETs. So even if the TLS431 is trying to turn the Fets hard off, the two FETS are still getting 3 volts in total across both their gates - so the TLS431 cannot turn the FETs hard off if they have minimum gate turn on voltages.

If the input voltage goes above 15V, I doubt there is any way the circuit could regulate. It is not a safe circuit.

It would be much easier to use a P Channel power FET (Drain and source swapped compared to the 5838 fet) with the gate going straight to the TLS431, and the IRF034 removed - and C2 removed as well. Now if the TLS431 is hard on, the FET will turn fully off.

The only catch is you need a bit more negative gate bias for the FET - 3v is probably not enough. If you are building the flyback supply powering this, there may be some tricks you can use to get a  few more volts out to bias a P Channel Fet gate. For example, a capacitor from the 15V flyback transformer output (before the current diode) followed by a voltage doubler diode circuit using 1n914s would probably give you enough volts. You would definitely get over -15V.

[Bored@Work]

I've been studying the few negative LDO's and the ones that do show a schematic show a NPN bjt as the pass device there must be a simple way to control an NFET on the negative rail using a TL431.

But that is the point. Negative linear voltage regulators are typically build as LDOs, because an NPN is easier to integrate. You get the LDO property for free when you buy a negative regulator. Even the 50 cent or maybe $1 LM7912 has a dropout voltage in the 1.5 V range. Your 15V input should be good enough for a 7912. 50 cent, maybe $1, and you are done.