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SC443
Applications Information (continued)
rent source and a 1mA discharge path is turned on to
pull down VOUT. When SS falls below 0.5V and VOUT
falls below 1V above VIN, the OVP detection latches are
reset and a new soft-start sequence is initiated to resume
normal operation.
Thermal Shutdown (TSD)
If the thermal shutdown temperature of 150°C is reached,
a hiccup sequence is initiated where the boost converter
and all IO current sources are turned off. SS is discharged
by a 1.5μA current source, and a 1mA discharge path is
turned on to pull down VOUT. As temperature falls below
TSD trip point, the SC443 will retry once SS falls below
0.5V and VOUT falls below 1V above VIN.
PWM Dimming
The PWM input needs to be held high for normal opera-
tion. PWM dimming can be achieved by cycling the PWM
input at a given frequency where a “low ” on the PWM
input turns off all IO current sources and a “high” turns
on all IO current sources. The short and open detection
latches are blanked for approximately 2μs as the PWM
input transitions from low-to-high to prevent false fault
detection during PWM dimming.
The PWM pin can be toggled by external circuitry to allow
PWM dimming. In a typical application, a microcontroller
sets a register, or counter, that varies the pulse width on
a GPIO pin. The SC443 allows dimming over two decades
in frequency (50Hz-50kHz) in order to allow compatibil-
ity with a wide range of devices including the newest
dimming strategies that avoid the audio band by using
high frequency PWM dimming. In this manner, a wide
range of illumination can be generated while keeping
the instantaneous LED current at its peak value for high
efficiency and color temperature.
Furthermore, advanced lighting effects such as backlight
dim-on can be implemented as the SC443 can resolve
PWM from 10% to 90% duty at its highest frequency.
Additionally, PWM dimming offers customers the ability
to reduce in-rush current to the output capacitor. Simply
apply the PWM signal to the device at 10% duty for a
? 2009 Semtech Corp.
millisecond or two, and in-rush current is reduced to less
than 50mA. This dim time will vary based on the number
of LEDs and the size of the output capacitor, but can be
easily determined on the bench and programmed into
the μC firmware.
Parallel Operation
When two or more SC443s are operating in parallel for
a large-sized panel application, audible noise may be
observed due to a non-synchronous switching frequency.
The ripple voltage on the input voltage rail will be modu-
lated by the beat frequency resulting in audible noise.
This situation can be resolved by adding an input induc-
tor between the input voltage rail and the VIN pin and
can also be improved by adding more input decoupling
capacitors.
Inductor Selection
The inductance value of the inductor affects the convert-
er ’s steady state operation, transient response, and its
loop stability. Special attention needs to be paid to three
specifications of the inductor, its value, its DC resistance
and saturation current. The inductor ’s inductance value
also determines the inductor ripple current. The converter
can operate in either CCM or DCM depending on its work-
ing conditions. The inductor DC current or input current
can be calculated as,
I IN - Input current;
I OUT – Output current;
V OUT – Boost output voltage;
V IN – Input voltage;
η – Efficiency of the boost converter.
Then the boundary condition for CCM and DCM is,
V D = Forward conduction drop of the output rectifying
diode.
www.semtech.com 13
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