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Maxim > Design Support > Technical Documents > Application Notes > Microcontrollers > APP 2198
Keywords: LED driver, LED driving, open drain, GPIOs, port expander, LEDs, i/o expanders
APPLICATION NOTE 2198
Driving LEDs with Open Drain Port Expander
Outputs
Sep 12, 2003
Abstract: This application note discusses techniques for driving LEDs with the MAX6964, MAX6965,
MAX7313, MAX7314, MAX7315 and MAX7316 I/O expanders (GPIOs). These techniques can be applied
to other I/O expanders with open-drain outputs, as well as other ICs with open-drain logic outputs.
(Types like the 74HC06 and 74HC07 can be used for high current drive applications).
The Basics
The standard connection for driving an LED from a port is shown in Figure 1. The load is typically a
single LED, but can be dual LEDs in series (as shown), depending on choice of LEDs and supply
voltage. The series resistor R1 is necessary to limit the current through the LED. The value of resistor
R1 required to drive a current I
PORT
through the LED can be calculated using the formula:
R1 = (V
EXT
- V
PORT
- V
LED
) / I
PORT
Ω
where V
EXT
is the LED supply voltage
V
LED
is the voltage drop across the LED(s) at the required load current (usually in the range 1.8V to
2.4V for red LEDs, and 3V to 4.2V for blue, white, and high efficiency green LEDs)
V
PORT
is the voltage drop across the output port when sinking the required load current (for example
0.25V at 20mA for MAX6964)
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