LT3757 48VDC to 90VDC fails

Hi all,

I try to use the LT3757 to step up 48V to 88V in order to driver a 88V led string. I made a simulation in LTspice and it works fine. So i decided to make a prototype based on the circuit's datasheet and formulas as following:

Design:

Vcc (chip supply): 12V

Vin(at the inductor): 48V / 8A 

fswitch: 200kHz

Cout: 100µF

L: 600µH

Rsense: 0.04Ohms

Desired results:

Iout: 1000mA

Vout: 88V

When i switch on the circuit, without led string,i can see high output voltage that i can change with the feedback resistor divider. So i adjust the output voltage @ 88V. But, when i connect the led string, the output voltage drops down at 76V with only 10mA in the led string. Adjusting the Vfbx voltage has no effect on the output voltage which stays at 76V and Vfbx stays under 1.6V.

Please, c  ould you help and explain me why i get this issue ?

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