SC441A Datasheet, PDF (15/20 Pages) Semtech Corporation – High Efficiency Integrated Driver for 4-Strings of 150mA LEDs

SC441A
Applications Information (continued)
the needed dimming steps and resolutions, it is uncommon
to run into the above D_max before reaching 100%. While
most applications will not run into D_max, the designer
should be aware of possible parasitic elements from PWM
dimming interface to the PWM pin of SC441A. Usually,
simply checking signal D_max at PWM pin of SC441A is
sufficient.
5
4
3
2
1
Linear Dimming
The linear dimming control is available for SC441A
D _{by applying an external control voltage on IOSET pin}
through an external resistor-like circuit (shown below).
External environment brightness compensation can also
be achieved when the control voltage is generated by a
light sensor circuit.
C
IOSET
R_EXT
V_EXT
R_IOSET
B _{The IOSET voltage is 0.5V when linear dimming is used and}
the minimum IOSET current must be higher than 27µA
(i.e. 15mA per LED string). The external control voltage
slew rate must slow at 1V/10ms.
A ^{LED Strings Connection}
Generally, LED strings are connected to IO1 ~ IO4 pins
through a mechanical connector which, generally, can-
not su₅pport an ele₄ ctrical conn₃ ection ther₂eby resulting₁ in
significant noise. Consequently, the SC441A LED short-
circuit protection may false trip when the noise level is
large. Certain ceramic decoupling capacitor on pins IO1
~ IO4 to GND are useful to prevent the SC441A from noise
influence.
As a general guideline, the decoupling capacitance should
be limited as follows.
C_dcple

I _LED
* ^0.6uS
V_o
Where, I_LED is the LED current per string, Vo is the Boost
output voltage and C_dcple is the suggested decoupling
capacitor value.
For example, if I_LED=10mA, Vo=13.5V, the calculated
upper bound of C_dcple is about 444pF. One could use
390pF or less in the circuit. If I_LED=100mA, Vo=13.5V,
the calculated upper bound of C_dcple is about 4.44nF.
One may use 3.9nF or less in the circuit. In some appli-
cations, circuit designers tend to select the decoupling
capacitors in the range of (100pF ~ 1nF). For some low
LED current (e.g. 10mA) applications, it is recommended
to add 1M-10Mohm resistor from IO pin to GND in order
to reduce IO pin voltage during PWM dimming.
Parallel Operation
When two or more SC441As are operating in parallel for
a large-sized panel application, audible noise may be
D _{observed due to non-synchronous switching frequency.}
The ripple voltage on the input voltage rail will be mod-
ulated by the beat frequency resulting in audible noise.
This situation can be resolved by adding an input induc-
tor between input voltage rail and the SC441A VIN pin.
This situation can also be improved by adding more input
C _{decoupling capacitors.}
Inductor Selection
The inductance value of the inductor affects the convert-
er’s steady state operation, transient response, and its
B ^{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
A ^{can be calculated as,}
,_,1
9₂₈₇ ˜,₂₈₇
9_,1 ˜ Ș
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 duty ratio is,
' ^{9287  9,1  9'}
9₂₈₇  9_'
V_D – Forward conduction drop of the output rectifying
diode
When the boost converter runs in DCM ( L < L_boundary), it takes
the advantages of small inductance and quick transient
response while avoiding the bandwidth limiting instability
of the RHP zero found in CCM boost converters.
© 2009 Semtech Corporation
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