aoz1110 Alpha & Omega Semiconductor, aoz1110 Datasheet - Page 8

• Manufacturer Part Number: aoz1110
• Description: 4a Synchronous Ezbuck Regulator
• Manufacturer: Alpha & Omega Semiconductor

Detailed Description
The AOZ1110QI is a current-mode synchronous step
down regulator with complimentary MOSFET switches.
The operating input voltage range is 2.7V to 5.5V. The
output range can be adjusted to a minimum of 0.8V and
supplies up to 4A of continuous current. Features include
cycle-by-cycle current limiting, short circuit protection,
adjustable soft start and a power good output signal.
Enable and Soft Start
The AOZ1110QI has both internal and external 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 2.5V
and voltage on EN pin is HIGH. In the soft start, a 2μA
internal current source charges the external capacitor at
SS. As the SS capacitor is charged, the voltage at SS
rises. The SS voltage clamps the reference voltage of the
error amplifier, therefore output voltage rising time follows
the SS pin voltage. With the slow ramping up output
voltage, the inrush current can be prevented. If there is no
external capacitor connected to the SS pin, the internal
soft start will operate at 500μs.
Power Good
The output of power good is an open drain N-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
AOZ1110QI monitors the FB voltage: when the FB pin
voltage is lower than 85% of the target voltage or higher
than 115% of the target voltage, N-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 AOZ1110QI 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 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
Rev. 1.0 October 2010
www.aosmd.com
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 AOZ1110QI uses freewheeling N-MOSFET to
realize synchronous rectification. It greatly improves the
converter efficiency and reduces power loss in the
low-side switch.
The AOZ1110QI uses a P-MOSFET as the high-side
switch. It saves the bootstrap capacitor normally seen in
a circuit which is using an N-MOSFET switch.
Switching Frequency
The AOZ1110QI switching frequency can be selected by
FSEL pin. When the FSEL logic is tied to VDD, the
switching frequency will be 1.0 MHz. When the FSEL
logic is tied to GND, the switching frequency will be
0.5 MHz.
Output Voltage Programming
Output voltage can be set by feeding back the output to
the FB pin by using a resistor divider network. In the
application circuit shown in Figure 1. The resistor divider
network includes R
by picking a fixed R
R1 with equation below.
Some standard value of R
voltage values are listed in Table 1.
Table 1.
The combination of R
avoid drawing excessive current from the output, which
will cause power loss.
V
O
=
Vo (V)
0.8
0.8
1.2
1.5
1.8
2.5
3.3
5.0
×
⎛
⎜
⎝
1
+
R
------ -
R
1
2
and R
1
2
value and calculating the required
1
⎞
⎟
⎠
and R
R
1
1
4.99
12.7
21.5
31.1
52.3
2
1.0
, R
10
. Usually, a design is started
(kΩ)
2
2
should be large enough to
and most used output
R
AOZ1110
Page 8 of 16
s
open
10.2
11.5
10
10
10
10
(kΩ)