TL431: About the question when two TL431s are connected in series.

Bob Lee62837

Mastermind 7230 points

Part Number: TL431

Dear support member,

My customer used TL431.

I have a question.

(Question1)
Is it possible to use two TL431 connected in series to make 2.495 × 2V?

(Question2)
If it make 2.495 × 2V possible,
could you teach if there is something like a recommended circuit?

Best regard.
Bob Lee.

over 5 years ago

0 Trailokya Rai over 5 years ago

TI__Genius 11351 points

Hi Bob

You can get 2.49*2V by adjusting gain cuircuit.

By putting R1 = R2 in the given circuit , you can get 2.49*2V.

Regards

Trailokya

0 Bob Lee62837 over 5 years ago in reply to Trailokya Rai

Mastermind 7230 points

Dear Trailokya Rai,

Thank you very much for reply.

May I ask a add question?

(Question)

Can I use two TL431s in series?

Best regard.
Bob Lee.

0 Trailokya Rai over 5 years ago in reply to Bob Lee62837

TI__Genius 11351 points

Hi Bob

You can use 2 TL431 in series with cathode to ref shorted . You will get 2*Vref Volts , when you are forcing more than minimum current.

Regards
Trailokya

0 Bob Lee62837 over 5 years ago in reply to Trailokya Rai

Mastermind 7230 points

Dear Trailokya Rai,

Thank you very much for reply.

May I ask add a question?

（Question）
Q1.
My customer want to use following schematic.
Is there problem?

Q2.
Does the current change and cause problems such as oscillation?

(Specification condition)
Input:15V
Output:5V

Best regard.
Bob Lee.

0 Trailokya Rai over 5 years ago in reply to Bob Lee62837

TI__Genius 11351 points

Hi Bob

In the given schematic , Vka will depend on current taken by load.
Vka = 15 - Iload * R_series (given that Ioff is few nA)

Don't connect to resistance to bottom TL431. It may give issue. When load is taking current , V_anode will become more than VKA for top TL431 , which is not recommended operating condition. The circuit will work fine without connecting a resistor to bottom TL431.

Regards
Trailokya

0 Bob Lee62837 over 5 years ago in reply to Trailokya Rai

Mastermind 7230 points

Dear Trailokya Rai,

Thank you very much for reply.
I mistook schematic.
The correct circuit diagram is following schematic.

May I ask add a question again?

（Question）
Q1.
My customer want to use following schematic.
Is there problem?

Q2.
Does the current change and cause problems such as oscillation?

(Specification condition)
Input:15V
Output:5V

Best regard.
Bob Lee.

0 Trailokya Rai over 5 years ago in reply to Bob Lee62837

TI__Genius 11351 points

Bob

If you are connecting resistor , you have to take extra care while choosing the resistor to satisfy KCL at all nodes.
Instead you can avoid using the resistor for bottom TL431 , the circuit will work fine.

Regards
Trailokya

0 Bob Lee62837 over 5 years ago in reply to Trailokya Rai

Mastermind 7230 points

Dear Trailokya Rai,

Thank you very much for reply.

There is question about your answer.

What mean "satisfy KCL"?
What mean "take extra care"?
Is there include any problems?

Best regard.
Bob Lee.

0 Trailokya Rai over 5 years ago in reply to Bob Lee62837

TI__Genius 11351 points

BOB

If you are connecting the resistor to bottom TL431 , it will sink IKA + (15 - 2.5)/ R current. As given in the schematic if you want IKA to be same for both TL431 , avoid using the bottom resistor. It will save some power also for your 15V source.

Regards
Trailokya

0 Bob Lee62837 over 5 years ago in reply to Trailokya Rai

Mastermind 7230 points

Dear Trailokya Rai,

Thank you very much for reply.

Is there a problem with a circuit diagram using the bottom resistor?
You mean there is a problem, right?

As TL431 controls the voltage by IKA,
Can control a circuit diagram using the bottom resistor?

Best regard.
Bob Lee.

0 Trailokya Rai over 5 years ago in reply to Bob Lee62837

TI__Genius 11351 points

BOB

If you are using bottom resistor , IKA will not be same for both TL431 as shown in the schematic.

TL431 controls the voltage by IKA, but its variation is not much because of less ZKA (>0.5 ohms).

Regards
Trailokya

0 Bob Lee62837 over 5 years ago in reply to Trailokya Rai

Mastermind 7230 points

Dear Trailokya Rai,

Thank you very much for reply.

May I ask add question?

How much is voltage variation of ZKA?

Althought I will return to the first question,
Can this circuit be guaranteed to operate stably at 5V output ?

Best regard.
Bob Lee.

0 Trailokya Rai over 5 years ago in reply to Bob Lee62837

TI__Genius 11351 points

Bob

ZKA < 0.5 ohm for TL431. The voltage variation will be very less. Both the scheme (With and without bottom resistance) will give you stable output.
Regards
Trailokya

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TL431: About the question when two TL431s are connected in series.