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Date: Sat, 18 Jan 2003 09:44:25 -0700
From: Jim Drew
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Subject: Re: PIC controlled variable power supply
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Spehro Pefhany wrote:
>>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.
Ok, but 5ma is way to much current draw for idle times. Is there a
specification for the minimum requirement?
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