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TPS3840: Response to Manual Reset

Part Number: TPS3840
Other Parts Discussed in Thread: LM393

Hey E2E Design Support,

Recently we have started testing with TPS3840 for RESET management and Voltage Monitoring.

Experiment 1:

We have LM393 for Fine-tuned voltage management. It has Open Drain output.

We are using TPS3840 in Open Drain output mode as you can see from the Serial Number. This enables us to give precise control for our System on Module.

SHUTDOWN_REQ signal is essential used to request for a SHUTDOWN. Internally it's pulled by 100kOhm resistor to 5V level (essentially POWER_IN for the SOM). SOM controls this signal for about 10us and then does not drive it. Hence, satisfying 300ns requirement for MR signal.

Whenever I get SHUTDOWN_REQ from HIGH to LOW

BLUE = SHUTDOWN_REQ connected to MR

RED = /RST output

Here's the magnified scope reading.

Here's the long duration scope reading. We see a glitch and then 1.246s of LOW.

Experiment 2:

Manually driving Manual Reset pin. Here's what I observe on /RST output:

Note that Red = /RST; Blue = MR

When the MR is manually triggered It's LOW for more than 300ns. I'd expect to see about 1.2 seconds delay before I observe the /RST being driven HIGH.

Can you help me determine if there's anything I'm missing in this setup? This setup is similar to Application Notes circuit in the datasheet. If everything is correct, is this part operating out-of-spec?

  • Hi Atharva,

    The issue that I see is that the expected reset time delay is 1.2 seconds using the 2.2uF capacitor on the CT pin.  The general rule of thumb, which is sort of mentioned in datasheet and is shown below, is that the /MR low state or any fault condition must be at least 10% of the programmed reset time delay.  Therefore, if the expected time delay is 1.2 seconds, the /MR low state must be at least 120ms long so the discharging of the CT capacitor (2.2uF) can be fully discharged down to ~0V.



  • Thanks for responding! This certainly helps :)

    I, somehow, didn't notice that statement in feature description. I'd think this would be the case for any reset controllers out there, as charging the capacitor makes sense. 

  • Hi Atharva,

    Glad to be of help.  Good luck to you!