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Digitally selected outputs for LM317


Guest Zeppelin

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Guest Zeppelin

I was thinking of using digitally selected outputs for the LM317 regulator. IC's Datasheet has the circuit attached which uses discreet transistors; though for a large number of outputs this is not practical :(
Is there anyway to get around this without using discreet transistors?

Thank you
Zeppelin

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Hi Zeppelin, the resistors need something to drive one with 10mA, so a Cmos output won.t be strong enough.
Think about driving the transistors with a binary group of signals, the 4 transistors would give 16 voltages, 5 transistors would give 32 and 6 transistors would give 64 voltages etc.

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You could use something like a UNL2003 darligton driver chip. It has 7 outputs.

Darlington transistors don't saturate so have a pretty high voltage drop when turned on. A transistor array IC would be better because a transistor can saturate with a very low output voltage. Then the current in the voltage-adjustment resistors is predictable.
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We came up with the design and it seems to work well.

Good. I just didn't want somebody to make it with a darlington array then discover that it has a problem with temperature change when the output voltage is set low. An ordinary saturating transistor array won't have a temperature problem.
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