SC644 datasheet(13/26 Pages) SEMTECH

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SC644

Applications Information (continued)

Main and Sub Backlight Bank Configuration

The six LED backlight drivers can be configured as a single

bank or as two independent banks — one dedicated for

a main display and the other for a sub display. This feature

allows the device to drive two sets of LEDs with different

settings so different current and fade settings can be

used.

The Register Map contains two separate control registers

for main and sub currents. Register 0h contains the

current setting code for the main bank, and register 02h

contains the setting code for the sub bank. There are also

three bits in register 0Ah that control which drivers are

assigned to each display. The default setting assigns all

six LED drivers to the main display control register. In this

scenario, the current control settings for each LED driver

come from register 0h. Other settings are available that

allow the groupings to be defined so that any number

from to 6 drivers can be grouped as the main display

backlight drivers, with the remaining drivers assigned to

the sub display backlight by default. See Table 8 of the

Backlight Fade-in and Fade-out Function

each display (main and sub). When enabled, the fade

function causes the backlight settings to step from their

current state to the next programmed state as soon as

the new state is stored in its register. For example, if the

backlight is set at 25mA and the next setting is the off

state, the backlight will step from 25mA down to 0mA

using all 29 settings at the fade rate specified by the bits

in register 09h. The same is true when turning on or

increasing the backlight current — the backlight current

will step from the present level to the new level at the

step rate defined in register 09h. This process applies for

both the main and the sub displays. The state diagram in

Figure describes all possible conditions for a fade opera-

tion. More details can be found in the Register Map

section.

Write FADE=0

Write new

bright le�el

FADE=0

No change

Write

FADE=0

FADE=1

Fa�e

proce��ing(1�

Write new

bright

le�el

Write new

bright le�el

Fa�e i� re�irecte�

towar� the new

�alue from current

�tate

Fa�e

begin�

Fa�e

en��

Write

FADE=1

FADE=0

Imme�iate

change to

new bright

le�el

Fa�e=0

Write

Fa�e=1

change

Write

FADE=1

change

Imme�iate

change to new

bright le�el

Write

new fa�e

rate

Continue

fa�e u�ing

new rate

Note:

(1� When the �ata in backlight

enable regi�ter 00h i� not 00h

Figure 1 — State Diagram for Fade Function

Fade-In from Off State

When the initial state of the main or sub backlight current

state is accomplished by following the steps listed in

Table . Following these steps explicitly will ensure that

the fade-in operation will proceed with no interruption at

the rate specified in the Main/Sub Backlight Fade register

(09h). This procedure must be followed regardless of

which backlight grouping configuration is being used.

Note that it is only necessary to set the BLEN bits for the

main or sub display that is required to fade.

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