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Is paralleling 3 LDO regulators a bad idea?

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AndersJ:
I’m testing the protypes of a new board.
Unfortunately I made a power supply mistake.

I have 3 LT1762 5V LDO regulators.
One of them should have been larger, 500 mA.
Not a big issue for the next revision,
but I would like a not too difficult workaround for the prototypes.

Is it a mistake to parallell the LDO outputs?

m3vuv:
as far as i know its not the done thing but using resistors on the outputs works by sharing the current,ive not tried it tho.

Siwastaja:
Yes, it practically won't work unless by very good luck. The output voltages typically do not match closely enough, so the highest voltage device does all the work. Adding series resistance after each regulator improves current sharing, but now it stops being a regulated supply. You can kind of adjust the resistances to find some compromise between the current sharing, and regulation. Or you can try to hand-pick regulators to make the current sharing better to begin with. Or you can derate massively, like parallel ten 150mA regulators to get 500mA, hoping that at least three of them happen to be quite close to each other (and maybe some of the rest contributing even slightly).

Note that even if you measure to verify you were lucky, thermal coefficients and aging-related drift make that unreliable.

langwadt:

--- Quote from: Siwastaja on June 07, 2020, 05:14:15 pm ---Yes, it practically won't work unless by very good luck. The output voltages typically do not match closely enough, so the highest voltage device does all the work. Adding series resistance after each regulator improves current sharing, but now it stops being a regulated supply. You can kind of adjust the resistances to find some compromise between the current sharing, and regulation. Or you can try to hand-pick regulators to make the current sharing better to begin with. Or you can derate massively, like parallel ten 150mA regulators to get 500mA, hoping that at least three of them happen to be quite close to each other (and maybe some of the rest contributing even slightly).

Note that even if you measure to verify you were lucky, thermal coefficients and aging-related drift make that unreliable.

--- End quote ---

you can help by putting current sharing resistors before the sense point

ConKbot:
When you start paralleling them, you also face the risk of oscillation. I've had bodged 7805s with 0.5 ohm series resistors oscillate at 80+ MHz under certain load conditions.

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