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LM79: Does the LM7915CT have a dV/dt limit for power up?

Part Number: LM79

Recently i got some issues with a competitor LM7915 part. It stays at 0V output voltage, if input voltage is higher then 27V and turned on rapidly. Rapidly means input voltage rises faster then the output voltage, creating a short period of time where input voltage is the differential voltage across Pins for Vin to Vout.

This limit is not the short circuit current limit from internal SOA circuitry. Because this limitation typically results in a reduced output voltage. However here it goes exactly to no output current at all. It's like the pass element is comletely turned off. (Not biased correctly? - Vout is never above GND...)

Thus my question, does the TI LM7915CT have a similar issue or is it more tolerant to rapid turn on through all production lots?

Some guaranteed statements to such a condition would be helpfull, as i couldn't find anything regarding this in datasheet for many kinds of this LM7915 variation.

  • Hi Thomas,

    I am not aware of any specific dV/dt limitations with our device.  With rapidly increasing VIN, it is important to make sure that the absolute maximum ratings of the device are not violated as shown below:

    Is there any chance that these limits are being violated as VIN ramps quickly? This may occur as a result of the inductance in the input power supply leads. 

    If so, some ideas to mitigate this could be to add some resistance in series with VIN or maybe a Zener between VIN and VOUT to try to keep the device below the absolute maximum ratings. 

    Do you have a local sales contact to help get you free samples to test out?

  • Hi John,

    thank you for the quick reply, that helps a lot. This latching behavour doesn't occure when the input voltage is at its maximum. But you might be right, i'm goning to look a bit closer to the input pins, if there's some kind of local overshoot, which might trigger this.

    Channel1: Output Voltage, Channel 2: Input Voltage, Channel 3: Input Voltage at power switch input, Channel 4: ADJ Pin

    Currently i don't have a local direct TI contact. However we just ordered some LM7915CT/NOPB from a catalog distributor. That might help to.
    However one thing still exists. As some of the competitor device show this behaviour and others not, it's not clear if samples can represent all possible states of various produciton lots.

    I'll just keep on watching out for this effects, see if there're some unintended high voltage peaks somewhere and see which devices perform better throughout the production cycles.

    Thank you very much for your good hints and support.

  • Hi Thomas,

    You are welcome, I agree a few samples may not be a good indicator as performance can vary lot to lot, However, with TI devices, if you do not violate the absolute maximum ratings of the device, they do tend to be very robust.

    Thank you,

  • Hi John,

    That's very promising, as this would solve the issue within minutes.

    Thank you.