AIC1647 データシートの表示（PDF） - Analog Intergrations

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AIC1647

White LED Step-Up Converter in SOT23

Analog Intergrations 

AIC1647

APPLICATION INFORMATION

Inductor Selection

A 10µH inductor is recommended for most

AIC1647 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 AIC1647 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 AIC1647

applications.

Diode Selection

Schottky diodes, with their low forward voltage

drop and fast reverse recovery, are the ideal

choices for AIC1647 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 1.2MHz

switching frequency of AIC1647. An Schottky

diode rated at 100mA to 200mA is sufficient for

most AIC1647 applications.

LED Current Control

LED current is controlled by feedback resistor

(RFB in Fig. 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. AIC1647 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. A zener diode

can be used at the output to limit the voltage on

SW pin (Figure 1). 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 ways of dimming control

circuits as follows:

1. Using a PWM Signal

PWM brightness control provides the widest

dimming range by pulsing the LEDs on and off at

full and zero current, respectively. The change of

average LED current depends on the duty cycle of

the PWM signal. Typically, a 0.1kHz to 1kHz

PWM signal is used. Two applications of PWM

dimming with AIC1647 are shown in Figure 19

and Figure 20. One, as Figure 19, uses PWM

signal to drive SHDN pin directly for dimming

control. The other, as Figure 20, 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 are dimmed by FB pin and

turned off completely by SHDN .

2. Using a DC Voltage

For some applications, the preferred method of a

dimming control uses a variable DC voltage to

adjust LED current. The dimming control using a

DC voltage is shown in Figure 21. Cautiously

selecting R1 and R2 is essential so that the

current from the variable DC source is much

smaller than the LED current and much larger

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