Rev. 1.1 August 2011 www.aosmd.com Page 1 of 13
AOZ1280
EZBuck™ 1.2 A Simple Buck Regulator
General Description
The AOZ1280 is a high efficiency, simple to use, 1.2 A
buck regulator which is flexible enough to be optimized
for a variety of applications. The AOZ1280 operates from
a 3 V to 26 V input voltage range, and provides up to
1.2 A of continuous output current. The output voltage is
adjustable down to 0.8 V. The fixed 1.5 MHz PWM
switching frequency reduces inductor size.
The AOZ1280 comes in a SOT23-6L package and is
rated over a -40 °C to +85 °C operating ambient
temperature range.
Features
3 V to 26 V operating input voltage range
240 m internal NMOS
High efficiency: up to 95 %
Internal compensation
1.2 A continuous output current
Fixed 1.5 MHz PWM operation
Internal soft start
Output voltage adjustable down to 0.8 V
Cycle-by-cycle current limit
Short-circuit protection
Thermal shutdown
Small size SOT23-6L
Applications
Point of load DC/DC conversion
Set top boxes
DVD drives and HDD
LCD Monitors & TVs
Cable modems
Telecom/Networking/Datacom equipment
Typical Application
Figure 1. 1.2 A Buck Regulator
LX
VIN
VIN
VOUT
FB
EN
GND
C2
10µF
C3
R1
R2
C1
4.7µF
L1 2.2µH
AOZ1280
AOZ1280
Rev. 1.1 August 2011 www.aosmd.com Page 2 of 13
Ordering Information
AOS Green Products use reduced levels of Halogens, and are also RoHS compliant.
Please visit www.aosmd.com/web/quality/rohs_compliant.jsp for additional information.
Pin Configuration
Pin Description
Part Number Ambient Temperature Range Package Environmental
AOZ1280CI -40 °C to +85 °C SOT23-6L Green Product
RoHS Compliant
Pin Number Pin Name Pin Function
1 BST Bootstrap voltage input. High side driver supply. Connected to 10 nF capacitor between
BST and LX.
2 GND Ground.
3 FB Feedback input. It is regulated to 0.8 V. The FB pin is used to determine the PWM output
voltage via a resistor divider between the output and GND.
4 EN Enable pin. The enable pin is active high. Connect EN pin to VIN through current limiting
resistor. Do not leave the EN pin floating.
5 VIN Supply voltage input. Input range from 3 V to 26 V. When VIN rises above the UVLO
threshold the device starts up.
6 LX PWM output connection to inductor.
LX
VIN
EN
1
2
3
BST
GND
FB
SOT23-6L
(Top View)
6
5
4
AOZ1280
Rev. 1.1 August 2011 www.aosmd.com Page 3 of 13
Absolute Maximum Ratings
Exceeding the Absolute Maximum Ratings may damage the
device.
Note:
1. Devices are inherently ESD sensitive, handling precautions are
required. Human body model rating: 1.5 kΩ in series with 100 pF.
Recommended Operating Conditions
The device is not guaranteed to operate beyon d the
Recommended Operating Conditions.
Electrical Characteristics
TA = 25 °C, VVIN = VEN = 12 V. Specifications in BOLD indicate a temperature range of -40 °C to +85 °C. These specifications are
guaranteed by design.
