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BQ24075: sysoff is high logic in case of battery supply only

Irem Donmez
Prodigy 10 points
Part Number: BQ24075 
In my project that I shared the schematic, we separated the output of the bq24075 from the load with the switch.
 Supervisor(active low/threshold 3.2V) and NOR gate driving our sysoff signal. Sysoff input is default low logic level.
 This is what we would normally expect it to work in.
 
Case 1 :However, when we feed our system only with battery and switch is the open state, the sysoff is at low level as we expect.
 Then,when we closed the switch and connect the output of bq24075 to the load, sysoff goes to a high level. 
The output of the BQ24075 cannot feed the load.
 
Case 2 :When we feed our system only with USB or USB and battery are plugged in at the same time, our system works well.It doesn't matter in this case ,if the switch is off or on.
 
Case 3 :While The switch is off state in our system , USB and battery are plugged in, our system works well.
if we remove the USB at run time, our system maintained as work well and the sysoff remains at the low level as expected.
 
What could be the reason for this situation?

 

 

 

 

 

 

 

 

 

 

  • 0
    TI__Mastermind 23200 points

    Hi Irem,

    Welcome to E2E!

    It sounds like when you expect the SYSOFF to be pulled low but are seeing high (Case 1), the SYSOFF is actually being left floating. The SYSOFF has an internal pullup to VBAT with a large resistor (5MOhm). You may need to adjust the resistor pulldown that you have on the SYSOFF pin. 

    Best Regards,

    Anthony Pham

  • +1 in reply to Anthony P Pham
    Prodigy 10 points

    Hello, when the sysoff battery voltage is above 3.2, sysoff pulls to low level. There is a 10K pull-down resistor connected to the sysoff in our system.

     

    When we disconnected the output of BQ24075 from the load, the system worked well. So we suspected that the system was drawing excessive current at starting.

    To observe that draws the inrush current, we disconnected the system from the load and installed a 2200uF capacitor as the load. The system did not work and the Bq24075 lowered Vout Voltage. We added a series resistor to the capacitor to limit current. The system started to work properly.

     

    When we connected a series resistor to the load to limit the excessive current and fed it back from the battery, the system worked in Case1 well. Thus, we have verified that there is an overload in the system.

    We figured out the cause of our problem.

    We are investigating an inrush current limiter circuit to solve the problem.