AIC1648 datasheet(7/10 Pages) AIC

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AIC1648

PIN DESCRIPTIONS

PIN 1: SW

- Switch

pin.

Connect

inductor/diode here. Minimize

trace area at this pin to reduce

EMI.

PIN 2: GND

- Ground pin. Tie directly to local

ground plane.

PIN 3: FB

- Feedback

pin.

Reference

voltage

95mV.

Connect

cathode of lowest LED and

resistor here. Calculate resistor

value to obtain LED current

according to the formula:

RFB = 95mV/ILED

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 10

µH 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. 1

µF 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

(RFB in Figure 1). The feedback reference voltage

is 95mV. The LED current is 95mV/ RFB. In order

to have accurate LED current, precision resistors

are preferred (1% recommended). The formula for

RFB selection is shown below.

RFB = 95mV/ILED

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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