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PTR08060W: Inhibit Pin - Open Circuit Voltage Not as Expected

paul petersen
Prodigy 10 points
Part Number: PTR08060W

Datasheet says this modules Inhibit pin open circuit voltage is typically 2.8V.  With Vin at 12V, we see 1.7V on this pin when disconnected from our circuitry.  Is this normal?

Also, What's the reasoning behind the stated limitation to not tie multiple modules Inhibit pins together?  If Vin is the same for a bank of modules and the Inhibit pin is pulled internally to Vin, it seems to us that tying the Inhibit pins together should be ok.  We just want to understand what about these modules precludes a common Inhibit pin control.

Thanks so much!

  • 0
    TI__Mastermind 30950 points

    Hello Paul, 

    2.8V is typical you can see less than that.

    connecting to multiple modules may affect the functionally of the device. Please follow the recommendation of the datasheet.    

    thanks

    Tahar

  • 0 in reply to Tahar Allag
    Prodigy 10 points

    I checked four modules out of 30.  All four measure 1.7V at the disconnected Inhibit pin.  The response said we may see less than the typical.  Why is that?

    And, the second part of the question went unanswered.  We know the recommendation.  We are trying to learn what about the circuitry led to that recommendation.

  • 0 in reply to paul petersen
    TI__Mastermind 30950 points

    Hello Paul, 

    You should be seeing some variation from the 2.8V from unit to unit. If you send a SCH I can take a look.  The typical value that we expect for this device is 2.8V, but we can see some devices that have lower than that, How other device's measurements look like, how far they are from the 2.8V?  

    Regarding the Inhibit pin, please follow the recommendation. Internally is connect to other blocks for soft start, internal pull up and controller regulating the output  and the timings for soft start. The interaction between these bocks if connected together with other devices will not lead the device function properly.

    Unfortunately, We can not share internal circuitry externally.     

    Thanks! 

    Tahar

  • 0 in reply to Tahar Allag
    TI__Mastermind 30950 points

    Hello Paul, 

    Are able to share the SCH for my review? 

    Here is more detail I can share on the  Inhibit pin. The structure of the pin input of this device has a Darlington transistor followed by a diode in parallel with a 1kΩ pulldown resistor through an N-channel MOSFET. Once the Enable threshold has been achieved, the voltage on the inhibit pin will be self-clamped. When that happens, one controller can meet its Enable VIH requirement before the other(s) (Part to Part variation), and it clamps the voltage on the common Enable node. This pulls the Enable voltage on the other part(s) below their respective Enable thresholds and can prevent them from turning on. 

    Thanks! 

    Tahar

  • 0 in reply to Tahar Allag
    Prodigy 10 points

    Here's the schematic.

    With R510 installed or removed, we measure 1.7V on the /INHIBIT pin during operation.  We've tested over 30 parts. 

    Our concern is the datasheet shows this voltage as 2.8V minimum.  

    We really want to understand the difference between our use and the datasheet.

  • 0 in reply to paul petersen
    TI__Mastermind 30950 points

    Thanks Paul for sharing the SCH.

    I do not see any issue with the SCH. You just need to make sure the small low leakage MOSFET is used to control the INHIBIT pin 

    For the CLAMP voltage, it can be as low as 1V. where do you see 2.8V as min in the datasheet? If it is left open circuit, the module operates when input power is applied. The open pin voltage is 2.8 V (typical). 

    Thanks! 

    Tahar

  • 0 in reply to Tahar Allag
    Prodigy 10 points

    The older datasheet we have showed the voltage as 2.8V to 6V.  I see the latest datasheet shows this 2.8V as a typical value.

  • 0 in reply to paul petersen
    TI__Mastermind 30950 points

    Thanks for clarifying, please use the latest datasheet.