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Author Topic: Low voltage LED Matrix control  (Read 1026 times)
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StefDrums
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« on: December 08, 2017, 05:15:58 05:15 »

Hello to all,
I would like to share with you a difficulty which I have been struggling with for days:
I'm developing a circuit that controls a 3x6 led matrix display via microcontroller, here are some data:
- the LEDs are 20 mA @ 2.1 V
- the PINs of the uC provide a maximum of 1.5 mA
- 3x6 LED matrix
- the circuit must have a low power consumption
- power supply voltage: I can choose it from 2.2 V to 3.0 V (but low consumes less)

I initially thought to connect them in this way (picture attached), but I have freedom to overturn the circuit.

For the control, since the microcontroller does not supply enough current, I thought of 3 different alternatives:
- use a shift register for "Control K" and 3 P-MOS for "Control A"
- use a buffer (or a hex-inverter) for "Control K" and 3 P-MOS for "Control A"
- use a driver LED for "Control K" and 3 P-MOS for "Control A"
all 3 possibilities unfortunately lead me to a dead end, because I can not find components that can work at voltages of 2.3V and source 20 mA per channel (with a total Icc of at least 120 mA).

I've run out of ideas, I'm giving up, but I'm sure some of you will be able to suggest a brilliant solution!
Thank you!
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PICker
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« Reply #1 on: December 08, 2017, 06:27:58 06:27 »

You can think about a LED Driver IC.
MAX6958/MAX6959, for example (IC-compatible 2-wire serial interface), can drive up to 36 individual LEDs with a power voltage between 3 and 5V:
https://datasheets.maximintegrated.com/en/ds/MAX6958-MAX6959.pdf
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bogdantk
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« Reply #2 on: December 08, 2017, 06:42:36 06:42 »


http://www.instructables.com/id/Make-a-24X6-LED-matrix/
very simple to use Maxim 7219
https://www.maximintegrated.com/en/products/power/display-power-control/MAX7219.html
http://www.electronics-lab.com/project/24x6-led-matrix-control-circuit/
and this one with Arduino
I used Max7219 with monolitic LED matrix, serial daisy chain connected and controlled with a PIC small board
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vern
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« Reply #3 on: December 08, 2017, 10:40:47 10:40 »

What do you want to display, multiple LED's with multiplexing or only one led at a time?
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StefDrums
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« Reply #4 on: December 08, 2017, 11:05:00 11:05 »

Hello, I have to display multiple LED's with multiplexing.
I think I have to use the lower Vdd I can because of series resistor power consumption.
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vern
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« Reply #5 on: December 08, 2017, 11:36:45 11:36 »

if you are multiplexing your drivers need to handle higher current, because the LEDs need probably more than 20mA since they are on only 1/18th of the time.
You can use a TPL7407L on the low side and and 3x  FDC6331 or similar on the high side, they are both 2V compatible
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StefDrums
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« Reply #6 on: December 10, 2017, 04:53:28 04:53 »

Thank you vern,
so I think that with low side and high side driver my circuit will be something like this (attached picture)

Can I have some problem in switching when both low side and high side driver are turned off?
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Signal
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« Reply #7 on: December 10, 2017, 09:33:41 09:33 »

... since they are on only 1/18th of the time.
I'd say 1/3d

- the circuit must have a low power consumption
If you are not stuck with certain 6x3 LED matrix device consider new generation of LEDs - they are much effective than ones from XX century, and thus at 1 mA are brighter than old ones at 20 mA. That could be a useful option for low power consumption - to begin from choosing energy efficient device.
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PICker
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« Reply #8 on: December 10, 2017, 11:27:22 11:27 »

StefDrums defined a specific Power Supply:
- power supply voltage: I can choose it from 2.2 V to 3.0 V (but low consumes less)
in the datasheet of the MAX7219/MAX7221  you can read:
"For applications requiring 3V operation or segment
blinking, refer to the MAX6951 data sheet."
https://datasheets.maximintegrated.com/en/ds/MAX6950-MAX6951.pdf
these device are similar to MAX6958/MAX6959.
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vern
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« Reply #9 on: December 10, 2017, 01:56:57 13:56 »

Signal:
Quote
I'd say 1/3d
of course you are right, what was I thinking

PICker: although I had a recommendation for  drivers for multiplexig  I would try another solution for a led count of 18 if you are free to connect them in a different way:
I would use 3   8 bit shift registers like for example 74HC595 and and not a matrix.
The advantage is static display, low current for diving the LEDs and very simple programming. You can even control the LED Brightness with the output enble input via PWM.
And it runs with 2V.
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Signal
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« Reply #10 on: December 10, 2017, 02:33:47 14:33 »

Just to make things fun Wink

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PICker
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« Reply #11 on: December 10, 2017, 11:53:22 23:53 »

vern: I understand (and agree with) your solution.
See this similar approach with Arduino: https://diyhacking.com/arduino-led-matrix/

My last post was just about the differences between MAX7219 (proposed by bogdantk) and MAX6951 (with a lower power supply voltage).
 Smiley
« Last Edit: December 10, 2017, 11:58:44 23:58 by PICker » Logged
StefDrums
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« Reply #12 on: December 11, 2017, 11:33:34 11:33 »

I read the 74HC595 datasheet and i saw that the maximum Icc current is 70 mA, so I can't drive 6 LEDs toghether @ 20mA.
I didn't found any shift register that can source or sink more than 75 mA total.
I think that I'm going to destroy the 74HC595 with 120 mA continuos current.
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PICker
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« Reply #13 on: December 11, 2017, 11:56:56 11:56 »

Just for "not reinventing the weel" I suggest you this discussion"
http://www.avrfreaks.net/forum/high-current-versions-74hc595
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Wilksey
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« Reply #14 on: December 11, 2017, 06:58:03 18:58 »

Don't drive them direct with a shift register then! Use the shift register to drive a transistor for example.
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vern
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« Reply #15 on: December 12, 2017, 02:12:13 02:12 »

Of course you have to use drivers, the 74HC595 will run with 2V but will not be able to sink 20mA. Recommendation by TI is 5.2mA per output.
You should use transistors or mosfets with a low gate voltage.
But in my experience 5mA is plenty with modern LED's, most of the time even to much.
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Signal
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« Reply #16 on: December 12, 2017, 09:38:43 09:38 »

Important note. One can not rely only on absolute maximum ratings regarding output current.
VOL and VOH significantly depends on power supply voltage especially at current levels near to maximum and supply voltage near to minimum. It could end with current significantly lower than expected and/or VOL significantly higher than expected.


Reducing supply voltage to leave less for current limiting resistor can cause this resistor to be ineffective in his job of handling changes of supply voltages, temperature, variation of parameters.
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