LM51551-Q1: LM51551 boost unstable output

We tried at no load, Output voltage can reach 28V

We already removed R662, changed R211 to 0R, change R217 but no help

Thanks,

Ty

Hi support,

Do you have any update on this?

Thanks,

Ty

Hi Ty,

Thank  you for designing with the LM5155.  Could you show some key waveforms like SW, CS vs Vout and Vin?   Also, I see your compensation network at the COMP pin uses two capacitors, one of 10nF and the other 8.2nF.  These two are too close.  The 8.2nF is too close to 10nF, making your zero less effective to boost the phase margin. It should be at least 10x smaller, i.e. 820pF or smaller.   Did you check your part selections against our design calculator at the following link? If not, please run the calculation and make necessary changes on your circuit.

https://www.ti.com/lit/zip/snvc224

Thanks,

Youhao Xi, Applications Engineering

Hi Youhao,

When we applied Load, Gate control stopped switching, SS pin went LOW

Please note that in PCB layout, we want to measure exactly current so we connect PGND and CS pin from sense resistor

Unfortunately, we disconnected pin 4 from Epad so PGND pin has long ground loop from sense resistor

Is this PCB design still OK?

Thanks,

Ty

Hi Ty,

It is really hard to trace the signals on your posted layout picture. When you applied load, how much is the load? Could you take some scope pictures of the MOSFET drain, input voltage and output voltage when the circuit is switching?

Thanks,

Youhao