On 3/21/2011 9:50 AM, Dwayne Reid wrote:
> At 01:30 AM 3/21/2011, IVP wrote:
>> Hi all,
>>
>> could I have suggestions for controlling an LM317 please
>>
>> IF I'm right assuming that ADJ can't be directly voltage-controlled
>> (correct me if I'm wrong)
>
> Sure it can.  You just have to use a stiff output on your voltage
> source such as an op-amp.
>
> You can think of a LM317 as a beefy power transistor with an accurate
> 1.25V offset between the ADJ pin and the output.  You need the stiff
> output just to control the bias current coming out of the
> pin.  Doesn't need to handle much current (hundreds of uA, IIRC) -
> just needs to be stiff.
>
> My early gell-cell battery chargers dating back from the late '80s
> uses LM317 regulators as pass devices in exactly that
> configuration.  Some of those are still in use to this day.
>
> dwayne
>

One thing to keep in mind about the LM317 is the minimum load current=20
needed to keep it happy.  It regulates well with 10mA or more load=20
current and this is typically supplied with the resistor between the=20
output and the adjust pin.  But it doesn't have to flow through the=20
adjust network.  The minimum load current can be supplied through a=20
resistor or some other current sink.

The adjust pin needs around 50uA of bias current, check your datasheet.=20
  This is the current your control circuit needs to supply.
--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.