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LMR14030-Q1: Low dropout Voltage condition

Kuramochi Tadahiko
Guru 28715 points
Part Number: LMR14030-Q1
Other Parts Discussed in Thread: LMR14030

Hi,

My customer is considering to use LMR14030-Q1.

Their usage is low dropout voltage condition as below.

VIN=10.8V(min)~, Vout=10V, fsw=400kHz, Iout=1.5A

Is there any minding point when designing external component on low dropout voltage condition?

Best Regards,

Kuramochi

  • 0
    TI__Genius 9550 points
    hi Kuramochi,
    in datasheet page 6, EC table, Dmax is 97% (typical) at fsw=200khz. We can get Dmax is 94%(typical) at fsw=400khz. (Toff_min time is fixed)
    for your application, VIN 10.8V, VOUT 10V, Duty cycle needs ~92.6% . The margin is little.
    One potential risk need to notice for your application.
    For non-synchronous buck, when VIN-VOUT comes near & light load, buck will enter "LDO mode" (no eough power to turn on high side mosfet). Please look into application note. "Methods to Improve Low Dropout Operation"
    www.ti.com/.../slva547a.pdf
  • 0 in reply to Jason Wang (BSR-MV)
    Guru 28715 points

    Hi Jason-san,

    Thank you for your answer.

    I have an additional question.

    Is LDO mode applied LMR14030-Q1?

    I'm understanding that it is not applied because LMR14030 has the low side FET to recharge BOOT capacitor.

    Best Regards,

    Kuramochi

  • 0 in reply to Kuramochi Tadahiko
    TI__Mastermind 27045 points
    Hi,

    Small Low side FET is dedicated for Cboot refresh so that high side FET can be keeping driven with largest duty cycle as possible. without this FET, the output will osicilate with very big ripple.
  • 0 in reply to Andy Chen
    Guru 28715 points

    Chen-san,

    Does LMR14030 enter into "LDO mode"?
    I'm understanding that it doesn't enter because the low side FET recharges the boot cap.
    Is it correct?

    Best Regards,
    Kuramochi

  • 0 in reply to Kuramochi Tadahiko
    TI__Mastermind 27045 points
    Hi,

    We call it "Low Drop-out mode" instead of LDO mode on LMR14030 data sheet, normally high side FET of buck regulator cannot works at linear conduction mode like a real LDO. I think the LDO mode means the duty cycle is very very close to 100%.
  • 0 in reply to Andy Chen
    Guru 28715 points
    Andy-san,

    Thank you for your reply.

    >normally high side FET of buck regulator cannot works at linear conduction mode like a real LDO.
    Yes, I'm understanding.

    I'm minding follow Jason-san's comment.
    >For non-synchronous buck, when VIN-VOUT comes near & light load, buck will enter "LDO mode" (no eough power to turn on high side mosfet).
    > Please look into application note. "Methods to Improve Low Dropout Operation"

    I think that above comment is not applied for LMR14030 because it has the low side FET for recharging.
    Is it correct?

    Best Regards,
    Kuramochi