AIC1648 Datasheet, PDF (7/10 Pages) Analog Intergrations Corporation – Built-in OVP White LED Step-Up Converter

PIN DESCRIPTIONS
PIN 1: SW
PIN 2: GND
- Switch
pin.
Connect
inductor/diode here. Minimize
trace area at this pin to reduce
EMI.
- Ground pin. Tie directly to local
ground plane.
PIN 3: FB
- Feedback pin. Reference
voltage is 95mV. Connect
cathode of lowest LED and
resistor here. Calculate resistor
value to obtain LED current
according to the formula:
R_FB = 95mV/I_LED
AIC1648
PIN 4: _SHDN - Shutdown pin. Tie to higher than
1.5V to enable device, 0.3V or
less to disable device.
PIN 5: OVP
- Overvoltage protection. When
VOUT is greater than 27V, the
internal MOSFET turns off.
PIN 6: VIN
- Power input pin. Bypass VIN to
GND with a capacitor sitting as
close to VIN as possible.
APPLICATION INFORMATION
Inductor Selection
A 10µH inductor is recommended for most
AIC1648 applications. Although small size and
high efficiency are major concerns, the inductor
should have low core losses at 1.2MHz and low
DCR (copper wire resistance).
Capacitor Selection
The small size of ceramic capacitors makes them
ideal for AIC1648 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. 1µF input capacitor with 1µF output
capacitor are sufficient for most AIC1648
applications.
Diode Selection
Schottky diodes, with their low forward voltage
drop and fast reverse recovery, are the ideal
choices for AIC1648 applications. The forward
voltage drop of an 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 1.2MHz
switching frequency of AIC1648. An Schottky
diode rated at 100mA to 200mA is sufficient for
most AIC1648 applications.
LED Current Control
LED current is controlled by feedback resistor
(R_FB in Figure 1). The feedback reference voltage
is 95mV. The LED current is 95mV/ R_FB. In order
to have accurate LED current, precision resistors
are preferred (1% recommended). The formula for
R_FB selection is shown below.
R_FB = 95mV/I_LED
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. AIC1648 will
then switch to a high duty cycle resulting in a high
output voltage, which may cause SW pin voltage
to exceed its maximum 33V rating. Connect built-
in OVP (Over Voltage Protection) pin to output
terminal to prevent the damage resulting from an
open circuit condition.
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