digital pots, what's their limits?

I'm wanting to make a voltage regulator that I can control wtih a
digital pot, so I'm thinking about using an lm317 and throwing in a
digital pot so I can vary my voltages between 1.25-30V.... but I was
wondering, would this kill a typical digital pot? with my application
my voltage regulator will be hooked up to a 82ohm resistve load (a
heater basically) and I've been flippin through some digital pot
datasheets but haven't found what kind of voltages I can put across
the pot... I was also wondering, if there's any kind of recommened
resistor value to use on an lm317 between the adj and Vout pins?

much thanks!

 

LM317's Vref is 1.20(min)-1.25(typ)-1.30(max).
So depending on a particular device, you may not be able to achieve
output voltage down to 1.25V precise. But that is a small thing.

Since generally digital pots can not withstand very high voltage,
you can not directly place the digital pot between the LM317's
ADJ pin and GND. For example, I sometimes use the Xicor(Intersil) X9116
digi-pot. http://www.intersil.com/data/fn/fn8160.pdf
In this data sheet in page-7, they show a sample circuit where this
device is being used with the LM317 in order to control the output
voltage. This circuit is ok as long as its output voltage is about
6.7V or less.

Instead of using LM317, I would use a pass transistor and op-amp to
form a simple linear regulator, where the op-amp is in non-inverting
configuration with some gain, and its input voltage is created with
some reference voltage source and digi-pot(as a part of voltage divider).
Another thing to consider is the granurality of the voltage setting.
Digital pots do not adjust their value linearly. So it depends on
how finely you have to be able to adjust the output voltage.

Atsunori

 

you'll need a difital pot that can handle a 27V drop which could be
hard to find.

if it's just for a resistive heater use PWM instead.

 

Correction.
This is when one actaully drives LM317's ADJ input with an external
op-amp. If one forms simple linear regulator with a pass transistor
and an op-amp, the inverting input is driven with the feedback voltage,
of course.

Atsunori

 

Which digital pot ?

You can always place it between the Vref and Vout pins with a series R to
keep the voltage across it low.

What's wrong with an ordinary pot ?

Graham

 

Second Jason's suggestion to go with PWM. Heaters are natural low pass
filters and a PWM is frequently used in this situation (no pun
intended).

WRT the value of R1: 240 ohms is often used. This gives about a 5 mA
load current through the adjustment resistors, thereby meeting the
minimum load requirements. I.e., it stays in regulation even if the main
load is disconnected.

 

Use an op amp that accepts 30V power, and the digital pot
on 5V power to make an attenuator (output of
attenuator being in the 0...5V range). Set the op amp to
voltage gain of 6, put the LM317 into the op amp
output circuit as a follower.

You still get the thermal and short-circuit limiting of the
LM317 regulator, but your reference voltage is the 5V
supply (which most digital pots will require anyhow).

There are simpler circuits that would work if you had a
four-terminal adjustable regulator instead of the LM317,
like LM2941 or the old uA78G series. The 'reference' pin
on those always balances at low voltage, suitable for the
direct connection of a digital pot.