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TXB0108: Control 'Enable' with TPS3808 RESET out

Ishay Shavit24
Intellectual 310 points

Part Number: TXB0108
Other Parts Discussed in Thread: TPS3808, SN74AUP1G34

Hello, 

Our customer asked to keep the level shifter disabled while the system is in reset, mainly to ensure no leak during power-up.

Can I connect an open-drain output (pulled to 1.8V) of a system-reset generator (e.g. TPS3808) to the TXB0108 ENABLE pin? 
Or that there is a need to have a pull-down as is specified in the spec. 
If there's a need to have a pull-down - how can I enable the level shifter when power is on? 

Here's my schematic.  For clarity, removed the I/O signals.

 

  • 0
    TI__Guru* 97666 points
    Hello Ishay,
    The OE pin of the TXB0108 must be held LOW to be put into high-impedance mode.

    If you want this device to be in high-impedance mode during power-up (ie while the supply is ramping up), then a pull-down resistor is a common way to achieve this. That won't work together with a pull-up resistor, as it seems you have already determined.

    You have two options here:
    (1) Do not place a pull-down resistor at all and keep the circuit the way you have it. The TXB0108 will not be in high-impedance during the initial power supply ramp. This may not be a problem for your system.

    (2) Place a push-pull buffer (such as SN74AUP1G34) between the open-drain device and the TXB0108's OE pin. This will allow for a pull-down resistor at the OE pin to force the device into high-impedance during startup, and will transmit the signal from H_RESET during operation.