SC443ULTRT Semtech, SC443ULTRT Datasheet - Page 11

• Manufacturer Part Number: SC443ULTRT
• Description: TVS
• Manufacturer: Semtech

Specifications of SC443ULTRT

• 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: 16-UFQFN Exposed Pad
• Operating Temperature: *
• Current - Output / Channel: *
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: SC443ULTRTTR

© 2008 Semtech Corp.
Applications Information
SC443 Detailed Description
The SC443 contains a high frequency, current-mode
boost regulator and three 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 approxi-
mately 30mA.
Operation
The SC443 regulates the boost converter output voltage
based on instantaneous requirements of the three 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 .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 SC443 is a LED current programmable regulator. The
LED current set point is chosen using an external 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
LED
is the output current of each string in mA.
I
IOSET
LED
=20/R
is in kΩ.
IOSET
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 5µA 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 increased and clamped by the SS pin voltage.
When the SS pin voltage reaches its switching threshold
0.8V(typical), the SC443 starts to switch and the output
voltage increases.
Each internal LED current source (IO ~ IO3) tries to regu-
late the LED current to its set point. While the output volt-
age 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
its UVLO, SC443 will stay at shutdown mode, drawing less
than μA 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 SC443
runs in standby mode, drawing 3mA (typical) from the
input power supply. Under this condition, soft-start is initi-
ated 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 the device, the COMP pin voltage will
rise as quickly as it can since it is not being limited by the
SS pin. A proper capacitance (0nF ~ 00nF) 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 while SC443 is at normal operation,
SC443 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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SC443