Parameter Rating
Supply Voltage (VIN) 30 V
LX to GND -0.7 V to VVIN+ 2 V
EN to GND -0.3 V to 26 V
FB to GND -0.3 V to 6 V
BST to AGND VLX + 6 V
Junction Temperature (TJ) +150 °C
Storage Temperature (TS) -65 °C to +150 °C
ESD Rating(1) 2 kV
Parameter Rating
Supply Voltage (VIN) 3.0 V to 26 V
Output Voltage Range 0.8 V to VVIN
Ambient Temperature (TA) -40 °C to +85 °C
Package Thermal Resistance (JA)
SOT23-6L 220 °C/W
Symbol Parameter Conditions Min. Typ. Max. Units
VVIN Supply Voltage 326V
VUVLO Input Under-Voltage Lockout Threshold VVIN Rising
VVIN Falling 2.3
2.9 V
V
UVLO Hysteresis 200 mV
IVIN Supply Current (Quiescent) IOUT = 0, VFB = 1 V, VEN > 1.2 V 11.5mA
IOFF Shutdown Supply Current VEN = 0 V 8 A
VFB Feedback Voltage TA = 25 ºC 784 800 816 mV
VFB_LOAD Load Regulation 120 mA < Load < 1.08 A 0.5 %
VFB_LINE Line Regulation Load = 600 mA 0.03 % / V
IFB Feedback Voltage Input Current VFB = 800 mV 500 nA
ENABLE
VEN_OFF
VEN_ON
EN Input Threshold Off Threshold
On Threshold 1.2
0.4 V
V
VEN_HYS EN Input Hysteresis 200 mV
IEN Enable Input Current 3A
MODULATOR
fOFrequency 1.2 1.5 1.8 MHz
DMAX Maximum Duty Cycle 87 %
TON_MIN Minimum On Time 100 ns
ILIM Current Limit 1.5 2 A
Over-Temperature Shutdown Limit TJ Rising
TJ Falling
150
110
°C
°C
TSS Soft Start Interval 400 s
POWER STATE OUTPUT
RDS(ON) NMOS On-Resistance VIN = 12 V 240 m
RDS(ON) NMOS On-Resistance VIN = 3.3 V 380 m
ILEAKAGE NMOS Leakage VEN = 0 V, VLX = 0 V 10 A
AOZ1280
Rev. 1.1 August 2011 www.aosmd.com Page 4 of 13
Block Diagram
GND
VIN
EN
FB
Regulator
Enable
Detect Softstart
PWM
Logic
Error
Amplifier
–
+
0.8V
–
+PWM
Comparator
OSC CLK
+
Ramp
Generator OC
Current
Sense
Driver
BST
LDO
LX
BST
AOZ1280
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Typical Performance Characteristics
Circuit of Figure 1. VIN = 12 V, VOUT = 3.3 V, L = 4.7 H, C1 = 10 F, C2 = 22 F, TA = 25 °C, unless otherwise specified.
Load Transient Test
(I
OUT
= 0.2A to 0.8A)
Steady State Test
(I
OUT
= 0.5A)
Short Circuit Protection Short Circuit Recovery
Start-up Through Enable No Load
Start-up Through Enable with IOUT = 1A
Resistive Load
200s/div 500ns/div
2ms/div 2ms/div
1ms/div 1ms/div
Vo ripple
50mV/div
Vo
1V/div
Vlx
10V/div
Vlx
10V/div
lL
1A/div
Ven
5V/div
Vo
2V/div
Vlx
10V/div
Vo
1V/div
lL
1A/div
IL
1A/div
Ven
5V/div
Vo
2/div
Vlx
10V/div
IL
1A/div
IL
1A/div
Io
1A/div
Vo ripple
20V/div
Vlx
10V/div
IL
500mA/div
AOZ1280
Rev. 1.1 August 2011 www.aosmd.com Page 6 of 13
Typical Performance Characteristics (Continued)
Circuit of Figure 1. VIN = 12 V, VOUT = 3.3 V, L = 4.7 H, C1 = 10 F, C2 = 22 F, TA = 25 °C, unless otherwise specified.
