Re: 300 mA from a microcontroller pin

Tomás Ó hÉilidhe wrote:
Tomás Ó hÉilidhe:


I'll post a picture of the relevant part of the circuit now in a few
minutes.



http://img266.imageshack.us/img266/4861/ledsetupug6.jpg

As you can see, I've pins coming from microcontroller going to the shift register.

The simple NPN/PNP buffers will auto-quasi-tristate, provided you can disable any pullups on the uC port (ie truly float the port).
There will be some turn-off delays, so allow some dead time between column selects would help.
A resistor from B-E would help reduce those delays. 2k2 ~ 4k7 region.

You only show one column drive polarity, but there are trimmed lines
on the drawing so I presume there is a similar PNP drive scheme, and
hence your 1:16 duty cycle.

A better shift register choice would be one with a latch, and OE,
so you can drive only ONE device, and control dead-times.
Look at HC4094, or HC595 devices.
A HEF4094 has lighter drive, so you could go straight to the darlington
bases with that.

The darlingtons will need to be large, as the column resistance should be much less than the series R's, to avoid unwanted brightness modulation effects.

-jg





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