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LM6211: Latchup by power up

Part Number: LM6211

I use the LM6211MF as a buffer for my reference voltage. It is supplied with ±5V from a DC-DC converter. On the output there is a 2.2uF and a 100nF . This is workin for years, now we change the input in DC-DC and the 

supply is rising much slower. In this case the output and input of LM6211 are going to +5V. 

How can I solve the Problem?

Do I need to change the OP AMP. This is the simplest solution or do I need to adjust the circuit?

The body of LM6211 is SOT23-5.

Thank you for help.

Rubin Müller

  • Hello Rubin,

    5V in and 5V out does violate the input common mode voltage, but 5V can be a correct output for that input. Are you sure the LT part isn't making the 5V?

    Does the -5V become positive voltage at any time?  Can you capture the +5V, -5V, pin 1, pin 3 of op amp during power up?

  • Hello Ron,

    I have disconnected the line between LT and OP AMP and the OP has also in this case 5V at the inputs and outputs.

    If I change the supply voltage the circuit is ok. It depends directly on the slew rate of the supply voltage.

  • Rubin,

    I think the input (IN+) is just close to 5V.  If true, the op amp can get stuck at 5V if output is directly connected to IN-. There are input to input diodes that will prevent IN+ from being able to go low. However I would expect IN+ to be closer to 4.3V (with Vref connected). 

    A resistor can be added to feedback, so Vref can pull down the IN+ and release the latched state. Pick a resistor large enough that Vref has enough sink current to get IN+ low. For stability add a 10pF or high capacitor in parallel with the new resistor. 

  • Hello Ron,

    thank you for help.

    I have done a test on one board and it is working. So I will close this issue as resolved.

    best regards