Lm3478 switching regulaor output voltage drops on loading

[themaccabee]

Hi,

I'm trying to build a boost converter with LM3478.It should convert 28V to 85V/10mA required output.
Ive used the design tool from National got the design values from the same.

While testing the board , im seeing that the output is 75V ,with no load.As soon as im providing some slight load this output is again decreasing.Like for 100K it is 67V.
probing the gate pulse i ve got the switching frequency as 588kHz( i used 27k instead of 26.7k) & duty cycle is no where near 71% its below 50% & with variation in load this duty cycle is not changing.
on the feed back pin im not getting the 1.26V as required in the data sheet, its only about some 0.93V.
I probed across the drain of MOSFET also , and it is switching.But what i feel is like the duty cycle is not getting adjusted as when im introducing any kind of load.

Can some body help me
Thanks & regards

 

[dick_freebird]

Begin with the ground. As in, which ground. Pick one, I recommend the chip
analog ground, and stick with it. Then decide where your feedback error
comes from, whether its ratio is right and see that it follows linearly the
output voltage.