From: Spehro Pefhany
Subject: Re: PIC controlled variable power supply
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Date: Sat, 18 Jan 2003 19:34:13 GMT
NNTP-Posting-Date: Sat, 18 Jan 2003 14:34:13 EST
On Sat, 18 Jan 2003 09:44:25 -0700, the renowned Jim Drew
>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.
What do you mean by "too much"? Is this battery powered? Then you
probably should use a different method.
> Is there a specification for the minimum requirement?
See the data sheet. 5mA is about right, taking into account
temperature, unit-to-unit variations, Vref variations, and resistor
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