Max current on header 5/3.3V supply

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1

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!

2

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.

3

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

4

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

5

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

6

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!!

7

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

Gerald

8

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