SC643EVB datasheet(13/26 Pages) SEMTECH

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SC643

Applications Information (continued)

Backlight Quiescent Current

The quiescent current required to operate all five back-

lights is reduced when backlight current is set to 4.0mA

or less. This feature results in higher efficiency under light-

load conditions. Further reduction in quiescent current

will result from using fewer than five LEDs.

Main and Sub Backlight Bank Configuration

The five LED backlight drivers can be configured as a

single bank or as two independent banks — one dedi-

cated 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 set-

tings can be used.

The Register Map contains two separate control registers

for main and sub currents. Register 01h 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

five LED drivers to the main display control register. In

this scenario, the current control settings for each LED

driver come from register 01h. Other settings are avail-

able that allow the groupings to be defined so that any

number from 1 to 5 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 Register Map section for more details.

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 1 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 level

FADE=0

No change

Write

FADE=0

FADE=1

Fade

processing(1)

Write new

bright

level

Write new

bright level

Fade is redirected

toward the new

value from current

state

Fade

begins

Fade

ends

Write

FADE=1

FADE=0

Immediate

change to

new bright

level

Fade=0

Write

Fade=1

change

Write

FADE=1

change

Immediate

change to new

bright level

Write

new fade

rate

Continue

fade using

new rate

Note:

(1) When the data in backlight

enable register 00h is 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 1. 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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