Thanks, those are great ideas, I'll try em out this evening...

On 4/24/06, Dave Hylands <dhylands@gmail.com> wrote:
Hi Eric,

>  Quick little problem to deal with here: does anyone have some
> recommendations on a simple circuit to supply 5V to a gumstix/robostix
> stack?
>
>  Currently I'm using a 7805 regulator with some caps and a fuse connected to
> a +9V battery in.  It supplies the correct voltage, ~+5.044V, when the
> gumstix isn't plugged in, but when it is plugged in, the voltage drops to
> +4.7V and starts causing segmentation faults on the robostix when I run
> i2c-io.  So I tried ramping the input voltage up to +12V, since the
> regulator should be able to take this, but the 7805 gets extremely hot in a
> very short time.  I checked the current when it's running, and the
> gumstix/robostix stack (and a wifi cf card) seems to draw ~450mA, way too
> much for this circuit.
>
>  What are you guys using?  Anyone have some some suggestions for something
> else than a 7805, which seems like it can't supply enough current?  Any help
> would be greatly appreciated...

The robostix has onboard regulators already. +12 is a bit on the high
side, but +9 should be OK.

5v is too low (since the voltage regulators which are on the board try
to output 5v but need a bit more for headroom.

In particular, the MIC5219 regulator on the board can drop as much as
600mV depending on the amount of load. So you need 5.6v or higher.

There are a total of four voltage regulators on the robostix. One of
them is dedicated for the gumstix. However its only rated at 500 mA,
so if your CF card will draw 450mA then that's too much.

What you can do is to take the input to your 7805 and connect it to
the robostix power and take the output of your 7805 and connect it to
the power on the cfstix. This way you'll get 500mA from the robostix
power supply, and whatever your 7805 is rated at from it.

You want your input to the 7805 to be a minimum of 7.2v (so it has
2200mV dropout), but as close to 7.2v as you can. Perhaps 7 1.2v
batteries for a nominal 8.4v.

Better yet, use a low-dropout regulator like the LM2940 which has a
500mV dropout voltage, and is available in the same package as the
7805. Then you could use 6 1.2v NiMH or NiCAD batteries.

--
Dave Hylands
Vancouver, BC, Canada
http://www.DaveHylands.com/


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