SN74LVT245B: Ensuring high-impedance state during start-up, power up

RuggedEngie

Part Number: SN74LVT245B

Hi,

That datasheet says the following:

"To ensure the high-impedance state during power up or power down, OE should be tied to VCC through a pullup resistor; the minimum value of the resistor is determined by the current-sinking capability of the driver."

What driver is this referring to when saying "current-sinking capability of the driver?" Can we get more detail on calculating this pull-up resistor?

We are seeing issue with these signals during start-up so I am questioning if these signals are staying in a high-impedance condition during power on.

Edit: I should add more details about our application:

The B-side is connected to LED cathodes and the A-side is connected to microcontroller PWM pins which control LED status and brightness. The DIR pin is pulled high, and /OE pin is pulled high by default through a 10K resistor. The /OE is later toggled by a microcontroller output pin.

Why do we see the LED flicker once during startup when the microcontroller code is not running and the /OE pin is pulled high??

over 4 years ago

0 Clemens Ladisch over 4 years ago

Guru 244360 points

The driver is what whatever device is forcing the OE signal low, usually a microcontroller's GPIO.

In practice, you never use a minimum-value resistor because that would waste power. See the application report Choosing an Appropriate Pull-up/Pull-down Resistor for Open Drain Outputs (SLVA485).

Can you show an oscilloscope trace of OE and the output?

0 RuggedEngie over 4 years ago in reply to Clemens Ladisch

Intellectual 375 points

Okay, I thought that was odd. We pull the /OE pin high through 10K resistor. Then /OE is later toggled by a microcontroller GPIO pin. I added more application data in the original post. We still see the outputs allowing current to flow before the microcontroller has toggled (enabled) the /OE pin.

0 Dylan Hubbard over 4 years ago in reply to RuggedEngie

TI__Mastermind 42670 points

Hi,

Can we get a schematic and some scope shots of the start up?

0 RuggedEngie over 4 years ago in reply to Dylan Hubbard

Intellectual 375 points

I am sorry I cannot provide those at this time. I do want to know what would happen at the outputs of this device when it is in isolation mode and the outputs are connected to LED cathodes? Would current flow through the LEDs?

0 Dylan Hubbard over 4 years ago in reply to RuggedEngie

TI__Mastermind 42670 points

Hi,

Sorry I didn't mean to mark that as resolved. The outputs will go into Hi-z in which there will be a leakage current spec'd in the datasheet. It's possible for that current to go through the LED however it would be in the uA range.

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SN74LVT245B: Ensuring high-impedance state during start-up, power up