SC443ULTRT Semtech, SC443ULTRT Datasheet - Page 12

• 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

Applications Information (continued)
If PWM pin voltage goes low while the SC443 is at nor-
mal operation, the device 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
SC443 is a programmable frequency, peak current-mode
boost switching regulator with an integrated A (typical),
power transistor. The switching frequency is program-
mable at the FSET pin. Referring to the Block Diagram on
Page 0, 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 cur-
rent is summed with the slope-compensating ramp and
fed into the modulating ramp input of the PWM compara-
tor. 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 A (the minimum 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)
The SC443 includes an external programming over-volt-
age 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
© 2008 Semtech Corp.
LIM
will set
the OVPIN pin. If the voltage at the OVPIN pin exceeds
.5V, the boost converter will shut off and a 0.8mA pull
down current source will be applied to the VOUT pin to
quickly discharge the over-voltage capacitor. This addi-
tional 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
6). The relationship can be described as follows:
Where OVPIN_TH is .5V typical.
An OVP event causes a fault which disables the boost
converter and enables the strong pull down, the soft-start
capacitor is discharged. When the soft-start capacitor
voltage falls below 0.5V and VOUT falls below V above
VIN, the SC443 enters a soft-start process. The OVPIN pin
is sensitive to noise and a proper decoupling cap, (nF ~
0nF) is required.
LED Open-Circuit Protection
If any LED is detected as open circuit, that string IO pin-
voltage will be pulled low and less than 0.2V. When LED is
opened, the output current is decreased at once ,but the
COMP pin voltage can’t be pulled low at the same time,
the boost converter duty cycle will be maintained causing
VOUT to rise. Because of the open string, VOUT will con-
tinue to rise until it reaches the programmed OVP level.
When OVP is reached, the voltages on the IO pins are
monitored. If any IO voltage is less than 0.2V, that string
will be identified as open and will be latched off. Only
VIN falling below UVLO, recycle EN signal, and thermal
shutdown will reset this latch. When a hiccup cycle is
initiated the SS is discharged slowly with a .5μA cur-
OVP_
TRIP
=
V
OVPIN_TH
*
R2
R4
+
R4
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SC443
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