SC662EVB datasheet(13/30 Pages) SEMTECH

13 / 30 page

SC662

Charge Pump Efficiency

Efficiency of the charge pump is defined as

OUT

The input current is equal to the output current multiplied

by the charge pump mode plus the quiescent current I

IN =

OUT x Mode + IQ, and the output current is equal to the sum

of all backlight currents.

Mode

BLn

OUT

OUT, IOUT, VIN, IIN, IQ, and IBLn are terms from the electrical

characteristics section. “Mode” is the active boost ratio of

the charge pump, equal to 1, 1.5, or 2. Efficiency plots in

the Typical Characteristics section provide charge pump

efficiency data labeled with “Charge Pump”.

Efficiency of the power conversion to the LEDs is defined as

BLn

)

(

F1 through VF6 are the forward voltages of the LEDs. IBL1

through I

BL6 are the regulated backlight sink currents

flowing in the LEDs. Efficiency plots in the Typical

Characteristics section provide LED backlight efficiency

data labeled with “Backlight”.

Backlight Quiescent Current

The quiescent current required to operate all backlights is

reduced when each backlight current is set to 5.0mA or

less. This low-current mode feature results in improved

efficiency under light-load conditions, saving approxi-

mately 350µA of bias current. Low-current mode disables

and bypasses the internal LDO when the charge pump is

in 1x mode, connecting the LED anodes to the supply at

IN. 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

six 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 SC662 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 SC662

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 SC662 contains register bits that control the fade state

of the main bank. When enabled, the fade function causes

the main backlights to change brightness by stepping the

current incrementally until the target backlight current is

reached. Fade begins immediately after the target back-

light current is stored in its register. Fade may be enabled

for the main bank only. Sub and third banks do not fade.

In addition to the 32 programmable backlight current

values, there are also 75 non-programmable current steps.

The non-programmable steps are active only during a fade

operation to provide for a very smooth change in backlight

brightness. Backlight current steps proceed at a program-

mable fade rate of 2, 4, or 6ms.The exact length of time used

to fade between any two backlight values is determined by

multiplying the fade rate by the number of steps between

the old and new backlight values. The fade time can be cal-

culated from the data provided in Table 1 on page 15.

Figures 2 through 6 on page 16 provide additional infor-

mation about the fade process. Each figure represents

one linear segment of the overall fade range shown in

Applications Information (continued)

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