LT3080 Voltage adjustment

[echen1024]

For simpler MCU control, and output down to 0v, I have decided to move away from the LM317 to the LT3080. However, I have some questions about how voltage is adjusted on the 3080. On the 317, I know that it tries to form a 1.25v voltage reference between V_out and V_set, but I have read that the 3080 simply outputs the DAC voltage after an Op-Amp. Please help me as I am new to Op-Amp theory and analog circuits in general. I have a MCP4725 on hand and a 10k 256 wiper digital pot as well.

[Bored@Work]

It is all in the datasheet, really. The LT3080 has a (weak) current source. The current is driven through an external (programming) resistor. This establishes the reference voltage on the set pin. Since the current source is weak, it can be overridden by an external voltage source driven into that pin.

Sounds good? Well, the LT3080 and some related regulators have a very ugly failure mode. If the voltage difference between the set and the output pin becomes to high, the thing blows up. Such situations can easily happen during power-up, power-down, short circuit, or when rapidly changing the programming voltage.

[IanJ]

Just a note:

LT1083 adjustable regs (similar operation to LM317) also have a problem whereby if you pull adjust pin to zero V suddenly and then release back to some voltage then output doesn't return. You have to reset the device by power down the input. Not much good when designing a CC psu.

Ian.

[echen1024]

Huh... So what would be a good LDO that can easily be Digitally controlled, in any way possible, and preferably down to 0v...

[kizzap]

Just use any form of LDO, and drive it with an op-amp that will be able to drive the adjust pin to your negative voltage. I'm considering the LT1085

-kizzap