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AOZ1021
(Rev.:2007)

EZBuck™ 3A Synchronous Buck Regulator

Alpha and Omega Semiconductor 

AOZ1021

Detailed Description

The AOZ1021 is a current-mode, step down regulator with

integrated high-side PMOS switch and a low-side NMOS

switch. It operates from a 4.5V to 16V input voltage range

and supplies up to 3A of load current. The duty cycle

can be adjusted from 6% to 100% allowing a wide output

voltage range. Features include enable control, Power-On

Reset, input under voltage lockout, output over voltage

protection, active high power good state, fixed internal

soft-start and thermal shut down.

The AOZ1021 is available in an SO-8 package.

Enable and Soft Start

The AOZ1021 has an internal soft start feature to limit

in-rush current and ensure the output voltage ramps up

smoothly to regulation voltage. A soft start process

begins when the input voltage rises to 4.1V and voltage

on EN pin is HIGH. In the soft start process, the output

voltage is typically ramped to regulation voltage in 4ms.

The 5ms soft start time is set internally.

The EN pin of the AOZ1021 is active HIGH. Connect the

EN pin to VIN if the enable function is not used. Pulling

EN to ground will disable the AOZ1021. Do not leave it

open. The voltage on the EN pin must be above 2V to

enable the AOZ1021. When voltage on the EN pin falls

below 0.6V, the AOZ1021 is disabled. If an application

circuit requires the AOZ1021 to be disabled, an open

drain or open collector circuit should be used to interface

to the EN pin.

Comparing with regulators using freewheeling Schottky

diodes, the AOZ1021 uses freewheeling NMOSFET to

realize synchronous rectification. It greatly improves the

converter efficiency and reduces power loss in the

low-side switch.

The AOZ1021 uses a P-Channel MOSFET as the high-

side switch. It saves the bootstrap capacitor normally

seen in a circuit which is using an NMOS switch. It allows

100% turn-on of the high-side switch to achieve linear

regulation mode of operation. The minimum voltage drop

from VIN to VO is the load current x DC resistance of

MOSFET + DC resistance of buck inductor. It can be

calculated by the equation below:

V O_MAX = V IN – I O × R DS(ON )

where;

VO_MAX is the maximum output voltage,

VIN is the input voltage from 4.5V to 16V,

IO is the output current from 0A to 3A, and

RDS(ON) is the on resistance of internal MOSFET, the value is

between 97mΩ and 200mΩ depending on input voltage and

junction temperature.

Switching Frequency

The AOZ1021 switching frequency is fixed and set by

an internal oscillator. The practical switching frequency

could range from 350kHz to 600kHz due to device

variation.

Steady-State Operation

Under steady-state conditions, the converter operates in

fixed frequency and Continuous-Conduction Mode

(CCM).

The AOZ1021 integrates an internal P-MOSFET as the

high-side switch. Inductor current is sensed by amplifying

the voltage drop across the drain to source of the high

side power MOSFET. Output voltage is divided down by

the external voltage divider at the FB pin. The difference

of the FB pin voltage and reference is amplified by the

internal transconductance error amplifier. The error

voltage, which shows on the COMP pin, is compared

against the current signal, which is sum of inductor

current signal and ramp compensation signal, at the

PWM comparator input. If the current signal is less than

the error voltage, the internal high-side switch is on. The

inductor current flows from the input through the inductor

to the output. When the current signal exceeds the error

voltage, the high-side switch is off. The inductor current

is freewheeling through the internal low-side N-MOSFET

switch to output. The internal adaptive FET driver

guarantees no turn on overlap of both high-side and

low-side switch.

Output Voltage Programming

Output voltage can be set by feeding back the output to

the FB pin by using a resistor divider network. See the

application circuit shown in Figure 1. The resistor divider

network includes R1 and R2. Usually, a design is started

by picking a fixed R2 value and calculating the required

R1 with equation below:

VO

=

0.8

×



1

+

R-----1- 

 R2

Some standard value of R1, R2 and most used output

voltage values are listed in Table 1.

VO (V)

0.8

1.2

1.5

1.8

2.5

3.3

5.0

R1 (kΩ)

1.0

4.99

10

12.7

21.5

31.1

52.3

R2 (kΩ)

open

10

11.5

10.2

10

10

10

Rev. 1.1 November 2007

www.aosmd.com

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