BQ25750EVM: Not switching Load from Vin to BATT

Hello Alex,

Thanks for working with this.

The default factory settings on the BQ25750 have not been updated and we currently do not plan to update those settings.

To help me with your question, what's the voltage on VIN and BAT when you do the power path switch over?

Best Regards,
Ethan Galloway

Hello Ethan,

My input source is a 4-Series LiFePO4 Pack. ACUV is set to 11Vdc and ACOV is set to 16Vdc.

The BATT input is another 4-Series LiFePO4 Pack. Its BMS is set to operate down to 11Vdc as well.

I am not having any problems charging the battery or switching under 300mA. I believe the problem is the FET drive.

The chipset is trying to turn on the BatFets, but is unable to. I can hear an audible tick when the chipset attempts to turn on the FETs. If i remove the load, then the chipset is able to turn on the FETs.

I encountered a similar problem with the BQ76200 FET driver when using FETs with too high gate capacitance, and too small of a charge pump capacitor.

This is only my theory at this point and I look forward to your feedback.

I have also attached the BQ file from the EVAL module I am using.

* Created: Tue Jun 11 09:00:54 EDT 2024
*
* Format: Register Name  tab Character,\t  Register Address  tab Character,\t  Hexadecimal register value.
* BQZ Container: Charger_1_00-bq25750.bqz
*
Charge Voltage Limit	00	10
Charge Current Limit	02	640
Input Current DPM Limit	06	640
Input Voltage DPM Limit	08	348
Reverse Mode Input Current Limit	0A	640
Reverse Mode System Voltage Limit	0C	3E8
Precharge Current Limit	10	140
Termination Current Limit	12	A0
Precharge and Termination Control	14	0F
Timer Control	15	1D
Three-Stage Charge Control	16	00
Charger Control	17	C9
Pin Control	18	C0
Power Path and Reverse Mode Control	19	20
MPPT Control	1A	20
TS Charging Threshold Control	1B	96
TS Charging Region Behavior Control	1C	57
TS Reverse Mode Threshold Control	1D	40
Reverse Undervoltage Control	1E	00
VAC Max Power Point Detected	1F	00
Charger Status 1	21	03
Charger Status 2	22	80
Charger Status 3	23	02
Fault Status	24	00
Charger Flag 1	25	61
Charger Flag 2	26	E0
Fault Flag	27	82
Charger Mask 1	28	00
Charger Mask 2	29	00
Fault Mask	2A	00
ADC Control	2B	60
ADC Channel Control	2C	02
IAC ADC - Use Field View	2D	00
IBAT ADC - Use Field View	2F	00
VAC ADC	31	00
VBAT ADC	33	00
VSYS ADC	35	00
TS ADC	37	00
VFB ADC	39	00
Gate Driver Strength Control	3B	00
Gate Driver Dead Time Control	3C	00
Part Information	3D	02
GM Adjust	61	30
Reverse Mode Battery Discharge Current	62	02

Hello Ethan,

I have confirmed my theory that the BATDRV output was not capable of driving the BATFETs when switching under load.

I took a BQ76200 driver (thats what I had in the lab) and removed R9 from the Eval module. I used the BATDRV to drive the BQ76200's input and put the output directly to the BATFETs.

This method fixed my switching issues, although not elegantly.

What can I do, to make the BQ25750 drive the BATFETs under load without having an external driver?

-Thanks

Hello Alex,

The BQ25750 has built-in over-current protection for the BATFETs. If the discharge current exceeds 8A, the BQ25750 will turn off the BATFETs. There are a few ways to work with this.

The 1st is to use an RC filter to slow down the BATFET turn-on. This will reduce the surge current from BAT to SYS. I also recommend using FETs with a high SOA like the AON6276.

The 2nd is to use the BQ25750's auto reverse mode. Auto Reverse mode will automatically use the converter to provide power to SYS from battery once VAC is removed. The response is really fast and it takes about 50µs for the converter to start working.

Also, I looked through your registers. If you are using the ICHG register to program ICHG, make sure to set WATCHDOG to disabled.

Let me know if you have any questions about this.

Best Regards,
Ethan Galloway

Hello Ethan,

The 8A discharge limit is a problem for my needs. I plan to run the backup battery at 150W so around 13Amps continuous, and I was hoping to use the chipset in stand-alone-mode so I wouldn't have to have it programmed.

I couldn't find the 8Amp limit in the datasheet. Pg.15 states that the max range of the bat sense resistor is ±20Amps, so I assumed that was the max current of the BAT line.

Is there any adjustment for that value or is it a non-changeable setting? Could I change the BAT current sense resistor to possibly get more current?

Hello Alex,

Thanks for being patient with the delay. I was out of office Thursday and Friday.

We are working on adding the 8 amp discharge limit to the datasheet. The max charge current through the converter is still 20A. The discharge current limit through the BATFET can't be changed.

For your application, would it be possible to use auto reverse mode for your application? Auto reverse mode uses the buck-boost converter to send power to the system. The buck-boost converter can handle a 13A discharge.

Best Regards,
Ethan Galloway