I need to regulate a +24 supply down to +22, 0.8 amps maybe. There's not much headroom for +24 drift, so I need an LDO. Few if any LDOs can tolerate +24 in. We stock LM2941, adjustable LDO, which for some reason is limited to an output range of +5 to +20.
I suppose I could design my own reg, but that's silly. Anybody know of an affordable HV LDO chip?
Microchip makes its MIC2941A without the LM2941 output restrictions. I could use that, with the LM as a backup source, I guess.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Just wondering... why not use a zener to float the gound up +12V or so, and use a 12v LDO?
I guess I could do something like that. I could park an LM2941 on top of, say, a 5 volt rail, to overcome its +20 max output limit. It would think it's a 17 volt regulator.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Most 3-terminal adjustable regulators are rated for the drop from the supply pin to the lowest voltage on any regulator pin.
Or is it that all the three-terminal adjustable regulators seem to be high-dropout "LDO"s?
(I hate what they've done to linear regulator terminology...)
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com I'm looking for work -- see my website!
Be careful you don't create a service problem where this depends on that which depends on this.
I'm trying to fix a TEK TDS-540 power supply without proper documentation. This depends on that which depends on the other that has tentacles into everything else. Fixing the part that is smoked results in it getting smoked again.
Yeah, all linear regs are "LDO" now.
I really like the LM1117, 3 pins, which I call an MDO regulator. For some reason, it's only rated for 20 volts max input. It actually dies at 60.
I just tested an LM2941, which has a specified output range of 5 to
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
So, use it! 24V on the input terminal, 22V on the output terminal, and 20.75V on the 'adjust' terminal. Not a high voltage difference anywhere in sight.
There are a few. LTC's LT3086 is rated 40V and 2A. Nice. Intersil's ISL80138 is rated 40V. And TI's 36V TPS7A4901. Others have 30V ratings. Sorry, only 150mA, but you could add an NPN follower in the loop. The LT3066 is rated at 45V and 500mA.
The danger in that approach is what happens if the output is shorted. Gross!!
Remember, your copy of AoE III is your friend. Figure 9.113, page 698, shows an enhancement-mode MOSFET helping a low-voltage LDO, etc., to work at high voltages. Figure 9.104, page 693, can also be inspiring. Depletion-mode MOSFETs will simplify the circuit in Figure 9.113. There are many useful HV regulator configurations you can explore, based on depletion-mode MOSFETs.
--
Thanks,
- Win
I want to regulate +24 +-5% down to +22. That only leaves me 0.8 volts across the regulator.
LM317 is PNP+darlington. 2 volt dropout.
LM1117 is PNP+NPN. 1.2 dropout.
LM2941 is pure PNP. 0.4 dropout.
None of them will do what I want legally.
PMOS would be nice.
Depletion-mode MOSFETs will
I have about 10 supply rails to regulate, so I'm not eager to design my own LDO out of parts. I guess I'll use the LM2941 and push the max Vout spec a couple of volts. It tests fine.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Why not? If you want +22V +-5%, the LM1117 is good to go.
I've solved this problem before (given some provisios of course, but that's nothing that can't be solved with just one additional tiny chip, hardly "make your own" level). But seeing as it's for commercial application, I don't feel it would be appropriate posting it here.
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website:
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A zener across the regulator will protect the regulator, but if it were e.g. a 15V zener then it likely wouldn't last very long when the output is shorted. Choosing the lowest-voltage zener that won't normally conduct enough to pull up the output would help. Perhaps a polyzen would be even better, to reduce the input current when the zener gets hot. Hopefully the polyswitch part of the polyzen is rated to a much higher voltage than the zener part.
There's the TPS7A4701 that will work here but it comes in a weird package and is on the pricey side in small quantity.
LT3080? (there are others in the LT308X series) Is cost important?
George H.
The reference in the MC part is about 3% lower than the TI part. Speculatin g on the output voltage range limitation on the TI part, my guess is that i t may be related to stability more than process withstanding voltage, so so mething you might want to look into like break the loop and sweep it in tem p chamber over 0.1 to 1.5 loading- LOL.
If the output voltage limitation is a stability issue then a quick fix would be to place a stiff voltage in series with R1 to GND in the diagram on page 1 , simplfd schem :
Please view in a fixed-width font such as Courier.
. . . . OUT -----+------+------+--> . | | | . | | | . [R2] | --- . | | --- . | | | . ADJ-----+ [RB] -+- . | | /// . | | . [R1] | . | | . | | . +------ . | . | + . \-\ VZ . ^ . | - . | . | . --- . /// . . R1 . VOUT= VREF x ( 1 + -- ) + Vz . R2 . . VOUT-VZ . RB= ------- . 10mA or so . .
That's a little marginal on dropout maybe. The 24 is from a wall-wart and could be down a bit.
I hate power supplies.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
itto:
one of these?
Bye Jack
That's a cute part with a really interesting architecture. Too bad about the package.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
By all means keep it to yourself.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
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