Shift Register Issue

Suraj143 · Apr 22, 2019

I'm doing a 8x8 LED matrix design.Rows driving via B562 & columns driving directly from 74HC595 (via a resistor). Doing a 1/8th scanning method.

The problem is only NXP brand 74HC595 is surviving & cheap Aliexpress SN74HC595 ICs are heating (only working few minutes) & getting stuck & heating too much.

How to avoid above situation?

Ian Rogers · Apr 23, 2019

Each pin CANNOT sink 240mA.. There is no current limit so your LED's are maxing out.. Normally we use a ULN2003a or similar to drive the LEDS BUT!! you still need a resistor....

Suraj143 · Apr 23, 2019

Hi I have a resistor 27Ohms. You can see that near every pin on 595 IC.

Pommie · Apr 23, 2019

Suraj143 said:
I have a resistor 27Ohms.

Your diagram says 27R.

Note, the datasheet states "max current through Vcc or Gnd" = 70mA. The current from all columns has to go through Gnd.

Mike.

rjenkinsgb · Apr 23, 2019

The 74HC series can only source or sink very low currents and are not at all suitable for direct driving a LED matrix.

Looks at the MIC5841YN - that is a combination of a shift register plus high current darlington driver transistors:
https://ww1.microchip.com/downloads/en/DeviceDoc/mic5841.pdf

There is also a 5891, which is a "high side" version, switching its loads to a positive supply (anything up to 50V) rather than ground.
https://ww1.microchip.com/downloads/en/devicedoc/20005638a.pdf

Ian Rogers · Apr 23, 2019

Even a 270 ohm will get too hot.... Each led will consume 30mA (ish) 8 of them.. 240mA through a 270 ohm resistor.... No wonder there's a bit of heat..

Suraj143 · Apr 23, 2019

Hi I used 27R resistors.Not 270R.

Nigel Goodwin · Apr 23, 2019

Suraj143 said:
Hi I used 27R resistors.Not 270R.

Which is even worse, the shift register isn't a high power driver, you can't stick that much current through it.

As already mentioned, you normally use a ULN2003a or similar to drive the LED's

ChrisP58 · Apr 23, 2019

rjenkinsgb said:
The 74HC series can only source or sink very low currents and are not at all suitable for direct driving a LED matrix.

Looks at the MIC5841YN - that is a combination of a shift register plus high current darlington driver transistors:
https://ww1.microchip.com/downloads/en/DeviceDoc/mic5841.pdf

There is also a 5891, which is a "high side" version, switching its loads to a positive supply (anything up to 50V) rather than ground.
https://ww1.microchip.com/downloads/en/devicedoc/20005638a.pdf

Nice parts. But the ~2.3 V saturation voltage on the MIC5891 might make it a no-go for use on 5V rails.

rjenkinsgb · Apr 23, 2019

ChrisP58 said:
But the ~2.3 V saturation voltage on the MIC5891 might make it a no-go for use on 5V rails.

We use them for 24V system outputs, I'd not consider them for as low as 45V.

The 5841 outputs are fine for things at 5V etc., no different to any other general darlington driver output.

Mike - K8LH · Apr 23, 2019

How about the TPIC6C595 8-bit serial-to-parallel sinking driver IC?

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