AIC1896 데이터 시트보기 (PDF) - Analog Intergrations
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AIC1896 Datasheet PDF : 15 Pages
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AIC1896
APPLICATION INFORMATION
Inductor Selection
A 15µH inductor is recommended for most
AIC1896 applications. Although small size and
high efficiency are major concerns, the inductor
should have low core losses at 1.4MHz and low
DCR (copper wire resistance).
Capacitor Selection
The small size of ceramic capacitors makes them
ideal for AIC1896 applications. X5R and X7R
types are recommended because they retain their
capacitance over wider ranges of voltage and
temperature than other types, such as Y5V or
Z5U. A 4.7µF input capacitor and a 1µF output
capacitor are sufficient for most AIC1896
applications.
Diode Selection
Schottky diodes, with their low forward voltage
drop and fast reverse recovery, are the ideal
choices for AIC1896 applications. The forward
voltage drop of a Schottky diode represents the
conduction losses in the diode, while the diode
capacitance (CT or CD) represents the switching
losses. For diode selection, both forward voltage
drop and diode capacitance need to be
considered. Schottky diodes with higher current
ratings usually have lower forward voltage drop
and larger diode capacitance, which can cause
significant switching losses at the 1.4MHz
switching frequency of AIC1896. A Schottky diode
rated at 100mA to 200mA is sufficient for most
AIC1896 applications.
LED Current Control
LED current is controlled by feedback resistor (R1
in Fig. 1). The feedback reference is 1.23V. The
LED current is 1.23V/R1. In order to have
accurate LED current, precision resistors are
preferred (1% recommended). The formula for R1
selection is shown below.
R1 = 1.23V/ILED
(1)
Open-Circuit Protection
In the cases of output open circuit, when the LEDs
are disconnected from the circuit or the LEDs fail,
the feedback voltage will be zero. AIC1896 will
then switch to a high duty cycle resulting in a high
output voltage, which may cause SW pin voltage
to exceed its maximum 30V rating. A zener diode
can be used at the output to limit the voltage on
SW pin (Fig. 20). The zener voltage should be
larger than the maximum forward voltage of the
LED string. The current rating of the zener should
be larger than 0.1mA.
Dimming Control
There are three different types of dimming control
circuits as follows:
1. Using a PWM signal
PWM brightness control provides the widest
dimming range by pulsing LEDs on and off at full
and zero current, repectively. The change of
average LED current depends on the duty cycle of
the PWM signal. Typically, a 0.1kHz to 10kHz
PWM signal is used. Two applications of PWM
dimming with AIC 1896 are shown in Fig 21. One,
as fig. 21(a), uses PWM signal to drive SHDN
pin directly for dimming control. The other, as fig.
21(b), employs PWM signal going through a
resistor to drive FB pin. If the SHDN pin is used,
the increase of duty cycle results in LED
brightness enhancement. If the FB pin is used, on
the contrary, the increase of duty cycle will
decrease its brightness. In this application, LEDs
9
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