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TXB0108: TXB0108 Input Current

user4646623
Prodigy 175 points
Part Number: TXB0108


Hello,

I have to drive a Telit Module (XE-910) from an STM32 Controller.

Now  Telit states that it can deliver only1 mA output current.

The STM32 has 3.6 volts logic levels and the Telit 1.8 Volt

The datasheet of the TXB0108 has a remark that the device, driving the TXB0108 must have a strenght of at least 2 mA.

So that doesn't match the requirement.

But what is the real input current? 

Figure 6, from the TI TXB0108 datasheet , tells that the input current is Vt/4k

So in fact the input current @ 1.8 Volt domain is only 450 uA??

Is the 2mA from the datasheet neccessarry in this case?

Regards Remco

P.S. this circuit is working fine now for years, and 2k units. But I want to be sure that design is within spec.

  • 0
    TI__Guru 62560 points
    Remco,

    The datasheet recommends 2mA considering the worst case Vcc of 5.5V and includes some guard banding to ensure signal integrity.( variation of resistor, process, temp etc)
    For lower voltages, the current requirement reduces which is why the device is working without issues until now.
  • 0
    TI__Guru* 97066 points
    Hello,
    You are thinking along the correct lines, but I think you're missing one important aspect here.

    What's the V_OH of your Telit module at 1mA? If the short circuit current is 1mA (ie V_OH = 0V), then it may not be able to drive enough current to over-power the TXB0108's 4k drivers, but if the 1mA rating is at V_OH = 1.6 V, then it's likely to work with no problems.
  • 0 in reply to Emrys Maier
    Prodigy 175 points

    Thanks!

    I will ask Telit, cause it is not in the datasheet.

    And I was thinking, that the current is even less than I thought, because it is defined by Vth and not Vcc.

    So my vcc(a) is 1.8 Volts, so Vth is approx 0.9 V

    So, the real input current is 0.9/4k= 225uA

    Is this Right?

  • 0 in reply to user4646623
    TI__Guru 62560 points
    Hi ,

    I will go by the 2mA requirement for the worst case of 5V.
    Scaling down to the 1.8V Vcc level, the drive strength required will be
    2mA * 1.8/ 5V = 0.72mA ; plus any margin for the 1.8Vcc may be necessary. With 1mA drive strength it should work fine.