# Current sourcing octal buffer/line-driver for 7+1 segment LED multiplexing?

I would like to multiplex 8, 7-segment LED displays with two, 8-bit shift-registers. The required current for one segment is 20mA. I have no desire to overdrive the leds with 8 times the specified current, that will shorten their precious lifetime.

Since the displays are common cathode, the TPIC-6B-595 is suitable for sinking current, but I did not find any current-sourcing shift registers, so I need to use an octal buffer/line-driver. I prefer octal-buffers over Transistor/FET arrays.

The 74**541 and 74**244 datasheets are not clear about the current-sourcing parameters, to me it looks like they are only sinking and cannot handle 7+1 (dot) * ~30 mA = 240 mA.

This is the circuit diagram:

Does anybody knows any solution to my problem?

• I think you're calculating wrong. Each gate would only be sourcing the current for one segment, so only 20mA. – Majenko Mar 1 '15 at 21:19
• Yes, but the whole device has to take 8*~30=240mA (worst case) which is far beyond any octal-buffer specs. – Gábor Dani Mar 1 '15 at 21:22
• Majenko is right. You are wrong. The whole device only has to produce 30 mA at any given instant. That said, 74**S is not a good way to source current. You "prefer octal-buffers over Transistor/FET arrays", but that just means you need a high-side driver. – WhatRoughBeast Mar 1 '15 at 21:29
• If all the 8 led, rated ~20mA each, (7 segment + 1 dot) is under power, how can the whole thing be only ~20-30 mA? – Gábor Dani Mar 1 '15 at 21:34
• Ah you mean the whole device current - ok, yes that can be 240 max for your arrangement. – Majenko Mar 1 '15 at 21:36

In the "for what it's worth" category, this resource I found below lists quite a few latching constant-current LED drivers (8 and 16 bit). http://www.altera.com/literature/an/an286.pdf

Some of them actually have shift outputs so you can gang them together and write one long stream to the first device in the daisy chain.