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MAX232E: Minimum short circuit current

Ryuuichi machida
Guru 12715 points
Part Number: MAX232E

Tool/software:

Hi,

I would like you to confirm specification of minimum value of Isc.
According to datasheet, typical value of this spec is defined as "+/-10mA".
However there is no description about min spec. Could you disclose min spec of Isc ?
The background of this question is customer will try to estimate bandwidth vs drive output current following by below application note (Figure 4)
https://www.ti.com/lit/an/slla037a/slla037a.pdf

So, if you can not disclose min spec of Isc, it is OK to show how we can consider load cap and bandwidth from other aspect(parameter).

Best Regards,

  • 0
    Guru 317370 points

    The RS-232 specification requires a driver to output at least ±5 V when connected to a load of 5 kΩ ± 2 kΩ. The MAX232E conforms to this specification.

    The MAX232E deliberately limits the output current to limit the slew rate to 30 V/µs, and is guaranteed to support 250 kbit/s even with the highest allowed load of 2500 pF.

  • 0 in reply to Clemens Ladisch
    Guru 12715 points

    Hello

    Thank you for your reply.

    For following your answer :
    >The MAX232E deliberately limits the output current to limit the slew rate to 30 V/µs, and is guaranteed to support 250 kbit/s even with the highest allowed load of 2500 pF.
    According to figure 4 of "Interface Circuits for TIA/EIA-232-F (Rev. A)", it seems that TIA/EIA-232-F driver output current show up to 10 kbit/s with 2500pF.

    And also, according to datasheet of MAX232E, this device follows TIA-EIA-232-F. So, could you please tell me how user can understand above highlighted yellow sentence from datasheet ?

    BR,

  • 0 in reply to Ryuuichi machida
    Guru 317370 points

    The speed of 250 kbit/s is guaranteed by section 5.8 of the datasheet. (Actually, it's not, because only the typical value is specified.)

  • 0 in reply to Clemens Ladisch
    Guru 12715 points

    Hi,
    Yes, I understand about below. However, there is no condition about acceptable capacitance. To understand this, I refered following application note as I said.
    >The speed of 250 kbit/s is guaranteed by section 5.8 of the datasheet. 
    https://www.ti.com/lit/an/slla037a/slla037a.pdf

    Again, could you please comment why you mentioned that 250kbit/s can be used under 2500pF condition ?

    BR,

  • 0 in reply to Ryuuichi machida
    Guru 317370 points

    The RS-232 requires 2.5 nF, and the MAX232E conforms to the specification. Also, this load is a test condition for SR(t). (The datasheet indeed is not very clear about this.)

  • 0 in reply to Clemens Ladisch
    Guru 12715 points

    Hi,

    >The RS-232 requires 2.5 nF, and the MAX232E conforms to the specification. Also, this load is a test condition for SR(t). (The datasheet indeed is not very clear about this.)

    Yes, as you said, I could understand test condition for SR is under 2.5 nF cap load from figure 6-3.
    However, I could NOT understand test condition for data rate is under 2.5 nF cap load.

    And also, if test condition for data rate(250kbit/s) of MAX232E is under 2.5 nF cap load though any description about this does not exist in datasheet, I'm not sure how user can distinglish information between "">www.ti.com/.../slla037a.pdf" and your answer.

    Is it possible to comment about above ?

    BR,

  • 0 in reply to Ryuuichi machida
    Guru 12715 points

    Hi,

    Could you please give your feedback ?

    BR,

  • 0 in reply to Ryuuichi machida
    Guru 12715 points

    Hi,

    From following spec, we calculated as shown below.

    * Vo = 7V (Vpp = 14V)
    then required rising/falling time is approx 4.7us. So, this can realize up to approx 50kbps...
    Again, is my understanding that you can NOT disclose min spec of "Isc" correct ?

    BR,

  • 0 in reply to Ryuuichi machida
    Guru 12715 points

    Hi,

    Could you please give your feedback ?

    Best Regards,

  • 0 in reply to Ryuuichi machida
    TI__Genius 13952 points

    Hi Ryuuichi,

    Do you still have questions remaining about MAX232E?

    I apologize for the delay in TI's response as Bobby has been out of the office.

    Best regards,

    Ethan

  • 0 in reply to Ethan Sempsrott
    Guru 12715 points

    Hello Ethan,

    Here is my latest question. If possible, please follow up.

