BQ77915: Protection for CHG and LD pin

Shivani Ghorpade

Part Number: BQ77915

Hello,

As per the application information, it is recommended to put a PMOS FET in series of the CHG pin. Here why PMOS FET is used instead of NMOS FET. Any specific reason for using PMOS FET.

Regards,

Shivani Ghorpade

over 5 years ago

0 Matt Sunna over 5 years ago

TI__Guru* 81034 points

Hi Shivani,

Page 10 and 11 of this app note discuss this circuit in more detail: https://www.ti.com/lit/an/slua749a/slua749a.pdf . This is for a different device but is using the same protection circuit. It should be a PMOS FET and the app note describes the different phases of operation and how the circuit will respond.

Best regards,

Matt

0 Shivani Ghorpade over 5 years ago in reply to Matt Sunna

Prodigy 190 points

Hello,

After going through the specified application note, I am still not clear why PMOS is used and not NMOS. Request to please brief on this usage and its specific application in the circuit for bq77915.

Regards,

Shivani Ghorpade

0 Matt Sunna over 5 years ago in reply to Shivani Ghorpade

TI__Guru* 81034 points

Hi Shivani,

Can you explain your reasoning for using an NMOS so I can understand the point of confusion?

Thanks,

Matt

0 Shivani Ghorpade over 5 years ago in reply to Matt Sunna

Prodigy 190 points

Hello,

As per my understanding, we are using FET here to protect CHG pin from exposing themselves to a voltage lower than their absolute max rating. This protection can be provided by using any MOSFET with some modifications in the circuit.

But there must be some specific reason why TI recommended PMOS FET, which is what I wanted to know. Also I think since PMOS FET can conduct by applying low voltage to their gate can be the reason for using them. Just wanted to confirm on the reasoning.

Regards,

Shivani

0 Matt Sunna over 5 years ago in reply to Shivani Ghorpade

TI__Guru* 81034 points

Hi Shivani,

Yes, the PMOS allows for the use of BAT- as the gate reference. When the CHG pin goes low (to turn off the CHG FET), the PMOS FET will also turn off.

Matt

Power management

Power management forum

BQ77915: Protection for CHG and LD pin