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TRSF3221E: Output resistance

Wilfried Hermann2
Prodigy 60 points
Part Number: TRSF3221E


Hello,
back again ...


in the data sheet on page 6 I found the following values for the
"Output resistance":
MIN: 300
TYP: 10M
Unit: Ohm
@Vo=+-2V
What does 10M mean (M=Mega ...)
i am confused ...

Under short circuit condition the chip delivers min. 35mA max. 60mA:
Ok, that is what the charge pump maybe can deliver.

Asuming no more voltage doubling would happen, the virtual output  resistance would be 3.6V/(35-60mA) < 100R.
Are there better diagrams regarding the driver output?

Does it really means that in short circuit the driver has 100R, with 2V output 300R and over 2V "high impedance" ?
MOSFET - Source follower or what?

BR
Wilfried

  • 0
    TI__Guru 74352 points

    Wilfried,

    An applications expert has been notified of your post and will respond accordingly. Thank you for your patience.

    Regards,

    Eric Hackett

  • 0
    TI__Guru 53575 points

    Hi Wilfried,

    Welcome back!

    The Output Resistance specification is refering the high-impedance state of the output pin while the device is unpowered (Vcc, V+,and V- = 0V). In this state, the leakage path from any voltage present on the output line (Vo = +-2V) will be limited by this impedance value. And yes, the 'M' represents Mega-ohms. 

    The short circuit current is limited by the capabilities of the driver which, due to the charge pump, are more complicated than a simple RDS(on) of a FET. The charge pump will continue to try to charge the V+ and V- rails to their proper voltage, but the storage capacitors will discharge very quickly due to the high current demand of the driver. For a good visualization of how the charge pump may work in this state, check out Hao's post about charge pump circuitry in RS-232 transceivers. 

    Let me know if this is clear and if you have any more questions.

    Regards,
    Eric Schott

  • 0 in reply to Eric Schott1
    Prodigy 60 points

    Hi Eric,
    then I ask the question differently.
    Which series resistor do I have to install if I want to drive a longer RS232 cable which has e.g. 85 Ohm impedance.

    I want to have as few reflections as possible and not cause ringing.

    If the driver has a dynamic resistance of 20 ohms, it would be 68 ohms.

    BR
    Wilfried

  • 0 in reply to Wilfried Hermann2
    TI__Guru 74352 points

    Wilfried,

    Thanks for your patience here, we'll get back to you tomorrow about this.

    Regards,

    Eric Hackett

  • 0 in reply to Wilfried Hermann2
    TI__Guru 53575 points

    Hi Wilfried,

    It's not necessary (or advisable) to put series resistors on an RS-232 connection. Series resistors will introduce a voltage drop across the line from the transmitter and result in a more attenuated signal at the receiver. Each RS-232 receiver should be terminated internally to dampen signal reflections on this line (TRSF3221E does this).

    Are you possibly referring to series resistors on the digital input/output pins (pins  9 and 11)? Series resistors here can help decrease the impact of corss-talk from fast digital signals on the same board. Is this what you're referring to?

    Regards,
    Eric Schott

  • 0 in reply to Eric Schott1
    Prodigy 60 points

    Hi Eric,
    sorry for the late answer. My simulation shows, that in case of a driver impedance - lower than the Z of the line - causes overshoots. This short overshoots results in a higher RF-spectrum.
    The better way is to higher the driver impeadene over the z of the line.
    The prototype design is ready, and so i will test ist by myself.


    BR
    Wilfried