Max current on header 5/3.3V supply

[Robert P]

Hi everyone,  I'm trying to figure out if it's safe to power an xbee off the 3.3V header on the bbb  ( P9_3 / P9_4 ) .. I need 250ma-300ma MAX

is this safe and stable?  It would seem like it, but I want to confirm.. thank you!

[PLyttle]

When you read the service reference manual, section 8.6, you find that the VDD_3V3B at P9 pins 3 and 4 can source 250 mA max. 

[Charles Steinkuehler]

Those numbers are for each pin, so 500 mA total.

--
Charles Steinkuehler
cha...@steinkuehler.net

[PLyttle]

I doubt that. The pins are connected together. Also:

6.1.9.2  VDD_3V3A Rail

The VDD_3V3A rail is supplied by the TPS65217C and provides the 3.3V for the

processor rails and can provide up to 400mA.

6.1.9.3  VDD_3V3B Rail

The current supplied by the VDD_3V3A rail is not sufficient to power all of the 3.3V

rails on the board. So a second LDO is supplied, U4, a TL5209A, which sources the

VDD_3V3B rail. It is powered up just after the VDD_3V3A rail.

U4 is capable of sourcing 500 mA, but on-board it also powers nand, ethernet and uSD. Whatever is left for externals is less than 500 mA, 250 mA left as is documented seems reasonable. 

LP

[Robert P]

thanks everyone, so I guess the answer is yes?  xbee uses 215ma max, so it looks like it's safe

[graff...@gmail.com]

If I tie the two 3.3V pins together, can I draw 0.333mA  or just 250mA??

I'm using 3 thermistor sensors at 0.11mA each so I'm wondering if I'm going to blow my board.

Many thanks for some knowledgable input!! 

[Gerald Coley]

Tying them together will not increase current capacity. They both come from the same place on the board, the power supply device itself..

Gerald

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[Graham]

Graff:

I think you are confusing Amperes (A) and milliAmperes (mA).

You need to understand the difference and then re-run your calculations.

I doubt that three thermistors, each drawing a fraction of a milliAmpere would overload a 250 milliAmpere supply.

--- Graham

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