aoz1020 Alpha & Omega Semiconductor, aoz1020 Datasheet - Page 7

• Manufacturer Part Number: aoz1020
• Description: Synchronous Buck Regulator
• Manufacturer: Alpha & Omega Semiconductor

Detailed Description
The AOZ1020 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 2A 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 AOZ1020 is available in an SO-8 package.
Enable and Soft Start
The AOZ1020 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 4ms soft start time is set internally.
The EN pin of the AOZ1020 is active HIGH. Connect the
EN pin to V
EN to ground will disable the AOZ1020. Do not leave it
open. The voltage on the EN pin must be above 2V to
enable the AOZ1020. When voltage on the EN pin falls
below 0.6V, the AOZ1020 is disabled. If an application
circuit requires the AOZ1020 to be disabled, an open
drain or open collector circuit should be used to interface
to the EN pin.
Power Good
The output of Power-Good is an open drain N-channel
MOSFET which supplies an active HIGH power good
stage. A pull-up resistor (R3) should connect this pin to a
DC power trail with maximum voltage no higher than 6V.
The AOZ1020 monitors the FB voltage; when FB pin
voltage is lower than 90% of the normal voltage,
N-channel MOSFET turns on and the Power-Good pin is
pulled LOW, which indicates the power is abnormal.
Steady-State Operation
Under steady-state conditions, the converter operates in
fixed frequency and Continuous-Conduction Mode
(CCM).
The AOZ1020 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
Rev. 1.0 March 2008
IN
if the enable function is not used. Pulling
www.aosmd.com
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.
Comparing with regulators using freewheeling Schottky
diodes, the AOZ1020 uses freewheeling NMOSFET to
realize synchronous rectification. It greatly improves the
converter efficiency and reduces power loss in the
low-side switch.
The AOZ1020 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 V
MOSFET + DC resistance of buck inductor. It can be
calculated by the equation below:
where;
V
V
I
R
between 97m and 200m depending on input voltage and
junction temperature.
Switching Frequency
The AOZ1020 switching frequency is fixed and set by
an internal oscillator. The practical switching frequency
could range from 400kHz to 600kHz due to device
variation.
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 R
by picking a fixed R
R
V
V
O
O_MAX
IN
DS(ON)
1
O
O _MAX
is the output current from 0A to 2A, and
with equation below:
is the input voltage from 4.5V to 16V,
=
IN
is the maximum output voltage,
0.8
is the on resistance of internal MOSFET, the value is
to V
=
O
V
is the load current x DC resistance of
1
IN
+
–
R
------ -
R
1
2
I
and R
value and calculating the required
1
2
O
R
2
DS ON
. Usually, a design is started
AOZ1020
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