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Anonymous
October 13, 2016
Question

Driving peripheral chips VCC directly from MCU

  • October 13, 2016
  • 3 replies
  • 671 views
Posted on October 13, 2016 at 14:29

Hi all,

I'm going to design an ultra low power circuit which should work with battery for years, so i have to decrease the power consumption as much as possible, i'm using some peripheral chips like EEPROM(m24128), accelerometer, magnetometer and ...

i know those chips have ultra low power standby mode down to 1-3 uA consumption, but i'm thinking to disconnect the VCC of them when not needed to avoid that consumption too !

for example EEPROM is used so much less than others, or magnetometer should work once every 5minute i.e ...

My MCU is Stm32L0 series, my question is can i drive those chips directly from MCU output pin ? (without on/off transistor)

and maybe connect 100nF or even 10uF caps to their VCC (so MCU out pin too).

will this make problem ?
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    3 replies

    valentin
    valentin
    Associate III
    October 13, 2016
    Posted on October 13, 2016 at 21:10

    I don't see why it shouldn't work - as long as the periph ics Vcc is the same as the stm32's and their current consumption doesn't exceed the pin's limits (individual pin and total sum current limit!).

    mark239955_stm1
    mark239955_stm1
    Visitor II
    October 14, 2016
    Posted on October 14, 2016 at 05:33

    In principle, yes, it's possible - although I wouldn't be intentionally directly driving  any 100nF caps off an MCU output.  A small series resistor to limit current into the capacitor to something reasonable might be advisable.

    mckenney
    mckenney
    Associate
    October 14, 2016
    Posted on October 14, 2016 at 13:57

    Yes, I've done this.

    When you do your power estimation, be sure to account for the startup time/current. EEPROMs start up pretty quickly, but some sensors (particularly ''smart'' ones) can take a long time (often at elevated current) to get going. More than once I've concluded that, for my target duty cycle, keeping it in sleep mode was the better bargain. If you need a bypass cap, you may already be headed down this road.

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