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LP3966: /SD pin threshold operatin question

Gabor Szoke
Prodigy 510 points
Part Number: LP3966
Other Parts Discussed in Thread: TPS7A37

Dear Team

Our customer has the following question:

We have noticed that we have an issue with the /Shuntdown pin for the LP3966-ADJ. We are controlling this pin from the uP and it has a 100k resistor to Vin.

The problem is that when we reset the regulator it doesn’t turn back on and we saw that in the datasheet that the voltage threshold for the /SD pin is Vin-0.3V. Vin in this case is 5V so the voltage threshold for the regulator to turn on should be 4.7V (according to the datasheet).

After we set the uP pin to high-Z the linear regulator turned on after shutdown but the voltage on regulator’s /SD pin in 3.9V, which according to the datasheet it shouldn’t work. This behavior was seen on 3 different PCBs

My question is: Why does the regulator start at 3.9V when Vin is 5V and is it enough to surely start the regulator every time?

Could you please review and assist?

Kind Regards
Gabor

  • 0
    TI__Genius 10490 points

    Hi Gabor,

    The shutdown pin is a digital input. The "Shutdown Threshold" spec defines the pin's logic high and logic low levels. If the voltage is in the logic high range, all LP3966 devices are ensured to be regulating the output. If the voltage is in the logic low range, all LP3966 devices are ensured to be in shutdown mode. Like all digital pins, the actual threshold will be in the indeterminate region between logic high and logic low. 

    In your application's case, an input voltage of 5 V means that any LP3966 will be regulating with a shutdown pin voltage of 4.7 V or higher. 3.9 V may not be enough to bring every regulator out of shutdown. 

    There is more information on this topic in this similar thread regarding the TPS7A37: 

    Thanks,

    Gerard Copeland