SN74AVC4T245: SN74AVC4T245RSVR

xujunfeng

Part Number: SN74AVC4T245
Other Parts Discussed in Thread: TXU0204

For SN74AVC4T245RSVR, VCCB is powered by 3.3V, and VCCA is suitable for 1.2V power supply. Can the I/O on the VCCB side withstand 3.8V voltage? The approximate application block diagram is as follows. Please help confirm.

5 months ago

0 Clemens Ladisch 5 months ago

Guru 325980 points

See the absolute maximum ratings and the recommended operating conditions. Inputs are overvoltage tolerant up to 3.6 V; outputs are not.

0 xujunfeng 5 months ago in reply to Clemens Ladisch

Intellectual 300 points

Hello, the information I gathered from the specification is that the input and output have the same limits. It's not a simple case of failing if it exceeds the voltage; rather, the judgment should be based on the current level. This has left me unsure about the specific criteria for determination. Could you please help list them out clearly?

0 Joshua Salinas 5 months ago in reply to xujunfeng

TI__Mastermind 41050 points

Hello Xujunfeng,

Correct me if I'm wrong here but from your original message the part is not in a Hi-Z or power-off state? If this is the case we should be using the below parameter.

VO Voltage range applied to any output in the high or low state

That parameter states any voltage applied to B-port output cannot surpass VCCB + 0.5V.

Regards,

Josh

0 xujunfeng 5 months ago in reply to Joshua Salinas

Intellectual 300 points

Note 2 states that overvoltage is acceptable as long as the input and output current does not exceed the rated value. Does that mean my application should be fine?

0 xujunfeng 5 months ago

Intellectual 300 points

Is the LSF0204x suitable for my block diagram? What are the VIH and VIL levels for the I/Os of the LSF0204x? I couldn't find them in the datasheet.

Or are there other recommendations?

0 Clemens Ladisch 5 months ago in reply to xujunfeng

Guru 325980 points

Note that current ratings are specifieds only for negative voltages. Inputs do not have clamping diodes and do not allow larger voltages. Also note that 3.8 V is above the limit of 3.6 V.

The LSF0x0x have open-drain I/O pins and always tolerate voltages up to 5.5 V.

0 xujunfeng 5 months ago in reply to Clemens Ladisch

Intellectual 300 points

Please help review the schematic diagram.

0 Joshua Salinas 5 months ago in reply to xujunfeng

TI__Mastermind 41050 points

Hello Xujunfeng,

LSF devices are supported by one of my colleagues. Please allow time to review. Thanks!

Regards,

Josh

0 Jack Guan 5 months ago in reply to Joshua Salinas

TI__Guru 60151 points

Hello Xujunfeng,

A pullup resistor is missing on B3. Aside from this I do not see any other immediate concerns.

Regards,
Jack

0 xujunfeng 5 months ago in reply to Jack Guan

Intellectual 300 points

Must a pull-up resistor be added to the B side? Doesn't this chip support push-pull? B3 is the reset signal, transmitted from A3. I used a pull-down resistor on A3 to default it low, intending to make the reset signal on B3 take effect by default.

0 Jack Guan 5 months ago in reply to xujunfeng

TI__Guru 60151 points

Hello,

In the event that A3 is driving a high state, the output B3 will not be able to see 3.3V as the output of this device is open drain based, therefore the pullup is needed for B3 to achieve a logic high state of 3.3V.

Regards,

Jack

0 xujunfeng 5 months ago in reply to Jack Guan

Intellectual 300 points

The voltage on the B side is required to be 3.3V. If it's an output from the B side, a pull-up resistor to 3.3V is always required, right? Similarly, the voltage on the A side is required to be 1.2V. If it's an output from the A side, is a pull-up resistor to 1.2V also always required?For example, A1 and A2.

0 xujunfeng 5 months ago in reply to Jack Guan

Intellectual 300 points

According to the previous schematic, A3 is pulled down to GND through a 1kΩ resistor, and B3 is pulled up to 3.3V through a 4.7kΩ resistor. If no other devices are connected at either end, what are the theoretical voltages at A3 and B3, respectively? Does the chip recognize the signal direction as A3 -> B3 or B3 -> A3?

0 xujunfeng 5 months ago in reply to Jack Guan

Intellectual 300 points

Is the TXU0204RUT better suited for our application? Because we have two inputs and two outputs.

0 Clemens Ladisch 5 months ago in reply to xujunfeng

Guru 325980 points

The LSF should have pull-up resistors at all its outputs.

The LSF does not recognize signals directions. When any I/O pin has a voltage below Vref_A, then this channel works as a bidirectional analog switch and directly connects the A and B pins.

For unidirectional signals, a unidirection translator like the TXU0204 indeed is simpler and more robust.

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SN74AVC4T245: SN74AVC4T245RSVR