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AAT1106

600mA Step-Down Converter

Skyworks Solutions 

Control Loop

The AAT1106 is a peak current mode step-down con-

verter. The current through the P-channel MOSFET (high

side) is sensed for current loop control, as well as short

circuit and overload protection. An adaptive slope com-

pensation signal is added to the sensed current to main-

tain stability for duty cycles greater than 50%. The peak

current mode loop appears as a voltage-programmed

current source in parallel with the output capacitor. The

output of the voltage error amplifier programs the cur-

rent mode loop for the necessary peak switch current to

force a constant output voltage for all load and line con-

ditions. Internal loop compensation terminates the

transconductance voltage error amplifier output. For the

adjustable output, the error amplifier reference is fixed

at 0.6V.

Enable

The enable pin is active high. When pulled low, the

enable input forces the AAT1106 into a low-power, non-

switching state. The total input current during shutdown

is less than 1μA.

Current Limit and

Over-Temperature Protection

For overload conditions, the peak input current is limit-

ed. To minimize power dissipation and stresses under

current limit and short-circuit conditions, switching is

terminated after entering current limit for a series of

pulses. Switching is terminated for seven consecutive

clock cycles after a current limit has been sensed for a

series of four consecutive clock cycles. Thermal protec-

tion completely disables switching when internal dissipa-

tion becomes excessive. The junction over-temperature

threshold is 150°C with 15°C of hysteresis. Once an

over-temperature or over-current fault conditions is

removed, the output voltage automatically recovers.

DATA SHEET

AAT1106

600mA Step-Down Converter

Dropout Operation

When the input voltage decreases toward the value of

the output voltage, the AAT1106 allows the main switch

to remain on for more than one switching cycle and

increases the duty cycle until it reaches 100%.

The duty cycle D of a step-down converter is defined as:

D = TON · FOSC · 100% ≈

VOUT · 100%

VIN

Where TON is the main switch on time and FOSC is the

oscillator frequency (1.5MHz).

The output voltage then is the input voltage minus the

voltage drop across the main switch and the inductor. At

low input supply voltage, the RDS(ON) of the P-channel

MOSFET increases and the efficiency of the converter

decreases. Caution must be exercised to ensure the heat

dissipated does not exceed the maximum junction tem-

perature of the IC.

Maximum Load Current

The AAT1106 will operate with an input supply voltage

as low as 2.5V; however, the maximum load current

decreases at lower input due to the large IR drop on the

main switch and synchronous rectifier. The slope com-

pensation signal reduces the peak inductor current as a

function of the duty cycle to prevent sub-harmonic oscil-

lations at duty cycles greater than 50%. Conversely, the

current limit increases as the duty cycle decreases.

Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com

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201970B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013

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