linear regulator design, darlington, 5V dropout @ 2.5A !? Answered
The question says it all, really. I built up a linear power supply shown below. My unregulated 12V supply can fall as low as 14.7V @ ~ 2-3 A but if I end up being unable to go higher than 9.5V at 3A. Because I'm driving a motor as a load, the current drawn is very dirty. At 10V I see small dips that correlate to dips in the supply voltage when it goes just under 15V.
So what can I do to help alleviate this, as I was under the impression that I would see a absolute maximum of 3V of dropout, not a whopping 5V. (p.s. the maximum gate current into the S8050 is 10mA, but currently set to 6.77mA w/ a 10 ohm resistor)
p.p.s: MY design uses an LM324, which is really not the best, as it's slow and cannot sink much current. To alleviate the first issue I simply added current mirrors to aid the op amp in sinking current in a simalar fashion as a current source used for an open collector outputs.
What op amp should I use instead for better performance? I suspect the LM324 due to it's low slew rate and low GBP may cause poor high frequency rejection (esp. if I choose to use a switching pre-reg) and poor high frequency load regulation (causes poor transient response on the output)
Op amps I can choose from:
LM324; // slow, from my testing, this one has inputs that work to ground :D
LM224; // ditto, but in a weird package
LM386; // slow, but from my testing, this one also has inputs that work to ground.
LM741; // ancient relics of the past, not rail to rail by any means, sucks IMO.
UA1458TC // ancient relics of the past, I don't have much knowledge of them. Inputs do not work to ground.
LM318H; // its in a fancy metal case :D
TL072CP; // JFET inputs, but I think the output current is limited and the output can't go below the noninverting input.
OP37; // These things can't even give a stable output by themselves!
OP27; // ditto
LT1007; // have not messed with it much
LT1363; CRAZY fast, 1000V/uS!!! Maybe too fast...