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BQ25731EVM: How to use PTM

Nate Martin
Prodigy 220 points
Part Number: BQ25731EVM
Other Parts Discussed in Thread: BQ25731, USB2ANY

Tool/software:

Hello,

I am trying to use PTM on the BQ25731. In the datasheet it doesnt give much information on how to properly use it.

When I try simply enabling PTM by setting the EN_PTM register through the Analog EVM online GUI it does nothing.

What is the procedure for turning on and off PTM?

Thanks,

Nate

  • 0
    TI__Mastermind 20490 points

    Hello Nate,

    PTM is used to improve the efficiency during Buck-Boost operation, input voltage=output voltage.

    Nate Martin said:
    What is the procedure for turning on and off PTM?

    PTM is turned on/off by disabling and enabling the EN_PTM bit.

    Nate Martin said:
    When I try simply enabling PTM by setting the EN_PTM register through the Analog EVM online GUI it does nothing.

    The LODR1/LODRV2 should be disabled during PTM mode. I recommend checking if  LODR1/LODRV2 go low or remain switching when you toggle EN_PTM bit If LODR1/LODRV2 go low when EN_PTM=1, this means PTM mode is working.

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    Prodigy 220 points

    Hi Christian,

    I have the development board in a 4S configuration, with 15V applied to the battery screw terminals from a bench power supply. I then connect to the board using the USB2ANY Analog EVM with Auto read set to every 1 second. When I then click on the EN_PTM checkbox in the charging configuration tab, nothing happens. The output voltage on VBus stays the same, and all fets remain off (LODRV1, LODRV2, HIDRV1, an HIDRV2).

  • 0 in reply to Nate Martin
    Prodigy 220 points

    I think I understand the issue.

    PTM mode is designed to only work while charging, and not work for discharging. PTM works when I attach the power supply to the VBUS input and leave VBAT floating, but not the other way around.

    Is there any way to enable pass through mode without voltage present on VBUS?

    Thanks,

    Nate

  • +1 in reply to Nate Martin
    TI__Mastermind 20490 points

    Hello Nate,

    Nate Martin said:
    PTM mode is designed to only work while charging, and not work for discharging. PTM works when I attach the power supply to the VBUS input and leave VBAT floating, but not the other way around.

    Yes, VBUS needs to be present.

    Nate Martin said:
    Is there any way to enable pass through mode without voltage present on VBUS?

    I'm confused on your question. Are you trying to discharge the battery through system or through VBUS using OTG mode?

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    Prodigy 220 points

    Hi Christian,

    USB C specification allows pass through of a direct battery voltage across a USB C cable upon negotiation. I was hoping I could use PTM on the BQ25731 to pass through the pack voltage to an attached USB C port. Putting the BQ25731 into its most efficient active state for USB PD devices that can make use of direct battery pass through from 13V to 16.8V.

    Does this answer your question?

    The datasheet was somewhat vague on whether PTM could work for both charge and discharge of the pack.

    Is there no way to place the BQ25731 into pass through mode for discharging and keep the current limits in place? Ie, if the battery DCL is reached, it turns off HIDRV1 and HIDRV2?

    Thanks,

    Nate

  • +1 in reply to Nate Martin
    TI__Mastermind 20490 points

    Hello Nate,

    Thank you for the clarification, 

    Unfortunately, PTM only works in forward mode when input voltage is applied to VBUS.

    Best Regards,

    Christian

  • 0 in reply to Christian Moyer
    Prodigy 220 points

    Does TI have an suggestions for passing out battery voltage to the USB C port?

  • 0 in reply to Nate Martin
    TI__Mastermind 20490 points

    Hello Nate,

    I recommend referring to section 9.3.9 USB On-The-Go(OTG) in the datasheet:

    Best Regards,

    Christian.