TL431: How TL431 can generate 2.5V reference voltage from a input voltage

Hi Van,

Thanks for your question. Can you please name the pins for on TL431 in the schematic for clarification? My thoughts below

• What is the collector and emitter current of Q1 when this event occurs?
• TL431 is being used in an open loop format. I would recommend this reviewing these articles in the open loop implementation of TL431

  • Appnote: https://www.ti.com/lit/an/slva987a/slva987a.pd

  • ADJ: https://www.ti.com/lit/an/slyt808/slyt808.pdf

Best,

Abhishek

Hi Abhishek,

Thank you so much for your feedback. In the schematic, pin 6 is Anode, pin 7 is Cathode and pin 11 is REF pin for TL431.

And why the current through Q1 is important for stability? I thought that current through TL431 is important (should be higher than 1mA).

Also, could you please explain the principle for TL431 to generate a 2.5V reference based on its schematic in the original question? I think I need to understand it in order to understand the root of the problem.

Thank you so much. Regards,

Hi Van,

Thanks for letting use know the pin assignment of TL431 in the schematic. Can you please be specific about the load conditions in which TL431 does this? I see that your implementation of TL431 is the one shown in the datasheet's Fig 34. We can clearly see from the current graph that the current being provided to TL431's cathode is not sufficient, >1mA. There are few ways we can try fixing this issue. 

1. Reduce R4 and see if we can provide more current to TL431 Cathode
2. Replace TL431 with ATL431 to see if it gets the job done. We will need to adjust the R4 accordingly.

Hope this helps in debugging the issue.

Best,
Abhishek

Hi Abhishek,

Thanks a bunch for your reply. My load is a Gate driver circuit for SiC Mosfet with a given switching frequency.

By the way, it is mandatory to have at least 1mA for Cathode current of TL431?

Since in some simulations, I saw that with Cathode current less than 1mA, the output is still well regulated. Any disadvantage in this case?

Thanks a lot. Regards,

Also, could you please let me know the method to calculate Cathode current of TL431 (formula relating input and output voltage, and resistance values)? For example, using Fig. 34 in the datasheet below. Thanks a lot! 

Hi Van,

Great points. Below are the answers:

• It is mandatory to have at least 1mA for Cathode current of TL431? -> Yes, that is the minimum cathode current we need to provide. You can see this in the footnote of Fig 34 also.

• Since in some simulations, I saw that with Cathode current less than 1mA, the output is still well regulated. Any disadvantage in this case? -> If the current is lower than 1mA, then the reference voltage will drop and will not maintain.

Best,

Abhishek

Hi Abhishek,

Thank you so much for your feedback. Can I know the formula to calculate the Cathode current?

For example in the Fig. 34 about, can I say that Cathode current: Icathode = VI(BATT)/Rb? 

Since when the output transistor of TL431 turns on, Rb is the only resistor connecting between VI(BATT) and GND.

Thank you so much. Regards,

HI Van,

It should be Icathode =  ((VI - (Vbe - Vo))/Rb - IB) where, IB is the npn base current, Vbe is the base-emitter voltage, and Vo is the output voltage = 5V in this case. Vbe is generally defined in the datasheet for npn transistors.

I will also recommend looking into IE, the emitter current ie the sum of IB and IC (collector current). IE should be high enough to drive the feedback resistors. IE >= Vo/(R1 + R2)

I hope this answers your questions. Please let me know if I can help with anything else. 

Best,

Abhishek