BQ25798: BQ25798 PMID PATH

Part Number: BQ25798
Other Parts Discussed in Thread: BQ25731

Tool/software:

I am planning to use this part for An application where in VBUS voltage is 15V 

I have the battery of 2s2p 8.4 volt max all the configuration is programmed based on 2s2p with charging current of 2.5A and safe discharging of 5A

now CPU and other loads are connected to a VSYS ,I have a requirement of charging a E-cap to 15V for a LED circuit During 15V charger plug in I am able to use the PMID pin voltage to charge the E-CAP 

When charger is removed can i boost the battery voltage to 15v  and get this in PMID pin ? what is the limitation of the current when USB charger is plugged and during this backup boost mode how to configure and use this mode please help 

  • Hi Justin,

    During 15V charger plug in I am able to use the PMID pin voltage to charge the E-CAP 

    From my understanding of this application, there is a 2S2P battery connected to BAT, and an E-CAP connected to PMID? The converter will not be able to regulate current from the charger plugging in to the PMID pin. If there is too much inrush current, this could damage the device.

    When charger is removed can i boost the battery voltage to 15v  and get this in PMID pin ? what is the limitation of the current when USB charger is plugged and during this backup boost mode how to configure and use this mode please help 

    The charger can boost the battery voltage to 15V on the PMID pin. I do not think the PMID load current will be regulated, so the current limitation will be on the safe discharge limit for the battery, in your case 5A. Since the voltage is boosting, the current limitations on the PMID pin will be lower than 5A.

    Best regards,
    Michael

  • Hi Michael ,

    Thanks for the response 

    The converter will not be able to regulate current from the charger plugging in to the PMID pin. If there is too much inrush current, this could damage the device. 

    For direct path from charger plugged in USB to PMID need to limit the current at the PMID pin externally 

    But for the VBAT to PMID boost the current is limited to battery discharge limit circuitry got this clarified 

    Now let say I have VSYS powering the system and Vbat supplied by battery PMID used for charging a cap using a charger BQ25731(which i think can limit input current)

    The PMID power for Vsb path is always present but when VBUS is not present the charger can cut VBUS PATH by disabling the external VAC  input mosfet and can boost the voltage to 15v ,will this transaction from vbus SOURCE  path to vbat SOURCE BOOSTED path is seamless or will have delay my vbus can vary from 15v 3a to 5v 3a.what is the procedure from the host to do this ?or this process can be automatically from charger ?

     

  • For direct path from charger plugged in USB to PMID need to limit the current at the PMID pin externally 

    This is correct if there is a load on the PMID pin.

    will this transaction from vbus SOURCE  path to vbat SOURCE BOOSTED path is seamless or will have delay my vbus can vary from 15v 3a to 5v 3a

    This can be seamless if you set EN_BACKUP=1 and add a little more capacitance to the pin. Take a look at section 9.3.7.2 Backup Power Supply Mode in the datasheet for more information.

    Best regards,
    Michael

  • Regarding the discharge current limitation of 5A need some clarity 

    I have a battery 8.4V which is 1C discharge capable which is 10.5A capable now during this mode I have two power paths when Vbus is absent from VBAT 

    one is Vsys path from Vbat path and other one is PMID path the discharge limitation applies only to PMID path is that correct ? for direct path there is no limitation .When the charger is unplugged what is the max Boost voltage i can get in PMID pin ? 

    Can I get 22V 2A constant current at the PMID pin when its in this mode ? + VSYS OUT 1.5A 7.4 or what ever battery voltage at that point in vsys pin at the same time 

  • Regarding the discharge current limitation of 5A need some clarity 

    I have a battery 8.4V which is 1C discharge capable which is 10.5A capable now during this mode I have two power paths when Vbus is absent from VBAT 

    one is Vsys path from Vbat path and other one is PMID path the discharge limitation applies only to PMID path is that correct ? for direct path there is no limitation .When the charger is unplugged what is the max Boost voltage i can get in PMID pin ? 

