SC441AEVB datasheet(12/20 Pages) SEMTECH

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SC441A

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Applications Information

SC441A Detailed Description

The SC44A contains a high frequency, current-mode

boost regulator and four programmable current sources.

The LED current source value is set using an external

resistor while the PWM controller maintains the output

voltage at a level keeping the current regulated through

the LEDs. Since the SC44A receives feedback from all of

the LED current sources, all LED strings can be turned on

at any given time. A typical application would use 3-0

backlight LEDs for each string, driven up to 50mA.

Operation

The SC44A controls the boost converter set point based

on instantaneous requirements of four current sources.

Therefore, only a single inductor and power switch is

needed to provide power to the entire lighting subsystem,

increasing efficiency and reducing part count. A digital

interface to output control is high-bandwidth, supporting

digital PWM dimming at 50Hz to 50kHz dimming

frequency, while aggressively shutting the entire supply

current down to 3mA (typical), when all LED strings are

turned off.

High frequency switching provides high output power

using a tiny .0mm high inductor, maximizing efficiency

for space-constrained and cost-sensitive applications.

Additionally, both converter and output capacitor are

protected from open-LED conditions by over-voltage

protection.

LED Current Programming

The SC44A features programmable LED current

regulators. The LED current set points are chosen using

external resistors tied to the IOSET pin. The relationship

between the programming resistor value and the output

current set point can be described as follows:

IOSET = (0.26V) / ILED

Where R

IOSET is in kΩ. ILED is the LED current in Amperes.

The four output channels have the same output current.

Start-Up

During start-up, when the VIN pin voltage reaches its

UVLO threshold and both the EN and PWM signals are set

to high, the SS pin begins to source 4.5µA as its voltage

begins to rise from 0V to its end value of 2.5V. The output

voltage of the internal trans-conductance error amplifier

(COMP), increases and clamps to the SS pin voltage. When

the SS pin voltage reaches its switching threshold, output

voltage increases. Proper decoupling is required on the

VIN pin, especially for a lower input voltage condition. A

22µF, 6.3V rated X5R ceramic capacitor is recommended

for a 5V input system.

The internal LED current source (IO ~ IO4) helps to

regulate the LED current to its set point while the output

voltage increases; a suitable amount of error information

will be generated on the internal error amplifier.The COMP

pin voltage keeps rising and once the LED current reaches

its set point, the error information will not be generated

by the LED current source. The COMP pin voltage stays

level while keeping the LED current in its set point.

If the EN pin voltage is pulled below 0.4V, the SC44A will

stay in shutdown mode drawing less than µA from its

input power supply.

During the normal operation, when PWM pin is pulled

below 0.4V, the device operates in standby mode,

drawing 3mA (typical) current from the input. Under this

condition, since the EN pin is pulled high, soft-start is

initiated and the SS pin voltage is raised to its end value.

Following this, when the PWM signal goes high to enable

the SC44A, the COMP pin voltage will rise quickly since

it is not limited by the SS pin. A proper capacitance (0nF

~ 00nF) is required to prevent output voltage overshoot

on the COMP pin and its external RC network.

Shut Down

If the VIN pin voltage falls below its UVLO, or the voltage

on the EN pin goes low, the device will run in shutdown

mode as the internal switch and the LED current sources

will immediately turn off. The SS capacitor is discharged

by the internal current source of the SS pin. The SS pin

voltage decreases to 0V while the output voltage falls to

the same level as the input voltage.

If the PWM pin voltage goes low while SC44A is in

normal operation, then the SC44A will run in standby

mode. The Internal switcher and the LED current source

will immediately turn off.

NOTE–

The PWM signal does not affect the SS pin nor its final value.

Main Power Operation

SC44A is a 700KHz fixed-frequency, peak current-mode

step-up switching regulator with an integrated 2.5A

(minimum) power transistor.

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