SC668 datasheet(13/46 Pages) SEMTECH

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SC668

General Description

The SC668 is optimized for handheld applications sup-

plied from a single cell Li-Ion and includes the following

key features:

Eight matched current sinks — BL1, BL2, BL3,

BL4, BL5, BL6, BL7, and BL8 regulate LED back-

lighting current, with 0mA to 25mA per LED.

Four adjustable LDOs — LDO1, LDO3, and LDO4

are adjustable with 15 settings from 1.5V to 3.3V.

LDO2 is adjustable with 7 settings from 1.2V to

1.8V.

ALS with a sigma-delta ADC that can also be used

for general purpose ADC functions.

PWM with an internal digital low-pass filter

I2C Bus fast mode and standard mode

LED Backlight Current Settings

The backlight current is set via the I2C interface. The current

is regulated to one of 32 values between 0mA and 25mA.

The step size varies depending upon the current setting.

The first three steps are 50µA, 100µA, and 200µA. Between

0.5mA and 5mA, the step size is 0.5mA. The step size

increases to 1mA for settings between 5mA and 21mA.

Steps are 2mA between 21mA and 25mA. The variation in

step size allows finer adjustment for dimming functions in

the low current range and coarse adjustment at higher

current settings where larger changes are not visible. The

settings are psuedo-logarithmic. A zero setting also disables

the current sink, providing an alternative to the enable bit.

LED Backlight Current Sinks

Backlight current is independent of forward voltage mis-

match (∆V

F) between LEDs. When two or more backlight

sinks are set to the same target current, their currents will

match, even if the LED voltages are different. The backlight

current sinks are designed with a low dropout voltage (typi-

cally 59mV for a bank of 6 LEDs at 20mA) to optimize run-time

when the LED anode voltage is provided by a battery.

LED Anode Supply

In the typical application circuit, the battery voltage sup-

plies the LEDs. An alternative to this configuration is to

connect the LED anodes to a second DC supply as shown

in Figure 1. Such a connection is especially useful when

an alternate voltage that is slightly higher than the

•

forward voltage of the LEDs is available. The resulting

efficiency in this scenario would be optimal. To achieve

best accuracy, the current sink amplifier requires the

LED sink pin (BLn) to be within the operational range of

DO ≤ VBLn ≤ 4.2V. When the sink is off, VBLn may float as

high as 5.5V.

SC668

GND

BL3

BL7

BL1

CIN

BL2

BL4

BL5

BL6

VIN

VBL1

VBL2

VBL3

VBL4

VBL5

VBL6

VBL7

BL8 VBL8

Figure 1 — Anode Supply

Unused Backlight Current Sinks

The backlight LEDs default to the off state upon power-

up. For backlight applications using fewer than 8 LEDs,

any unused output must be left open or grounded and

the unused LED must remain disabled. When writing to

the backlight enable register, a zero (0) must be written

to the corresponding enable bit of any unused output.

Backlight Quiescent Current

The quiescent current required to operate the backlights

is reduced when backlight current is less than 8.0mA. This

feature results in higher efficiency under light-load con-

ditions. Further quiescent current reduction will result

from using fewer LEDs.

Backlight Configuration into Banks

The eight LED backlight drivers can be assigned to a single

bank or divided among up to four independent banks —

refer to the Register Map section for more details. The

independent banks can each be configured with different

settings for backlight current and fade operation.

Bank Configuration into Groups

The four backlight banks can be assigned to two groups

(group #1 and group #2). Each group provides indepen-

dent settings for the fade and breathe effect rate options.

Each group also provides independent settings for target

time and start time, which are used to customize the

Applications Information

SC668 Datasheet(PDF) 13 Page - Semtech Corporation

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