SC440AEVB datasheet(11/21 Pages) SEMTECH

11 / 21 page

SC440A

www.semtech.com

Applications Information

SC440A Detailed Description

The SC440A contains a high frequency, current-mode

boost regulator and six string LED current sources. The

LED current for all strings is programmed by an external

resistor and the PWM controller operates to maintain the

output voltage at a level which will keep the current of

each string regulated. A typical application would use 3-8

backlight LEDs for each string, driven with approximately

30mA.

Operation

The SC440A regulates the boost converter output voltage

based on instantaneous requirement of the six string

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 logic interface to output control circuit has

high-bandwidth, and supports PWM dimming with 50Hz

to 50kHz dimming frequency while the entire supply

current is reduced to 4.5mA (typical) when all LED strings

are off.

High frequency switching provides high output power

using a 1.0mm height inductor, maximizing efficiency

for space-constrained and cost-sensitive applications.

Additionally, the converter and output capacitor are

protected from open-LED conditions by programmable

over voltage protection.

LED Current Programming

The SC440A is a LED current programmable regulator. The

LED current set point is chosen using external an resistor

connected to the IOSET pin. The relationship between the

programming resistor value and the LED current set point

of each string can be described as follows:

IOSET

LED

Where, R

IOSET

is in k

Ω.

LED

is the output current of each string in mA.

Start-Up

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

threshold, and both EN and PWM signals are set to high,

the SS pin begins to source 6

μA to the SS capacitor and its

voltage begins to rise from 0V to its end value (2.6V). The

output voltage of the internal error amplifier (COMP) is

increases and clamped by the SS pin voltage. When the SS

pin voltage reaches its switching threshold, the SC440A

starts to switch and the output voltage increases.

Each internal LED current source (IO1 ~ IO6) tries 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 as the COMP pin

voltage keeps rising. Once each LED current reaches its

set point, the error information is not generated by the

LED current source. The COMP pin voltage stays at a level

which keeps the LED current at its set point.

If the EN pin voltage is pulled below 0.4V and VIN reaches

to its UVLO, SC440A will stay at shutdown mode, drawing

less than 1μA from the input power supply.

If the PWM pin voltage is pulled below 0.4V when the EN

pin is pulled high and VIN reaches its UVLO, the SC440A

runs in standby mode, drawing 4.5mA (typical) from the

input power supply. Under this condition, soft-start is

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

since the EN pin is pulled high. After that, when PWM

signal goes high to enable SC440A, the COMP pin voltage

will rise as quickly as it can since it is not being limited

by the SS pin. A proper capacitance (10nF ~ 100nF) is

required for the COMP pin and its external RC network in

order to prevent output voltage overshoot.

Shut Down

When the VIN pin voltage falls below its UVLO or EN

pin voltage goes low, the SC440A will run in shutdown

mode. The internal switch and LED current sources will

be immediately turned off. The SS capacitor is discharged

by SS pin internal current source and the SS pin voltage

decreases to 0V. The output voltage falls to the same level

as the input voltage.

SC440AEVB датащи(PDF) 11 Page - Semtech Corporation

SC440AEVB датащи(HTML) 11 Page - Semtech Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

датащи скачать

ссылки URL