MIC5270 negative LDO misbehaving

The sharp edge of the sawtooth actually happens in two goes, about 2 us apart and of about equal size. So whatever is going wrong seems to be happening at very roughly 500 kHz.

It does it over the full range of loads, ~300 uA to 100 mA where the current limit kicks in.

I also tried reducing the input voltage and feedback ratio to match the one that works on the other board--no change to the instability. This morning I'll try running it off a bench supply and see if that fixes it.

I really miss dumb chips sometimes. :(

Cheers

Phil Hobbs

Nah, just resistors at the moment.

The oscillation frequency goes up because the cap bleeds down faster with a heavier load, but no other noticeable change.

Cheers

Phil Hobbs

Mine's a 527-00, which is discontinued but still available. There are lots of older ones on the used market for cheap, but AFAICT they mostly take the smaller-diameter eraser refills, which are unobtainium. The

Cheers

Phil Hobbs

Nah, doesn't have a grid leak. ;)

Cheers

Phil Hobbs

The input bypassing would affect that sort of oscillation.

Oh, I have an AC-powered electric eraser and an electric pencil sharpener. That sketch would make a superb blue-line copy, but it's a lot easier to photograph with a cell phone camera, drawing lying on the floor. We're still working mostly by email.

I have a long USB cable on my phone that lets me shoot drawings and my whiteboard without disconnecting from my PC. It won't quite reach my scopes.

I like to draw on vellum, at my drafting table in the big window. It's functional enough.

I'm going to re-do a lot of that later this morning. Too many possibilities. Someone pointed out the the optional OCXO can pull a full amp during warmup. Fixing that of course cascades in all directions. We have a lot of switched +5 available, so I guess I'll do a home-made LDO to make 3.3Q, the quiet 3.3 supply.

Power supplies are pretty close to grunt work, with hazards. There's no eureka-mode upside to getting them right.

If an opamp doesn't like a capacitive load, there is the option to add a small series resistor to the c-load and split the feedback into AC and DC paths. I use that with TCA0372 a lot, with a 1 ohm series resistor.

It's not current or thermal limited either.

Oh well. If it can't drive that then there really is something wrong.

Sounds like the control loop has a pole in the wrong place for whatever reason. I notice the weasel words "designed for ceramic capacitors" and "typically only requires 1uF" on the output for stability. I'd be tempted to put a small fast decoupling capacitor in close either side.

What's the input? You have a couple of negative rails mentioned, that could be candidates.

This chip DOES have a thermal limiter; though I doubt it's very fast, there's no reason for IT to be stable - not in IT's job description.

Rated at ~160mW with Tlead ~ 85C.

RL

Didn't Intel once try putting switching regs on their CPU chips? I vaguely remember something like that.

Just looking at the data sheet for this thing - it describes a non-existed enable line. The unused pins are supposed to be NC.

I'd look for similar parts that DO have an enable line, identify the pin (only pin1 is left in the adjustible version), and treat it as such on this part, wiring it 'on' (even if it doesn't have one). Somebody may be cutting down on binning costs.

Three terminal regulators have also demonstrated sensitivity to the value of resistors in the feedback string. Micrel/Microchip doesn't specify a prefered value on specs, but you might see what an order of magnitude does.

RL

Turned out to be the spikies on the input after all. A 1-ohm resistor and a 4.7 uF cap fixed it right up. Since the spikes are nearly-symmetric damped 100 MHz with about 4 cycles visible, I don't know why the 330 ohm bead/1uF cap didn't do an even better job, but apparently it didn't.

Thanks to all for the help.

Cheers

Phil Hobbs

Tell me about it. ;)

Cheers

Phil Hobbs

No ringing in RC :) GH

Exactly. Ferrite beads are tricky. An RC at the input of a regulator is a reliable lowpass filter. Sometimes you can size the resistor to absorb some of the power dissipation too.