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TCA9800: Using TCA9800 with push pull outputs ?

Finbarr long
Prodigy 100 points
Part Number: TCA9800
Other Parts Discussed in Thread: SN74LVC1G34, SN74LVC3G34, SN74LVC2G34

Hi 

I was wondering if it is possible to use the TCA9800 in parallel with "push pull" outputs, to supply "additional" current to charge capacitive loading.

I have a SIM card interface with excessive capacitive loading on it. I had intended using the TCA9800 (B side) to provide the additional current to the

bidirectional Open drain I/O as mentioned before in a previous "post".  I need to add large Pull down resistors on the on the B side of the TCA9800 due to possible floating pin issues.

I am aware that the values of the pull downs need to such that Iext-i and Iext-o are not exceeded (something in the order of 50K is required)

The SIM card signals may be at either 1.8V or 3V levels. 

I have similar capacitive loading issues on the RST and CLK push pull output lines (from the modem to the SIM card) of the SIM card interface. So I was wondering once more if I could use the TCA9800 to supply the additional current to charge these capacitive loads .

Maybe there is a better solution from TI to do this. I need to drive capacitive loads in the order of 200pF. The input signals (from the modem card interface) can be either 1.8V or 3V levels. There is a negotiation phase between the modem and the SIM card which determines what the signal levels will be. I need to meet 1us rise and fall times. 

If you have nay queries let me know.

Finbarr Long

  • 0
    TI__Mastermind 19435 points

    Hello Finbarr,

    It sounds like you need a buffer that is designed for push-pull.  We have lots of buffers that can do this and I will push this post to the appropriate forum.

    -Francis Houde

  • 0 in reply to fhoude
    Prodigy 100 points

    fhoude said:

    Hello Finbarr,

    It sounds like you need a buffer that is designed for push-pull.  We have lots of buffers that can do this and I will push this post to the appropriate forum.

    -Francis Houde

    Hi Francis

    That's fine thanks, but the solution needs to work for both 1.8V and 3V supplies voltages.

    Finbarr

  • 0 in reply to Finbarr long
    TI__Mastermind 19435 points
    Hello Finbarr,
    I thought I missed this to translation group. I hope that this isn't too late. Sorry.
    -Francis Houde
  • 0 in reply to fhoude
    TI__Guru* 96681 points

    Hi,
    The 1 us rise time should be fairly easy to achieve with a logic buffer.

    Given a 200 pF load, the rise time is just a function of RC, with R being the r_on of the driver. using 4*RC as a 'rise time', I was able to quickly calculate that a 1.25 kohm r_on is required.

    This gets a tiny bit more complex, however. The majority of logic device have outputs close to 25ohms, so now you have to watch out for over-current situations. With a 3.3V supply, a buffer will drive up to ~132 mA (3.3V/25ohm)... which is more than any one buffer can handle. This can be easily fixed by adding a series resistor on the output of 330 ohms, which drops the output current to a very managable 10 mA (max), and keeps your rise time well within the defined limit.

    How many channels do you need? The SN74LVC1G34 (single buffer) can work from 1.65V up to 5.5V. There's also a dual and triple version, SN74LVC2G34 and SN74LVC3G34, respectively.

    If there are other requirements, please let me know. From the post, it sounds like there might be bidirectional communication - in that case we might have to look at other options.