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LM139A: Output transistor surge current.

Michael Hoffman
Prodigy 30 points
Part Number: LM139A
Other Parts Discussed in Thread: LM139

I see that for this component the maximum continuous output current is 20 mA.
In my design I have a zener diode regulating the output voltage of the device to 12V. Upon switching off, the zener diode discharges causing a 50 mA surge current through the output transistor of this device (transistor shown below).
Could this surge cause any harm to the component?
Any assistance is much appreciated!!

  • 0
    TI__Guru 52776 points

    Hi Michael,

    Is the output being pulled below ground?

    The Abs-Max is 20mA, so, yes, forcing 50mA could damage the device - either immediately or over time..

    The output will current limit at about 12mA (when powered).

    Did you attach a schematic?

  • 0 in reply to Paul Grohe
    Prodigy 30 points

    Hi Paul,

    That's odd, the picture didn't post. I'll try again below, however in case it doesn't work I'm looking at the output transistor on the functional block diagram in the data sheet (section 7.2).

    Thank you,
    Michael H.

  • 0 in reply to Michael Hoffman
    TI__Guru 52776 points

    Hi Michael,

    You cannot paste images into the editor window (Yeah, I know...).

    You have to upload and attach them using the 'Media' icon above (the picture with the plus, next to the paperclip icon).

    The output transistor will self current limit at about 12-16mA. You can see that in Figure 3 as a sharp uptick after 10mA.

    Technically, you should not see 50mA *IF* the current is being properly sunk. If you are forcing the output below ground, this will damage the die as there are not dedicated ESD structures and the die "body diode" and an assortment of reversed bias junction are conducting. Since the paths are not designed to carry current, damage can occur if the current is high enough.

    See the LM139 family appnote. While it talks about taking the inputs below ground, essentially the same thing will happen to the output.

     Application Design Guidelines for LM339/LM393/TL331 Family Comparators

  • 0 in reply to Paul Grohe
    Prodigy 30 points

    Paul,

    Ahh I see that now.

    Thank you very much, that answers my question perfectly.

    Regards,
    Michael H.