Fez Panda External Power Source

Question

I have 24 Leds on a MAX7219 that I'm connecting to my Fez Panda 2. The Leds should pull around 160ma which I don't want to supply directly off the Fez 5V pin. It should handle it but I have other things pulling from the controller as well. The AC adapter I have is a switched 9V 2amp.

I have an extra L7805CV voltage regulator lying around so I was thinking I could split the Power cable, then power both the Fez Panda and the L7805CV with the same AC adapter. I could then use the L7805CV to power the LED matrix. This should save me from having to incorporate another 5V AC adapter. I take it I would have to tie the grounds together after the regulators as well?

Does this sound like a workable design, or is there a better alternative?

Thanks!

Answer 1

• Summary: Looks good.

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Max7219 LED driver datasheet here

L7805CV regulator data sheet here

Driver will sink 5 mA /segment minimum x 24 = 120 mA min so 160 mA sounds about right.

At 5V out regulator dissipation is about (9-5) x 0.16 = 0.64 W. Thermal resistance is 100 C/W in D-Pak (~= 65C rise)
down to 50 C/W in TO220 (~= 30C rise) so a smidgeon of heatsinking is in order for the D-Pak and you'd lightly burn yourself on an unheatsunk TO220.

Total driver dissipation including LEDS at 5V supply is 5 x 0.160 = 800 mW.
LED disspation depends on colour.
Red LEDs will take about 2V x 0.16 =~ 0.3 W off the driver
and White LEDs will take say 3V x 0.16 =~ 0.5W
leaving the driver dissipating 500 mW with red LEDs and 300 mW with white LEDs.

Regulator efficiency is 5V/9V = 55%.
A switching regulator may give you say 85% here so regulator losses would go down from (9-5) x 0.16 = 640 mW to (15/85) x (5 x 0.16) =~ 140 mW - a reduction in regulator dissipation of 500 mW. This would keep the regulator happily cool without any heatsinking in most cases but is unlikely to be worthwhile except where energy budget matters. (If the regulator efficiency changes from 55% to 85% the losses drop from 640 mW to 140 mW so smps losses are only 140/640 =~ 22% of linear regulator losses.

You can get drop-in smps regulators to replace the 7805xx if energy budget matters eg

Murata 78xxSR series - claim about 90% efficient in this application.

CUI V78xx-2000 =- claimed over 90% efficient in this application

CUI V78xx-500 Claimed around 95% efficient in this application.

All available from Digikey.

So, yes, this sounds OK.