TPS61088: TPS61088 - About Min and Max values of Soft start(tss)

The Webench gives out a draft value of softstart time. You can change it by chaing the ss cap value . or you can choose a time, then webench will give out the ss cap value. Since in the real applciation, the ceramic cap itself has tolerance/ the IC itself has toleracne , so the real ss time will be 'around' the time you set in the webench.

Hi Helen-san,

Thank you for your prompt reply.

I have additional question.

I understand that TPS61088 can operate normally even if they set the soft start time(tss) to “less than 0.5ms” or “more than 100ms”.

So, the value of tss does not have the limitation.

Is this information correct?

Regards,

Kanemaru

Hi Helen-san,

Thank you for your advice.

I understand. Thank you very much.

Regards,

Kanemaru

Hi Helen-san,

I have additional question.

[Q1]

Our customer would like to set “tss=1.1us (Css=4.7pF)”.

I understand that this time is very short but we would like to know whether they can use TPS61088 at this condition.

Could you please let us know if you have any concern?

[Q2]

They would like to know the delay time from "EN is changed from Low to High" to “the soft-start charging current(Iss) begins to flow".

Could you please let us know if you have any information?

Regards,

Kanemaru

1. you can sset css=4.7pF. but why you set the tss such quick, could you please tell us the stroy?

2. The duration from EN high to Vo begin to rise(Iss from off to on) is around 4ms.

Hi Helen-san,

1. you can sset css=4.7pF. but why you set the tss such quick, could you please tell us the stroy?

(Answer)

According to our customer, they have to set tss=around 1.1us (css=4.7pF) for their application system.

2. The duration from EN high to Vo begin to rise(Iss from off to on) is around 4ms.

(Answer)

I apologize for any confusion.

The soft start time which they set is very short.

Therefore, they would like to confirm this delay time.

(If this delay time is more than 1.1us, I guess that they can’t set tss=1.1us).

By the way, I simulate TPS61088 by TINA-TI.

As a result, the soft start time was around 180us so the soft start time don’t meet 1.1us.

Is this correct operation?

Regards,

Kanemaru

From the datasheet, you can see tss means: The soft-start phase is completed after the SS pin voltage exceeds the internal reference (1.204 V). So if the Css is very small, SS pin voltage ramp up very quickly to 1.204V. then Conveter works in the "full duty cycle" untill the Vo reaches the target Vo. So Tss not equal  to Vo rise time.  small Css can make the Vo rise quicker, but the Vo rise time is also depend on the load current, output capacitance.

So the simulation from the TI-TiNA is correct!

Hi Helen-san,

Thank you for your prompt reply.

I have one more question.

I understand that the following delay time is “less than 200ns”.

Is this information correct?

Regards,

Kanemaru

Hi Helen-san,

Thank you for your kind support.

I understand. Thank you very much.

Regards,

Kanemaru