STM32F0 Discovery Board 5V Output Current Limit

[jacky]

Posted on July 08, 2018 at 10:31

Hi all,

I have a STM32F0 Discovery board and would like to use its 5V power pin (pin 1) to power external devices (LEDs).  The user manual UM1525 section 4.3 says that the power consumption must be lower than 100mA.  However, when I look at the schematics on pg38, the 5V pin is connected to the USB Vcc through a BAT60JFILM diode, which is a 1A part.  Anyone know why the power consumption is limited to 100mA?

Thanks.

[Tesla DeLorean]

Posted on July 08, 2018 at 15:12

The diode doesn't supply 1A, it is a maximal draw rating, the amount that can flow through the component, the current has to flow from some where else.

For USB applications there is a maximum total budget of 500mA once the system has negotiated connectivity, but initially the limit at startup is 100mA

http://www.ti.com/lit/an/slyt118/slyt118.pdf

 

[jacky]

Posted on July 09, 2018 at 00:33

Thanks for your reply.  So the limit is due to the USB port instead of the evaluation kit itself?  If the USB port is able to supply 500mA or 1A without enumeration, is it OK to use the 5V pin to supply current > 100mA?

I am thinking about the situation when the board is powered by a USB battery pack.  

Thanks.

[T J]

Posted on July 09, 2018 at 01:47

The inference being that you have to charge the battery pack ?

thereby requiring  a higher current ?

is that what you are asking ?

USB always had the 100mA limit, as a fault detector.

[jacky]

Posted on July 09, 2018 at 02:42

Hi T J,

No, I am referring to powering the discovery board through the mini USB connector using a battery pack, which does not have a 100mA limit.

I am intended to use the 5V on the discovery board to power some LEDs which will exceed 100mA.  So if understand correctly based on the schematics, the 5V pin on the discovery board is connected directly to the USB supply through a diode.  So no extra current limitation imposed by the discovery board, except the max current spec of the diode.

I am not sure if it is true that USB always had the 100mA limit.  A lot of the ports do not enforce this limit and allow > 100mA without enumeration.

Thanks.

[T J]

Posted on July 09, 2018 at 02:48

There are so many aspects in designing PCB Systems

If you draw 1 Amp as the connector is being inserted or removed,

you will get burning on the connector, and it will fail after too many insertions.

usually you would make a separate board that powers the LEDS from the Battery Pack, then a cable to the USB port.

[jacky]

Posted on July 09, 2018 at 02:55

Hi TJ,

Thanks for the comments.  The LEDs are off during insertion and I am not going to drive a full 1A current, more like 200mA.  And the mini USB connector are normally rated to a couple of amps.  But I take your points.  Thanks.