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From: "Roger Hamlett"
References: <3E27B2A3.email@example.com> <firstname.lastname@example.org> <3E2830B5.email@example.com>
Subject: Re: PIC controlled variable power supply
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Fri, 17 Jan 2003 17:45:43 -0000
NNTP-Posting-Date: Fri, 17 Jan 2003 17:46:01 GMT
Organization: ntl Cablemodem News Service
"Jim Drew" wrote in message
> 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
> > | |
> > x--------------x
> > |
> > R2
> > |
> > Op -------x
> > amp
> > buffer
> > 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. The adjust pin only needs to provide a relatively low current. For the
LM317, it is specified by the data sheet as 'Adjustment pin current', with a
maximum value of 100uA, and a 'change' for the output range of 10mA to the
maximum output current, at a maximum of 5uA, and a 'typical' value of just
Hence you can use resistors as shown to control the regulator, with only a
very small change in output under different loads. Ideally you want the
current in the divider to be a lot larger than the current to the adjustment
pin (to minimise the voltage change that any change in output current will
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