From: Bob Blick <bobblick@outlook.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Wed, 12 Feb 2020 10:46:09 -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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Hi Jason,
RB pinches off the base of the final transistor and makes the Darlington co=
nfiguration faster and more linear. RA is called a base stopper and destroy=
s the Q of what could otherwise be a resonant circuit due to combination of=
 the output load on the regulator, interelectrode capacitances in the trans=
istors, and the output gain. Emitter followers are quite susceptible to osc=
illation. C1 adds local feedback to the TL431 to reduce its gain long befor=
e the phase shift of the output transistors can cause a positive feedback s=
ituation.
Basically, those three components all are there to give stability. Dependin=
g on the type of load, they may not be needed. The TL431 by itself is a pre=
tty 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. Just =
because it's marked TL431 doesn't mean it is anything like one from TI. Inc=
luding the pinout :)
Friendly regards, Bob

________________________________________
From: piclist-bounces@mit.edu <piclist-bounces@mit.edu> on behalf of Jason =
White=20
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

--=20
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