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ADD5203ACPZ-RL 데이터시트(Datasheet) 17 Page - Analog Devices

부품명 ADD5203ACPZ-RL
상세내용  8-String,White LED Driver with SMBus and PWM Input for LCD Backlight Applications
PDF  24 Pages
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제조사  AD [Analog Devices]
홈페이지  http://www.analog.com
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ADD5203
Rev. 0 | Page 17 of 24
EXTERNAL COMPONENT SELECTION GUIDE
Inductor Selection
The inductor is an integral part of the step-up converter. It stores
energy during the switch-on time and transfers that energy to
the output through the output diode during the switch-off time.
An inductor in the range of 4.7 μH to 22 μH is recommended.
In general, lower inductance values result in higher saturation
current and lower series resistance for a given physical size.
However, lower inductance results in higher peak current, which
can lead to reduced efficiency and greater input and/or output
ripple and noise. Peak-to-peak inductor ripple current at close
to 30% of the maximum dc input current typically yields an
optimal compromise.
The input (VIN) and output (VOUT) voltages determine the
switch duty cycle (D), which in turn can be used to determine
the inductor ripple current.
OUT
IN
OUT
V
V
V
D
=
Use the duty cycle and switching frequency (fSW) to determine
the on time.
SW
ON
f
D
t
=
The inductor ripple current (ΔIL) in a steady state is
L
t
V
I
IN
L
ON
×
=
Δ
Solve for the inductance value (L).
L
IN
I
t
V
L
Δ
×
=
ON
Make sure that the peak inductor current (that is, the maximum
input current plus half of the inductor ripple current) is less
than the rated saturation current of the inductor. In addition,
ensure that the maximum rated rms current of the inductor is
greater than the maximum dc input current to the regulator.
For duty cycles greater than 50% that occur with input voltages
greater than half the output voltage, slope compensation is required
to maintain stability of the current-mode regulator. The inherent
open-loop stability causes subharmonic instability when the
duty ratio is greater than 50%. To avoid subharmonic instability,
the slope of the inductor current should be less than half of the
compensation slope.
Inductor manufacturers include Coilcraft, Inc., Sumida
Corporation, and Toko.
Input and Output Capacitors Selection
The ADD5203 requires input and output bypass capacitors to
supply transient currents while maintaining a constant input
and output voltage. Use a low effective series resistance (ESR)
10 μF or greater capacitor for the input capacitor to prevent noise
at the ADD5203 input. Place the input between the VIN and
GND, as close as possible to the ADD5203. Ceramic capacitors
are preferred because of their low ESR characteristics.
Alternatively, use a high value, medium ESR capacitor in
parallel with a 0.1 μF low ESR capacitor as close as possible to
the ADD5203.
The output capacitor maintains the output voltage and supplies
current to the load while the ADD5203 switch is on. The value
and characteristics of the output capacitor greatly affect the
output voltage ripple and stability of the regulator. Use a low
ESR output capacitor; ceramic dielectric capacitors are preferred.
For very low ESR capacitors, such as ceramic capacitors, the
ripple current due to the capacitance is calculated as follows.
Because the capacitor discharges during the on time (tON), the
charge removed from the capacitor (QC) is the load current
multiplied by the on time. Therefore, the output voltage ripple
(ΔVOUT) is
OUT
ON
L
OUT
C
OUT
C
t
I
C
Q
V
×
=
=
Δ
where:
COUT is the output capacitance.
IL is the average inductor current.
Using the duty cycle and switching frequency (fSW), users can
determine the on time with the following equation:
SW
ON
f
D
t
=
The input (VIN) and output (VOUT) voltages determine the
switch duty cycle (D) with the following equation:
OUT
IN
OUT
V
V
V
D
=
Choose the output capacitor based on the following equation:
(
)
OUT
OUT
SW
IN
OUT
L
OUT
V
V
f
V
V
I
C
Δ
×
×
×
Capacitor manufacturers include Murata Manufacturing Co.,
Ltd., AVX, Sanyo, and Taiyo Yuden Co., Ltd.
Diode Selection
The output diode conducts the inductor current to the output
capacitor and loads while the switch is off. For high efficiency,
minimize the forward voltage drop of the diode. Schottky diodes
are recommended. However, for high voltage, high temperature
applications, where the Schottky diode reverse leakage current
becomes significant and can degrade efficiency, use an ultrafast
junction diode. The output diode for a boost regulator must be
chosen depending on the output voltage and the output current.
The diode must be rated for a reverse voltage equal to or greater
than the output voltage used. The average current rating must
be greater than the maximum load current expected, and the peak
current rating must be greater than the peak inductor current.
Using Schottky diodes with lower forward voltage drop decreases
power dissipation and increases efficiency. The diode must be
rated to handle the average output load current. Many diode




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