Date: Sat, 18 Jan 2003 09:42:11 -0700
From: Jim Drew
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Subject: Re: PIC controlled variable power supply
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Roger Hamlett 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. 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
So, would it be possible to drive the adjust pin directly with an R2R
ladder off of the PIC, or does the adjust pin use current and not
voltage to adjust the output voltage of the regulator?