Apply voltage to IC's current source pin? - Page 1
Forum section: electronics / beginners

author: MaxSimmonds
author_rank: Contributor
timestamp: September 23, 2021, 02:31:35 pm
content: Hi guys,I'm looking at a few different circuit ideas to drive a fan, that's proportional to temperature. One method I've been thinking is by using an NTC resistor, with some amplifying electronics, to drive the input of an LT3080 SET pin (https://www.analog.com/media/en/technical-documentation/data-sheets/lt3080.pdf)However, this will only be possible if I can provide a voltage across the SET pin and ground, rather than using a current set resistor like you're supposed to. The current set works by an internal 10uA current source, which then drops a voltage across the resistor. This, obviously, drops a voltage across this resistor which is then used as a reference for the internal op amp.Can I simply drive it with a voltage? In simulation, this works, but I'm worried I might kill something in real life. Here's a simulation attached to show the idea (there's also a pregulator switch mode too, since the input is over twice the output of the LDO, to keep things cool).I'd appreciate any help, if this is possible, I think it's a nice cool circuit!

author: Terry Bites
author_rank: Super Contributor
timestamp: September 23, 2021, 03:07:46 pm
content: Looking at the data sheet, there seems to be no reason you cannot override the Iset current source. The maximum input current to this pin is 10mA.There are pwm controllers deigned for use with an NTC eg MAX31740 or TC646B/TC648B/TC649B at a quarter of the price of an LT3080.

author: perieanuo
author_rank: Frequent Contributor
timestamp: September 23, 2021, 03:14:43 pm
content: this simple one works perfectly

author: David Hess
author_rank: Super Contributor
timestamp: September 23, 2021, 03:22:39 pm
content: There is nothing wrong with driving the SET pin with a voltage. The regulator then operates as a voltage follower.The same applies to older regulators like the LM317; their adjustment pin can be driven with a voltage to produce an output of Adj+1.25 volts or whatever.Note that in both cases, the minimum output current requirement of the regulator still applies. For the LT3080, this is 1 milliamp.

author: magic
author_rank: Super Contributor
timestamp: September 24, 2021, 06:15:32 pm
content: There are two problems, both related to absolute maximum ratings.Firstly, SET voltage can't exceed the input voltage to the regulator. Depending on overall circuit design, this may or may not become a problem when some upstream supply rail fails.Secondly, there may be a limit on the differential voltage between SET (IN+ of the internal voltage follower amplifier) and OUT (IN- of the same). For LT3080, it's specified as only ±0.3V. This becomes a problem when the output gets shorted to ground, for example, and defeats the short circuit immunity that you would normally expect from such regulator.Both problems can be mitigated by putting a bit of resistance before the SET pin in order to limit fault current flowing into the pin's internal clamping diodes to less than 10mA. (This limit is also specified in the DS). Note that SET pin's current will flow through that resistance and produce a voltage offset.