SC446TETRT Datasheet, PDF (14/19 Pages) Samtec, Inc – High Efficiency Integrated Driver for 3-Strings of 100mA LEDs

SC446
Applications Information (continued)
elements from PWM dimming interface to the PWM pin of
SC446. Usually, simply checking signal D_max at PWM pin
of SC446 is sufficient.
5
4
3
2
1
Linear Dimming
The linear dimming control is available for SC446 by
D _{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
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
D_{When two or more SC446s are operating in parallel for a}
large-sized panel application, audible noise may be ob-
served due to non-synchronous switching frequency. The
ripple voltage on the input voltage rail will be modulat-
ed by the beat frequency resulting in audible noise. This
situation can be resolved by adding an input inductor
C_{between input voltage rail and the SC446 VIN pin. This}
situation can also be improved by adding more input de-
coupling capacitors.
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.
LED Strings Connection
A _{Generally, LED strings are connected to IO1 ~ IO3 pins}
through a mechanical connector which, generally, cannot
support an electrical connection thereby resulting in sig-
nifican⁵ t noise. Con⁴ sequently, ³the SC446 LE² D short-circ¹uit
protection may false trip when the noise level is large.
Certain ceramic decoupling capacitor on pins IO1 ~ IO3 to
GND are useful to prevent the SC446 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 applica-
tions, circuit designers tend to select the decoupling ca-
pacitors in the range of (100pF ~ 1nF).
Inductor Selection
B^{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
A^{can operate in either CCM or DCM depending on its work-}
ing conditions. The inductor DC current or input current
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.
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