From: Harold Hallikainen <harold@mai.hallikainen.org>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Tue, 11 Feb 2020 18:42:19 -0800
Subject: Re: [EE] TL431+NPN as rated series regulator? Circuit questions.
Thread-Topic: [EE] TL431+NPN as rated series regulator? Circuit questions.
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I think C1 provides a "soft start" by pulling the input to the error
amplifier up on power up. I've seen a similar trick used on POLA modules.
It's interesting that the data sheet shows this capacitor but does not
explain it.

I think the resistor from the base to emitter of the second transistor
ensures that the second transistor does not conduct due to leakage of the
first.

Harold


> Hello everyone,
>
> The TL431 datasheet shows the attached circuit has an example of how
> to create a series regulator.
>
> I would like some help understanding the purpose of C1, RA, and RB.
>
> I assume RA is to prevent oscillation?
> What is C1's purpose? Why is C1 from cathode to reference? (to prevent
> oscillation?)
> What is RB's purpose? (to improve turn off time?)
>
> --
> Jason White
> --
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