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TPS3895: Source of SENSE_OUT power loss & output logic

Joyce Chen
Expert 3820 points

Part Number: TPS3895
Other Parts Discussed in Thread: TPS3896, TPS3898

Hi team,

My customer met below two questions about TPS3895, pls help on this, thanks.

The simplified power supply is as below, there is two power desperately for VCC abd SENSE pin. Power1 is for VCC, and Power2(battery) is for SENSE. 

  1. Pls check if below logic is right?
    1. power 2 (BAT)'s voltage is from 3.6V to 3.0V, the SENSE_OUT is high.
    2. When power 2 (BAT)'s voltage less than 3.0V, the SENSE_OUT is low.

image.png

2. Due to the considerations of POWER2 loss, pls help check which power will support for SENSE_OUT's power loss? Power 1 or power 2(BAT)?

image.png

Thanks.

Joyce

  • 0
    TI__Expert 3800 points

    Hi Joyce,

    In the schematic it appears that they are using TPS3896 (active-high output), but you mention TPS3895 (active-low output). Can you confirm the full part number, including the last letter (A or P)?

    1. Going by the schematic and assuming they are using TPS3896, their understanding is generally correct. When the BAT voltage drops below the threshold, the SENSE_OUT pin will assert and go to logic high. Additionally, can the customer confirm the purpose of the extra resistor from SENSE_OUT to SENSE? Typically we see this when using an active-low device in order to add additional hysteresis, but when using an active-high device this may not work as expected.
    2. Since both TPS3896 and TPS3895 are push-pull devices, the SENSE_OUT logic high voltage will be the same as the voltage on VCC (Power1), as shown in the block diagram provided.

    -Henry

  • 0 in reply to Henry Naguski
    TI__Expert 3820 points

    Hi Henry,

    Yes, customer will use TPS3895 with extra resistor from SENSE_OUT to SENSE. Do you have recommended values on this resistor? Thanks.

    Joyce

  • +1 in reply to Joyce Chen
    TI__Expert 3800 points

    Hi Joyce,

    Thank you for confirming the part number. With TPS3895, when the BAT voltage drops below the threshold, SENSE_OUT will assert and go to logic low. When BAT rises above the positive-going threshold, SENSE_OUT will de-assert and go to logic high.

    The value of the hysteresis resistor (resistor from SENSE_OUT to SENSE) depends on the desired amount of hysteresis and values of the other resistors in the circuit. The app note Adding Hysteresis to Supply Voltage Supervisor describes the calculations to be done. Note that the example given uses an open-drain device, so there is an additional resistor Rp that is not present when using push-pull devices like TPS3895.

    -Henry

  • 0 in reply to Henry Naguski
    TI__Expert 3820 points

    Hi Henry,

    Just some questions from principle about the hysteresis resistor:

    1. Double check that hysteresis resistor can be added to TPS3895 for higher hysteresis, is this correct?

    2. How could this hysteresis resistor work for hysteresis on push-pull device TPS3895? Is it the same principle with open-drain device? Could you share some principles about this?

    Thanks.

    Joyce

  • 0 in reply to Joyce Chen
    TI__Expert 3800 points

    Hi Joyce,

    1. Yes, the extra hysteresis resistor can be used with TPS3895.
    2. Adding additional hysteresis resistance on a push-pull output is the same principle as on an open-drain device. The difference is that there is no pull-up resistor since VDD is connected to the output via a transistor.

    When calculating the trip point using the app note, V2 will be the VDD of the device, and Rp is essentially 0.

    Note that we do have an open-drain version of this device, TPS3898, which can be used to set V2 and Rp independently.

    -Henry