Efficiency
Shut-down Through Enable No Load
Shut-down Through Enable with IOUT = 1A
Resistive Load
1ms/div 1ms/div
Ven
5V/div
Vo
2/div
Vlx
10V/div
IL
1A/div
Ven
5V/div
Vo
2/div
Vlx
10V/div
IL
1A/div
Efficiency (V
IN
= 12V) vs. Load Current
5.0V OUTPUT
3.3V OUTPUT
100
90
80
70
60
50
40
00.2 0.4 0.6 0.8 1.0 1.2
Load Current (A)
Efficieny (%)
Efficiency (V
IN
= 24V) vs. Load Current
5.0V OUTPUT
3.3V OUTPUT
100
90
80
70
60
50
40
00.2 0.4 0.6 0.8 1.0 1.2
Load Current (A)
Efficieny (%)
Efficiency (V
IN
= 5V) vs. Load Current
5.0V OUTPUT
3.3V OUTPUT
100
90
80
70
60
50
40
00.2 0.4 0.6 0.8 1.0 1.2
Load Current (A)
Efficieny (%)
AOZ1280
Rev. 1.1 August 2011 www.aosmd.com Page 7 of 13
Detailed Description
The AOZ1280 is a current-mode step down regulator
with integrated high side NMOS switch. It operates from
a 3 V to 26 V input voltage range and supplies up to 1.2 A
of load current. Features include: enable control, under
voltage lock-out, internal soft-start, output over-voltage
protection, over-current protection, and thermal shut
down.
The AOZ1280 is available in SOT23-6L package.
Enable and Soft Start
The AOZ1280 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 a voltage higher
than UVLO and the voltage level on the EN pin is HIGH.
In the soft start process, the output voltage is typically
ramped to regulation voltage in 400 s. The 400 s
soft start time is set internally.
The EN pin of the AOZ1280 is active high. Connect the
EN pin to VIN if the enable function is not used. Pulling
EN to ground will disable the AOZ1280. Do not leave EN
open. The voltage on the EN pin must be above 1.2 V to
enable the AOZ1280. When voltage on the EN pin falls
below 0.4 V, the AOZ1280 is disabled.
Steady-State Operation
Under steady-state conditions, the converter operates
in fixed frequency and Continuous-Conduction Mode
(CCM).
The AOZ1280 integrates an internal NMOS as the
high-side switch. Inductor current is sensed by amplifying
the voltage drop across the drain to the 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
voltage is amplified by the internal transconductance
error amplifier. The error voltage is compared against the
current signal, which is sum of inductor current signal
plus 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 external Schottky diode to
output.
Switching Frequency
The AOZ1280 switching frequency is fixed and set by an
internal oscillator. The switching frequency is set
internally 1.5 MHz.
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 R1 and R2. Usually, a design is started
by picking a fixed R2 value and calculating the required
R1 with equation below.
Some standard values of R1 and R2 for the most
commonly used output voltage values are listed in
Table 1.
Table 1.
The combination of R1 and R2 should be large enough to
avoid drawing excessive current from the output, which
will cause power loss.
Protection Features
The AOZ1280 has multiple protection features to prevent
system circuit damage under abnormal conditions.
Over Current Protection (OCP)
The sensed inductor current signal is also used for over
current protection.
The cycle-by-cycle current limit threshold is set normal
value of 2 A. When the load current reaches the current
limit threshold, the cycle-by-cycle current limit circuit
immediately turns off the high-side switch to terminate
the current duty cycle. The inductor current stop rising.
The cycle-by-cycle current limit protection directly limits
inductor peak current. The average inductor current is
also limited due to the limitation on peak inductor current.
When cycle-by-cycle current limit circuit is triggered, the
output voltage drops as the duty cycle decreasing.
Vo (V) R1 (kΩ)R2 (kΩ)
1.8 80.6 64.2
2.5 49.9 23.4
3.3 49.9 15.8
5.0 49.9 9.53
VO0.8 1 R1
R2
-------
+
=
AOZ1280
Rev. 1.1 August 2011 www.aosmd.com Page 8 of 13
The AOZ1280 has internal short circuit protection to
protect itself from catastrophic failure under output short
circuit conditions. The FB pin voltage is proportional to
the output voltage. Whenever the FB pin voltage is below
0.2 V, the short circuit protection circuit is triggered. As a
result, the converter is shut down and hiccups. The
converter will start up via a soft start once the short circuit
condition is resolved. In the short circuit protection mode,
the inductor average current is greatly reduced.
