Maxim 7221 LED Driver Circuit Help: Driving Higher Voltage Displays

[Rocky77]

I need a little assistance sorting a couple of things out. I want to build an LED scoreboard and I am trying to figure out the circuitry to actually drive the LED's with a MAXIM 7221 Display Driver.

It's a pretty straightforward problem with smaller 7 segment LED's because they don't require a higher voltage than the MAX7221 can display. But I want to use larger (2.3") LED's for my display for visibility. There is an app note for the MAX7221 that describes a circuit for driving higher voltage displays from the MAX7221 but it's not clear to me if I need 1 of the circuits or 2.

First, here's some info
http://datasheets.maxim-ic.com/en/ds/MAX7219-MAX7221.pdf"
http://www.maxim-ic.com/app-notes/index.mvp/id/1196"
http://www.futurlec.com/LED/7SR23011BS.shtml" .

I'll try to outline what (I think) I know and hopefully someone can tell me if I am thinking correctly.

1) The datasheet for the MAX7221 specifically addresses driving a 2.3" LED (on page 10) but seems to ignore the fact that the LED requires 8V.

2) The app note addresses using the additional circuits to drive higher voltages and/or currents than the MAX7221 can provide, but I don't think I fully understand the implications of the app note. Here is what I am thinking...

a) I need to drive higher voltage LED's. I can use the circuit if FIG 3 of the app note with a 9V supply to get the desired voltage to the anode.

b) Because the LED forward current is only 25mA, I don't need to boost the current to the segment drivers (as shown if FIG 4 of the note) (Is this correct?)

c) Because I am using the circuit in FIG 3, I have to use a common anode LED instead ot the common cathode LED that the MAX7221 is designed to drive. (This comes directly from the app note).​

3) I am not 100% clear on how the 7221 drives the LED's. Assuming a 5v LED that doesn't require additional driver circuits...The anode would be at +5V and the cathode at GND to light up a segment...now when a segment is turned off...does the anode go to GND or does the cathode go to +5v?

 

[vk6kro]

Common cathode drivers can't directly drive common anode displays.

This type of driver produces an output voltage which drives a LED between this output and ground.

The LED display is connected from a power source to the output of the common anode driver and it is lit when the driver output becomes a low resistance path to ground.
This is called "current sinking".

So, you can see that the two types of driver operate in completely different ways.

It is probably possible to use either type to drive the other kind of display, by using extra components, but it is much better to just use the correct type of display and driver.

 

[Rocky77]

Common cathode drivers can't directly drive common anode displays.

This type of driver produces an output voltage which drives a LED between this output and ground.

The LED display is connected from a power source to the output of the common anode driver and it is lit when the driver output becomes a low resistance path to ground.
This is called "current sinking".

So, you can see that the two types of driver operate in completely different ways.

It is probably possible to use either type to drive the other kind of display, by using extra components, but it is much better to just use the correct type of display and driver.

Thanks for the reply vk.

I understand the distinction between CC and CA drivers but the real issue here is that the device that I am using can only drive up to 5V. The app note gives examples on driving higher voltage LED's (which is what I want to do) but you have to insert some additional circuitry. The circuitry added inverts the drive polarity (requiring one to use a CA LED for this CC driver).

I don't think there is a device that can drive multiple 7 segment 8V LED's directly so I have to apply the technique in the app note but it's not completely clear to me which of those circuits I need to use. I know I need one of the Digit Driver circuits, but it's not clear to me if/why I need the Segment Driver circuit (Fig 4 in the app note).

 

[vk6kro]

The chip can only supply 5 volt output because it comes from the chip's own power supply. This can't be more than 5 volts so the output can't be more than 5 volts either.

None of the application notes are for common anode displays.

The output of this chip is sufficient to turn on the base of a transistor (via a suitable base resistor) and the transistor could switch whatever voltage LEDs you like.

Multiplexing means that the chip drives all the segments in parallel but only the required LED display gets a supply voltage. So, only that LED display has its segments light up.

The following diagram may give you an idea of how complex this circuit is going to get:

[PLAIN]http://dl.dropbox.com/u/4222062/common%20anode.PNG
For an 8 digit 7 segment display, you would need 8 digit drivers and 7 segment drivers. I have just shown 2 of each.

I have shown optocouplers for the digit drivers. This is because the bases of the PNP transistors would be at a voltage 0.6 volts below the display power voltage and this would exceed the ratings of the driver chip.