Digital control of a LM350/333

[dwurmfeld]

I am doing research on digitally controlling a linear regulator. I guess there are a number of methods to use, I would like to prototype a circuit using a "Digital Pot". The problem I see is the current into the adjust pin can be as high as 100uA, typically 50. Looking at the specs for some popular digital pots I see maximum ratings in terms of volts, but no rating of how mych current the "pot" can safely handle. The Intersil X9C103 looks to be a likely part, but I have no experience using digital pots.

The other approach is to "link in" resistors in parallel using an octal darlington array like the ULN2803A. The problem there is trying to select the resistor "set" that will will give a reasonably linear control over the supply.

To complicate matters more, I would like to make the supply adjustable down to zero volts, which necessitates bring the adjustment pin negative to compensate for the 1.2V reference. I found no digital pot that would tolerate a negative voltage on the wiper pins w.r.t ground.

Anyone out there have any ideas? anyone actually get a LM317 or LM350 to work with a digital control of any kind?

Thanks,
David, Melbourne Florida

 

[hjames]

In order to "shutoff" a regulator, you need to take its feedback pin above it's reference voltage. For small voltage deviations, the concept is called "voltage margining" - the general idea is to inject a current into the feedback divider of the regulator to get the output to move. The easiest way of "calculating" how much current to inject is to just have an ADC on the output watching what the voltage is, and increment/decrement accordingly.

Note that the current source is just a resistor connected to a voltage ouput DAC - or just a simple resistor DAC (google for "R-2R DAC" and you'll see a lot of microcontroller circuits). You'll probably want to use a nice high resolution (10+bits) though.

 

[Hero999]

The problem I see is the current into the adjust pin can be as high as 100uA, typically 50. Looking at the specs for some popular digital pots I see maximum ratings in terms of volts, but no rating of how mych current the "pot" can safely handle.

100uA isn't much, I wouldn't worry about it.

The problem there is trying to select the resistor "set" that will will give a reasonably linear control over the supply.

What's the voltage range/acuracy?
How many bits?

You could always connect an array of resistors in series and use MOSFETs to short them out.