BQ25606: Charging battery via USB Port while in DCP mode

Hey Jobby,

Two things here:
1) Because you have the D+/D- lines shorted together, the charger will always detect as a DCP with 2.4A. If you want to detect the correct current limit for a USB port with a 500mA current limit, I would suggest keeping the D+/D- lines tied to the appropriate pins on the USB connector.

2) If you are going to keep the charger in DCP mode no matter the input source type, there is another feature of the charger called VINDPM. On this part, VINDPM is set to the higher of 4.3V and VBAT+300mV. Now, I believe the lower limit the USB 2.0 spec is 4.75V or 4.5V. If VINDPM does not catch this, when the charger attempts to pull more than 500mA from SDP type USB ports, the USB will shut off due to a violation in the power consumption. It should not damage your USB port, but it may not charge.


Regards,
Joel H

Hi Joel,

Thanks for the reply. I'm also dealing with another issue. Currently the IC is set to DCP mode and supposed to charge the battery at 5V/1A via a wall-adapter. When the charging started the charging current was approximately 890mA, but after ~30min it dropped down to 550mA and stayed at that rate for a few hours until the current reduced to 0A while the battery got fully charged. This significantly affected the recharging time. Any reason why the charge current doesn't stay around 1A longer than 30 minutes?

Thanks,
Jobby

Hey Jobby,

One possible reason is thermal foldback or regulation. If the IC itself gets too hot, it is possible the charger will reduce the charging current as a protection mechanism such that the heat generated inside the IC is reduced. You may want to check the case temperature of the IC if possible.

I also don't know if you have a system load attached to your device.

One other possible reason is your battery itself. Older batteries can have a larger ESR and push the charger into CV mode much sooner than normal. And even a newer, good battery can also see if this it charges quickly. However, it is odd that it stayed at 550mA. How is this being measured?


Regards,
Joel H

Hi Joel,

The IC temperature is approximately around 38°C at peak charging current (around ~800mA) and then the temperature decreases to 28°C when battery is fully charged. There is no load attached to the device while battery is charging.

Ok, I will try a new battery and see if the problem still exists. The current is being measured using a ammeter in series with the battery holder. At what battery voltage does the charger switch from CC to CV mode? If V_Batt < V_sys_min (3.5V) will charging be disabled?

Thanks,
Jobby

Hey Jobby,

The only other thing I can think of is the charger hitting VINDPM. If the current and line resistance from your adapter to the charger is high enough, the charger can enter VINDPM where it will reduce the charging current in order to regulate the input voltage. Can you try this test with a bench power supply with something like 6V input voltage and 1A current limit?

To your questions, the battery enters CV mode just as the battery voltage as measured on the BAT pin of the charger approaches the regulation threshold you set using the VSET pin. Charging is not disabled when VBAT is less than VSYSMIN.


Regards,
Joel H

Hi Joel,

I have connected a power supply in place of an USB adapter. However, I am unable to set the current limit to 1A on the power supply. Every time I increase the current limit past 700mA the voltage drops to 4.1V and then the input current decreases to 60-100mA & voltage resets to 6V. Essentially the power supply stops operating in CC mode and switches to CV mode. It seems like it is the VINDPM affecting the charging circuit and causing the current to be limited. If the input voltage is below 4.3V does the IC start to reduce the charging current? What can cause the input voltage to be pulled down to 4.1V?

Thank You,
Jobby

Hey Jobby,

When the charger hits VINDPM, the charge current is reduced. On the BQ25606, the VINDPM threshold is the higher of wither 4.3V OR VBAT+300mV. It will reduce the charge current so much so that the input voltage is regulated to that threshold.

VINDPM is there to protect your adapter from overloading when you do not have a fixed or known current limit.

So I find it odd that the input current suddenly drops to 60-100mA when the current limit is increased past 700mA.

Lets measure some voltages at different points before we go start looking at waveforms:
Below 700mA input current limit setting, measure:
1) VBUS at the pin of the charger
2) VBAT at the pin of the charger
3) IBAT, or the current flowing into the battery
4) IBUS, or current flowing out of the power supply

Above 700mA input current limit setting, measure:
Same as 1-4 above.


Regards,
Joel H

Hi Joel,

Thank you for your reply. I have measured the following pins you requested here are the details.

Power Supply set to CC Mode: Current - 660mA, Voltage - 4.3V [attempting to set current limit to approximately 700mA]
1) VBUS Pin: 4.08V ( and slowly rising)
2) VBAT Pin: 3.66V
3) IBAT: 650mA
4) IBUS: 667mA

Power Supply switches to CV mode: Current - 450mA, Voltage - 5V [attempting to set current limit above 700mA]
1) VBUS Pin: 5.01V
2) VBAT Pin: 3.60V
3) IBAT: fluctuated from 10mA to 126mA
4) IBUS: 10-25mA

Does the Battery FET only turn on AFTER the VBAT reaches VSYS_MIN? Meaning does the charger only enter CC charging mode once the VBAT is higher than VSYS_MIN?

Thanks,

Jobby

Hey Jobby,

A few comments and questions:

So when your power supply is "set to CC mode", this is due to it hitting the constant current limit, correct? You are not programming it as a DC current source, correct? You should be programming it as a 5V DC source, and then set a constant current limit of 700mA. When it would naturally drop to the VINDPM threshold of the charger as it is overloaded. This is what you are doing correct?

For the second case, you are setting your power supply as a 5V DC source as well, just with a higher constant current limit, correct?

The BATFET is operated in LDO mode when the battery is below VSYSMIN and is fully on when the battery is above VSYSMIN. However, in both of these phases, the charger is in CC charging mode. Only until VBAT reaches the regulation voltage does it change to CV mode.


I think we need to see a waveform here of the VBUS, VSYS, VBAT, and the IBAT (charge current) on an oscilloscope to see if something is occurring transiently.


Regards,
Joel H

Hi Joel,

Yes, the power supply is set to 5V DC source then set a constant current limit of 700mA. For the second instance the power supply is set to 5V DC and current limit is set above 750mA, however the voltage resets to 5V and the current fluctuates between 10mA-120mA until the current limit is reduced to below 700mA. I have attached the waveforms for the pins 5V DC and 700mA current. 

If the VBUS voltage falls below the VINDPM range does the charger automatically set the charge current to the termination current regulation? Does stat pin only enter LOW state once the V_BATT is equal to V_SYS_MIN (3.68V)?

Thanks,

Jobby

Hey Jobby,

The input voltage looks to be tracking with the battery voltage. We cannot see the oscillation due to the timescale. This would be valuable to see. However, from the moments of no oscillation, VBAT and VBUS are very close in value.

In this case, the charger would have hit the SLEEP condition which disables the converter. When VBUS falls below the VINDPM range, the converter will stop providing power to the output and if deep enough the aforementioned SLEEP threshold, the converter and charging will be disabled.

STAT pin is only LOW when charging. It will also oscillate at 1Hz when there is a fault present.


Regards,
Joel H

Hi Joel,

Thank you for your suggestion. I will continue testing to see if the issue still arises. I also had some questions regarding the V_BAT_min for BQ25606. Is minimum V_BAT to operate the system 3.0V? Will the system boost the output voltage to 3.6V once the V_Bat falls below V_SYS_MIN?

Thanks,
Jobby