WM8255 datasheet(31/49 Pages) CIRRUS

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WM8255

Production Data

PD, Rev 4.7, August 2013

LEDRFLAG and subsequently ILIMITFLAG will be set and LED current will be reduced to: 68 mA * (1-

0.25) = 51 mA

This is within the safe operating area of the LED to be driven.

In the case of a safe operating area test fail, the LED current will be reduced by 25% to give a

maximum limit of up to 39.75 mA

Should this LED current drive be insufficient during operation it may be calibrated until a target is met.

The algorithm used to control the calibration of the LED DAC current is user specific but it has access

to the LED Flags, ILIMITFLAG and the LED current DAC register values.

A binary incremental or binary weighted search may be used to increase the LED DAC current to the

absolute current maximum limit. An indication of how to perform a binary incremental search is

mentioned below.

A SUGGESTED BINARY INCREMENTAL SEARCH IMPLEMENTED BY THE

USER

When the LED maximum current is detected, the LED current can be trimmed back by either 12.5%

or 25%, depending on ILIMITDEC. At 25% this will guarantee the LED current is below the LED DAC

absolute maximum current limit. The LED current can then be incremented by an LSB of the LED

current register. The LED current will continue to be incremented until ILIMITFLAG is again set.

Figure 24 shows this process graphically.

The LSB of the current is 0.4% of the full LED current DAC full scale range. As a result the LED

current DAC may trim the LED current drive to within +/-0.2% of target.

If the coarse LED current limit is 35mA to 53mA when LEDIMAX=1, this means the trip point accuracy

will be:-

Min. Limit – ((Min. Full Scale Range/255)/2) >> Max. Limit + ((Max. Full Scale Range/255)/2)

In this case LEDRNG = 11 so Min. FSR is 50 and Max FSR is 68.

LEDIMAX=1 so Min. Limit is 35 and Max. Limit is 53.

Therefore:-

35 - ((50/255) /2) >> 53 + ((68/255) /2) gives

35 - (0.196/2) >> 53 + (0.266/2)

= 34.9 >> 53.13

or 44 +/-20.75%.

During the binary incremental search, the LED current will continue to be incremented until

ILIMITFLAG is again set. At this point it is desirable to disable the reduction of the LED current.

Different options to disable the reduction of current can be achieved through the register bit ILIMITEN.

Refer to Table 4 in the Blank Period Section.

WM8255 Datasheet(PDF) 31 Page - Cirrus Logic