TL431LI: TL431 Unexpected voltage thershold

RFScott

Part Number: TL431LI
Other Parts Discussed in Thread: TL431, TLV431

With the TL431 circuit below the TL431 diode stops conducting ~3.0V (voltage at J7), but it was expected to turn off at 2.5V + (Iref * R82) = 2.5V + (0.4uA * 1kohm) the voltage drop across R82 is not expected to be significant.

The 1kohm is needed to limit reverse voltage current.

Any insights on why does the diode stop conducting 3.0V instead of 2.5V?

Thank you in advance for any help.

over 4 years ago

0 RFScott over 4 years ago

Prodigy 95 points

Here is the schematic

0 Marcoo Z over 4 years ago in reply to RFScott

TI__Mastermind 19550 points

Hi RFScott,

What are the maximum and minimum voltage on J7?

The TL431 still needs a IKA(min) voltage to function properly in operate in open loop mode. R81 is a very large value that might limit the current.

The way I see is, Vth = 2.5V then Vka = 2V. So R81(max) = Vth - Vka / Ika(min) which is much smaller than 100k.

0 RFScott over 4 years ago in reply to Marcoo Z

Prodigy 95 points

Hello Marcoo Z,

Thank you for the reply, the maximum voltage at J7 is 3.6V and the purpose of the circuit is to be 2.5V under-voltage lock-out. It is a battery operated device and the standby current that needs current needs to be minimized.

Best regards,

Scott

0 Marcoo Z over 4 years ago in reply to RFScott

TI__Mastermind 19550 points

Hi Scott,

If the goal is to reduce current I have two suggestions.

I would recommend to switch a supervisor such as the TPS3840DL25DBVR which can have <1uA monitoring with a 2.5V threshold.

If not, then I would use a TLV431 because its Ika(min) is <100uA so you can use a larger resistor than you can on the TL431.

0 RFScott over 4 years ago in reply to Marcoo Z

Prodigy 95 points

Thank you for the suggestion I will look into that.

Scott

0 RFScott over 4 years ago in reply to RFScott

Prodigy 95 points

Hello Marcoo Z,

TPS3840DL25DBVR will be used in a voltage protection circuit to protect the device we would like to use a low Vf diode to provide reverse polarity protection. The leakage current of the diode may allow a negative voltage to build, does the TPS3840DL25DBVR have a protection for weak negative voltages?

Thank you,

Scott

0 Marcoo Z over 4 years ago in reply to RFScott

TI__Mastermind 19550 points

Hi Scott,

What do you mean by weak negative voltages?

The ABS max table shows that the device can handle down to -0.3V. Any more negative and you will need protection.

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TL431LI: TL431 Unexpected voltage thershold