From: "Roger Hamlett"
References: <3E27B2A3.firstname.lastname@example.org> <email@example.com> <3E2830B5.firstname.lastname@example.org> <3E2983E3.email@example.com>
Subject: Re: PIC controlled variable power supply
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Date: Sat, 18 Jan 2003 17:50:44 -0000
NNTP-Posting-Date: Sat, 18 Jan 2003 17:58:35 GMT
Organization: ntl Cablemodem News Service
"Jim Drew" wrote in message
> 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
> > LM317, it is specified by the data sheet as 'Adjustment pin current',
> > maximum value of 100uA, and a 'change' for the output range of 10mA to
> > maximum output current, at a maximum of 5uA, and a 'typical' value of
> > 0.2uA.
> > Hence you can use resistors as shown to control the regulator, with only
> > 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
> > pin (to minimise the voltage change that any change in output current
> > produce).
> 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?
The pin draws a _little_ current, but the amount changes slightly with
output load. The 'best' solution (which then retains the thermal shutdown,
and current limiting of this regulator), would be to drive the pin from an
op-amp, using the incoming 'unregulated' supply to feed this (assuming the
op-amp is rated for the voltage concerned). You can have the +ve input pin
of the op-amp fed from either an R-2R ladder, or a PWM source, allowing it's
voltage to be controlled. Then the feedback connection linked to a resistor
divider, taking (perhaps) 1/3rd the output from the regulator. You will have
to be a little careful about the stability of this (the combined gain may
lead to an unstable solution). The input impedance of the op-amp, can be
nice and high, allowing you to use high resistor values. Setting the
non-inverting input to (say) 2v, then gives 6v out of the regulator.