revisiting the UART board

Werner Almesberger werner at
Sun Feb 6 18:59:56 EST 2011

Ron K. Jeffries wrote:
> 1) What's your estimate of the (approx) max power draw
> that is safe when external Ben Dongle leeches power from Ben?

Heh, I knew this question would come :-) I don't really know. But
let's see what the limits are along the path:

First, Q4, next to the 8:10 card slot: it's a TM2301FN [1] with a Ron
of about 0.1 Ohm and a maximum current of 2.3 A. So it's not really
an obstacle for anything.

Q4 is supplied from +V3.3, which comes from the U2 regulator, an
EUP3406 [2]. It has a nominal output current of 600 mA, peak 800 mA.
Again, quite a high limit.

The schematics say the inductor L1 is rated at 1 A, so it should be
good for a regulator output current of about 833 mA.

Q4 is supplied from +VBAT. If USB-powered, +VBAT is fed via D2 from
+VUSB. D2 is an MSCD104 [3] with a maximum average current of 1 A.

+VUSB passes through BD6, a PZ1608D221TF [4], which the data sheet
rates at 800 mA (Adam, [6] shows a rating of 1 A ?).

Of course, you could have looked up all this yourself with schematics
[5] and the data sheet links in the inventory [6] :-)


So if the upstream port can handle it, the Ben can draw a maxmimum of
about 600 mA at its 3.3 V rail if the other rails have no significant
load. I don't know the Ben's own peak consumption, but I would
estimate it's below 200 mA. So you may be able to draw up to 400 mA,
provided that your upstream USB port can handle it.

I wouldn't recommend to design things to go over 100 mA, which would
leave a 500 mA port with ~100 mA for the 8:10 card (a bit less), maybe
200 mA for the total Ben, 110 mA for charging the battery, and a bit
of tolerance.

Unfortunately, there is no proper current limiter in the path. But
nothing bad happened on those occasions when I accidently shorted
8:10 card power.

> 2) Given recently discovered lower than optimal power
> to Ben from external charger during Ben operation
> (not an issue when Ben is powered down) what do you estimate
> to be a steady state allowable dongle power draw that will
> not cause battery to discharge?

The low charge current limit only has a peripheral effect on the
current available at the 8:10 card interface. After all, the charge
current goes to the battery, not the 8:10 card. However, the battery
would start discharging if you overload the USB upstream port to the
point of it shutting down.

> p.s. I hope you appreciate my suggested
> OLD  ubb = "universal breakout board"
> NEW UBbb = "Universal Ben breakout board."

I very much hope at even its successor would offer the same interface.
So it would have to be something like the Universal NanoNote Breakout
Board, UNNBB, which seems a little awkward.

If you like triplicate letters, you'll appreciate the naming of my
helper library:

But I assure you, typing libbbd.c a lot of times or spotting the typo
when you get a "no such file" can drive you nuts ...

- Werner

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