It won't be a simple linear or buck, as there's no way to get -15V at some current from -15V, if there's a pass element controlling the current.
If you'd be satisfied with a few millivolts less than -15, then it's a trivial linear task.
see a.b.s.e. transfer1
RL
I'm just guessing here, 'cuz you haven't said what you're trying to do with the 100uA output, but I'm going to guess that it's for an on/off control pin on a switcher module or some such...at least I've been faced with 'zactly that problem (and solved it with a very similar circuit as what has been posted). Does the output have to stop at
-10Vmax and -15Vmin? Or is -10 the minimum allowable for that state? Can it be -0.05V instead? If it's to a control pin, is the current from that pin known to be in one direction? If all that's true, you may be able to simplify further, and probably not need a connection to the +5 at all: drive the base w/ a voltage divider between the input and -15, and also replace the 20k w/ a short if the output is OK at nom. 0V. Presumably, you don't need _exactly_ -15 and -10V outputs; what is the allowed range, and what it the output current max and min at each? You could probably use a (depletion-mode, of course) P-channel JFET, source grounded, gate driven directly by the logic input, drain driving that 20k-10k divider (or maybe even just a 10k pulldown). Note that a 10k pulldown in the original circuit or with the jfet will allow a 1 volt drop with 100uA: the output could be only
-14 in that case. Though a jfet would eliminate a couple resistors, unfortunately, use of jfets is deprecated.
This is such an easy task that I'm surprised that you posted with "impossible" in the subject...the table of desired values immediately tells you you need an inversion, and you get that with a common-emitter/common-source stage. Then it's just a matter of where you tie the emitter/source, and how you drive the base/gate to get the device to switch. Where you tie the emitter/source also gives a strong hint which polarity device to use: is the e/s above or below the output range you want to use? Things like, "I need to sink some current, but don't have to source any," or "I need to source current but don't have to sink any more than a couple microamps" can affect how you do the job.
Cheers, Tom
snipped-for-privacy@gmail.com wrote:
People usually scream latch-up with the slightest little substrate current, but your circuit would look something like this: View in a fixed-width font such as Courier.
. . . . . in >-------. ----. . | | | . | | --- . [22k] | /// . | |< . +----| pnp . | |\\ . | | . | | . | [10k] . | | . [110k] +------>out . | | . | [4.7k] . | | . '------+ . | . --- . -15V . .
Not to be too picky, but how many (rail to rail) op-amps do you know of that produce good outputs when driven to the rail.
-- JosephKK Gegen dummheit kampfen die Gotter Selbst, vergebens. --Schiller
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