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TPS3840: Autoreset microcontroller

JOSE MANUEL PAC
Prodigy 30 points
Part Number: TPS3840

Tool/software:

We have an issue with this reset generator.

Below you can see the TPS3840 is always powered at a constant 3.3V. The 12V is always on, thus the 3.3V zener always works.

The MCU is at 3.3V and commands the /MR pin. When the MCU sets a high level, the N-MOSFET conducts and /MR is low for about 275 microseconds.

I have a CT capacitor of 10uF to fix the delay time at 6 seconds, but it does not work as I expect because the /RESET level is only low during 275 us like the /MR. Seems the Delay Time is not assured.

The ON_3.3V signal powers down the LDO that supplies the MCU.

It's curious because the first time I power up the 12V and the Zener at 3.3V. the /RESET is low and takes 6 seconds to high level before powering up the LDO at 3.3V and the MCU.

But once the MCU commands the /MR then the /RESET just stays low during 275 microsecs. in spite of the CT capacitor of 10 uF. I would expect the 6 seconds.

I tried to use a CT capacitor smaller of 2.2uF, but same result.

Could you give me any clue?

Thanks.

Jose

 

  • +1
    TI__Genius 9640 points

    Hi Jose,

    I tested TPS3840-Q1 with 10uF CT capacitor on our EVM. Please find my bench result in below. 

    The td reset delay =  6.8 seconds. ( This is the delay from VDD goes higher threshold to de-assert the reset)

    The td_MR delay = 7.1 seconds. (This is the delay from release MR to de-assert the reset.) 

    You should get similar delay as in above. 

    If you go lower CT capacitor, you should see a better accuracy in the td and td_Mr delay. I would recommend testing with another TPS3840-Q1 for diagnostic purposes. 

    Best,

    Sila