aoz1092d Alpha & Omega Semiconductor, aoz1092d Datasheet - Page 7

• Manufacturer Part Number: aoz1092d
• Description: Ezbucktm 3a Simple Buck Regulator
• Manufacturer: Alpha & Omega Semiconductor

Detailed Description
The AOZ1092D is a current-mode step down regulator
with integrated high side PMOS switch and low side
Schottky diode. 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 range of output voltage. Features include enable
control, Power-On Reset, input under voltage lockout,
fixed internal soft-start and thermal shut down.
The AOZ1092D is available in 4x5 DFN-8 package.
Enable and Soft Start
The AOZ1092D has 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 4.0V and voltage
on EN pin is HIGH. In soft start process, the output
voltage is ramped to regulation voltage in typically 2.2ms.
The 2.2ms soft start time is set internally.
The EN pin of the AOZ1092D is active high. Connect the
EN pin to VIN if enable function is not used. Pull it to
ground will disable the AOZ1092D. Do not leave it open.
The voltage on EN pin must be above 2.0 V to enable the
AOZ1092D. When voltage on EN pin falls below 0.6V,
the AOZ1092D is disabled.
Steady-State Operation
Under steady-state conditions, the converter operates
in fixed frequency and Continuous-Conduction Mode
(CCM).
The AOZ1092D 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
voltage, the high-side switch is off. The inductor current is
freewheeling through the internal Schottky diode to
output.
The AOZ1092D uses a P-Channel MOSFET as the high
side switch. It saves the bootstrap capacitor normally
Rev. 1.3 February 2009
www.aosmd.com
seen in a circuit which is using an NMOS switch. It allows
100% turn-on of the upper switch to achieve linear regu-
lation mode of operation. The minimum voltage drop from
V
MOSFET plus DC resistance of buck inductor. It can be
calculated by equation below:
where;
V
V
I
R
between 40m
junction temperature, and
R
Switching Frequency
The AOZ1092D 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 with a resistor divider network. In the
application circuit shown in Figure 1. The resistor divider
network includes R
by picking a fixed R
R
Some standard value of R
output voltage values are listed in Table 1.
The combination of R
avoid drawing excessive current from the output, which
will cause power loss.
V
O
V
O_MAX
IN
IN
DS(ON)
inductor
1
O
is the output current from 0A to 3A,
O_MAX
with equation below.
is the input voltage from 4.5V to 16V,
to V
=
V
0.8
is the maximum output voltage,
O
is the on resistance of internal MOSFET, the value is
is the inductor DC resistance.
O
0.8
1.2
1.5
1.8
2.5
3.3
5.0
(V)
is the load current times DC resistance of
=
×
Ω
⎛
⎜
⎝
V
and 70m
1
IN
+
–
R
------ -
R
1
2
I
and R
1
2
O
value and calculating the required
1
⎞
⎟
⎠
Ω
and R
×
R
depending on input voltage and
(
1
1
R
4.99
12.7
21.5
31.1
52.3
2
, R
1.0
10
(k
. Usually, a design is started
DS ON
2
Ω
2
should be large enough to
and most commonly used
(
)
)
+
R
inductor
AOZ1092D
R
2
open
Page 7 of 16
11.5
10.2
10
10
10
10
(k
Ω
)
)