SC668 datasheet(22/46 Pages) SEMTECH

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SC668

Applications Information (continued)

Protection Features

The SC668 provides OT (Over-temperature) protection

and LDO current limiting to safeguard the device from

catastrophic failures.

Over-Temperature Protection

The OT protection circuit prevents the device from over-

heating and experiencing a catastrophic failure. When the

junction temperature exceeds 165°C, the device goes into

thermal shutdown with all outputs disabled until the junc-

tion temperature is reduced. All register information is

retained during thermal shutdown. Hysteresis of 30°C is

provided to ensure that the device cools sufficiently before

re-enabling.

LDO Current Limit

The device limits current at each LDO output pin. The

typical limit is 400mA, with a minimum limit rating of

200mA. The LDOs may be used for up to 200mA without

tripping the current limit.

Thermal Management

A junction temperature calculation should be performed

for each new application design to ensure the device will

not exceed 125°C during normal operation. The first step

is to determine how much power can be dissipated by the

SC667 in the application. The following formula approxi-

mates the maximum dissipation. This formula can be used

to sum the maximum internal power dissipation required

of each LDO and each backlight sink.

BLm

LDOn

)

(

The resulting power dissipation can then be used in the

calculation for maximum junction temperature.

where,

A = Maximum ambient temperature rating in °C.

JA= Thermal resistance, from junction to ambient, equal

to 35°C/W for a optimum circuit board layout.

General Purpose ADC

The ADI pin may also be used for general purpose ADC

functions. For example, a linear temperature sensor may

be added to the application circuit, and the SC667 may

provide temperature data or an over-temperature warning

flag. In this case, registers 13h, 14h, and 15h can be used

to store the ADC reading and set thresholds that will

trigger and interrupt output if the reading does not remain

between them.

Programmable LDO Outputs

Four LDO (low dropout) regulators are included to supply

power to peripheral circuits. Each LDO output voltage

setting has ±3.5% accuracy over the line, load, and operat-

ing temperature ranges. Output current greater than

specification is possible at somewhat reduced accuracy

(refer to the typical characteristic section of this datasheet

for load regulation examples). LDO1, LDO3, and LDO4

have identical specifications, with a programmable output

ranging from 1.5V to 3.3V. LDO2 is specified to operate

with programmable output ranging from 1.2V to 1.8V. All

of the LDOs are low noise and can be used with noise sen-

sitive circuits.

LDO4 is internally connected to the ADC (Analog to Digital

Converter) to provide the reference voltage for the ADC.

LDO4 must be enabled for the ADC to function. When the

ALS function is used, LDO4 may also be used to provide

power to the external ALS circuit.

Shutdown Mode

The device is disabled when the EN pin is held low for the

shutdown time specified in the electrical characteristics

section. All registers are reset to default conditions at shut-

down. Typical current consumption in this mode is 0.1µA.

Sleep Mode

Sleep mode is activated when all backlights are off. This is

a reduced current mode that helps minimize overall

current consumption. In sleep mode, the I2C interface

continues to monitor its input for commands from the

host processor. All registers retain their settings in sleep

mode. Typical current consumption in this mode is 90µA.

SC668 Datasheet(PDF) 22 Page - Semtech Corporation

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