TPS62740: Maximum source impedance on the EN pin

Hi Steven,

it's my pleasure to support you.

I have not really understood the configuration you described. Could you kindly draft and share a schematic representation?

Hi Emmanuel,

Thanks for getting back to me.  I have attached a schematic here.  In this schematic, R1 is 1 MOhm.  When the jumper is present, the DC/DC converter is disabled.  When the jumper is removed, the DCDC converter is enabled.  I am trying to minimize the leakage through R1 when the jumper is present.  What is the maximum permissible value for R1?  Can I use a 10 MOhm resistor instead of 1 MOhm?  Are there any parasitic leakage and/or noise issues if using a large value resistor?

Hi Emmanuel,

The jumper is intended to be an externally connected jumper. There are a couple purposes. One purpose is an "on-off" switch. The other purpose is to place the unit into a "ship" mode so that the product can sit on the shelf for a while. The products incorporate a primary battery that is encapsulated with the electronics. So, nothing is accessible internally after they are encapsulated.

I was thinking that the default condition should be that the unit is in the "off" state. That is, with nothing physically connected, the device is off by default. That would suggest a pull down on the enable pin instead of a pull up. However, to address safety, I am trying to not expose any connections that could cause fault conditions that would cause high discharge currents if they were accidentally short circuited to one another. Any connections that are exposed need to be current limited. So, exposing +BATT on the external connector needs to be done with care so that it cannot cause a fault and/or accidentally discharge the battery.

Thank you for the feedback,
Steve

Hi Emmanuel,

I guess there is also the issue of static for the externally exposed connection. I suppose there should be a series resistor with the Enable pin to minimize static discharge current?

Steve

Hi Emmanuel,

What about something like this?

Steve

Hi Emmanuel,

I do not think the other part you suggested will be suitable ... the Iq is way to high for my battery powered IoT application.

Regarding the TPS62740, I have observed some behaviour that is concerning.  I presently have a 1 Mohm pull up on the EN pin.  If I briefly connect the EN pin to ground and then release it, the output shuts down and does not start again.  The device appears to latch itself in the off state.  When it is in this state, connecting EN to ground and releasing it for less than about 3 seconds does not enable the output.  This suggests that there is some sort of internal delay associated with the EN pin that prevents the device from starting up.  I can find nothing in the documentation regarding this "feature".  Can you please explain why this is happening?

Also, I have noticed on several occasions that the device seems to drop out randomly when someone gets physically close to the circuit with their hands.   What I think is happening is that human body static charge is affecting the enable pin and causing the device to drop out.  Can you confirm if this is an issue with this component and suggest a mitigation?  

Thank you,

Steve