LM5158: Adjustable output

Hi,

I am using sepic topology.

I am trying to switch the output voltage from 30V to 15V, using mosfet. But i as add the feedback resistor. The Vfb starts to increase. Once it reaches above 1.4V the Pgood goes zero and SS is grounded. OVP/PGOOD forces a Soft Start after about a 4ms delay, the switching is stopped.

1. When we switch from 30V to 15V, need to change the feedback from 1.76K to 3.46K
2. During this transition as output voltage is at 30V, Vfb reaches to 1.9V
3. As Vfb crosses the VUVTH (1.45V) and (Vref of 0.99-1.01V), Pgood become zero
4. SS goes to zero and switching is stopped
5. Output and feedback voltage starts to fall
6. Once Vfb fall below 0.9V, SS start to rise again
7. Switching starts after 8ms
8. Output reaches to 15V after 14ms

I need the transition to be faster and delay less than 1-2 ms.

I have the verified the same using the TI pspice simulation, just added a MOSFET to change the feedback resistor. (LM5158x-Q1 PSpice Transient Model (Encrypted))

Any possible solution or way around to reduce delay.

Thanks.

Harshad Dhuri