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SN75188: IC temp is too high

Shu-Cheng LIN
Guru 11785 points
Part Number: SN75188
Other Parts Discussed in Thread: LT1030, TRS3237E,

Hi Sirs,

Sorry to bother you.

May i know what is SN75188D normal working temp?
Because we get the temp is to 69.5 when TA is 25.
The IC is very hot.

Our V+ and V- is connect 12v.


Is normal? if not, have any idea to fix?

  • 0
    TI__Intellectual 2090 points
    Hello Shu-Cheng,

    Just to clarify, is the V+ pin is connected to +12V, and is the V- pin is connected to -12V? I just want to make sure that both pins are not connected to +12V together. Connecting both supply pins to +12V would be to operate outside of the recommended operating range for the V- pin.
    What is your application for this device?

    Best Regards,
    Max Megee
    TI Transceiver Applications
  • 0 in reply to Max Megee
    Guru 11785 points

    Hi Sirs,

    Thanks for your reply.

    Our schematic as below

    We think we aren't over IC spec.

    Our application is IPC COM MB.

    So, have any experience on this phenomenon?

  • 0 in reply to Shu-Cheng LIN
    TI__Intellectual 2090 points

    Hi Shu-Cheng,

    After some investigation, it appears that your temperature measurements may be expected from the datasheet parameters provided.  The datasheet gives a theta_JA (thermal resistance) for the D package of 86 degC/W.  Using the formula provided in Note 3 of the Absolute Maximum Ratings Table on Page 3 of the datasheet, we may calculate an estimated power dissipation for the device:

    PD = (TJ - TA)/Theta_JA      

    Since you measured 69.5C at the package for an ambient temperature of 25C, then TJ = 69.5 and TA = 25.

    PD = (69.5 - 25) / 86

    PD = 517mW

    This seems like a reasonable value for power dissipation when the VCC rails are +/- 12V, since the datasheet specifies a maximum power dissipation of 576mW for an unloaded unit at those voltage rails.  Therefore, it is plausible that the measured temperature is expected.

    For your consideration, the SN75188 device is a very old device for RS-232 communication.  You may want to consider a newer device.  It looks like the LT1030 device is comparable to your current solution and provides better thermals.  Even better thermals can be achieved with the TRS3237E device.  This device is a 5 driver, 3 receiver RS-232 device.  I know that you don't specifically need 3 receivers for your current implementation, but the package sizes are comparable and the thermals of the TRS3237E should be significantly lower than the SN75188.  You can reference the datasheets for these two devices here:

    http://www.ti.com/lit/ds/symlink/lt1030.pdf

    http://www.ti.com/lit/ds/symlink/trs3237e.pdf

    Best Regards,

    Max Megee