SC657ULTRT Semtech, SC657ULTRT Datasheet - Page 11

• Manufacturer Part Number: SC657ULTRT
• Description: Backlight Driver For 5 LEDs With SemPulseTM Interface
• Manufacturer: Semtech

Applications Information (continued)
Backlight Quiescent Current
The quiescent current required to operate all backlights is
reduced when the 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 the maximum number of
LEDs.
LED Banks
The LEDs can be grouped in up to three independently
controlled LED banks. Using the SemPulse interface, the
five LED drivers can be grouped as described in the
Backlight Grouping Configuration subsection. The banks
can be used to provide up to three different current
options. This can be useful for controlling keypad, display,
and auxiliary backlight operation from one SC657 device.
The LED banks provide versatility by allowing backlights
to be controlled independently. For example, applications
that have a main and sub display may also need to supply
an indicator LED. The three bank option allows the SC657
to control each function with different current settings.
Another application involves backlighting two displays
and a keypad, each requiring different brightness settings.
A third scenario requires supplying different brightness
levels to different types of LEDs (such as RGB) to create
display effects. In all applications, the brightness level for
each LED can be set independently.
Backlight Fade-in / Fade-out Function
The SC657 contains bits that control the fade state of the
main bank. 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 settings at the fade rate
specified by the bits in register 04h. 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 04h. This
process applies to the main display only.
The fade rate may be changed dynamically when a fade
operation is active by writing new values to the fade reg-
ister. When a new backlight level is written during an
ongoing fade operation, the fade will be redirected to the
new value from the present state. An ongoing fade opera-
tion may be cancelled by disabling fade which will result
in the backlight current changing immediately to the final
value. If fade is disabled, the current level will change
immediately without the fade delay.
The state diagram in Figure  describes the 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 backlight current regis-
ter is 00h (the data value for 0mA), fading to an on 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 Fade register (04h). This procedure
must be followed regardless of which backlight grouping
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=0
fade using
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
toward the new value
bright level
bright level
Write new
Write new
bright level
No
Write
Immediate
Fade is redirected
from current state