TXS0108E / to hold the port level

K.Yaita

Guru 29690 points

Other Parts Discussed in Thread: TXS0108E

Hi Team,

My customer considers to use TXS0108E for his application.

He would like to hold input level of certain port.

1) to hold "H" level

Is external pull-up resistor necessary to hold "H" level?

I believe it isn't required because TXS0108E has 4kΩ pull-up.

2) to hold "L" level

I believe external driver or pull-down is required to hold "L" level.

I would like the recommended value for pull-down resistor.

Best Regards,

Yaita / Japan disty

over 9 years ago

0 Junjie Mai over 9 years ago

TI__Expert 4705 points

Hi, Yaita-san

Thanks for using our part, here is the feedback for your concerns. any unclear, please let me know.

1) Yes, you are right, the TXS0108E does have the integrated pull up reisitor, so, if hold "H" needs, no externall pull-up resistor need to add.

2) to hold "L" level , it is a little complicated, could you please clarify the detail application scenario for the hold "L" ?

For example, when power on, what the detail required, or when power off, what the detail required, or when normal working, what the detail required etc. also will it transfer the bi-directional signal or the uni-direction signal? and the transfered signal is open drain or push pull ?

The main reason is, if it is the bi-directional signal, you can not add the stronger pull down resistor, in case you output VOH has much drop.

While , this weaker pull down can not pull the output to low when the driver stops to drive the TXS0108E and the TXS0108E just has the High output before.

That is the reason I say it is a little complicate to hold "L" level, whether it can be implement or not, it based on the application itself.

BR
Junjie

0 K.Yaita over 9 years ago in reply to Junjie Mai

Guru 29690 points

Hi Junjie-san,

Thank you for your support.

2) My customer' s application scenario is the following.

--------------------------------------------------------
-interface between +3.3V I/O (from MCU) and +5V I/O (Motor driver).
-transfer uni-directional only( +3.3V I/O -> +5V I/O)
-the transfered signal is push-pull
--------------------------------------------------------

He would like to hold "L" level during power-up sequence because MCU output is high-impedance(reset state).

I believe adequate pull-down resistor is required that satisfy VOL during power-up and MCU can drive normally under static operation.

Is my understanding correct?

Best Regards,

Yaita

0 Junjie Mai over 9 years ago in reply to K.Yaita

TI__Expert 4705 points

Hi, Yaita,

Yes, what your understanding is correct. I just have the sample on hand and did some test and then provide the feedback to you.

1. Since you need have the "L" at the 5V side (B side) when power-up sequence, I suggest you have below design.

2. Put the stronger pull down at the input 3.3V side (A side), less than 1K. like 500ohm, while it needs you check your MCU, whether it has higher current drive capability, because dirving 500ohm pull down to high , it needs at least 3.3/500 =6.6mA. if 1K, it needs 3.3mA.

3. Put the weak pull down resistor at the output 5V side(B side), because, for our part, if one of Vcc is about 0V, the IO will be in high-Z state, so when at the power up duration, the A side low can not be transfered to B side. so you need to put the weak pull down at the B side before the power up sequence finish. like 150K or 200K.

4. While , you know power up duration and power down duration , no part can make sure the transfer function. so for making sure no unexcepted glitch at power up or power down. so suggest to have the VccB power on and power down firstly since your B side is connectted to the receiver.

any unclear, please let me know.

BR
junjie

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TXS0108E / to hold the port level