DC DC Buck convertor design using L6986F not working on PCB Board.

[sendhilkumar23]

hi all,

i have designed DC DC Buck convertor using L6986F convertor for 3.3V, 5V and 12V output respectively with input voltage of 24V DC. we tested the circuit and observed that at 3.5V input we are getting same 3.5V at the output port of 3.3V,5V and 12 V respectively. when we increase input voltage above 3.75V to 24V we are getting only 3.3V output voltage. but remaining output ports ie 5V and 12V we are not getting any voltage. If we switch off and  24V input supply we are not getting 3.3V also. 

we need to vary the input supply from 2V to 24 V that time only we are getting 3.3V. the design is already working in our other application.

please suggest the possible faults. so that its helpful to me.

[Peter BENSCH]

Welcome @sendhilkumar23, to the community!

You have taken a lot of trouble to describe your problem in detail. However, in order to be able to help you, important information is missing.

It would be best if you could insert the schematics of the individual buck regulators here as a readable picture (.PNG), as well as the relevant section of the layout of these regulators. The parameters of the passive components would also be helpful, e.g. inductance and the saturation current of the coil(s) and the capacitors (value and type, like MLCC, electrolytic, etc). The connected loads should also be known for assessment.

Regards
/Peter