SC440MLTRT Semtech, SC440MLTRT Datasheet - Page 11

• Manufacturer Part Number: SC440MLTRT
• Description: IC LED DRVR WHITE BCKLGT 24-MLPQ
• Manufacturer: Semtech
• Type: Backlight, White LEDr

Specifications of SC440MLTRT

• Constant Current: Yes
• Topology: PWM, Step-Up (Boost)
• Number Of Outputs: 6
• Internal Driver: Yes
• Type - Primary: Backlight
• Frequency: 640kHz ~ 960kHz
• Voltage - Supply: 4.5 V ~ 21 V
• Voltage - Output: 40V
• Mounting Type: Surface Mount
• Package / Case: 24-MLPQ
• Current - Output / Channel: 30mA
• Internal Switch(s): Yes
• Efficiency: 91%
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Operating Temperature: -
• Other names: SC440MLTR

Applications Information
SC440 Detailed Description
The SC440 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 SC440 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 3mA (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 SC440 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:
Where, R
I
© 2010 Semtech Corp.
LED
I
LED
is the output current of each string in mA.
R
IOSET
IOSET
60
is in kΩ.
NOT RECOMMENDED FOR NEW DESIGNS: SEE SC440A
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 4µA 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 SC440
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, SC440 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 SC440 runs
in standby mode, drawing 3mA (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 SC440, 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 SC440 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.
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SC440
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