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LP38798 PSRR

Par Johnsson
Prodigy 180 points
Other Parts Discussed in Thread: LP38798

Hi My customer is looking to replace 2pcs LP2989-5.0 LDO's on a board due to the PSRR become too bad at the low dropout they are operating on. They are now considering the LP38978 instead.

 

However the datasheet on LP38978 only has PSRR plots with a Vin-Vout difference of 0.5V

 

They are looking at operating on Vin 5.3x and Vout 5.0 and a current of 250-300mA.

 

Could you help with PSRR data for this case ?

 

I guess this should be safe as dropout at 125C at 300mA is about 120mV according to plots.

 

Reason for the low Vin-Vout out is they have 5.5V spec Switchers on the same rail.

 

Looking forward to your swift reply.

  • 0
    TI__Mastermind 22610 points

    I suggest that you use the existing PSRR plot with the Vin-Vout difference of 0.5V, just use the 800mA data line. The operating condition for the 800mA data ends up being similar, just slightly less operating headroom (Vin - (Vout + Vdo)) than your described condition.

    Rds(on) of LP38798 pass element is (420mV/800mV)= 0.525Ω.

    (5.50V - (5.0V + (800mA x 0.525Ω.))) = (5.50V - 5.42V) = 0.08V

    (5.30V - (5.0V + (300mA x 0.525Ω.))) = (5.30V - 5.16V) = 0.14V