torsdag 10. september 2015

Shift register led drivers

I've tried to do a little survey of viable solutions for the led dial. Preferably I'd like to not have any resistors or transistors on the lines connected to the LEDs as it s easier to solder one IC than several small smd parts, even if the cost would be quite a bit less with more parts.

The need for transistors is removed in two ways: 

On the positive side of the diodes it is possible to use a line driver such as the uln2803 transistor array. This does however make a "local" solution where both positive and negative sides of the LED are controlled by a single shift register (see post earlier) impractical, but a single driver may power a whole bunch of diodes on several dials. The number of lines running between the dials will increase somewhat though.

On the negative side of the diode the transistor may be omitted if one uses an open drain led driver/shift register.

The resistors are still needed with the open drain version, but switching to a "constant current" led driver lets you control the current using a single, shared resistor that sets up a reference current.

I've done a little list of available 12bit and 16bit LED drivers. It is by no means exhaustive, but probably covers most of the common ones.

Open drain LED drivers 
(low side, meaning they should be connected to the LEDs negative pole to sink current)

12 stage:
TLC6C5912
Hef4894b 
Npic6c4894



Constant current

12 channel:
Max6979
Bd18377

16 channel:
Max6969
Max6971, high voltage 
A6282
A6276 (obsolete)
Stp16c596
Dm133
Tlc59281x
Tlc59025
Cat4016
Tlc5927
Pca9952
Pca9955

24 channel:
Stp24dp05


So far the cheaper solution seems to be:
Tlc59281 - 16bit, 0.987€@100



These are normal shift registers often used for LED scanning in the DIY world. They are neither open drain nor constant current and just mentioned here to remember them:

74hc4094 (can this drive a led from its outputs? Some people seem to do it, letting it sink current without transistors).
74hc164

74hc595 in combination with the uln2803 (row scanner)

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