SC440MLTRT Semtech, SC440MLTRT Datasheet - Page 12

• 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 (continued)
If PWM pin voltage goes low while SC440 is at normal
operation, SC440 will run in standby mode. The internal
switcher and the LED current sources will be immediately
turned off. The SS pin will not be affected by the PWM
signal and remains at its final value.
Main Power Stage Operation
SC440 is an 800kHz fixed-frequency, peak current-mode
boost switching regulator with an integrated 2.5A (typical),
power transistor. Referring to the Block Diagram, the clock
from the oscillation section resets the latch and turns on
the power transistor. Switch current is sensed with an
integrated sense resistor. The sensed current is summed
with the slope-compensating ramp and fed into the
modulating ramp input of the PWM comparator. The latch
is set and the power transistor conduction is terminated
when the modulating ramp intersects the error amplifier
output (COMP).
The current-mode switching regulator is a dual-loop
feedback control system. In the inner current loop, the EA
output (COMP) controls the peak inductor current. In the
outer loop, the error amplifier regulates the output voltage
to keep the LED current in set point. The double reactive
poles of the output LC filter are reduced to a single real
pole by the inner current loop, allowing the simple loop
compensation network to accommodate a wide range of
input and output voltages.
Over-Current Protection
If the switch current exceeds 2.5A (the typical current-
limit trip point), the current-limit comparator I
the latch and immediately turn off the internal power
switch. Due to separate pulse-width modulating and
current limiting paths, the OCP trip point is not affected
by slope compensation (i.e., trip point is not affected by
switching duty cycle).
Over-Voltage Protection (OVP)
SC440 includes an external programming over-voltage
protection circuit to prevent damage to the IC and output
capacitor in the event of an open-circuit condition. The
output voltage of the boost converter is detected at the
OVPIN pin. If the voltage at the OVPIN pin exceeds 1.5V,
© 2010 Semtech Corp.
NOT RECOMMENDED FOR NEW DESIGNS: SEE SC440A
LIM
will set
the boost converter will shut off and a 1mA pull down
current source will be applied to the VOUT pin to quickly
discharge the over-voltage capacitor. This additional level
of protection prevents a condition where the output
capacitor and Schottky diode must endure high voltage
for an extended period of time, which can pose a reliability
risk for the system. The total resistance of the divider for
the OVP protection should be more than 200kΩ.
The output over-voltage trip point can be programmed
by R2 and R4 resistor divider (see the schematic on page
17). The relationship can be described as follows:
Where OVPIN_TH is 1.55V typical.
An OVP event causes a fault which disables the boost
converter and enables the strong pulldown and the FFLAG
pin will also go high. Meanwhile, the soft-start capacitor
is discharged. When the soft-start capacitor voltage falls
below 0.5V, SC440 enters a soft-start process.
The OVP detection circuitry provides a disconnect feature
during the shutdown state to prevent any leakage from
the output. The external OVP resistor divider should be
connected between VOUT and OVPRTN while its central
tap is connected to OVPIN. If this disconnect function
is not desired, just connect the end of the OVP resistor
divider directly to GND. The OVPIN pin is sensitive to noise,
a proper decoupling cap (1nF ~ 10nF) is required.
LED Short-Circuit Protection
If one or more LEDs are detected as shortened, that
corresponding string will be latched off if SCP_EN is
floating. The voltages on all internal LED current sources
(IO pins) are monitored to see if any exceeds 2.3V. (The IO
voltage on abnormal LED string will rise earlier than other
floating LED strings). If any IO pin voltage exceeds 2.3V,
that IO current source will be latched off and the FFLAG
will go high. The latch can be reset if VIN falls below UVLO
or recycle EN signal. Other normal LED strings remain at
their normal operation. The protection will be disabled if
SCP_EN is tied to GND.
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SC440
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