LM79: Expected phenomenon during OUT-GND pin short circuit

Hi Hiromu-san,

• First, how each circuit works:
  • Thermal overload protection:
    • There is a temperature sensor on the die, and when the temperature on that rises above a threshold it collapses the output, preventing current from passing through the device, until the temperature falls below a falling threshold and it turns back on again. It has hysteresis. In a real system, if the Vin and Load are too high for the ambient temperature, it will repeat this endlessly, cycling into and out of thermal shutdown.
  • Short circuit current limiting:
    • The current limit part of the circuit scales back the current it limits the device to when it detects a sufficiently low voltage on Vout. 
    • UA78M Output short quirks
      • ...
    • LM79 Output Short quirks
      • The device also has safe operating area protection which may scale back short circuit current even further in certain cases
• Output short to ground event:
  • Current will be limited to ISC
  • Power Dissipation = VIN*ISC (because VOUT=0 due to short)
  • The device will heat up
  • Depending on system, the device may overheat to beyond the thermal shutdown threshold temperature
    • Current will be limited to zero
    • The device will cool off to below the falling thermal shutdown threshold
    • The cycle will repeat if it is still shorted
    • Note: This is hard to measure on output because VOUT=0 but you can observe the phenomena by measuring current into the device
  • Otherwise, if it reaches an equilibrium below that thermal shutdown threshold (Depends on RθJA,ambient temperature, and VIN)
    • Device will continuously draw current at ISC
• Effects:
  • These protection features are pretty good at preventing damage from overcurrent and overtemperature conditions themselves
  • However, high temperature and high inrush current from starting repeatedly when thermal cycling will cause an accelerated rate of normal failures 
  • Expect the chip temperature to hover just under the thermal shutdown threshold and less than abs. max

Best,

Gregory Thompson

Hi Hiromu-san,

There is no real risk of ignition or smoke if temperature and other values are staying within abs. max.

 UA723CN/UA723:

This device does not appear to have thermal shutdown but does have a form of output short circuit protection (the part does not appear to have been significantly altered since the 1999 version of the datasheet; the short circuit condition isn't as robust as our modern specs, but it should still limit current to well below its rating in the event of a true short). It may be damaged due to power dissipation increasing temperature beyond safe levels. 

UA7905CKCS/UA7900:

This device has both thermal overload protection and short circuit current limiting, so these overload conditions should not damage the device.

The 

Best,

Gregory Thompson