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Topic: What is the impact of running a MAX7219 at higher voltages than recommended? (Read 874 times) previous topic - next topic

AZWoodWorker

Background:  I'm developing a custom clock.  The display will be in a 7 segment format using LEDs in a frame that I've 3d printed.  I'm using a Wemos D1 Mini to connect to the NTP servers to pick up the time and a MAX7219 to run the LEDs in the 7 segment arrays.  At the moment I'm using 3 red LEDs in each segment due to the size of the clock.

The MAX7219 is well suited for running two LEDs in series for each segment, but when you get to three LEDs (or more), the voltage required becomes higher than the 5V maximum (and 6 volt extreme maximum) limit on the datasheet.

I'd like to run the segments at 7.5 volts (and more if I decide to make the segments have 4 LEDs per segment).  I've seen a few solutions that facilitate this, and I set up a circuit with this solution, but I was getting pretty visible ghosting from digit to digit, and I was unable to find any combination of pull-up or pull-downs that would eliminate it.

I've currently installed a voltage level shifter between the D1 Mini and the MAX7219, and I'm running the MAX7219 and the LEDs at 7.5 volts.  This is obviously outside the recommended voltage range, but it is within the max power range.  I'm wondering if anyone has run a MAX7219 at higher than recommended voltage and what the results were, or whether someone knows what the potential failure mode would be if I did so.  My guess is that the MAX7219 would experience a reduced lifespan, but I'm not sure. 

Any opinions?

markd833

I've only skimmed it but this app note may help you out.

It's a Maxim app note for the MAX7219 called: "Using the MAX7219/7221 to Drive Higher Voltage or Current".

vaj4088

Using any part beyond its Absolute Maximum ratings is quite risky.  The app note suggested has ways to drive LEDs that do not involve exceeding the ratings.


srnet

Were you expecting someone in the forum to say;

"Sure you can ignore the manufacturers data sheet, they dont know what they are talking about and its perfectly safe and reliable to exceed what they recommend"

AZWoodWorker

Were you expecting someone in the forum to say;

"Sure you can ignore the manufacturers data sheet, they dont know what they are talking about and its perfectly safe and reliable to exceed what they recommend"
In a word, possibly.

Here's why.  Any component will have a set of parameters above which it always fails.  Below those levels, the life span of a component is determined by the combination of inputs, some variability in the manufacturing process, and the environment in which it's operating.  In order to provide a component with long life spans, the manufacturer will provide recommended ratings that should deliver an extended lifespan.  Above the recommended values you're just playing with lifespan.

I was hoping that perhaps someone had tried the same thing I proposed and had found it to work; alternatively, perhaps someone had tried it and found that the IC failed after a week at a given voltage.  Either way, by asking the question, and having someone reply who had similarly experimented previously, the knowledge of the community grows.

vaj4088

The knowledge of the community grows for that one part.  Manufacturing variations occur across a wafer.

Exceeding Absolute Maximums could result in a life span measured in nanoseconds.  I am happy for you that you exceeded this.


CrossRoads

If asked, I would have suggested using a TPIC6B595 or TPIC6C595 to sink the current for each segment.
50V rated, 150mA and 100mA rated outouts.
I have a board with a '328P and 12 TPIC6B595 chips that can drive up to 12 digits.
Use SPI and shift data into them.

Here's a video of a display running a sped-up clock test:
https://www.youtube.com/watch?v=6HZ0Mr51jUY

Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

PaulRB

@Crossroads, your link is broken, I can't fix it.

+1 for the tpic6b595 suggestion. That's how my Wemos/NTP clock works. I used cut lengths of 12V led strip to make my segments. Ok, you need 1 chip per digit, but in a clock with large digits, space is probably not a problem. Plus this type of clock will often need to be seen from a distance, possibly in bright daylight, so not multiplexing, resulting in a brighter display, is another advantage. You can dim the displays if needed by connecting an arduino pwm pin to the output enable pins of the tpic chips.

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