TXB0102: some questions about the internal resistor

Lin_Charles

Part Number: TXB0102
Other Parts Discussed in Thread: SN74AVC2T245, SN74AVC4T245

Hi team,

My customer is using this device to realize voltage level translator for UART between MCU and slave device. VCCA=1V8, VCCB=3V3, and they find that when A1=0V, B1 will about 1.5V, this will cause data error in this application. We suspect that this is because we have internal resistor 4kOhm in the output, when customer have pull up resistor (4.7k for example) in the B side, this will cause B1 still have about 1.8V when A1=0. We may need team's help below,

Is the internal resistor cause this issue? Why we have this resistor?
Is it possible if we use B1/B2 to send the data as well as driving LED load?

over 5 years ago

0 Clemens Ladisch over 5 years ago

Guru 292890 points

The internal resistor is needed to make the output weak enough so that external devices can override the voltage level. This is required for the auto direction sensing.

The outputs cannot drive a load larger than a high-impedance CMOS input (see sections 8.3.3 and 8.3.5 of the datasheet).

For two unidirectional channels, the best solution would be a direction-controlled translator (with real buffers), such as the SN74AVC2T245.

0 Dylan Hubbard over 5 years ago in reply to Clemens Ladisch

TI__Mastermind 44955 points

Hi Charles,

To add to what Clemens has stated, here's an app note that goes into details on adding external pull-ups to this device.

http://www.ti.com/lit/an/scea054a/scea054a.pdf

0 Lin_Charles over 5 years ago in reply to Clemens Ladisch

TI__Expert 4260 points

Hi Clemens,

If SN74AVC4T245 is used, and the external pull up resistor is 2k, will it affect the output signal? Thanks.

0 Clemens Ladisch over 5 years ago in reply to Lin_Charles

Guru 292890 points

The additional current of 1.65 mA is no problem.

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TXB0102: some questions about the internal resistor