In the following figure, the LED array consists of three common anode LEDs connected together.
The PWM signal created by the controller is amplified by a transistor prior to being applied to the array. The amplitude of 5V is too low to drive the array directly. If the array size is increased, additional paths must be connected to the multiplexer and additional transistors may possibly be required for amplification.
This circuit is from an OSRAM App Note: Driving LEDs with a PIC Microcontroller