AOZ1081AI datasheet(9/16 Pages) AOSMD

9 / 16 page

AOZ1081

Rev. 1.1 April 2009

www.aosmd.com

Page 9 of 16

off the high side switch immediately to terminate the

current duty cycle. The inductor current stop rising.

The cycle by cycle current limit protection directly limits

inductor peak current. The average inductor current is

also limited due to the limitation on peak inductor current.

When cycle by cycle current limit circuit is triggered, the

output voltage drops as the duty cycle decreasing.

The AOZ1081 has internal short circuit protection to

protect itself from catastrophic failure under output short

circuit conditions. The FB pin voltage is proportional to

the output voltage. Whenever FB pin voltage is below

0.15V, the short circuit protection circuit is triggered.

As a result, the converter is shut down and hiccups at a

frequency equals to 1/8 of normal switching frequency.

The converter will start up via a soft start once the short

circuit condition disappears. In short circuit protection

mode, the inductor average current is greatly reduced

because of the low hiccup frequency.

Output Over Voltage Protection (OVP)

The AOZ1081 monitors the feedback voltage: when the

feedback voltage is higher than 330mV, it immediate

turns-off the PMOS to protect the output voltage over-

shoot at fault condition. When feedback voltage is lower

than 240mV, the PMOS is allowed to turn on in the next

cycle.

Power-On Reset (POR)

A power-on reset circuit monitors the input voltage. When

the input voltage exceeds 4V, the converter starts opera-

tion. When input voltage falls below 3.7V, the converter

will stop switching.

Thermal Protection

An internal temperature sensor monitors the junction

temperature. It shuts down the internal control circuit and

high side PMOS if the junction temperature exceeds

150°C.

Application Information

The basic AOZ1081 application circuit is shown in

Figure 1 and 2. Component selection is explained below.

Setting the Maximum LED Current

The AOZ1081 features a programmable LED current

using a resistor RFB at the end of the primary chain of

LEDs.

Input Capacitor

The input capacitor (C1 in Figure 1) must be connected

to the VIN pin and PGND pin of the AOZ1081 to maintain

steady input voltage and filter out the pulsing input

current. A small decoupling capacitor (Cd in Figure 1),

usually 1µF, should be connected to the VIN pin and

AGND pin for stable operation of the AOZ1081. The

voltage rating of input capacitor must be greater than

maximum input voltage plus ripple voltage.

The input ripple voltage can be approximated by equa-

tion below:

Since the input current is discontinuous in a buck

converter, the current stress on the input capacitor is

another concern when selecting the capacitor. For a

buck circuit, the RMS value of input capacitor current can

be calculated by:

if let m equal the conversion ratio:

The relationship between the input capacitor RMS

current and voltage conversion ratio is calculated and

shown in Figure 2. It can be seen that when VO is half

of VIN, CIN is under the worst current stress. The worst

current stress on CIN is 0.5 x IO.

Figure 2. ICIN vs. Voltage Conversion Ratio

ILEDMAX

250mV

RFB

-------------------

ΔVIN

fCIN

-----------------

VIN

---------

–

⎝⎠

⎜⎟

⎛⎞

VIN

---------

ICIN_RMS

VIN

--------- 1

VIN

---------

–

⎝⎠

⎜⎟

⎛⎞

VIN

---------

0.1

0.2

0.3

0.4

0.5

CIN_RMS(m)

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