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SN74LVCC3245A: How will it work when VCCA drops from 3.3V to 0V

Hung Ching Hsu
Expert 3086 points
Part Number: SN74LVCC3245A
Other Parts Discussed in Thread: SN74LVC8T245

Hi Team 

My customer is considering a situation that when 3.3V power on VCCA has a malfunction that becomes 0V. Then only 5V on

VCCB is supplied to SN74LVCC3245A.

The customer wants to know 

1. how much leakage current(mA), leakage voltage (mV) will occur to slave side?

2. If signal is input from master side(5V power supplying normally), what will the status of A1~A8 pin be?

Thanks.

  • 0
    TI__Mastermind 42600 points

    Hey Hung,

    Unfortunately, the answer to both questions will be unknown in this scenario. VCCA supplies the control circuity so the state of the device will be unknown.

  • 0 in reply to Dylan Hubbard
    TI__Expert 3086 points

    Hi Dylan

    Could you tell me the specific reason why the answer to both questions will be unknown?

    Does one of below apply?

    1. When losing 3.3V power, the status of DIR and OE pin will be unstable, so the direction of signal output became unpredictable.

    2. When losing 3.3V power, the power of internal direction control circuity also lost, so became unable to control.

    Finally, is there any measures to know how the status of the device will be even if 3.3V power is lost?

    Thanks.

  • 0 in reply to Hung Ching Hsu
    TI__Mastermind 42600 points

    Hey Hung,

    Both of those actually apply, but I'm more focused on case 2 since case 1 can be solved with pull-downs/pull-ups. The VCCA supplies the control circuitry so there is no way to know how they control circuitry will behave in this scenario. For this reason, the data sheet recommends powering up the VCCA side first.

    I can't really confirm if there will be any leakage from the 5 V side to the 3.3 V, though it doesn't see likely that this will be the case. If possible, I'd recommend using the SN74LVC8T245 since it has Vcc Isolation which disables the I/O when one of the supplies is at 0V (the data sheet provides a leakage spec in this state - Ioff). It's mostly p2p where the NC pin is traded for a supply pin. Using this device will eliminate any uncertainty during this scenario.