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Original question:

TPS7A94: Fast start up control

TPS7A94: Regarding fast sharge up switch threshold setting method

teritama
Expert 2255 points
Part Number: TPS7A94


Hi team,

In the below thread, the statement that “fast charge-up switches to a 150 µA internal current source when PG_FP reaches 93% of the intended Vout value” is a typo.

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1351826/tps7a94-fast-start-up-control?tisearch=e2e-sitesearch&keymatch=tps7a94#

 

My understanding is that this means the switch threshold is determined by the Rfb resistor divider.

I would like to know if it is possible to set this threshold to 1.0 V when Vout is set to 1.0 V.
And if we set this condition, is it possible to reduce the t2 time in Figure 8-13 to almost 0 seconds?

According to the datasheet, this divider sets the current limit and the PG assert threshold (see Section 8.1.5 “Power-Good Feedback (FB_PG Pin) and Power-Good Threshold (PG Pin)” and Section 8.1.6 “Adjusting the Factory-Programmed Current Limit”).

From my understanding, this seems difficult for reasons related to these sections.

Please share your opinion.

And in this device EVM, this resister divider is two 10kohm resisters.

This value is different from the value in Table 8-1 in datasheet.

What is the design intent?

 

Best regards,

 

teritama

 

Best regards,

teritama 

 

  • 0
    TI__Mastermind 37135 points

    Hi Teritama,

    I don't see the typo in the previous E2E thread.

    What you describe regarding the t2 time is possible under ideal conditions.  But it is not recommended to set the changeover voltage equal to the desired Vout due to tolerance analysis.  The tolerance of the internal 0.2V reference, resistor dividers, etc, may make your design overshoot the setpoint and not recover if Vout < changeover voltage.  We recommend 95% changeover voltage max (see table 8-1) to account for these tolerances (thus, for a 1V output you can set the changeover voltage to 0.95V).

    The 10k resistors on the EVM will allow you to obtain 100% current limit.  But please follow the datasheet recommendation (section 8.1.6) which offer updated resistor guidance for your design.

    Thanks,

    Stephen