8 bit addressable latch at 3.3V with open drain output

Question

I need to drive 40 leds from a 5V power source. These leds are controlled from a raspberry and I was thinking to use 5 TPIC6B259 but I need a logic level shifting from 3.3 to 5 V for the input of TPIC6B259. I was looking a 3.3V device similar to the TPIC6B259 but I didn't find anything: only addressable latch without power mos (ex 74HC259). Adding 40 power mos is not convenient. Is it possible that the TPIC6B259 is the only addressable latch with power output? Can you please suggest me other solutions? Since the 3.3 power is sourced from raspberry I need a low power addressable latch followed by an open drain mosfet to drive (at 5V) the loads.

Thanks Fabio

Answer 1

You can use a simple NPN transistor with a pull-up resistor on each input. The downside is that you need 2 resistor + 1 transistor for each input. If I undestood, you need 3 pins for each IC for addressing (so 5*3) plus 1 input for data and whatever logic signals you may need. That's 16 transistors and 32 resistors. It's possible, but that's a lot of glue circuitry (and there's level shifting ICs on the market to do that).

Another way of doing it would be replacing the addressable latch by a shift register like TPIC6B595. It's high-power just like your latch. The nature of the shift register reduces the number of control signals (and level converters) as you can chain infinite ICs together.

The downside is that information needs to travel in a serial fashion and there's no individual addressing. If you want to light up an LED at the end of the row, you'll need to send the data for every LED on the row.

It will take some time (milisseconds) and you can either let the data flow through the LEDs like a pipe or shut them off until you flush all the data.