Slow start time of TPS54550

Nishizawa-san,

The TPS54550 has not been characterized for any maximum value for the SS capacitor. SS times in the range of up to 20 msec should not pose any issue. How long of a SS time are you considering?

You do not need to connect any resistor to the SS pin. That is only for TPS54350.

Thank you for your answer!


I have new questions.

Question 1.

Your last answer,

The TPS54550 has not been characterized for any maximum value for the SS capacitor. SS times in the range of up to 20 msec should not pose any issue.

In datasheet p.14 Figure.21, SS time is 80mS.

Which is 80mS or 20mS correct?
Please tell us the reason.


Question 2.

If We use 0.2μF capacitor about SS time, SS time is 35mS .
SS time is from Vin(start) to reaching Vout.
Is this understanding correct?


Best regards,


Takahiro Nishizawa

Nishizawa-san,

The plot in figure 21 is just a graph of equation 3 on p. 8 of the datasheet.  That was done 12 years ago, so I do not remember all the details, but so far as I can recall SS was not tested over that entire range.  Theoretically, you should be able to extend the slow start time even beyond 80 msec, but most users have SS times in the range 0f 1 to 20 msec, so I use that as a general rule of thumb.  I haven't seen any issues with those times unless there is an over current condition or there is a large amount of output capacitance.  Slow start causes the output voltage to ramp up at a linear rate.  You are also charging the output capacitor bank.  the charge current required is I = C * dV / dt = Cout * Vout / tss.  If the required current is larger than the current limit.  The TPS54550 will eneter hiccup mode and continuously restart.

ss time is the time for Css to charge from 0 V to to 0.891 V. The SS cap will start to charge when both Vin is above the UVLO and EN is asserted.  Please bear in mind that the tolerance on the SS charge current is quite wide, 1.8 uA min, 5 uA typ, 10 uA max.

Thank you for your answer!


Question.1

SS time is from 0V to Vref(0.891V).
Is this understanding correct?


Question.2

Vin=12V
Vout=6V
Iout=4.7A
Css=0.2μF
Fsw=500kHz/250kHz

Please tell us the time from 0V to 6V.
(0→0.891 is SS time, 0V→6V is ?)
Can you show the Waveform about Start-up( Vout rising).


Question.3

Vin=12V
Vout=9V
Iout=3.6A
Css=0.2μF
Fsw=500kHz/250kHz

Please tell us the time from 0V to 9V.
(0→0.891 is SS time, 0V→9V is ?)
Can you show the Waveform about Start-up( Vout rising).

Best regards,

Takahiro Nishizawa

Nishizawa-san,
I don't have any hardware available that matches those specifications. Will simulations suffice? If so I will set the external UVLO to an approriate voltage above the output set point, say 10 V. Let me know.

Thank you for your answer!

If you don't have the data, simulations are no problem.
Please tell us the time from 0V to Vout(9V and 6V).
and please tell us the waveform or data if it is possible.

Vin=12V
Vout=6V
Iout=4.7A
Css=0.2μF
Fsw=500kHz/250kHz


Vin=12V
Vout=9V
Iout=3.6A
Css=0.2μF
Fsw=500kHz/250kHz

Best regards,


Takahoro Nishizawa

Nishizawa-san,
I'll get this in my queue. I should have something by the end of the week.

Thank you for your answer!

I understand that you will answer by this weekend.
I am waiting for your answer.


Best regards,


Takahiro Nishizawa

How is the situation about this question?
I am waiting for your answer!

Best regards,


Takahiro Nishizawa

Nishizawa-san,

I have not been able to get the simulation to run for the first case yet.  It will not complete without hanging up.  Now I am engaged in a customer emergency.  It is very likely I will not complete it this week.  I'll keep you posted if I have any progress.

Thank you for your answer!

We am waiting for your answer.
We hope as soon as possible.

Best regards,


Takahiro Nishizawa

Nishizawa-san,

I have tried this again yesterday. The simulation crashes when an external SS capacitor used. It looks like I cannot simulate it. I will order an EVM, and get you the SS time for 3.3 V output. I can extrapolate the results for other conditions. It will probably be a week at the earliest. Let me know if that will work for you.

Thank you for your answer!

If you can simulate when VOUT is 3.3V, we want to the check the data of SS start.
We are waiting for your answer!
And please tell us the speculation result if VOUT is 6V and 9V.

Please tell us the reason why you think that simulation is impossible about using external SS capacitor.


We hope as soon as possible.!


Best regards,


Takahiro Nishizawa

How is your situation?

I am waiting for your answer!
Please tell us the data of simulation as soon as possible.

Best regards,


Takahiro Nishizawa

Nishizawa-san,

I have ordered an EVM. When I receive it, I will measure the SS time. I expect it will be early next week.