BQ25180: C2 pin voltage question

Hi Roy,

Thanks for reaching out. I'm reviewing this and will follow up tomorrow, 11/4.

Best regards,

Alec

Hi Roy,

Two quick checks to separate the two cases:

1. Is charge enabled via I2C on all boards when VIN is applied and the battery is not connected?
2. Please share scope captures for both cases (C2 pin = 2V and 4V): VBAT, VIN, and VSYS as close as possible to the IC pins.

With VIN present, charge enabled, and no battery, VBAT behaves as shown in Figure 9-2 of the BQ25180 datasheet:

CBAT charges up to VBATREG --> Charge terminates and the BATFET is turned off --> CBAT begins to deplete --> VBAT falls below the recharge threshold (VRCH) --> New charge cycle begins --> Cycle repeats

The captures will confirm whether this is the behavior when the C2 pin is 4V.

Best regards,

Alec

Hi Alec,

The symptom seems to relate the capacitor used on the fuel gauge BQ27427‘s LDO output VDD pin. Here’s the schematic:

The charger measurement of :

 1. Remove BQ27427 and keep BQ25180 C2 pin capacitor=1uF

2. Remove BQ27427 and keep BQ25180 C2 pin capacitor=1+4.7uF

3. With BQ27427 VDD capacitor =2.2uF

4. With BQ27427 VDD capacitor =4.7uF(Current design)

BQ27427 uses VDD pin capacitor =4.7uF for solving the BQ27427 unexpectedly enter sleep mode issue.

The measurement result 1. and 2. Seems to be as expected. But with BQ27427 LDO connected, the “CBAT charges up to VBATREG --> Charge terminates and the BATFET is turned off --> CBAT begins to deplete --> VBAT falls below the recharge threshold (VRCH)” cycle disappears(measurement result 3. and 4.).

Also when BQ27427 uses VDD pin capacitor =4.7uF, the load effect of LDO output seems to be the root cause of this issue? Pleas provide your thoughts.

Not sure if you could  provide the module spice of both side BQ25180’s charger block and BQ27427’s internal LDO block for simulation this symptom?

Regards,

Roy

Hello Roy,

It looks like there is some interaction between the charger and the gauge which may be causing this behavior. I would recommend keeping the nominal capacitance and rating for the LDO for the gauge, it is very important for low power modes to maintain low IQ. Unfortunately we don't have any spice models for the chargers or gauges.

Can you clarify that the captures #3 and #4 are both without a battery? Can you share a zoomed-in version of #4 so we can look at the waveform of VBFG oscillating?

Sincerely,

Wyatt Keller

Hi Wyatt,

The experiment result 1,2,3 and 4 are all without battery.

Here’s the Zoom in waveform of 4. Could you please help to provide more insights about the interaction between the charger and the gauge? We have another product using the other vendor’s charger with BQ27427 and did not see this symptom.

Regards,

Roy

Hello Roy,

Do you happen to have the datasheet for the other charger device?

I think what may be happening is the gauge is turning on and consuming all 8mA of our IBATSC current when using the large capacitor for VDD, the loading may not be consistent for the gauge either since it is a MCU.

It looks like VSYS is supplied and stable, is their host active? This can also disable charge while there is no battery present to prevent any oscillations. I would also be curious if they have the register dump from our charger while this behavior is occurring?

Sincerely,

Wyatt Keller