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LT4250L Hot swap power problem


We design a custom board that use LT4250L as hot swap power controller. I attach our design as a PDF file.

When we do not montage C571 and C569, the PWRGD signal is not equal to -48V and we do not have 5V0 at output of U18, but Gate pin of LT4250 drive Q9 and we have -48V at the input of U18.

But when we montage C571 or C569, the PWRGD signal is  equal to -48V and we have 5V0 at output of U18.

I can not understand what are C569 and C571 doing? can you explain it to me?


  • Hi ,

    For LTC4250L , Please refer to Fig. 13 in datasheet that explains the two conditions required to set the PWRGD# . We need to ensure those conditions are met for setting the PWRGD# signal. 

    The time UV low to gate low ( tPHLUV) for LTC4250L is 1.5usec. The capacitor C571 increases this delay and adds filtering response. As soon as you DC/DC turns on , it seems you are hitting UV and not getting 5V out .By adding this capacitor C571 this situation can be avoided. 

    Similarly C569 adds delay between PWRGD# latch set and turn on of the DC-DC module.Not having either causes the part to either detect UV , or not latch the PWRGD at all as Vds might be high. Having either one of them causes delay in the turn-on of downstream DC-DC module and hence  normal behavior.  Also I have assumed R1078 and R1077 are populated .

    Let me know if you have any further questions.