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BQ25895: BQ25895 query

Bino Mani
Prodigy 90 points
Part Number: BQ25895

Tool/software:

Hello Ti Support Team,


I have a query related to BQ25895. I am designing a battery operated medical device and I want to know what is the use of SYS pin when VBUS is not connected and how should be the SYS pin connected when a Lipo Battery is used with 3V to 4.2V, 4000mAh. Please note that there won't be any VBUS supply.

Looking forward for a quick response.


Regards,
Bino Mani

  • +1
    TI__Mastermind 26190 points

    Hi Bino, 

    With no VBUS connected SYS pin output is powered directly by the battery through Q4 BATFET. SYS will be regulated a small amount below battery voltage due to the voltage drop across the BATFET. 

    If all other components in your design require 5V then they should be connected at PMID boost mode output rather than SYS. If you have components which can directly be powered by the battery then they should be connected to SYS. 

    Additionally, even if you have no components connected to SYS pin the 20uF output capacitor is still required. 

    Best Regards,

    Garrett 

  • +1 in reply to Garrett Kreger
    TI__Mastermind 26190 points

    Hi Bino, 

    I see you have marked the thread as not resolved, but did not reply to my answer. Do you have any additional questions? If not please consider this thread closed. 

    Best Regards,

    Garrett

  • 0 in reply to Garrett Kreger
    Prodigy 90 points

    Hello Garrett,

    SYS pin will reflect the battery voltage after the voltage drop across the BATFET. Please confirm.

    What is the use of inductor between SW and SYS pin? Is the inductor required if PMID is active? Please note that I will only connect VBUS while charging.

    What is the use of VREG pin? Is it a power output pin? If yes, what is the voltage out and current out?

  • 0 in reply to Bino Mani
    TI__Mastermind 26190 points

    Hi Bino, 

    Firstly, in general please familiarize yourself with the datasheet. It contains the vast majority of the information you ask about across multiple E2E posts. 

    Bino Mani said:
    SYS pin will reflect the battery voltage after the voltage drop across the BATFET. Please confirm.

    Yes with only battery connected SYS pin will reflect the battery voltage after the voltage drop across BATFET. When VBUS is connected for charging SYS is regulated above sys_min (default 3.5V).

    Bino Mani said:
    What is the use of inductor between SW and SYS pin? Is the inductor required if PMID is active?

    Yes the inductor is required. It is necessary for both buck mode charging and boost mode operation to PMID output. 

    Bino Mani said:
    What is the use of VREG pin? Is it a power output pin?

    The REGN pin is not designed to be a power output pin. Please refer to its description in the pin function table in the datasheet on page 5 and additional description in section 8.2.3.1. Its main purpose is to supply internal bias circuits as well as the HSFET and LSFET gate drive. 

    The REGN pin should only be connected to the required 4.7uF capacitor and RT1 of the TS pin resistor divider circuit. 

    Best Regards,

    Garrett