SN74LVC244A: What's the internal pull up and pull down resistor value on output pin?

Wesley He

Part Number: SN74LVC244A

Tool/software:

Hi team,

Customer had a question, during their design, they had 2x SN74LVC244A output pin connect to one bus.

And the input had no pull up or down to control the output logic before the MCU is power up. After the MCU power up, the conflict will be fixed by Software.

The worse case here will be one 244 output 1, and one 244 output 0. Power supply is 3.3V.

Question 1: What's the internal output pull up and pull down resistor value? This is to confirm what's the output voltage level, to check if this will impact the logic.

Question 2: What's the risk if 2x output connect together and one output high and the other one output low?

Thanks.

Regards,

Wesley

3 months ago

0 Clemens Ladisch 3 months ago

Guru 317000 points

1. At 3.3 V, the worst-case V_OL / I_OL is 0.55 V / 24 mA ≈ 23 Ω. The typical value is roughly half that.

2. Section 8.2.1 says: "Avoid bus contention because it can drive currents in excess of maximum limits." Both outputs will be degraded and might burn out.

In theory, you should have a pull-up at the /OE pins and force it low from the MCU after power up.

0 Wesley He 3 months ago in reply to Clemens Ladisch

TI__Expert 6205 points

Hi Clemens,

Quick question, the ~23Ω is the output resistor in output. What's the number for internal pull up and internal pull down ?

Thanks.

Wesley

0 Clemens Ladisch 3 months ago in reply to Wesley He

Guru 317000 points

(V_CC − V_OH) / I_OH ≈ 33 Ω.

Logic

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SN74LVC244A: What's the internal pull up and pull down resistor value on output pin?