WM8255 datasheet(30/49 Pages) CIRRUS

30 / 49 page

Production Data

WM8255

PD, Rev 4.7, August 2013

Figure 23 PWM LED Current Control Timing - Monochromatic Scan with PWM Duty Cycle and Blanking Period

Disabled

CURRENT ACCURACY AND ABSOLUTE MAXIMUM CURRENT LIMIT

To protect the LED when operated near its maximum operating current range, an accurate absolute

maximum current limit can be set.

An external resistor connected to the EXTRES pin must be provided to generate an accurate

reference current for the LED circuit. As the current DAC is designed for low compliance voltage, a

separate higher accuracy current detection circuit is provided.

At the start of every state change the Current DAC setting for that LED is measured (Red, Green or

Blue). If the state machine is at RED and the current, LEDIDACR, exceeds the absolute maximum

current limit, a register, LEDRFLAG will be set. If ILIMITEN is set to the appropriate mode, the current

will be automatically reduced and the current retested. Should this new current be within the safe

operating area the Red LED will be enabled. The LEDRFLAG register will remain set to indicate that

the reduction has been implemented.

The next time the state machine enters the RED state, the current value is measured again. If the

current, LEDIDACR is now an acceptable value (without a reduction) the LEDRFLAG will reset. This

is the same when in the GREEN and BLUE states, where LEDIDACG and LEDIDACB are measured

respectively. Note that for the initial current test of a new state, the machine always loads the Current

DAC register setting, not a reduced value previously used.

Throughout the DAC loading and current limit testing, the LED is disabled and the current is steered

through the power supply AVDD1. This prevents stress in the LED, by ensuring that it is not enabled

until the current is within the safe operating area.

A register bit ILIMITFLAG will get set when any of the LED Flags are set. All of the Flags will be reset

when applying a LED Software Reset.

For example:

The Red LED maximum current is 53mA in this example.

LEDIRNGR = 11 which gives an absolute maximum current of 68 mA.

LEDIMAX = 1 which gives a maximum limit of up to 53 mA.

ILIMITDEC = 1 which sets reduction to 25%

ILIMITEN = 01 which enables current reduction

WM8255 Datasheet(PDF) 30 Page - Cirrus Logic