Under Voltage Lock Out (UVLO)
An UVLO circuit monitors the input voltage. When the
input voltage exceeds 2.9 V, the converter starts
operation. When input voltage falls below 2.3 V, the
converter will stop switching.
Thermal Protection
An internal temperature sensor monitors the junction
temperature. The sensor shuts down the internal control
circuit and high side NMOS if the junction temperature
exceeds 150 °C. The regulator will restart automatically
under the control of soft-start circuit when the junction
temperature decreases to 100 °C.
Application Information
The basic AOZ1280 application circuit is shown in
Figure 1. Component selection is explained below.
Input Capacitor
The input capacitor must be connected to the VIN pin and
the GND pin of the AOZ1280 to maintain steady input
voltage and filter out the pulsing input current. The
voltage rating of the input capacitor must be greater than
maximum input voltage plus ripple voltage.
The input ripple voltage can be approximated by
equation below:
Since the input current is discontinuous in a buck
converter, the current stress on the input capacitor is
another concern when selecting the capacitor. For a
buck circuit, the RMS value of input capacitor current can
be calculated by:
if we let m equal the conversion ratio:
The relationship between the input capacitor RMS
current and voltage conversion ratio is calculated and
shown in Figure 2. It can be seen that when VO is half of
VIN, CIN is under the worst current stress. The worst
current stress on CIN is 0.5 x IO.
Figure 2. ICIN vs. Voltage Conversion Ratio
For reliable operation and best performance, the input
capacitors must have a current rating higher than
ICIN_RMS at the worst operating conditions. Ceramic
capacitors are preferred for use as input capacitors
because of their low ESR and high ripple current rating.
Depending on the application circuits, other low ESR
tantalum capacitor or aluminum electrolytic capacitor
may be used. When selecting ceramic capacitors,
X5R or X7R type dielectric ceramic capacitors are
preferred for their better temperature and voltage
characteristics.
Note that the ripple current rating from capacitor
manufactures are based on a fixed life time. Further de-
rating may be necessary for practical design
requirement.
Inductor
The inductor is used to supply constant current to output
when it is driven by a switching voltage. For given input
and output voltage, inductance and switching frequency
together decide the inductor ripple current, which is:
The peak inductor current is:
High inductance provides a low inductor ripple current
but requires larger size inductor to avoid saturation.
Low ripple current reduces inductor core losses and also
VIN IO
fC
IN
----------------- 1VO
VIN
---------
–
VO
VIN
---------
=
ICIN_RMS IOVO
VIN
---------1VO
VIN
---------
–
=
VO
VIN
---------m=
0
0.1
0.2
0.3
0.4
0.5
0 0.5 1
m
I
CIN_RMS
(m)
I
O
ILVO
fL
-----------1VO
VIN
---------
–
=
ILpeak IO
IL
2
--------
+=
AOZ1280
Rev. 1.1 August 2011 www.aosmd.com Page 9 of 13
reduces RMS current through inductor and switches.
This results in less conduction loss.
When selecting the inductor, make sure it is able to
handle the peak current without saturation at the highest
operating temperature.
The inductor takes the highest current in a buck circuit.
The conduction loss on inductor needs to be checked for
thermal and efficiency requirements.
Surface mount inductors in different shape and styles are
available from Coilcraft, Elytone and Murata. Shielded
inductors are small and radiate less EMI noise but cost
more than unshielded inductors. The choice depends on
EMI requirement, price and size.
Output Capacitor
The output capacitor is selected based on the DC output
voltage rating, output ripple voltage specification and
ripple current rating.
The selected output capacitor must have a higher rated
voltage specification than the maximum desired output
voltage including ripple. De-rating needs to be
considered for long term reliability.
Output ripple voltage specification is another important
factor for selecting the output capacitor. In a buck
converter circuit, output ripple voltage is determined by
inductor value, switching frequency, output capacitor
value and ESR. It can be calculated by the equation
below:
where,
CO is output capacitor value, and
ESRCO is the equivalent series resistance of the output
capacitor.
