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# TPS92682-Q1: TPS92682 really soft start time problem

Part Number: TPS92682-Q1

Hello expert,

About the datasheet application of CV 2 phase mode，I check the figure8-25&8-26 have some confused,figure 8-25 softstart time(COMP PIN from 0-1.9V)  is 8ms,

figure 8-26 softstart time(COMP PIN from 0-1.9V)  is 7.6ms,theoretical calculation like below，

1.about the figure 8-25 SSxdiv=0111,Fsw=200kHz，use the equation 15，we can get the max softstart time tss≤ 25.6ms，

2.about the figure 8-26 SSxdiv=1111,Fsw=200kHz，use the equation 15，we can get the max softstart time tss≤ 128ms，

Question1：As above，it seem if SSxdiv set 1111，soft start time will longer than 0111，but about the real test waveform figure 8-25（8ms）&8-26（7.8ms） is contrary to it，why？

Question2：I know use the equation 15 only can get the maximum softstart time，but I want to know what is the really softstart time（the time will infulence by other factor cause it become longer？），
base the datasheet application assume we need the limit soft start smaller than 6ms，whether need set the SSxdiv to 0010 or not？if set SSxdiv to 0010，the really soft time will smaller than 6ms？

soft start time

Thank you&best regard！

• Hello,

Figure 8-25 that you listed is 2mS/div and Figure 8-26 is 10mS/div so it's longer time for the 1111 setting of soft-start.  Since the TPS92682 can also be used as a constant current LED driver, the way we designed the Soft-Start is shown

• Hello,

Figure 8-25 that you listed is 2mS/div and Figure 8-26 is 10mS/div so it's longer time for the 1111 setting of soft-start.

Since the TPS92682 can also be used as a constant current LED driver, the way we designed the Soft-Start is show in section 7.3.14 from the specification below.  We clamp the COMPx pin to the output of the SSDAC.  This is different than a constant Voltage output IC where the output voltage is ramped up relative to the soft-start time.  Again, we do this for this part because of the Contant Current capability of the part.

This is different than just a constant Voltage output application where the output ramp up relative to the Soft-Start time.

Thanks Tuan

• Hi expert,

Thank you for your support, you mean that the CV mode ouput voltage ramp up time is represent the actual soft start time,CC mode  COMP pin ramp up time is represent the actual soft start time,right?

Additional question is when I use in CV mode,how to theoretical calcuate the actual soft start time?

• Hi John,

No, the TPS92682 CV mode is different than other voltage regulator as we explained above.  The SS clamps the COMPx pin so the actual output voltage does not follow the actual soft start time.

In CV mode for the TPS92682, the actual output voltage soft-start time is not the same as the programmed soft-start time.  This time is dependent on the power stage design along with the output caps.  Again, we have to do this since the part is also used as a constant current to drive LED's.

Thanks Tuan