• Sat 2019.12.28

    'It all depends how much current you need...'

    and that my friends is the secret. E = I/R and P = IE

    I originally made a comment #135 regarding powering external devices from the micro itself. My take is to allow the $1 part to take the heat dissipation, protecting the $30 micro. I made my forum comment and moved on.

    three hours later

    I remembered a comment made on the power dissipation when the voltage demand has a higher voltage input, that caused me to perform a Google search.

    forum #3 post Pico Supply Voltage


    Should I mis-interpret the datasheet, feel free to correct me. I'll own up and eat crow. Used to it as we have our share of road kill here in the mid-west. I grew up on tube theory and had to improve my TTL skills and transition into electronics with extra evening schooling. This was back in the, (gulp) seventies, a time before TTL, 7400's and $10 555's, $0.22/gal leaded gasoline, gourmet coffee was a nickel, beer was a dime, microwave ovens were just available under $1000, we had a black and white TV, color too expensive, cable was five years on the horizon, had a party line (shared with neighbor) with a wired pay-per-call phone, one could get a free C cell or 9V battery at Radio Shack monthly without an in-store purchase, CPM was king as the PC/DOS would make it's debut in a decade, the Internet wasn't even a concept, cell phone technology we read about in Dick Tracy comic books and twenty years into the future. I read George Orwell's 1984 a decade before it's title would become mainstream. I digress . . .

    off sidebar

    Now, my comments are not to dis-credit the efforts established by @allObjects, but rather to inform as to why checking the datasheet should be a normal part of our projects development. See the article in #2 post ' GPS powered by Espruino pin(s)' for an excellent overview of how to power an external module, using the GPIO pins of the Espruino device itself. As that article uses the Espruino Pico, we'll stick with that for the sake of this post discussion.

    p.60 and p.68

    ref http://www.espruino.com/datasheets/STM32­F401xD.pdf

    If we look at the online docs for a Pico,


    we are able to confirm we have a STM32F401CDU6 device. On p.132 we are able to deduce we have a 'U' type device which leads us to understand we able to dissipate a maximum of 625mW or 0.625 Watts.

    U = UFQFPN

    Performing some simple maths, we are able to connfirm:


    I = P/E 0.625 / 5v = 0.125A or 125mA

    0.625 / 12 = 0.052 or 52mA

    38 mA all peripherals enabled

    which conservatively is where @Gordon arrived at the 40mA draw. Post #4

    So, it can be seen that as the voltage across the device goes up, the ability for that device to supply the needed current goes down, at the maximum allowable current/power dissipation. At full output, this Pico would be passing a quarter of an amp or disspiating over half a Watt of power!!

    We all know that heat is a number one destroyer of compents, and I'd rather see (and unsolder a three pin device) the low cost regulator take the punishment, rather than risk the more expensive, more pins microcontroller.

    Design time and cost limits will finallize the design. Gravity and Mother Nature always win.  Ohm's law it is . . . .

    'It all depends how much current you need...'


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