BQ24259: VBUS connection recognition, boost enabled

Hey Chris,

So for boost mode, the generated OTG voltage is actually seen on the PMID pin; not VBUS. As such, the load would have to be attached to PMID.

However, when a valid power supply is attached to VBUS, they charger would exit Boost Mode.

Regards,

Joel H

Joel-

    If that is true, that is great, but according to the datasheet, boost output is on the VBUS line (see below from 8.3.1.5). Can you confirm that the boost output is indeed on the PMID line and that no voltage is present on the VBUS line when the boost is active (and nothing else is connected to VBUS)?

-Chris

8.3.1.5 Boost Mode Operation from Battery The device supports boost converter operation to deliver power from the battery to other portable devices through USB port. The boost mode output current rating meets the USB On-The-Go 1-A output requirement. The maximum output current is 1.5 A. The boost operation can be enabled if the following conditions are valid: 1. BAT above BATLOWV threshold (VBATLOWV set by REG04[1]) 2. VBUS less than VBAT + VSLEEP (in sleep mode) 3. Boost mode operation is enabled (OTG pin HIGH and REG01[5:4] = 10) 4. Thermistor Temperature is within boost mode temperature monitor threshold unless BHOT[1:0] is set to 11 (REG06[1:0]) to disable this monitor function 5. After 30 ms delay from boost mode enable In boost mode, the device employs a 1.5-MHz step-up switching regulator. Similar to buck operation, the device switches from PWM operation to PFM operation at light load to improve efficiency. During boost mode, the status register REG08[7:6] is set to 11, the VBUS output is 5 V and the output current can reach up to 1 A or 1.5 A, selected via I 2C (REG01[0]). In addition, the device provides adjustable boost voltage from 4.55 V to 5.5 V by changing BOOSTV bits in REG06[7:4] Any fault during boost operation, including VBUS over-voltage or over-current, sets the fault register REG09[6] to 1 and an INT is asserted.

Hey Chris,

It's through PMID. We need to update that datasheet.

Regards,

Joel H

Joel-

What is the state of RBFET/Q1 when in boost mode? If enabled, wouldn't the output also be available on VBUS? I'm basing this on the functional block diagram (section 8.2 in the datasheet).

Is there anything else that is a pending datasheet issue? Is there a dev board for this part (I'm not seeing one online).

-Chris

hey Chris,

I would like to rescind my previous comment about OTG mode through PMID only on the bq24259. I was considering a different part in our product line.

In actuality, that datasheet is correct. The boosted voltage is delivered from VBUS. As such, the Q1 is state is on when in boost mode.

For an EVM, I would consider using the bq24296M. They are Pin-2-Pin compatible chargers with similar performance up to 2A. There are rated for different charging currents. You can purchase it here. 

Regards,

Joel H

Joel-

Thanks - then my original question still stands - "If the boost is enabled on the bq24259 and the load is removed/replaced with a charging source, how does the device recognize that the charging source is now present? Or does external circuitry need to be added to do this?"

I'm assuming external circuitry needs to be added, but it would be great to find out otherwise.

-Chris

Hey Chris,

You are correct. External circuitry would be needed to implement this change.

I would consider using the bq24295, which ONLY provides the boosted voltage out of PMID (meaning Q1 is off). With this charger, plugging in a valid input source at VBUS will disable boost mode and start charging.


Regards,
Joel H

Joel-

     Follow up question regarding the bq24295 - I can place in a new post if you would prefer. Given the below except from the datasheet (8.3.3.3.1), if the host is powered through the bq24295 and the boost mode battery thermal protection is triggered, the host will be disabled when BATFET is shut down, correct? So the only way to recover the system in this case is via a transition on the QON pin or plugging in an adapter, correct? This doesn't seem like a particularly useful protection mode since in most applications the host will be powered through the bq24295 .. . 

"For battery protection during boost mode, the device monitors the battery temperature to be within the VBCOLDx
to VBHOTx thresholds unless boost mode temperature is disabled by setting BHOT bits (REG06[3:2]) to 11.
When temperature is outside of the temperature thresholds, the boost mode and BATFET are disabled and
BATFET_Disable bit is set (REG07[5] bit) to reduce leakage current on PMID. Once temperature returns within
thresholds, the host can clear BATFET_Disable bit (REG07[5]) or provide logic low to high transition on QON pin
to enable BATFET and boost mode."

-Chris

Joel-

One more question about the bq24295. Can you elaborate on the voltage rating selection for the caps on the dev board for the bq24295. I'm trying to understand if there is a real need for 25V parts, or if this was done for other reasons (large stock of 25V parts, etc). Given that this will be used in a space constrained design, if a lower voltage/smaller part is possible, then it will be advantageous to use it.

-Chris

Hey Chris,

The TCOLD / THOT faults will automatically disable OTG mode by latching the BATFET off.

And you are correct that either QON or plugging in an adapter would be the only ways to re-enable the BATFET.

Some of the newer generations of parts such as those in the bq2589x family and the bq2560x family do not turn off Q4 during an NTC fault.

In regards to the voltage rating of the caps on our EVMs, we typically pick ratings on par with the ABS max rating of the charger itself. We never know how customers will utilize the demo board, so we assume worst case scenarios. What I will say is that we typically pick passives rated at ~2x the max expected voltage on the rail. I think, for example, if you are confident VBUS will only ever see 5VBUS (i.e. no damaged adapters, unique power rails), then the PMID and VBUS caps could be rated for 10V.

Regards,
Joel H

Joel-

    Thanks - can you confirm that the BATFET is turned off when charging and the temp falls out of the Vltf to Vhtf range? Based on the datasheet, (again 8.3.3.3.1), it seems like charging is only suspended if the temperature falls out of range - it seems like boost temp faults are the only temp fault resulting in BATFET shutdown.

Thanks for all of your help!

-Chris

Hey Chris,

This is correct. In buck mode, we do not latch off the FET. We only prevent charge current from flowing into the battery. However, if the charger enters supplement mode, the discharging is not affected. As such, VSYS is still regulated above the battery voltage.


Regards,
Joel H