BQ25720: Integration with PD TPS65988 questions

Smart Battery Charger Specification: sbs-forum.org/.../sbc110.pdf

Hi David,

Not sure on the PD controller suggestion. BQ25720 can cover the input voltage and power range for USB PD SPR. One note is TPS65988 is a I2C device while BQ25720 is SMBus. Thus TPS65988 cannot directly communicate with the BQ25720.

1. We recommend the inrush snubber for this application.

2. You highly likely need a dedicated DCDC for this 5V rail. The VSYS is not a fixed 5V rail. I will follow the battery voltage when the battery voltage is charge to above VSYSMIN.

3. Yes, charge voltage can be adjusted in the charge voltage register to achieve this.

4. Not sure what "go flat" means here.

5. No, the charger does not have a setting for this. Most of the applications, this is done by the gauge

6. Yes, you should be able to. 

Hi Jing Zou, thanks for your support with this.

>Thus TPS65988 cannot directly communicate with the BQ25720

We will have a CPU hosting the i2c/smbus/pmbus, which should be able to communicate with both.

1. Ok this has been added to the circuit
2. We will have a dedicated 5V regulator as our system voltage. I was unsure whether this should supply the USBC OTG Via the dual PD device, or if it was better to use the BQ25720 OTG functionality. I am using the PD device at the moment.
3. Excellent
4. If the battery was discharged and the external power was not connected, would the BQ25720 registers be reset to defaults? If so, the next time USB C power is applied, while the processor is off, the battery will then charge to 100%. To avoid this we would need to power up the processor, update the registers and power down again.
5. Ok, the CPU could detect the level and power down the system, if we wanted this functionality.
6. Excellent. The Battey can also act as a Host and "broadcast its desired charging voltage and charging current to the Smart Battery Charger address (0x12) every 10s to 60s (typically 50 seconds). It will additionally broadcast the AlarmWarning() (which is similar to the BatteryStatus() register) to the SMBus Host (0x10) and to the Smart Battery Charger (0x12) addresses."   I need to understand this in greater detail to determine if we will use a multi-host or a single-host architecture. Does TI have an example of a multi-host architecture?

I have been reviewing https://sbs-forum.org/specs/sbc110.pdf and comparing it with the BQ25720 datasheet https://www.ti.com/lit/ds/symlink/bq25720.pdf.

I don't see commands 0x11, 0x13, 0x16 in the BQ25720 datasheet. These are required commands according to 6.2.1

SMART BATTERY CHARGER COMMANDS
5.1. Smart Battery Charger Slave Functions (battery or host-to-charger)
5.1.1. ChargingCurrent() (0x14)
5.1.2. ChargingVoltage() (0x15)
5.1.3. AlarmWarning() (0x16)
5.1.4. ChargerMode() (0x12)
5.1.5. ChargerStatus() (0x13)
5.1.6. ChargerSpecInfo() (0x11)

5.2. Smart Battery Charger Master Functions (charger-to-battery)
5.2.1. ChargingCurrent() (0x14)
5.2.2. ChargingVoltage() (0x15)

Does BQ25720 support these or is a multi-host architecture required?

I had the same question as in the following post.

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/916020/bq25710-where-to-connect-smart-battery-pack-t-pin

I plan to connect to a smart battery: https://www.rrc-ps.com/en/battery-packs/standard-battery-packs/products/RRC2040-2

The T pin has a 300R resistor between T and the negative terminal instead of a thermistor.

Could CMPIN detect the resistor, and then the host could read STAT_COMP on register 0x21 to determine battery presence?

Or, is polling the SMBus Device address 0xB reliable even when the battery is discharged?

If the T pin is connected to a weakly pulled-up NFET Gate, could the NFET be connected in parallel with the bottom resistor of the CELL_BATPRESZ potential divider? So when the battery is connected, the gate is pulled down and the NFET is off, but when the battery is removed the NFET will pull CELL_BATPRESZ to Gnd.

Hi David,

If the T pin is low when battery is connected and if it's high when battery is disconnected, then it is possible. 

Please refer to the EVM schematic where cell control is implemented. 

Best Regards,

Jibin

Thanks for your help. I have added it to the design.

Is it possible to have a review of the schematic?

Thanks, I completed the checklist. There are two C19 checks, I believe one of them should be C20.

I used the WE 7443340150 1u5 inductor elsewhere in my design. The datasheet looks like it is expecting 2u2 or 1u. Would 1u5 be okay with the default of 800kHz, or is it best to change to 2u2?

I could change to: WE 7443340220

2u2 @ 800kHz Default
Iripple_buck= 20x0.5*0.5/(2u2*800k)=2.84
Ichg_max=4A
Isat=5.42A

When calculating the inductor Isat, do I also add the VSYS current?