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LM317: Drop out mode operation

Poornanand Chittal
Prodigy 10 points
Part Number: LM317

Hi,

We are using LM317 IC for regulation purpose ,where input voltage is 24V . Now with the output resistor settings I am getting 20.5V as well as 23.5V. But the datasheet says minimun headroom of 3V has to be given between input and output for the the IC to operate in regulation mode if the headroom is not provided it works in drop out mode. My concern is if the IC is operating in drop out mode and operating for a long time what effect will it have on Thermal condition of the IC, Stability and in drop out mode how long is it possible to operate without any failure?

Please provide the details ASAP.

Thank you

  • 0
    TI__Guru 55940 points
    Hi Poornanand,

    Operating in dropout will not have any impact on the linear regulator's lifespan; however, it is important to note that since the output is not regulated, Vout will track Vin. Most of the performance depicted in the datasheet is for a regulated output, when in dropout you may not achieve the depicted performance. For example, PSRR will be severely degraded when in dropout.

    Very Respectfully,
    Ryan