Circuit:

I have 2 different multiplexed sets of LEDS. The first set is 3 7-segment leds blocks, each with 7 20mA segments plus 1 10mA dot. The other set is 8 10mA leds x 4. These leds are common cathode red/green dual 3 lead leds.

Multiplexing:

I am multiplexing using the common cathode of both sets. Common cathode is switched by a 74HC595 ( source 35mA max ) with a 2N2907 for 350mA max. The rows are sunk using 2 TPIC6C595 shift registers. Each one will sinc 100mA per row and 500mA total.

My thinking is that since my 7-segments are on 1/3rd of the time, max that would be (20mA * 7 + 10mA ) * 3 or a pulse of 450mA with a 1/3 duty cycle to get 100% brightness. First, is this correct?

Assuming that, then I am not going to get enough current out of my source register because it gives only 35mA, with 2n2907 transistor that is 350mA. But since I only need 450mA for 100% brightness then that is like 77% brightness at 350mA, so I can live with that.

The second mutiplex set is 8 x 4 10mA LEDs, or 320mA < 350mA, so I am fine. On the sinc side, each line can drain 100mA each and 500mA total, so I am also fine.

So now I have the current limiting resistors for each of the 20mA segments ( 1v drop). The 7 segment ones are on 1/3rd of the time, so I need 3 times the normal current or

1v/ 20mA * 3 = 16.6 ohms

and the 'dot'

1v/ 10mA * 3 = 33.3 ohms

and the 8x4 10mA LEDS

1/ 10mA *4 = 25 ohms

Am I doing this right?

As for the base resistors on the transistors, that would be:

4.3v / ( 20mA * 7 + 10mA ) = 28.6 ohms

and the base resistor for the single LEDs

4.3v / (10mA * 8 ) = 53.7 ohms

Does this check out?

Thanks

http://www.ti.com/lit/ds/symlink/tpic6c595.pdf