    * Vo = 7V (Vpp = 14V)
    then required rising/falling time is approx 4.7us. So, this can realize up to approx 50kbps...
    Again, is my understanding that you can NOT disclose min spec of "Isc" correct ?

    Best Regards,

  • 0 in reply to Ryuuichi machida
    TI__Genius 13952 points

    Thanks for your patience on this question Ryuuichi.

    I was able to find data on the min spec for I_OS (Driver short-circuit output current). Across 30 devices from -40 to 125C, the min was -21mA. We cannot guarantee this spec, but hopefully this estimate will help. 

    MAX232E meets or exceeds the TIA/EIA RS-232 standard as Clemens stated. Assuming the RS-232 guides are followed, this device will not have any difficulties delivering 250kbps at 2.5nF. 2.5nF or 2500pF is the maximum capacitance for the RS-232 bus according to the RS-232 standard. 

    Best regards,

    Ethan

  • 0 in reply to Ethan Sempsrott
    Guru 12715 points

    Hi,

    Thank you for your reply.

    >I was able to find data on the min spec for I_OS (Driver short-circuit output current). Across 30 devices from -40 to 125C, the min was -21mA. We cannot >guarantee this spec, but hopefully this estimate will help.
    Thank you for your information.
    I thought that this spec is defined as pos/neg. so, I thought your spec mean max spec not min spec. (I think "+/-21mA" is correct..)
    Is my understanding incorrect ?

    >MAX232E meets or exceeds the TIA/EIA RS-232 standard as Clemens stated. Assuming the RS-232 guides are followed, this device will not have any >difficulties delivering 250kbps at 2.5nF. 2.5nF or 2500pF is the maximum capacitance for the RS-232 bus according to the RS-232 standard. 
    => Umm, Clemens-san also said same thing. However, if so, we are not sure what following figure mean...
     
    https://www.ti.com/lit/an/slla037a/slla037a.pdf

    Root cause of our question come from above information (capacitive load vs Drive output current).
    From above figure, if we get 250kbps with 2500pF, it seems that over 100mA is required as output current..
    However, according to datasheet, typ Isc is defined as 10mA. So, I'm not sure why you said as shown below.
    "Assuming the RS-232 guides are followed, this device will not have any difficulties delivering 250kbps at 2.5nF.".

    Best Regards,

  • 0 in reply to Ryuuichi machida
    Guru 317370 points

    That figure assumes a rise time of 4 %. The MAX232E's rise time will be longer. At 250 kbps, the length of a bit is 4 µs. With a typical slew rate of 3 V/µs with a 2.5 nF load, the rise/fall time is 2 µs (measured between ±3 V).

  • 0 in reply to Ryuuichi machida
    TI__Genius 13952 points

    Ryuuichi,

    Yes, +/-21mA can generally be assumed as a worst case. But +/-10mA is still the typical value. 

    As Clemens mentions, that figure assumes a 4% rise time. Each RS-232 transceiver will have unique current consumption and should be tested for the most accurate current consumption value.

    Regards,

    Ethan

  • 0 in reply to Ethan Sempsrott
    Guru 12715 points

    Hello Ethan and Clemens,

    I understood that required rising time condition is different between MAX232E and application note example.
    In fact, case of MAX232E is "2us". On the other hands, if this device will meet 4% of unit interval, it should be "1.6us".
    So, as clemens-san said this is longer than application note example.
    (One thing, I think following "20ms" in datasheet is mistake of "20us".)




    However, it seems measurement condition under "3kohm with 2.5nF" of MAX232E is approx 25kbps.
    So, I can not still understand why you can mention below...

    >this device will not have any difficulties delivering 250kbps at 2.5nF. 

    Sorry for my poor understanding, but could you re-clarify about above ?

    Best Regards,

  • 0 in reply to Ryuuichi machida
    Guru 317370 points

    At 250 kbps, the unit interval is 4 µs. 4 % of that would be 0.16 µs.

    The RS-232 specification requires a transition time of less than 4 % of the unit interval, but at least 0.2 µs. (See figure 1 of SLLA083.) This means that it is not possible to use more than 200 kbps with a strictly conforming RS-232 system, and 200 kbps is possible only if the driver actually uses the maximum slew rate of 30 V/µs.

    The MAX232E uses a typical slew rate that is slower, and does not try to stay below the 4 % limit. It works in practice.