TXB0104: Shorting of Output pins for Higher Drive Strength

Hello Ahamed,

Can you help to provide details on what you are trying to achieve?

How many bits are you translating?  What input and output voltage? What is the directionality of the signals? What is the data rate you are trying to achieve? What drive strength do you want?

This will help to give the best recommendation for the application.

To address your questions directly, I would not recommend shorting the outputs of this device together. It has dynamic outputs that could cause odd behaviors if tied together. Shorting outputs together to get higher drive strength with other devices is possible, but there are some devices that may not require such a solution.

For example the SN74AXC4T774 device is a buffered translation device that can support a decent current drive and data rate. May this part work for your application?

Best,
Michael

Hi Michael,

Our requirement is SPI Interface with special requirement of drive strength > 60mA in each output lines. I/O Voltage requirement is 3.3V on A side & 1.2V to 5.5V on B side. This can be variable using VCCB IO supply 

We planned to 4 Channel translator for the 4 SPI signals (MISO, MOSI, CLK, CS). 

But we couldn't find the translator part with higher drive strength. So we decided to short two pins at input and output to have higher drive strength using 8 channel or 2No. of 4 channel translator.

The data rate has to be greater than >20 mbps.

Please provide your suggestion on this.

Thanks,

Ahamed Faize

The only translator that can buffer signals and supports this voltage range is the SN74LXC8T245-Q1. But at 1.2 V, its guaranteed drive strength is less than 1 mA.

Can you tell us why you need 60 mA, and why such a wide voltage range is needed?

Hi Michael,

Thanks. Really appreciate your help.

But I can't able to get this part (SN74LXC8T245-Q1) in online. Do you have any other parts with similar features that will be available in online?

Generally, whether we can short the two Input pins and Output pins of translators & driven by single input signal.

If we can short, 1) Whether it will affect the speed?

                          2) Any issues/Concerns while shorting the pins?

                          2) Why we should not short the TXB0104 pins?

Thanks,

Ahamed Faize

A TXB input requires the external device to have a drive strength of 2 mA. Connecting multiple pins together adds up these currents.

A TXB output has a drive strength of 20 µA. Connecting multiple pins together will not increase this value enough for it to be useful.

Thanks Clemens.

Do you have any inputs on the below mentioned Queries. Because it will be helpful for us to go with the shorting of O/P pins for High drive strength.

Generally, whether we can short the two Input pins and Output pins of translators & driven by single input signal?.

If we can short, 1) Whether it will affect the speed?

                          2) Any issues/Concerns while shorting the pins?

Thanks,

Ahamed Faize

Shorting pins is no problem for direction-controlled translators (those in the LVC/LXC/AVC/AXC families).

Thanks for the information.

So from the discussions, I understood that for the direction controlled translators, there will be no issues in shorting the pins, if single signal is applied.

Just clarify me the below use case,

Assuming the Output pin current of a translator or any IC is 24 mA. And in some cases, we have to connect/short the O/P directly to Ground or Supply.

1) Whether we can short the output pins to directly ground, while driving the output as Logic HIGH? or Whether we can short the output pins directly to supply, while driving the output as Logic LOW?

2) If We can short, whether IC will take care of limiting the current to 24mA.

3) If we should not short, Whether we have to limit the current to 24mA by using the resistor?

Thanks,

Ahamed Faize

Hi Ahamed,

I read through the above thread, but I didn't see a reason why you need so much power.

I've never seen an application where 60mA is required a 1.2V. Can you explain why this is a requirement?  Are you delivering power to a subsystem rather than translating a signal?

Hi Emrys,

Yes, You are correct. Generally, No SPI lines will need higher drive strength. But this is a special requirement from our customer.

They are going to drive an external analog system using SPI communication along with higher current drive (~60mA).

Thanks,

Ahamed Faize

Sorry, but you have not really explained why the high current is necessary. What problem is solved with this?

If that applications involves long cables or a noisy environment, differential signals (LVDS or RS-485) would be a much better solution.

Hi Clemens,

The External system has special use case as mentioned below. But exactly we don't have the External System specs.

1) The External system SPI pins have a fixed voltage level (say 10V along with internal pullup). We don't know why the pins have 10V.

2) But during SPI communication, we have to send the signals with I/O voltage of less than 10V (say 3.3V) from our board to external system.

3) So during SPI communication we have to send with 3.3V signals at higher drive current to over drive the 10V. (So here the effective voltage seen by the SPI pin is 3.3V & not 10V).

I hope, Now you can able to understand the scenario.

Thanks,

Ahamed Faize

Hey Ahamed,

I think you're going to find that no matter how many channels you parallel, a 1.2V device won't be able to pull down a 10V supply, and it will end up just destroying the 1.2V device.

You would need an external driver -- ie a dedicated power MOSFET that can sink the current required to drag that supply down.