SC643 datasheet(15/26 Pages) SEMTECH

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SC643

Applications Information (continued)

Shutdown Mode

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

the shutdown time specified in the electrical characteris-

tics section. All registers are reset to default condition at

shutdown. Typical current consumption is this mode is

0.1μA

Sleep Mode

When all backlights are off the charge pump is disabled,

and sleep mode is activated. This is a reduced current

mode that helps minimize overall current consumption.

In sleep mode, the SemPulse interface continues to

monitor its input for commands from the host. Typical

current consumption in this mode is 90μA.

Protection Features

The SC643 provides several protection features to safe-

guard the device from catastrophic failures. These features

include:

Output Open Circuit Protection

Over-Temperature Protection

Charge Pump Output Current Limit

LDO Current Limit

LED Float Detection

Output Open Circuit Protection

Over-Voltage Protection (OVP) at the OUT pin prevents

the charge pump from producing an excessively high

output voltage. In the event of an open circuit between

the OUT pin and all current sinks (no loads connected),

the charge pump runs in open loop and the voltage rises

up to the OVP limit. OVP operation is hysteretic, meaning

the charge pump will momentarily turn off until V

OUT

sufficiently reduced. The maximum OVP threshold is 6.0V,

allowing the use of a ceramic output capacitor rated at

6.3V with no concern of over-voltage damage. Typical OVP

voltage is 5.7V.

Over-Temperature Protection

The Over-Temperature (OT) protection circuit prevents the

device from overheating and experiencing a catastrophic

failure. When the junction temperature exceeds 165°C, the

•

device goes into thermal shutdown with all outputs dis-

abled until the junction temperature is reduced. All

Hysteresis of 30°C is provided to ensure that the device

cools sufficiently before re-enabling.

Charge Pump Output Current Limit

The device limits the charge pump current at the OUT pin.

When OUT is shorted to ground, the output current will

typically equal 300mA. The output current is also limited

to 300mA when over loaded resistively.

LDO Current Limit

The device limits the current at all LDO output pins. The

minimum limit is 200mA, so load current of greater than

the rated current can be used (with degraded accuracy)

without tripping the current limit.

LED Float Detection

Float detect is a fault detection feature of the LED back-

light outputs. If an output is programmed to be enabled

and an open circuit fault occurs at any backlight output,

that output will be disabled to prevent a sustained output

OVP condition from occurring due to the resulting open

loop. Float detect ensures device protection but does not

ensure optimum performance. Unused LED outputs must

be disabled to prevent an open circuit fault from

occurring.

Thermal Management

The device has the potential for peak power dissipation

equal to 2.7W when all outputs are simultaneously oper-

ating at maximum rated current and powered by a fully

charged Li-Ion cell equal to 4.2V. A calculation of the

maximum power dissipation of the device should be done

to identify if power management measures are needed to

prevent overheating. The MLP package is capable of dis-

sipating 1.85W when proper layout techniques are used.

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