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From: Spehro Pefhany
Subject: Re: PIC controlled variable power supply
References: <3E27B2A3.firstname.lastname@example.org> <email@example.com> <3E2830B5.firstname.lastname@example.org>
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Date: Fri, 17 Jan 2003 18:01:17 GMT
NNTP-Posting-Date: Fri, 17 Jan 2003 13:01:17 EST
On Fri, 17 Jan 2003 09:35:01 -0700, the renowned Jim Drew
>Spehro Pefhany wrote:
>> Use your PWM output or R-2R ladder to generate a voltage. Buffer the
>> voltage with a unity-gain op-amp and feed it to the adjust pin on an
>> LM317 as so.
>> 0---- IN Out -------x----->
>> adj 240R
>> | |
>> Op -------x
>> This gets you short circuit and overtemperature protection, and
>> the cost will be very low.
>I was under the impression from the data sheets on the LM317 that the
>adjust pin can sink up to the output level in current. Normally just
>resistors are used for the adjustment, and these are required to be
>values that can handle the power. Is this not correct?
No. You must sink a minimum current through the load or the output
voltage may not regulate, see the data sheet, it's a few mA. The 240R
resistor takes care of that by drawing about 5mA from the regulator
regardless of output voltage. The adjust pin itself doesn't sink or
source much current (microamps). So, in this case, the op-amp must
sink 5ma. In this case we want to cover 5-8 V output. Suppose the
op-amp covers 1-4V. Then we might make R2 such that the output voltage
is nominally 5V with 1.5V out, -> R2 is about 430R.
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