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Original question:

TXB0104-Q1: Port A/B Output Current Capability

TXB0104-Q1: Port A/B Output Current Capability

Mike Marlborough
Prodigy 20 points
Part Number: TXB0104-Q1
Other Parts Discussed in Thread: TXB0104,
I alos have a question regarding drive strength of TXB0104 as I don'tt see this specified in the TXB0104 data-sheet
Drive Strength of 20 uA is very low ... is this correct ?  it seems that this is the 'Test Condition' that was used for Voh and Vol as shown in section 6.5
The Parametrics table  for the TXB0104   ( see link  www.ti.com/.../description ) specifies the Drive Strength at 50 mA  , is this correct ? the data-sheet specifies the Continuous
output current at +/- 50 ma.
Please clarify the output drive strength range from TXB0104. Thanks.

  • 0
    TI__Guru* 97046 points
    Hi Mike,
    The TXB0104-Q1 is a pretty complex device, so it kind of depends on what you mean here.

    If you're looking for DC drive strength, then 20uA is the right value. There are 4kohm resistors in series with each output, preventing strong output driver. This is an absolute requirement for an auto-bidirectional translator, considering that when the direction of communication changes, the translator must be overpowered by the driver sending data.. and if it had a strong output driver, many controllers would not be able to overpower it.

    If you're looking for transient drive strength, that's going to be around 30 ohms (I'll have to check with our local expert on translation who is current out of the office for a few days to get the exact value).

    The value you mentioned of 50mA is referring to the absolute maximum current rating of the device. I think the way it's written is a bit misleading, but since TXB is so complex, I guess it's the best way to describe the worst-case current at the output. During switching, the 4kohm output resistor is temporarily bypassed and becomes relatively low impedance (about 30 ohms) for a short period of time. This provides a fast edge and signficant drive current during that time, but will shut off quickly when the one-shot expires.

    So, to summarize - the TXB isn't intended to drive a DC load. It's intended to do a CMOS voltage translation into a high-impedance load.
  • 0 in reply to Emrys Maier
    Prodigy 20 points

    I would like to know the transient drive strength (ma)  during switching and the duration of this higher current drive strength when 4K output Resistor is bypassed with a lower approx. 30 ohm resistance.

    Thanks, MIke

  • 0 in reply to Mike Marlborough
    TI__Guru* 97046 points
    Hi Mike,
    The one-shot fires for about 30ns.

    Current is directly related to resistance through Ohm's law:

    I = V/R

    For example, if you are operating the output at 5.5V, then the output current would be up to 5.5/30 = 183mA

    This of course ignores things like inductance and capacitance -- the real value is typically much lower.