LT8357 Sepic - not working at low voltage input

Hello Ohad,

Computing for your EN/UVLO divider resistor gives around 1.15V drop at 9V input. The drop should be >1.2V to turn on the device. Maybe try adjusting the UVLO resistors more and take in to account the hysteresis and tolerances.

Regards,

Jhun

Thanks for reply.
Even with EN/UVLO at 6V or EN tied to Vin, it still the same.  The converter cannot supply the needed power

Some more info:

At Vin > 10.8V, switching signal to gate is ~2MHz . At low load (~100mA) it has low duty cycle, and while increasing load to ~2A, duty cycle gets higher.

At Vin < 10.8V, when load is low, ~100mA, switching signal to gate is 2MHz, with low duty cycle, and while increasing load to ~2A switching frequency decrease to 750KHz and voltage output drops down.
If I cycle the load, after several cycles it switches to 2MHz and works fine.

Ohad

Hello,

Can you try removing the capacitors you have in parallel with your feedback dividers and see if this improves? Also you have a high DCR on your inductor, have you tried simulating your design? If not, please do so. Thank you.

Regards,

Jhun

Hello,

1. You have used a 40V FET -> in a SEPIC the Switch does see the sum of VIN and VOUT, hence 51V at 36V Vin -> You need a 60V FET. And the diode has to be a 60V type as well.

2 With my approach LTpowerCAD suggests a Sense resistor of 4.7mOhm. You have chosen 8mOhm, hence you limit the possible output power at lower VIN.