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LM5121: Use the UVLO of LM5121 as the remote shutdown

Wendy Wang1
Genius 16790 points
Part Number: LM5121
Other Parts Discussed in Thread: LMV431

Hi,

Our customer wants to use LM5121 as a part of the back-up power solution. When the Vmain is higher than 9Ｖ, the LM5121 stops. And when the Vmain is lower than 9V, the backup capacitor is discharged and LM5121 will boost the voltage of capacitor(5.4V) to 9V. And here below is the block diagram of the system. 

My question is how can we stop the LM5121 when Vmain is higher than 9V? If I build the schemetic diagram like the image below, when Vmain is higher than 9V, UVLO will be high. And the result will be reverse with what I want.

Looking forward to your reply.

Thanks&Best Regards,

Wenxiu

  • 0
    TI__Guru* 78060 points

    Thank  you for posting, and we will take a look into this and answer your question tomorrow.

    Best Regards,

    Youhao Xi, Applications Engineering

  • 0 in reply to Youhao Xi
    TI__Guru* 78060 points

    Hi Wenxiu,

    I would recommend you to sue a LMV431 to implement an input line drop monitor.  Use a resistor divider at the UVLO pin to set your regular UVLO threshold to monitor the Super Cap voltage.  And, use a second set of resistor divider and a LMV431 to monitor the 12V rail and connect the Cathode of the LMV431 to the UVLO pin.  Once the 12V rail voltage is higher than 9V, the Cathode of LMV431 will hold the UVLO pin low, and keep the LM5121 off. If the 12V rail voltage drops below 9V, the LMV431 becomes open, and the UVLO pin will exceed the threshold and start to operate the LM5121.

    Thanks,

    Youhao