From: Ryan O'Connor <rocifier@gmail.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Wed, 12 Feb 2020 11:19:40 -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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Thanks for that Bob! What you explain makes sense when you consider it
might be driving an inductive load? It would also explain why I have seen
C1 not installed before for basic loads.

Ryan


On Thu, 13 Feb 2020 at 7:49 AM, Bob Blick <bobblick@outlook.com> wrote:

> Hi Jason,
> RB pinches off the base of the final transistor and makes the Darlington
> configuration faster and more linear. RA is called a base stopper and
> destroys the Q of what could otherwise be a resonant circuit due to
> combination of the output load on the regulator, interelectrode
> capacitances in the transistors, and the output gain. Emitter followers a=
re
> quite susceptible to oscillation. C1 adds local feedback to the TL431 to
> reduce its gain long before the phase shift of the output transistors can
> cause a positive feedback situation.
> Basically, those three components all are there to give stability.
> Depending on the type of load, they may not be needed. The TL431 by itsel=
f
> is a pretty slow device, but the writers of the app note probably wanted =
to
> protect against a worst-case scenario.
> BTW, there are lots of TL431 variations from different manufacturers. Jus=
t
> because it's marked TL431 doesn't mean it is anything like one from TI.
> Including the pinout :)
> Friendly regards, Bob
>
> ________________________________________
> From: piclist-bounces@mit.edu <piclist-bounces@mit.edu> on behalf of
> Jason White
> Sent: Tuesday, February 11, 2020 9:30 AM
> To: Microcontroller discussion list - Public.
> Subject: [EE] TL431+NPN as rated series regulator? Circuit questions.
>
> 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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