TPS61175 soft start - Power management forum - Power management

TI__Guru**** 191445 points

Soft start for TPS61175 works differently than many converters. Normally the SS voltage will be a non invering input to the error amplifier and the output will track the lower of the reference voltage and the ss voltage. On the TPS61175, the SS is used to clamp the COMP pin voltage. So the peak cycle by cycle current is limited by the SS circuit. The datasheet suggest you use the table to choose the SS cap value.

0 Anthony F over 12 years ago

TI__Mastermind 35165 points

Hi Alan,

I recommend trying the search function before posting. Many times questions have been asked before and you can find the answer with a quick search. The answer is also in the link below.

https://e2e.ti.com/support/power_management/non-isolated_dcdc/int-non-isolated_dcdc/f/198/p/163168/593799.aspx#593799

Best Regards,
Anthony

0 Christophe MANON over 11 years ago in reply to Anthony F

Prodigy 20 points

Hi,

I also use the TPS61175 converter, and I want to calculate the soft start time. The link in the last post is broken, so I don't find the answer. The datasheet (page 9, table2) gives some values for C3 capacitor for several examples different of my own case.

I need two things:

1) How to determine the time minimum and maximum for the soft start time?

2) What is the maximum value for the soft-start capacitor (C3)?

Please help me.

Best regards.

C. MANON

0 Anthony F over 11 years ago in reply to Christophe MANON

TI__Mastermind 35165 points

Here's the response from the link:

Dear person who can support the Soft Start setting of TPS61175.

In the datasheet, the typical Soft Start bias current is specified as 6uA. However, in the table-2 which is describing the relation between soft start time and soft start capacitor seems not much to the specified bias current, soft start capacitor and threshold.

From the block diagram, the soft start bias current and threshold seems influenced by the COMP node bias and beta of the PNP transistor. Therefore, the caluculation of soft start may be much complecated. (may be influenced by load current, inductor, frequency, input voltage etc...)

If you have any specific equation which enables us to estimate the relation of softstart time and capacitor, can you advise?

As you correctly mention above, an equation would be very complicated. So, we did not derive one. My recommendation is to perform bench testing at the various startup conditions and adjust the SS capacitor until the output voltage rise time and inrush currents are within specifications.

0 Christophe MANON over 11 years ago in reply to Anthony F

Prodigy 20 points

Hi,

Thank you for reply, but it doesn't answer my question.

I can understand that the calculation of the soft-start is complicated, but I need some justification for the the choice of the soft-start capacitor. Can I find a simple formula which gives the link between the soft-start time and the soft-start capacitor charge?

My main constraint is a maximum input current of 300 mA, during the starting phase, whatever its duration.

Here are the main features of my power supply:

Input voltage: +6.5V to +15V

Output voltage : +20.5V

Maximum output current : 400 mA

Load at start : about 30µF, no resistive load

Switching frequency : about 900 kHz (R-FREQ=100kOhms)

I think that the worst case is for the minimum input voltage of +6.5V.

Which capacitor value can always ensure a maximum input current of 300 mA?

What is the maximum value for the soft-sart capacitor? Are there any risks tu use large capacitances?

Please help me.

Best regards.

C. MANON

0 JohnTucker over 11 years ago in reply to Christophe MANON

TI__Guru**** 191445 points

This is a pretty old thread. I don't directly support that device. I can;t commit to anything, but let me see if i can find the apps engineer for that device. He may have an answer. Alternately, I see there is a pspice transient model. You could use that to simulate your start up performance for different slow start capacitors. That is what I would do.