When low ESR ceramic capacitor is used as output
capacitor, the impedance of the capacitor at the switching
frequency dominates. Output ripple is mainly caused by
capacitor value and inductor ripple current. The output
ripple voltage calculation can be simplified to:
If the impedance of ESR at switching frequency
dominates, the output ripple voltage is mainly decided by
capacitor ESR and inductor ripple current. The output
ripple voltage calculation can be further simplified to:
For lower output ripple voltage across the entire
operating temperature range, X5R or X7R dielectric type
of ceramic, or other low ESR tantalum capacitor or
aluminum electrolytic capacitor may also be used as
output capacitors.
In a buck converter, output capacitor current is
continuous. The RMS current of output capacitor is
decided by the peak to peak inductor ripple current.
It can be calculated by:
Usually, the ripple current rating of the output capacitor is
a smaller issue because of the low current stress. When
the buck inductor is selected to be very small and
inductor ripple current is high, output capacitor could be
overstressed.
Schottky Diode Selection
The external freewheeling diode supplies the current
to the inductor when the high side NMOS switch is off.
To reduce the losses due to the forward voltage drop and
recovery of diode, a Schottky diode is recommended.
The maximum reverse voltage rating of the Schottky
diode should be greater than the maximum input voltage,
and the current rating should be greater than the
maximum load current.
Thermal Management and Layout
Consideration
In the AOZ1280 buck regulator circuit, high pulsing
current flows through two circuit loops. The first loop
starts from the input capacitors, to the VIN pin, to the
LX pin, to the filter inductor, to the output capacitor and
load, and then returns to the input capacitor through
ground. Current flows in the first loop when the high side
switch is on. The second loop starts from inductor, to the
output capacitors and load, to the anode of Schottky
diode, to the cathode of Schottky diode. Current flows in
the second loop when the low side diode is on.
In PCB layout, minimizing the area of the two loops will
reduce the noise of this circuit and improves efficiency.
A ground plane is strongly recommended to connect the
input capacitor, the output capacitor, and the GND pin of
the AOZ1280.
In the AOZ1280 buck regulator circuit, the major power
dissipating components are the AOZ1280, the Schottky
diode and the output inductor. The total power dissipation
of converter circuit can be measured by input power
minus output power.
VOILESRCO 1
8fC
O
-------------------------
+
=
VOIL1
8fC
O
-------------------------
=
VOILESRCO
=
ICO_RMS
IL
12
----------
=
Ptotal_loss VIN IIN
VOVIN
–=
AOZ1280
Rev. 1.1 August 2011 www.aosmd.com Page 10 of 13
The power dissipation in Schottky can be approximated
as:
where,
VFW_Schottky is the Schottky diode forward voltage drop.
The power dissipation of inductor can be approximately
calculated by output current and DCR of inductor.
The actual junction temperature can be calculated with
power dissipation in the AOZ1280 and thermal
impedance from junction to ambient.
The maximum junction temperature of AOZ1280 is
150 ºC, which limits the maximum load current capability.
The thermal performance of the AOZ1280 is strongly
affected by the PCB layout. Extra care should be taken
by users during design process to ensure that the IC will
operate under the recommended environmental
conditions.
Several layout tips are listed below for the best electric
and thermal performance.
1. The input capacitor should be connected as close as
possible to the VIN pin and the GND pin.
2. The inductor should be placed as close as possible
to the LX pin and the output capacitor.
3. Keep the connection of the schottky diode between
the LX pin and the GND pin as short and wide
as possible.
4. Place the feedback resistors and compensation
components as close to the chip as possible.
5. Keep sensitive signal traces away from the LX pin.
6. Pour a maximized copper area to the VIN pin, the
LX pin and especially the GND pin to help thermal
dissipation.
7. Pour a copper plane on all unused board area and
connect the plane to stable DC nodes, like VIN,
GND or VOUT.
