SC441CTETRT Semtech, SC441CTETRT Datasheet - Page 15

• Manufacturer Part Number: SC441CTETRT
• Description: TVS
• Manufacturer: Semtech

Specifications of SC441CTETRT

• 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: 20-TSSOP Exposed Pad, 20-eTSSOP, 20-HTSSOP
• Operating Temperature: *
• Current - Output / Channel: *
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: SC441CTETRTTR

D
C
B
A
Applications Information (continued)
interfaces, 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
SC441C. Usually, simply checking signal D_max at PWM
pin of SC441C is sufficient.
Linear Dimming
The linear dimming control is available for SC441C
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.
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.
LED Strings Connection
Generally, LED strings are connected to IO1 ~ IO4 pins
through a mechanical connector which, generally, can-
not support an electrical connection thereby resulting in
significant noise. Consequently, the SC441C 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 SC441C from noise
influence.
As a general guideline, the decoupling capacitance should
be limited as follows.
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.
© 2010 Semtech Corporation
5
5
V_EXT
4
4
C
dcple
R_EXT
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I
LED
*
0
3
3
6 .
V
uS
o
R_IOSET
2
2
IOSET
1
1
D
C
B
A
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 SC441Cs are operating in parallel for
a large-sized panel application, audible noise may be
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 SC441C VIN pin.
This situation 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
Then the duty ratio is,
V
diode
When the boost converter runs in DCM ( L < L
the advantages of small inductance and quick transient
response while avoiding the bandwidth limiting instability
of the RHP zero found in CCM boost converters.
IN
D
- Input current;
– Forward conduction drop of the output rectifying
V
V
I
η – Efficiency of the boost converter.
OUT
OUT
IN
– Input voltage;
– Output current;
– Boost output voltage;
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boundary
SC441C
), it takes
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