LM431: Vref regulation voltage 2.6V instead of 2.5V

Nadim Sakr

Part Number: LM431
Other Parts Discussed in Thread: TL431

Hello, I am using the LM431BIM3 in the control loop of a flyback converter. However instead of having a Vref of 2.5V, i am measuring 2.6V instead which makes my output regulates at 12.6V instead of 12V.

R2=4.7k, R1=18k, R3=2.2k, R4=1k, Cz=22n.

The voltage across R3=7V

The voltage across R4=1.1V

voltage across VAK of LM431=3.9V

Can someone help me understand why Vref=2.6V? and how to make it 2.5V? I guess Vref is current dependent..

Thanks

over 4 years ago

0 Marcoo Z over 4 years ago

TI__Mastermind 19370 points

Hi Nadim,

Is the output stable at 12.6V? Have you observed the output using a scope?

-Marcoo

0 Nadim Sakr over 4 years ago in reply to Marcoo Z

Prodigy 40 points

Hi Marco,

Yes i did. It is stable yes!

Nadim

0 Marcoo Z over 4 years ago in reply to Nadim Sakr

TI__Mastermind 19370 points

Hi Nadim,

The output is expected to be slightly higher than expected during a no load or low load condition. Can you apply a load and see if the output decreases?

-Marcoo

0 Nadim Sakr over 4 years ago in reply to Marcoo Z

Prodigy 40 points

I tested the flyback under no load condition and down to 22 Ohm (6.5W). The output is well regulated and equal to 12.6V under all conditions.

Input voltage range of the flyback is from 9 to 29Vdc. By changing the input voltage, the output remains at 12.6!

Weird right?

Thanks

0 Marcoo Z over 4 years ago in reply to Nadim Sakr

TI__Mastermind 19370 points

HI Nadim,

That is odd. I would remeasure the resistors to make sure there isnt one that is off value.

If you have spare LM431, switch it out for a new one and see if it persists.

0 Nadim Sakr over 4 years ago in reply to Marcoo Z

Prodigy 40 points

Thank you I will.

I just found TL431 Feedback Loop Error Calculation detailed here (page 20): http://www.ti.com/lit/ds/slvsdq6a/slvsdq6a.pdf

Using this equation with 1% accuracy resistors and LM431B parameters give the following:

Vout= (2.52+0.017)*(1+18*(1+1%)/(4.7*(1-1%)))+18e3*(1+1%)*(4uA+1.2uA) = 12.544 V

Which is close with what i measured.

I guess for best accuracy, LM431B should be replaced by TL431LIB and the 4.7k/18k 1% resistors by 0.1% 2.2k/8.2k. With this i hope having:

Vout= (2.507+0.017)*(1+8.2*(1+0.1%)/(2.2*(1-0.1%)))+8.2e3*(1+0.1%)*(0.4uA+0.3uA) = 11.956 V

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LM431: Vref regulation voltage 2.6V instead of 2.5V