L7987L Not working as expected

Shivalee · ‎2025-07-28

Hello All,

I am using L7987L to generate +15V from 24V. I have 10 same boards wherein I have used the same circuit (see below), I am able to generate 15V for 2 boards but in rest it is generating 1.22V only.

R191 is 6.8kohm and diode D36 is US1M-E3/61T.

SS pin is showing 0.07V only

Please note there was no change in layout for working as well as non-working boards. I have checked all individual resistors as well as replaced IC too. still the issue persists.

Peter BENSCH · ‎2025-08-06

It looks as if the L7987L is not starting up fast enough, probably because the SS capacitor was dimensioned too small and therefore the soft-start function gets stuck when discharging the capacitor. With the 6.9nF in your design, you specify a soft start in 1.1ms. I would increase this capacitor, i.e. your C98 and C95, to a total value of 68...100nF for a start-up time of about 11...16ms as a test.

Regards
/Peter

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