TPS22918: Usage of load switch output when the external input is applied

Hi Sathyasai, 

Thanks for reaching out on E2E! Alek has moved over the systems role in Power Switches. To introduce myself, I am the new Applications Engineer covering load switches, power mux, and low side switches.

The TPS22918 uses an NFET for the internal pass FET, whereas the TPS22917 uses a PFET. Depending on which type of FET you choose, the device will turn on differently and Ct will impact the turn-on different:

1. In the NFET architecture (TPS22918), we have CT connected to GND. In order for the NFET to work properly, the gate of the FET needs to be higher than the source of the FET (VOUT). In order to get Vgate higher than Vout, an internal charge pump charges Vgate. By introducing a CT capacitor connected from Vgate to GND, this slows down the charge time on Vgate.  
2. In the PFET architecture (TPS22917), CT is connected to VIN. In order for the PFET to work properly, the gate of the FET needs to be lower than the source of the FET (VIN). At the start, the voltage on Vgate is the same as VIN. As the device turns on, Vgate is pulled to GND via an internal switch. By adding CT from VIN to Vgate, this slows down the pull-down time from the internal switch.

Please let me know if you have any additional questions!

Thanks,

Arthur Huang

Hi Sathyasai, 

Yes, this will cause additional stress on the internal channels used in the CT connection. At lower CT values it won't stress the device as much, but in either case we recommend that the CT pin not be connected to GND on the TPS22917. 

Thanks,

Arthur Huang

Hi Sathyasai, 

Without the CT capacitor connected from VIN to the CT pin, the device will ramp up with the fastest timing. The speed that it ramps up will ultimately depend on your output load and VIN, you can approximate the turn on time from page 6 in the datasheet, or run the tests on your board for more accurate results.

Thanks, 

Arthur Huang