MCP 23018/23S18 for LED and Switch control

Question

I'm working on a project that involves about 200 LEDs and about 120 Switches. I'd like to use several MCP23S18 DIP chips across 2-3 buses to control everything. I'm a CS guy and just want to make sure I've got a decent idea of how this is supposed to come together.

The MCP23S18 datasheet.

Some general fact checking and advice on better ways to do this are all appreciated, I'm very new at this (for the record, this is not for class, just fun).

1) I've got LEDs for 2.2v at 20ma draw. I plan on using them in serial pairs, so the voltage drop should be 4.4 and the draw should be 20ma for each pair right?

2) According to the MCP23S18 datasheet (pg 33) the max Vin current draw is 125ma, and the max Vout is 400ma. This means I can safely power 6 pairs of LEDs with an approximate 4.4v chip input right? This means I should be able to use the remaining 11 pins an inputs as long as they are in the open drain configuration, thus avoiding significant draws right? Will it be safe to run all LEDs and have all switches open at the same time (assuming ample power supply)? What is the difference between "current sunk" and "clamp current"?

3) Does anyone have any ideas for a better alternative way to drive multiple leds? I know for some areas of the project that have groups of LEDs it will probably be easier to make a few cheap drivers so I can use all 16 pins as LED control. I've also considered using shift registers for this because I've found some with a 500ma Vin max, I could drive several easily this way. Perhaps a combination of the two, controlling the SR via an IO expander?

Answer 1

1. Yes. LEDs in series share the current, divide the voltage. Choose the resistor based on the combined Forward Voltage of the series LEDs.

2. Vout is not the right term. It's Vss, or Gnd. But yes, 400 mA is the max current that can go through the Ground pin, through the Open-Collector Outputs. 6 LED strings, at 20 mA each is only 120 mA, only a quarter of the max. A resistor is required per string. Trying to just match the voltage is not good.

3. The inputs are high impedance and not open-drain. They should only draw microamps, or fraction of a milliamp.

4. All 6 leds and 10 inputs at the same time, sure, as long as you keep within the specs.

5. The Clamp currents are the max current that can go through the clamp diodes, at a voltage higher than VDD or lower than VSS. Not relevant to your situation.

6. Current Sunk means current flowing into the device, typically through the open-drain output, through VSS/Gnd. Current Sourced is from VCC, through the voltage high output.

7. You can do up to 400mA, if your Power Dissipation is under 700 mW, so 500mA isn't significantly better. At 25 mA max per channel, when you want 20mA, you can already do 16 channels of 2 leds, per MCP. A shift register and a SPI device like the MCP23S18 are functionally quite similar as is. As for 500 mA through VIN, the difference would be wiring the in a common anode (To V+) or common cathode (To Gnd) manner. If you are using regular two pin LEDs, then that's not really a concern.

Typical Setup for LEDs:

Answer 2

These chips are rated to drive up to 25 mA per pin (times 16 pins = 400 mA max output current, as you mentioned), so they'll be able to pull 20 mA of current through your LED(s) without any problems. You'll be able to use all 16 channels simultaneously, as well.

I would ignore that 125 mA figure for maximum Vdd input current; it honestly makes no sense; since the chip has open-drain outputs, no appreciable current will flow into VDD (other than the rated 1 mA of current the device will pull during normal operation, as reported in the DC characteristics section of the datasheet). I suspect it might be a bad copy-paste job from another Microchip I/O port expander that has push-pull outputs.

...Having said all that, this chip has no intelligent current control; it's not designed to drive constant-current devices such as LEDs (unless you want to drive them at the current-limiting capabilities of the chip -- 25 mA). Because of this, you'll have to use series drop resistors connected to each channel, and any variances in the LEDs' current-voltage relationships will cause different brightness values across the LEDs.

Instead, I would recommend you use a true constant-current LED driver. The TLC59282 from Texas Instruments is similar to the MCP23018 in open-drain functionality, but it is less expensive and has a single IREF pin to set the current for all outputs.

The CAT4016 is a similar ON Semi part. The cheapest option is probably the ST ST16CPC26, which is similar to the other two parts mentioned.

But why stop there? Add in individual PWM brightness control with the infamous TLC5944, or the much more user-friendly (and easy-to-use) ISSI IS31F3216, or the venerable PCA9685, made by NXP. I prefer the NXP and ISSI solutions, as they have an i2C connection with internal PWM clock generation; the TI part requires a more "baby-sitting" from the MCU.

Aren't integrated circuits great? All of these parts are widely-available from your neighborhood electronic component distributor -- good luck!