• State Resolved
  • Locked Locked
  • Replies 1 reply
  • Subscribers 63 subscribers
  • Views 476 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS23757: Pull-up nad Pull-down resistor for CS pin

Takahiro Saito
Expert 5445 points
Part Number: TPS23757
Other Parts Discussed in Thread: , TPS23751

Hi Team,

In the TPS23757 datasheet, CS pin is used without pull-up to VB. Looking at TPS23757EVM user's guide, the CS pin is pulled up to VB. I could not find why pull-up is needed and how to set the pull-down and pull-up resistor value when CS pin is pulled up to VB in the datasheet.

Could you explain why TPS23757EVM uses pulled-up for the CS pin and how to set the pull-up and pull down resistor value?

I found other PoE device datasheet(TPS23751) describes about this in 8.2.2.10 Voltage Feed-Forward Compensation section. Does TPS23757EVM uses pull-up resitotor between CS and VB for voltage feed-forward compensation?

Regards,

Saito

  • 0
    TI__Genius 9230 points
    Hi Saito,

    This pull up was added in so that the peak current of 1.2A was reached when Vcs hits Vcsmax (0.55V in the datasheet). 1.2A was picked because it is the max peak primary current specified in the datasheet for the C1174-AL transformer.

    In order to size this feedforward pull up resistor, you calculate it similar to a voltage divider (top resistor R20, bottom resistor is R21 + R22). Using the 9.53k ohm resistor, an additional ~0.25V would be seen at the CS pin. At 1.2A peak primary current, ~0.3V would be dropped across R20. 0.3V + 0.25V = 0.55V (Vcsmax specified in the TPS23757 datasheet).

    Regards,
    Tom Amlee
    PoE PD Apps Engineer