TPS56121: Regarding the glitch and overshoot on start up on TPS56121

Can you share a schematic?    

The capacitor on EN/SS pin = 2.2nF.

when I put 22nF, the output is gone.

with 22nF on SS, I estimated the soft start should be 1.3ms which I believe you have discovered was much faster with the 2.2nF. 

Based on the ramp rate, I was wondering if the component was connected. But you discovered the value was 2.2nF,

so the soft start would be 0.1ms which is fast.    

If you need an additional help, just post

So do you want to add more capacitor on EM/SS pin ?

Sorry I forgot one thing - three 5V TPS56121 share this EN/SS signal. 

Is it related to overshoot ? 

Thanks for your advise.

Per your advice I cut the EN/SS traces of three 5V outputs and made them separate. 0.3uF is applied on EN/SS pin of each IC. 

They show slightly different shapes. I may use 0.47uF to make sure.

                                    <5V-1>                                                                     <5V-2>                                                                     <5V-3>      

[CH1: 5V output, CH2: 12V input]

Two questions are 1) do i need this much capacitance ?

                              2) This feature is related to the load current ?

I tried 22nF for EN/SS but shows overshoot and glitches - not good.

Could you advise me with my pictures?

Regards,

Moon

Ideally, you need to wait for the input voltage reach steady state of 12V before enabling the converters.   As input voltage rises the devices starts switching  once the input voltage is greater than uvlo and the device is drawing current causing the input voltage to drop and devices stops switching on 2 and 3, look at the sw node during the startup (place vout and vin on the same volt/div and it will be easier to see what is happening).     The current draw is greater when input voltage is low.   If the input voltage is 12V when the devices startup the current draw will be much lower than starting up at 5V. 

Starting up should not cause the input voltage to drop so much.     Another option is to Increase the soft start time use (220nF increase soft start time to 10ms each) to minimize the inrush current cuasing the input voltage to drop, are the converters staggering the timing for start up.     What is the source of the input voltage and the current capability, maybe the 12V needs a bulk capacitor to help during the start up.