TL431: Calculation consideration

Ajay R

Part Number: TL431
Other Parts Discussed in Thread: LM2903

Tool/software:

Hi,

I’m using the TL431 in one of my circuits, but I’m facing an issue where the output voltage isn’t matching my expected value. I suspect I might be missing something in the calculation.

I'm currently using resistors R164 = 1k, R130 = 330k, and R129 = 4.7k, and calculating the reference voltage using a basic voltage divider approach. However, the reference voltage doesn't seem to be what it should be.

Could you please help me with the correct calculation method? Also, is there any specific requirement for the TL431, such as minimum knee current or any other consideration that I should be aware of?

Thanks for your support!

2 months ago

0 Andrew Li 2 months ago

TI__Expert 3910 points

Hi Ajay,

Thanks for your question!

In the current setup with the reference pin connected to the cathode, VKA = 2.5V before going through the resistor divider of R130 and R129.

Is this your intended functionality? One example functionality for VKA > VREF is shown below.

I hope this helps!

Best Regards,

Andrew Li

0 Ajay R 2 months ago in reply to Andrew Li

Prodigy 65 points

Based on my calculations, the output should be 35 mV. However, when I measure it, it's consistently around 70 mV. What could be the exact reason for this discrepancy?

0 Ajay R 2 months ago in reply to Ajay R

Prodigy 65 points

Circuit used for generating reference to feed non-inverting input of LM2903.

0 Andrew Li 2 months ago in reply to Ajay R

TI__Expert 3910 points

Hi Ajay,

Thanks for the update!

Could you please also measure the voltage at the cathode (Vka)? This would be very useful in identifying how the system functions.

Best Regards,

Andrew Li

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TL431: Calculation consideration