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TPS22918-Q1: ON level higher as VIN

Part Number: TPS22918-Q1
Other Parts Discussed in Thread: TPS22918

Hello,

I have following schematic:

The transistor at ON pin is not populated and CT=100p.

And I get following measurement:

I'm surprised that level at ON pin go to ~2,6V when IN is 1,8V.

Could this be related to the charge pump inside the IC? Is it "normal" to have ~2,6V at the ON pin? Why did we have a "step" at the on pin voltage the first 2ms?  

And I have the same circuit with 3,3V input and I have also ~2,6V at ON pin!

Regards,

  • Hi Eric, 

    Thanks for joining E2E!

    The ON pin should not reach a higher voltage than the VIN pin if they are shorted together. There is no internal circuitry that could be bringing ON to that voltage.

    Is there additional circuitry at the npn transistor that would resemble some sort of source? 

    Or is it possible you are probing the CT pin? We do not recommend probing the CT pin as it could lead to current flow into your oscilloscope. 

    What is your intention with the npn currently? Are you hoping to drive the ON pin with an external control? If so, I don't see why you would need to short Vin and ON together. 

    Are you perhaps trying to alter the system to an enable-low structure? 

    Best Regards, 

    Elizabeth 

  • Hello Elizabeth,

    I double check and I have really measured the ON pin, not the CT pin. I also exchange the probe/scope channel used for CT with this for OUT to prevent a defect probe/ scope channel or wrong settings at the scope.

    The npn transistor is populated on the original board. It is to invert the polarity of the signal at low cost. The control signal of the npn is also used for other component. I only removed it for the measurement to find out the root cause of this not expect level at ON pin. The measurement in my previous message was with npn removed so only the pull up to VIN is connected to the ON pin.

    To see the effect I have replaced the 10k resistor by a 1k resitor. The basic effect is the same, only levels is different.

    So clearly it's not a voltage that is set but there is current that are driven in the pull up resistor. And the only part that is connected to the resistor is the ON pin (as the npn is removed). And I can also see in the TPS22918 block diagram that you have a charge pump:

    As ON pin voltage level is higher as VIN and VOUT, I so not see other source for this higher voltage at ON pin as this charge pump. Could you please double check my effect by reproduce my setting on a eval board?

    Thanks,

    Eric 

  • Hi Eric, 

    I will double check on an EVM to see if I can reproduce your output and get back to you later later this week, possibly tomorrow. 

    However, the charge pump should only affect the output voltage you see at the CT pin. 

    Best Regards, 

    Elizabeth

  • Hi Elizabeth,

    Additional information from my side: We replaced the IC by a new one but behavior is unchanged. It looks that it not a single IC failure.

    Regards,

    Eric 

  • Hi Eric, 

    Thank you for verifying that. I will send you an update by tomorrow EOD. 

    Best Regards, 

    Elizabeth

  • Apologies for the delay here Eric, I'll have to get back to you with my results Monday. 

    Best Regards, 

    Elizabeth

  • Eric, 

    Could you clarify your QOD to Vout connection? Is the "NB" resistor not populated, placed, or shorted? 

    Best Regards, 

    Elizabeth

  • Hi Eric, 

    This is what I've collected for VON, Vin, and Vout(pink) by shorting Vin and Von with a 10kohm resistor. I didn't observe any sort of jump in ON and no voltage on ON greater than Vin which can be seen in the second plot where the Vin(yellow) and Von(blue) plots are on the same vertical offset. 

    We would not expect a rise in Von outside of the limit of Vin when shorted together. Could there possibly be any surrounding circuitry bring ON higher?

    Best Regards, 

    Elizabeth

  • This was resolved offline where the issue was determined to be caused by a unseen connection to ON in setup and not the part itself. 

    Therefore, I will close this thread; please start a new thread if you have any further questions. 

    Best Regards, 

    Elizabeth