SC446TETRT Semtech, SC446TETRT Datasheet - Page 11

• Manufacturer Part Number: SC446TETRT
• Description: IC LED DRVR WHT BCKLGT TSSOP-16
• Manufacturer: Semtech
• Type: Backlight, White LEDr

Specifications of SC446TETRT

• Topology: PWM, Step-Up (Boost)
• Number Of Outputs: 3
• Internal Driver: Yes
• Type - Primary: Backlight
• Frequency: 700kHz
• Voltage - Supply: 4.5 V ~ 27 V
• Voltage - Output: 36V
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP Exposed Pad, 16-eTSSOP, 16-HTSSOP
• Current - Output / Channel: 100mA
• Internal Switch(s): Yes
• Efficiency: 90%
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Operating Temperature: -
• Other names: SC446TETR

Applications Information
SC446 Detailed Description
The SC446 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 SC446 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 3-10
backlight LEDs for each string, driven up to 100mA.
Operation
The SC446 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 SC446 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 2.6V. 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 ~ IO3) 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 SC446 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 SC446, 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 SC446 is in normal
operation, then the SC446 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
SC446 is a 700kHz fixed-frequency, peak current-mode
step-up switching regulator with an integrated 2.5A
(minimum) power transistor.
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SC446
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