TPS40210EVM: What is the R3 for?

Kazuhiko Hirano

Part Number: TPS40210EVM

Hello,

According to datasheet, Css si connected between SS pin and GND to adjust soft start.
However, EVM has an additional resistor R3 (10Kohm) in series.
What is the R3 for?

Best regards,

K.Hirano

over 2 years ago

0 Stefan Schauer over 2 years ago

TI__Mastermind 46971 points

Hi Hirano-san,

the R3 in this case is used to have better adjustments for the softstart time. Capacitor values have a quite big step from one value to the other whereas Resistors have much smaller steps available. So if you would like a time which is in-between two Capacitor values this could be adjusted this way.

Best regards,

Stefan

0 Kazuhiko Hirano over 2 years ago in reply to Stefan Schauer

TI__Genius 12603 points

Stefan,

Thank you for your response.
However, I have no idea how the R3 affect softstart time.
Regarding R3 and Css, do you have any equations or rule of thumb explanations to estimate softstart time?

Best regards,

K.Hirano

0 Stefan Schauer over 2 years ago in reply to Kazuhiko Hirano

TI__Mastermind 46971 points

HI Hirano-san,

please have a look at this picture in the datasheet: (chapter 7.3.1 Soft Start)

As you can see the softstart cap will be charge by an internal resistor. So adding another external resistor does influence the charging curve of the softstart cap. Also that generic calculation is described there

Best regards,

Stefan

0 Kazuhiko Hirano over 2 years ago in reply to Stefan Schauer

TI__Genius 12603 points

Stefan,

I see, thank you for your confirmation.

Best regards,

K.Hirano

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TPS40210EVM: What is the R3 for?