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TXB0102 as a regular buffer

Other Parts Discussed in Thread: TXB0102, TXS0102, SN74LVC2T45

Hello,

I made a design using the TXB0102. Originally, I made the design to use it for a RS232 interface, with voltage level translation (From 3.3V to 5V).

After the card was finished, the requirement changed, and the RS232 is not needed any more.
Instead, I need to drive 2 LEDs. 

I've problems to use it as a regular buffer, to drive a LED. If I use a serial resistor, it is affecting the current, and the led doesn't reach a good bright level. 
Since the TXB0102 includes a serial 4K resistor, I thought is is possible to use this resistor as current limiter. The problem is, that when I connect a LED directly to the B side, when the driver goes low, the output voltage get stack at 2V (the direct voltage of the LED).

Please look at the attached schematic. The A side is connected to a processor, with a serial resistor of 22 Ohm. 

Can you tell me if it is possible to use this device to drive a LED? Is it possible to find a solution with the current layout ? 

Is there any other device, pin compatible, that can do the job ?

Thank  you very much, 

Leonardo.

  • Leonardo,

    Connecting the output of the device to a resistor in series with an LED should work fine.

    I think the reason your circuit isn't working as intended is because there is a voltage divider on the OE pin. To ensure that the outputs are enabled, connect the OE pin to 3.3 V.

    James

  • Hi James,

    Thanks for you answer.

    The resistor divider is actually an option. The lower one, connected to GND, is not populated.

    So we have the OE connected to VCC.

    I tried several resistor values, and seems that the output stage of the TXB is not suitable for this kind of application. Seems that the device cannot deliver the required current to drive a LED.
    As it is wrote in an application note (scea043.pdf), we have to consider the internal 4K Ohm resistor in series with the load, to calculate the output current and output voltage. 

    I would appreciate comments or suggestions.

    Thanks,


    Leonardo.

  • Leonardo,

    Thank you for your feedback. I will ask some others in my group about your issue and respond early next week.

    James

  • I am moving this post to the Translation forum for more visibility.

    -Ryan

  • Hi, Leonardo

    Thanks for using this part.

    Actually, yes, the TXB0102 doesn't have high current dirve capability. you can see in the spec, the VOH and VOL were got whent the current is 20uA due to the internal 4Kohm serial resistor. the target receiver for the TXB0102 should have the high impendance for input pin.

    I guess you need the LOW level to drive the LED to be ON , while HIGH level to drive the LED to be OFF.

    how many current will the LED need to be ON, we have the TXS0102 maybe OK since you need the p2p replacement now. the TXS0102 has up to 1mA when LOW output ,  it may be OK for dirving your LED. since I have no idea what the LED you used, so I need have the sample and to do the test.

    Also, you don't mention what package you used, so you need check whether our TXS0102 has the same package.

    Another reminder is, the TXS0102 doesn't have high current capability when HIGH output, so if the LED is ON when HIGH, you can ignore my suggestion to use the TXS0102.

    Actually our SN74LVC2T45 is prefered for this applicaiton, but I don't it has the same package with TXS0102. while you can check.

    any further thing or unclear, pelase let me know.

    BR

    junjie

  • Hello Junjie,

    Thank you very much for your answer. 

    I see now that this part is not suitable for this application.

    I'll replace it by another buffer in the next card revision.

    Thanks,


    Leonardo.