SC441CTETRT Semtech, SC441CTETRT Datasheet - Page 12

• Manufacturer Part Number: SC441CTETRT
• Description: TVS
• Manufacturer: Semtech

Specifications of SC441CTETRT

• Constant Current: *
• Constant Voltage: *
• Topology: *
• Number Of Outputs: *
• Internal Driver: *
• Type - Primary: *
• Type - Secondary: *
• Frequency: *
• Voltage - Supply: *
• Voltage - Output: *
• Mounting Type: Surface Mount
• Package / Case: 20-TSSOP Exposed Pad, 20-eTSSOP, 20-HTSSOP
• Operating Temperature: *
• Current - Output / Channel: *
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: SC441CTETRTTR

Applications Information
SC441C Detailed Description
The SC441C 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 SC441C 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 3-12
backlight LEDs for each string, driven up to 150mA.
Operation
The SC441C 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 1.0mm 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 SC441C 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:
Where R
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
© 2010 Semtech Corporation
R
IOSET
IOSET
= (0.261V) / I
is in kΩ. I
LED
LED
is the LED current in Amperes.
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 (IO1 ~ 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 SC441C will
stay in shutdown mode drawing less than 1µA 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 SC441C, the COMP pin voltage will rise quickly since
it is not limited by the SS pin. A proper capacitance (10nF
~ 100nF) 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 SC441C is in
normal operation, then the SC441C 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
SC441C is a 700KHz fixed-frequency, peak current-mode
step-up switching regulator with an integrated 2.5A
(minimum) power transistor.
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SC441C
12