LT8316 and DC2781

Dear Sir/Madam, 

I am using LT8316 as a controller for a 600V to 12V buck converter in my system.

Basically I am just copying the design of DC2781 including the schematics and components selection, and just modify the layout according to my application.

My schematics and layout are shown below.

However, when I give input voltage (I use 25 V for initial test), there is no output voltage. But when I turn-off the input source, there will be a pulse of 12V last for a short moment.

And it is the same case for the gate signal of the MOSFET, when power on, no signal; at the instant of turning-off, Vgs appear for a short time period.

I feel like the circuit is only enabled when input has a falling edge somewhat, so the remained energy in input cap is released through the converter.

I tried the same input voltage 25V and same load 0.5W with the commercial demo board DC2781A, and everything works well.

So may I ask what can go wrong with my design, is there any suggestions?

I really appreciate your time and help.

Regards, 

Keyao

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