• State TI Thinks Resolved
  • Locked Locked
  • Replies 1 reply
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 305 views
  • Users 0 members are here
Support feedback
Options
Options
Related

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.

Original question:

TPS22975: TPS22975 DIODE CHARACTERISTICS

TPS22975

HABIB MOUSAVIZADEH
Prodigy 80 points
Other Parts Discussed in Thread: TPS22975, BQ25601

I had this question under battery charger bq25601 and I was requested to submit it under the TPS22975 section.

The question or my concern is regarding selecting the Cin cap: 

I went a bit more through the selecting the Cin cap for the load switch (TPS22975) section 10.1.3. And looks like the issue is regarding the body diode of the NMOS switch, which it says “ the bigger CL than Cin can cause Vout to exceed Vin when the system supply is removed”.

However since in our case we can not provide Cin as big as CL, as our CL is around 2mF, so wondering if this could be a serious problem of using this load switch.

  • 0
    TI__Genius 10855 points

    Hi Habib,

    Thanks for reaching out on E2E! I believe we are having a separate conversation/email discsussion on this topic, but I can address your concerns here as well.

    We generally recommend a 10:1 ratio of input capacitance – output capacitance for two reasons:

    1. As the load switch is enabled, you’ll have a sudden change in voltage (large dv/dt). Since I  = CLOAD* dv/dt, if you have a large amount of output capacitance, then you’ll have a large inrush current event as the switch is enabled. As this large inrush current event occurs, depending on the input supply it could dip due to the sudden change in load current. By having a 10:1 ratio, the large input caps can provide an immediate source of current for an inrush event, preventing dip on the input supply. However, for this configuration I’m assuming your CT value will be large enough to limit the inrush current. This shouldn't be an issue here, which I will explain below.

    2. The second benefit to a 10:1 ratio is if the input supply is suddenly removed or turned off while enable is on. If you have a larger ratio of capacitance on the output vs input, this could cause the voltage on VOUT to exceed the voltage on VIN during this event. This may result in current flowing from VOUT to VIN through the body diode, potentially damaging upstream circuitry or the load switch itself. Will there be a condition where VIN could suddenly be removed or turned off while enable is ON?

    For this application, you can minimize the impact of #1 by using a very large CT value. 

                                    Inrush Current = CL × dVOUT/dt

                                    Inrush Current = 2mF * 3.3V / dT

                    Depending on your inrush current requirements, you can calculate the dT value. Then you can choose a CT value that yields a rise time of roughly the dT time using this equation:

                                   

                                    Slew Rate (in us/V) = 0.43 * Ct + 26

    Using a large CT cap will help reduce your inrush current, which will also reduce the need for including a lot of input capacitance. 

    Thanks,

    Arthur Huang