    Can I get 22V 2A constant current at the PMID pin when its in this mode ? + VSYS OUT 1.5A 7.4 or what ever battery voltage at that point in vsys pin at the same time 

  • Hi Justin,

    The device will sense the battery discharge current. This will be both the direct discharge to SYS and the discharge to PMID. If the SYS is pulling too much current, the device will stop powering to PMID.

    PMID will likely be able to do 2A current at 22V, but the device will be in boost mode which means the battery current will be higher than 2A (in addition to the SYS current).

    Best regards,
    Michael

  • Thanks for the response .

    If i use 2s3p vs  3s2p 3s will have higher voltage and lower discharge current .. this current limitation that you mentioned does it have the limitation same across different cell voltage configuration ? 

  • Hi Justin,

    Actually, in the backup mode, there is a bit to disable the current limit on the battery discharge:

    Best regards,
    Michael Bradbourne

  • But its mentioned that this is in OTG mode ,I am using in Backup mode ?

    I am not sourcing to OTG (external USB port ) I am consuming through PMID is this still applicable .Also if let say its applicable for PMID as well as a part of boost converter then is this a constant current limiting mechanism or a fold back mechanism in case of load trying to sink more than programed limit 

  • Hi Justin,

    It will set the current for both OTG and backup mode:

    This is a battery protection mechanism that does not allow the battery to discharge too much current. This will regulate the battery current and reduce the input power which in turn limits the output power in backup mode.

    Best,
    Michael Bradbourne

  • Hi 

    Is there a proper backup mode entry sequence for Providing power to PMID pin when Vbus drops like which register to set and in which order ? 

  • The backup mode only can be enabled when VBUS is high, by setting EN_BACKUP = 1. When VBUS becomes low, the charger resets EN_BACKUP bit to 0.

    When Vbus not present do this back up mode work or don't work or this line in  datasheet is only during first time or this EN_BACKUP can be set ny the host any time after  ACFET1-RBFET1 detected at POR

  • Hi Justin,

    Is there a proper backup mode entry sequence for Providing power to PMID pin when Vbus drops like which register to set and in which order ? 

    You only need to set EN_BACKUP = 1 while the VBUS is present

    The backup mode only can be enabled when VBUS is high, by setting EN_BACKUP = 1. When VBUS becomes low, the charger resets EN_BACKUP bit to 0.

    This is correct.

    When Vbus not present do this back up mode work or don't work or this line in  datasheet is only during first time or this EN_BACKUP can be set ny the host any time after  ACFET1-RBFET1 detected at POR

    When VBUS is not present, backup mode is not necessary. Backup is for uninterrupted power supply and if VBUS is not present, there is no need to have backup enabled.

    EN_BACKUP can be set by the host after the device has detected ACFETs and RBFETs and VBUS is present.

    Best regards,
    Michael Bradbourne

  • When VBUS is not present, backup mode is not necessary. Backup is for uninterrupted power supply and if VBUS is not present, there is no need to have backup enabled.--

    when vbus is plugged during  power up the PMID path is from VBUS and since ACFET is available i can enable backup mode and I have disabled OTG mode so it wont source power to VBUS when Vbus is removed 

    But when Vbus is removed after power on means when Vbus is removed  will the power on the PMID pin will stop or it will be still in  sourcing power to PMID from SYS getting power in PMID and EN_BACKUP are there any relation ? if so when vbus is removed can i enable backup bit and still get power in PMID path 

  • when vbus is plugged during  power up the PMID path is from VBUS and since ACFET is available i can enable backup mode and I have disabled OTG mode so it wont source power to VBUS when Vbus is removed 

    This statement is correct.

    But when Vbus is removed after power on means when Vbus is removed  will the power on the PMID pin will stop or it will be still in  sourcing power to PMID from SYS getting power in PMID and EN_BACKUP are there any relation ?

    PMID will lose power when VBUS is removed unless EN_BACKUP has been set. VBUS powers PMID after power on. When VBUS is removed, SYS is powered by BAT through the BAFET. If EN_BACKUP was set before VBUS was removed, PMID will be powered by BAT through the switching network.

    if so when vbus is removed can i enable backup bit and still get power in PMID path 

    If EN_BACKUP is set before vbus is removed, then PMID should not lose power as long as the minimum capacitance requirement is met.

    Best regards,
    Michael