Understanding Operation of TL431..Purpose of diode....?

malli arjun1

Intellectual 610 points

Other Parts Discussed in Thread: TL431

Hi,

I want to understand the internal operation of TL431.

I always see 2 conditions, using resistor divider for REF pin

When REF > 2.5V, Internal BJT will be more ON taking more current

When REF <2.5V Internal BJT wil be less ON or OFF taking less current or no current

But, no where internal diode embedded in TL431 behavior was mentioned. what is the purpose of this internal diode ?

over 10 years ago

0 Ron Michallick over 10 years ago

TI__Guru*** 149226 points

Malli,

The diode is unavoidable with the process technology of TL431. The diode also makes the device similar to a Zener diode that passes current in a forward biased mode.

0 malli arjun1 over 10 years ago in reply to Ron Michallick

Intellectual 610 points

Hi,

I have never seen TL431 is used such a way that this diode is forward biassed like Cathode -ve voltage and Anode +ve voltage.

Figure 7 in datasheet Vka Vs Ika mentions the Breakdown voltage of TL431 ~ 2.5V, is this the breakdown Vz of Internal diode?

0 Ron Michallick over 10 years ago in reply to malli arjun1

TI__Guru*** 149226 points

Malli,

At 2.5V the output transistor turns on similar to the breakdown of a Zener diode. However, the TL431 has better regulation than a Zener diode.

0 malli arjun1 over 10 years ago in reply to Ron Michallick

Intellectual 610 points

So, actually in TL431, there is no Vz or breakdown, it is just BJT turning ON. am i correct? Terminology is too different..

0 Ron Michallick over 10 years ago in reply to malli arjun1

TI__Guru*** 149226 points

Malli,

Correct, no zener breakdown in TL431.

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Understanding Operation of TL431..Purpose of diode....?