herunterladen
Maxim > Design Support > Technical Documents > Application Notes > Microcontrollers > APP 2199
Keywords: LED driver, LED driving, push-pull, push pull, totem pole, GPIO, port expander
APPLICATION NOTE 2199
Driving LEDs with Push-Pull Port Expander
Outputs
Sep 22, 2003
Abstract: This application note discusses techniques for driving LEDs, using the MAX7300, MAX7301,
MAX7310, MAX7311, and MAX7312 I2C and SPI I/O expanders (GPIOs). These techniques can be
applied to other I/O expanders with push-pull outputs, as well as other ICs with standard CMOS logic
outputs, using bus drivers such as the 74HC24x and 74HC54x for high current drive.
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 the 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 or R2 = (V+ - V
PORT
- V
LED
) / I
PORT
ohms
where V+ is the I/O expander and 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 MAX7310)
Page 1 of 10
