LM3488 12-70V boost issue

I don't see that output 200 ohm resistor in parallel with 560uF capacitor. Also how is the PCB layout?

I forgot to say they acts as test load. I'll publish pcb layout asap

added pcb layout

Hi ,

Why there is no output capacitors?

How did you calcaulate the output? Why R4 is bigger than R5? they arre opposite, right?

Suggest to refer to EVM file to do layout.

www.ti.com/.../snva656a.pdf

yuchang

For 70 V you need to switch R4 and R5. The layout is iffy at best. I also suggest you take a look at the EVM file referenced above.

ok , thank you very much

I updated schematics and pcb layout, inverting R4 and R5 resistor, and trying to follow as much as possible AN-1204 layout ( i need to develop and etch pcb on my own , so no 4 layer pcb )

but it still doesn't work.

Hi Leonardo ,

When you say it still doesnot work , are you seeing no switching at the drain of the FET ?

Further can you please specify what capacitor you have chosen for C3 and C4 . Based on your Layout , the capacitor seems to be much smaller then 3.3mF , mentioned in the schematic , particularly for C4 .

Please make below changes in the schematic of the design and re evaluate it

1. Change COUT to 150uF,100V capacitor with ESR less than 100mohm
2. Change compensation R1 and C1 to 37k and 3.9nF
3. Change R2 to 26k for 550KHz operation .



---Ambreesh