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LM27761: During the power-on instant, the LM27761 chip momentarily pulls down the input supply voltage slightly, then allows it to rise and eventually stabilize.

Simon Li
Prodigy 10 points
Part Number: LM27761


During the power-on instant, the LM27761 chip momentarily pulls down the input supply voltage slightly, then allows it to rise and eventually stabilize.Why Does This Phenomenon Occur?

Even with a capacitor of over 200uF added at the front end of the chip, this phenomenon still occurs. Does this phenomenon need to be mitigated? The chip's output is connected to the negative power supply of the AD5423, and the load is very small. Is this phenomenon a normal behavior caused by the chip itself?

3300mV 4700nfdianrong.png3300mV 47uf+47uf+47uf+47uf+47uf+47uf+4700nfdianrong11.png

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    TI__Mastermind 34900 points

    Hello!

    We have received your case and will take some time. Thank you for your patience.

  • 0
    TI__Mastermind 34900 points

    thanks for reaching out in E2E. In the moment of startup the LM27761 is charging the output cap, which is causing a higher inrush current at the input. This is the normal behavior. The input supply must be well regulated and capable of supplying the required input current. If the input supply has a too high impedance then it can cause voltage drops even with a buffer cap of 200uF. If the input voltage drop is below the Undervoltage Lockout of the LM27761 then it will force the device into shutdown (see section 6.3.1 in the datasheet). This behavior does not damage the device, but a stronger supply would help for a stable start. What is the supply source for the LM27761 in this application?

  • 0 回复 Links
    Prodigy 10 points

    The supply source for the LM27761 is +3.3V from  LDO of AD5423.Now I connected the VIN pin and EN PIN of LM27761.Can I achieve a soft-start function by changing the voltage level of the chip's EN pin, thereby change the voltage drop phenomenon?

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    Prodigy 10 points

    Thanks for your reply. The supply source for LM27761 is LDO output of AD5423.Can I achieve a soft-start function by changing the voltage level of the chip's EN pin, thereby improving the voltage drop phenomenon? 

  • 0 回复 Simon Li
    TI__Mastermind 34900 points

    thanks for your feedback.

    The LDO output of AD5423 is limited to 30mA and the voltage dropping entirely at 50mA. So, I think this is no suitable supply for the LM27761. The LM27761 does not support any kind of soft-start function, but it might help to enable the LM27761 delayed when the 200uF cap is fully charged.

    However, is there some other power supply of the AD5423 (for example AVDD2) which could be used for the LM27761? What other supply rails are available in this application?