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PTN78020W Inhibit Not Working

Tim Hamilton
Prodigy 10 points
Other Parts Discussed in Thread: PTN78020W

I am trying to use the inhibit pin to disable the regulator output to save power on my system. I have a PTN78020W on two boards and they are both having the same issue.

The problem is that when I bring the inhibit pin to ground the output does not fall to 0VDC but about 1.6VDC. I have tried 3 different methods to bring the inhibit to ground, none of which turn the PTN78020W completely OFF.

First, I pull the inhibit pin to ground using an I/O pin on a STM32F407 micro-controller. Based on the PTN78020 datasheet I would expect the low level ground signal from the micro to pull the inhibit pin down.

Second I placed a N-FET from the inhibit pin to ground as shown in the datasheet example. I use the STM32F407 to drive the gate and also directly connected the gate to 3.3VDC to turn the N-FET ON and both had the same outcome.

Lastly I shorted the inhibit pin to ground and did not see the regulator output fall to zero, it's output was about 1.6VDC.

I am not sure what is happening here and I have run out of ideas. 

I have a 4.7uF input cap and the following values on the output, 220uF tant, 100uF tant & 1uF. The ADJ pin is connected to a digi-pot so that i can trim the output voltage but there is always a minimum 4kohm resistance from the ADJ pin to ground.

-Tim

  • 0
    TI__Mastermind 38230 points

    Hi Tim:

    It appears from  the information  above, you have either a pre-bais voltage present on Vout when teh PTN78020W  inhibit pin is grounded or'

    The digital pot has feedback   voltage  back through the adjusment pin to Vout.

    I recommend you add an low value 10 ohm resistor for bleeding Vout  to ground when Inhibit pin is grounded.  if  the Vout is set to about 3.3V. Otherwise , 50 ohm bleeder resistor to ground on Vout bus.

    Inhibit pin must be only controlled by an open drain FET , no external voltage applied at anytime.

    My e-mail address is tguerin@ti,com for any additoanl questions.

    Tom