TXU0104: VOH Specification & Behaviour when Outputs Connected to Damaged Ports

Bogdan Banik

Part Number: TXU0104
Other Parts Discussed in Thread: TCA9535

I am using this device in an application where I am connecting a 3.3V input device with 2 different 3.3V output devices (input and output voltages are from 2 different redundant power sources with a common ground).

I have 2 questions....

Question 1: If 1 or 2 of the 4 TXU0104 outputs are connected to damaged device inputs, what will happen to the TXU0104. Will the remining 2 to 3 of the 4 outputs continue to work properly?

Question 2: The Table under Datasheet para. 7.5 specified VOH as >/= 2.2V with a 3V VCCB. This is problematic for me since the device I am connecting the TXU0104 outputs to has a VIH specified at >/= 2.31V. Why is the TXU0104's VOH so low... many 3.3V devices specify VOH at >/= 2.5V to 2.6V. In addition, TXU0104 datasheet Figure 7.1 shows a nominal VOH of >/= 2.75V for IOH levels of up to 24mA. The devices I will be connected to has high impedance inputs so I will be operating at logic-high current levels of </= 1mA; hence expect VOH to be > 3.25V (nominal) as shown in Figure 7-1. This could be a show-stopper for me so please advise urgently.!

Best Regards...BJ Banik (bj.banik@rogers.com)

over 4 years ago

0 Clemens Ladisch over 4 years ago

Guru 316760 points

That depends on what kind of damage.
If the inputs have a lower impedance, some current will flow. As long as you do not exceed the recommended operating conditions, the device will work correctly.
The output voltages are specified for a certain output current. If the outputs are connected to high-impedance inputs, then the current is zero, and the voltage is at the supply rails. (See [FAQ] What is the output voltage (VOH or VOL) when the output current is X or the supply voltage is Y?)

0 Bogdan Banik over 4 years ago in reply to Clemens Ladisch

Prodigy 30 points

Thank-you Clemena.

I believe this resolves my second question (BTW the device that I am driving with the TXU0104 is a TI TCA9535 I/O Expander... high impedance inputs; hence very low Level-Shifter output current levels required).

As far as the first question is concerned, I assumed that the TXU0104 outputs would be current limited. Is that not the case? Does the fact that I am driving a normally high-Z I/O Expander Input help here? Please advise your comments.

+1 Clemens Ladisch over 4 years ago in reply to Bogdan Banik

Guru 316760 points

CMOS outputs usually are not current limited.

If you want to protect them, add a series resistor. (For connections to high-impedance inputs, this does not have much of an effect otherwise.)

0 Bogdan Banik over 4 years ago in reply to Clemens Ladisch

Prodigy 30 points

Thanks Clemens... this resolves my first question. Your help is very much appreciated! Cheers, BJ Banik

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TXU0104: VOH Specification & Behaviour when Outputs Connected to Damaged Ports