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BQ25173EVM: R15 Resistor

Part Number: BQ25173EVM
Other Parts Discussed in Thread: BQ25173-Q1, BQ25173

Hi,

Why was package resistor 0805 used for R15 (604K), and not 0603 like the others?

regards

Jose

  • Hi Jose

    We were looking for a  0.1% tolerance resistor, 604k ohm this was difficult to find in stock. But we were flexible on device size and used 0805. 

    For this spot a large value resistor is desired to keep discharge of capacitor small.  

    Also see the BQ25173-Q1 EVM it has a disconnect circuit you may want to consider.

  • Hi Bill,

    Thank you for your reply.
    Considering that our circuit uses the Mosfet to avoid discharging the supercapacitor, we see no reason to use the 0805 package given the space constraints on the PCB. We'll use package 0603 as for the other resistors.

    regards,

    Jose

  • Hi Bill,

    Our circuit will charge a supercapacitor with 5.6V.

    We've used a circuit similar to the one in the BQ25173-Q1 Evaluation Module, with the Mosfet to avoid discharging the supercapacitor.

    Considering that the maximum permissible voltage is 5.5V at the PG pin, isn't there a risk of damaging the PG pin?

    regards

    Jose

  • Hi Jose

    Yes the voltage at PG pin is limited to 5.5V.  If used with cell configuration above this point additional protection will be needed.   

    An update to the EVM circuit is in work.

  • Hello Bill,

    Thank you for your reply, but in the meantime we are going to change the circuit to use the 'Q1' version.

    Since we're going to reduce the capacitor charge voltage from 5.4V to 5.0V to protect the PG pin, we can now connect the FB resistors to the PG pin instead of Gnd, eliminating the Mosfet that prevented the reverse current. With the space issues on the PCB, it's certainly an alternative to consider.

    What is not clear to me, using the circuit described above, is what the leakage current will be when there is a power failure and the supercapacitor powers the circuit. I can't find this parameter clearly on the datasheet, can you help me identify it?

    Thanks.

    regards

    Jose

  • Hi Jose

    The PG pin can be used to disconnect voltage divider circuit and remove this current .  But you will see some current into the FB pin this will be in the 1uA to 2uA range.

  • Hi Bill,

    Thank you in advance for your reply, and for indicating the estimated current value.

    When we use the circuit recommended for TI with the Mosfet to prevent the supercap from being discharged by the resistors, aren't we creating a higher current via the PG pin?

    What happens in this situation?

    regards

    Jose

  • Hi Jose

    Yes, when the adapter is connected,  /PG is low and we will have current flow as you pointed out.  But this is a condition where the BQ25173 is powered and keeps the capacitors charged.  Leakage current is often not a concern during this time.  Current can be reduced by increasing RX.  

    During the unpowered condition with /PG high the circuit with have the greatest benefit to leakage current. 

  • Hi Bill,

    Thank you for your comments, which were very important to us.

    As one of the biggest concerns in our system is current consumption during normal operation, we're opting to use the PG pin instead of the Mosfet solution.


    The Mosfet solution would lead to higher consumption during most of the time the system has power, which would not be advantageous for us.

    regards

    Jose