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TPS5420: Random start-up problems

Part Number: TPS5420

I am using an open drain mosfet to control en (pin 5) on controller.  Sometimes the chip doesn't start - no output.  The mosfet I am using is a ROHM SM6K2. It is maximum leakage of 1uA at 60v.  I am turning him off with another SM6K2 pulled to ground.

I have 4 of these supply circuits and only 2 are giving me a problem.  The good units measure (enable pin)  about 5v to ground, while the units with no output hang round 1.3v. 

I am thinking to put a pull-up to the input voltage to try and help the enable pin go high.  Maybe a MEG ohm resistor to the 28V supply.  

Any thoughts  this?

  • Hello Bob,

    If the idea to add just a single 1Megohm pull-up to +28Vin may help for you to debug the issue with startup, but may not be wise in production because the max voltage spec is 7V on the Enable input.

    If compatible with the operating input voltage range; place two resistors in series to Vin, the midpoint tied to ENable input and the MOSFET. The midpoint voltage of the divider should not exceed 6V at high line, and remain above the enable voltage at low line.

  • Thank you Alan.

    I understand the maximum spec is 7v.  Would it be possible for TI to specify a maximum current into the pin? 

    As an example- often times there is a max voltage spec where you wouldn't want to put a voltage much higher than the supply voltage on the output of a driver chip.  There is a mosfet on the high side so the intrinsic diode in the fet would be forward biased.  It is not a problem to put a pull up on this type of output as long as current is limited.

    I am looking for an understanding of the circuitry so that I can apply similar logic, and/or a number of microamps that I may force into the pin without damage.  The board will get a spin before production so that I can put  voltage divider or a connection to 5v with a pull up to 5v. 

    In the mean time I will put a decade box on it and see how many microamps it takes me to drive pin to 5v or so.

    thank you.

  • I don't think it is wise for TI to spec an allowed current into the ENA input. Pulling the input above the abs max of 7V can be risky because there may be a structure that could cause device malfunction if that level is exceeded. I still recommend the two resistor approach where the midpoint is grounded by the Rohm MOSFET.

    I also notice the 60V 1uA rating on the Rohm transistor is just a room temperature spec I don't see a different graph that details an actual expected range across temperature.