SC658 datasheet(11/22 Pages) SEMTECH

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Applications Information (continued)

SC658

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

four 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 SC658 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 SC658

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 SC658 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.

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 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

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