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MAX232

Bruce Blizzard
Prodigy 40 points
Other Parts Discussed in Thread: MAX232, TRS232

Regarding the MAX232N device, wants to know if Ican leave the unused leads open or does he have to connect them to ground or positive.

Thanks B2

  • 0
    TI__Expert 5230 points

    It is best to tie unused inputs to a known logic state (tie low or pulled high) to minimize power consumption.

     

    Best Regards,

    Joe

  • 0 in reply to Joe Fockler
    Prodigy 40 points

    joe,

    I agree; however this chip has internal 400k and 5k pull-up resistors.  And the Maxum Datasheet for the same chip states it is not necessary to connect unused inputs.  I don't see this in the TI version datsheets

  • 0 in reply to Bruce Blizzard
    TI__Guru**** 151036 points

    The TI MAX232 has bias resistors on both TIN and RIN pins.

    It is not necessary to connect these pins.
    Alternatively you can connect unused RIN to ground and unused TIN to VCC for a modest ICC decrease.

    Regards,
    Ron M.

     

  • 0 in reply to Ron Michallick
    Prodigy 40 points

    Hello,

     

    I would like to use MAX232.

    The VS+ and VS- pins are connected to +8.5V & -8.5V respectively. Is it must to connect 8.5V to those pins? What is the reason behind this?

    Can I connect +5V and GND on VS+ and VS- pins?

     

    Imman

  • 0 in reply to Immanuel Rathinam
    TI__Guru**** 151036 points

    Hello Imman,

    Only 5V is needed to operate the MAX232. Add capacitors as seen in figure 4 of the data sheet.
    The +/-8.5V text in figure 3 is for information only. The MAX232 will generate this voltage automatically.

    Do not connect VS+ and VS- to any voltage.

    Regards,
    Ron Michallick.

  • 0 in reply to Immanuel Rathinam
    TI__Mastermind 28265 points

    Immanuel,

    the MAX232 needs to be powered with 5V and GND from an external power supply. Internally, the MAX232 has a charge-pump, thas is a voltage generator which uses the 5V supply voltage as an input to create the two output voltages of +8.5V and -8.5V. These outputs are connected internally to the drivers and also bonded out to allow voltage buffereing via external capacitors.

    the only components you should connect to the VS+ and VS- outputs are the 1uF bulk capacitors. I know that data sheet on page 7 shows arrows to +8.5V and -8.5V, which is not drawn correctly. It does not mean to connect the outputs to these voltages, but rather that the outputs can provide these voltage to other loads. This however, is not recommended unless you have very light (high-impedance) loads that consume only minute currents.

    Again, for best performance, only connect 1uF capacitors to these outputs.

    Regards,

    Thomas

  • 0 in reply to Ron Michallick
    Prodigy 50 points

    For the TI version of the MAX232, I'm confused as to whether it has what amounts to pullups on the TIN pins or not.  I've seen both responses from Ron M.!

    Is it dependent on a particular version or how do we know?

    Brad A.

  • 0 in reply to Brad A
    TI__Mastermind 28265 points

    Brad,

    what environment are you designing for? A clean, low noise, laboratory environment, or a noise, with possible transients occuring, industrial environment?

    for industrial designs I recomend pull-ups/downs in the range of 1K to 10k. I personally choose 4.7k.

    The RIN of our MAX232 has typical pull down resistor of 3k with a minimum value of 1.5k and a maximum value of 7k (this rane is due to process variations). This is a proper pull-down action and you do not need to do anything here.

    On the TIN side however, Bruce was mentioning a 400k pull-up. This is too much impedance for noisy environments, it's almost open if you like. In this case I recommend using a pull-up of 4.7k to Vcc. You do not want to use a direct connection in case you get heavy noise transients on the board which can destroy any high-impedance inputs such as signal inputs or Enable pins.

    So I hope this helps.

    regards, Thomas 

  • 0 in reply to Thomas Kugelstadt
    Prodigy 50 points

    I have an existing design with unused inputs (half of the MAX232 is unused), so I wanted to know if I could use the TI version MAX232.  The environment is industrial and there are no traces on the unused pins.  From the data sheet's Iis (Short-circuit input current) the apparent pullup could be as small as 25k, is this correct?  The data sheet really doesn't provide much information concerning the TIN circuitry.

    I agree with your guidance concerning the unused inputs, but this is an existing situation and I am hoping not to have to kludge something.  As long as it is not self-oscillating, then potentially it will be ok.  And as there are no traces to the inputs, pickup of transients should be minimal, correct?

    Thanks, Brad A.

  • 0 in reply to Brad A
    TI__Guru**** 151036 points

    Hello Brad,

    Sorry about the inconsistency. In this thread (Jan 2011), I tested the devices in the lab. In the other thread, I looked at the data sheet and missed the "short circuit input current spec". I personally do not like naming of that spec.

    The TI MAX232 (and TRS232) are direct equivalents to the Maxim MAX232. They are OK to use in your application.
    In all chips, the 400k resistor is not actually a true linear resistor. An actual resistor would take a lot of space, but a small MOSFET does a good job in very little space.

    I attached a sample curve that shows input current versus input voltage.
    In the sample data, the floating input will be greater than 4V and support over 10uA at VIH(min)=2V. 

     

    Let me know if you need any more data or further explanation.

    Regards,
    Ron Michallick