Pdiode_loss IO1D–VFW_Schottky
=
Pinductor_loss IO2Rinductor 1.1=
Tjunction
Ptotal_loss Pinductor_loss
–JA Tamb
+=
AOZ1280
Rev. 1.1 August 2011 www.aosmd.com Page 11 of 13
Package Dimensions, SOT23-6
E1
E
AA2
A1
e
e1
D
.010mm
0.80
0.95 0.63
2.40
b
L
c
Gauge Plane Seating Plane
0.25mm
θ1
1.20
Notes:
1. Package body sizes exclude mold flash and gate burrs. Mold flash at the non-lead sides should be less than 5 mils each.
2. Dimension “L” is measured in gauge plane.
3. Tolerance ±0.100 mm (4 mil) unless otherwise specified.
4. Followed from JEDEC MO-178C & MO-193C.
5. Controlling dimension is millimeter. Converted inch dimensions are not necessarily exact.
Symbols
A
A1
A2
b
c
D
E
E1
e
e1
L
θ1
Dimensions in millimeters
Min.
0.90
0.00
0.70
0.30
0.08
2.70
2.50
1.50
0.30
0°
Nom.
—
—
1.10
0.40
0.13
2.90
2.80
1.60
0.95 BSC
1.90 BSC
—
—
Max.
1.25
0.15
1.20
0.50
0.20
3.10
3.10
1.70
0.60
8°
Symbols
A
A1
A2
b
c
D
E
E1
e
e1
L
θ1
Min.
0.035
0.00
0.028
0.012
0.003
0.106
0.098
0.059
0.012
0°
Nom.
—
—
0.043
0.016
0.005
0.114
0.110
0.063
0.037 BSC
0.075 BSC
—
—
Max.
0.049
0.006
0.047
0.020
0.008
0.122
0.122
0.067
0.024
8°
Dimensions in inches
UNIT: mm
RECOMMENDED LAND PATTERN
AOZ1280
Rev. 1.1 August 2011 www.aosmd.com Page 12 of 13
Tape and Reel Dimensions, SOT23-6
Tape
Reel
Leader/Trailer and Orientation
Trailer Tape
300mm min. or
75 Empty Pockets
Components Tape
Orientation in Pocket
Leader Tape
500mm min. or
125 Empty Pockets
Unit: mm
Unit: mm
G
V
Feeding Direction
Tape Size
8 mm
Reel Size
ø180
M
ø180.00
±0.50
N
ø60.50
Min.
W1
11.40
±1.0
W
9.00
±0.30
Package
SOT-23
A0
3.15
±0.10
B0
3.27
±0.10
K0
1.34
±0.10
D0
1.10
±0.01
D1
1.50
±0.10
E
8.00
±0.20
E1
1.75
±0.10
E2
3.50
±0.05
P0
4.00
±0.10
P1
4.00
±0.10
P2
2.00
±0.10
T
0.25
±0.05
H
ø13.00
+0.50 / -0.20
K
10.60
S
2.00
±0.50
G
ø9.00
R
5.00
T
K0
B0
D1
A0 D0
P2
P1
P0
E1
E2 E
V
18.00
R
W1
N
W
M
S
K
H
AOZ1280
Rev. 1.1 August 2011 www.aosmd.com Page 13 of 13
Part Marking
AOZ1280CI
(SOT23-6)
2DAX
11
Week & Year Code
Part Number Code
Assembly Location Code
Assembly Lot Code
As used herein:
1. Life support devices or systems are devices or
systems which, (a) are intended for surgical implant into
the body or (b) support or sustain life, and (c) whose
failure to perform when properly used in accordance
with instructions for use provided in the labeling, can be
reasonably expected to result in a significant injury of
the user.
2. A critical component in any component of a life
support, device, or system whose failure to perform can
be reasonably expected to cause the failure of the life
support device or system, or to affect its safety or
effectiveness.
This data sheet contains preliminary data; supplementary data may be published at a later date.
Alpha & Omega Semiconductor reserves the right to make changes at any time without notice.
LIFE SUPPORT POLICY
ALPHA & OMEGA SEMICONDUCTOR PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL
COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS.
