sc643 Semtech Corporation, sc643 Datasheet - Page 13

• Manufacturer Part Number: sc643
• Description: Light Management Unit With 4 Ldos And Sempulse Interface
• Manufacturer: Semtech Corporation

Applications Information (continued)
Backlight Quiescent Current
The quiescent current required to operate all fi ve back-
lights is reduced when backlight current is set to 4.0mA
or less. This feature results in higher effi ciency under light-
load conditions. Further reduction in quiescent current
will result from using fewer than fi ve LEDs.
Main and Sub Backlight Bank Confi guration
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 diff erent settings so diff erent 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
fi ve 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 defi ned 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
Register 09h contains bits that control the fade state of
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 specifi ed 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 defi ned 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.
Fade-In from Off State
When the initial state of the main or sub backlight current
register is 00h (the data value for 0mA), fading to an on
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 specifi ed in the Main/Sub Backlight Fade register
(09h). This procedure must be followed regardless of
which backlight grouping confi guration 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.
Figure 1 — State Diagram for Fade Function
Fade=0
new bright
Immediate
change to
Write FADE=0
level
No change
change
No
Fade=1
Write
Fade
ends
fade using
FADE=0
Continue
new rate
FADE=0
Write
processing
FADE=0
FADE=1
Fade
FADE=1
FADE=1
new fade
Write new
Write
Write
bright
rate
level
begins
(1)
Fade
change
Note:
(1) When the data in backlight
enable register 00h is not 00h
change to new
FADE=1
bright level
value from current
Fade is redirected
bright level
Write new
Write new
bright level
No
Write
Immediate
toward the new
state
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