AOZ1280

EZBuck™ 1.2 A Simple Buck Regulator

General Description Features

The AOZ1280 is a high efficiency, simple to use, 1.2 A  3 V to 26 V operating input voltage range
buck regulator which is flexible enough to be optimized  240 mΩ internal NMOS
for a variety of applications. The AOZ1280 operates from
 High efficiency: up to 95 %
a 3 V to 26 V input voltage range, and provides up to
1.2 A of continuous output current. The output voltage is  Internal compensation
adjustable down to 0.8 V. The fixed 1.5 MHz PWM  1.2 A continuous output current
switching frequency reduces inductor size.  Fixed 1.5 MHz PWM operation

The AOZ1280 comes in a SOT23-6L package and is  Internal soft start

rated over a -40 °C to +85 °C operating ambient  Output voltage adjustable down to 0.8 V
temperature range.  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

VIN
C3
C1
4.7µF

VIN
L1 2.2µH
EN VOUT
LX
AOZ1280 R1
C2
FB 10µF

GND R2

Figure 1. 1.2 A Buck Regulator

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 AOZ1280

Ordering Information
Part Number Ambient Temperature Range Package Environmental
AOZ1280CI -40 °C to +85 °C SOT23-6L Green Product
RoHS Compliant

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

BST 1 6 LX

GND 2 5 VIN

FB 3 4 EN

SOT23-6L
(Top View)

Pin Description
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.

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 AOZ1280

Absolute Maximum Ratings Recommended Operating Conditions

Exceeding the Absolute Maximum Ratings may damage the The device is not guaranteed to operate beyond the
device. Recommended Operating Conditions.
Parameter Rating Parameter Rating
Supply Voltage (VIN) 30 V Supply Voltage (VIN) 3.0 V to 26 V
LX to GND -0.7 V to VVIN+ 2 V Output Voltage Range 0.8 V to VVIN
EN to GND -0.3 V to 26 V Ambient Temperature (TA) -40 °C to +85 °C
FB to GND -0.3 V to 6 V Package Thermal Resistance (JA)
BST to AGND VLX + 6 V SOT23-6L 220 °C/W

Junction Temperature (TJ) +150 °C

Storage Temperature (TS) -65 °C to +150 °C
(1)
ESD Rating 2 kV
Note:
1. Devices are inherently ESD sensitive, handling precautions are
required. Human body model rating: 1.5 kΩ in series with 100 pF.

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.

Symbol Parameter Conditions Min. Typ. Max. Units

VVIN Supply Voltage 3 26 V
VUVLO Input Under-Voltage Lockout Threshold VVIN Rising 2.9 V
VVIN Falling 2.3 V
UVLO Hysteresis 200 mV
IVIN Supply Current (Quiescent) IOUT = 0, VFB = 1 V, VEN > 1.2 V 1 1.5 mA
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 EN Input Threshold Off Threshold 0.4 V
VEN_ON On Threshold 1.2 V
VEN_HYS EN Input Hysteresis 200 mV
IEN Enable Input Current 3 A
MODULATOR
fO Frequency 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 150 °C
TJ Falling 110 °C
TSS Soft Start Interval 400 s
POWER STATE OUTPUT
RDS(ON) NMOS On-Resistance VIN = 12 V 240 mΩ
RDS(ON) NMOS On-Resistance VIN = 3.3 V 380 mΩ
ILEAKAGE NMOS Leakage VEN = 0 V, VLX = 0 V 10 A

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 AOZ1280

Block Diagram

VIN

Regulator

Current BST
+ Sense LDO BST
Enable Softstart
EN Detect
Ramp
OC
Generator

CLK
OSC
PWM
FB – Logic
– Driver
0.8V + Error + LX
Amplifier PWM
Comparator

GND

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 AOZ1280

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 Steady State Test

(IOUT = 0.2A to 0.8A) (IOUT = 0.5A)

Vo ripple
20V/div
Vo ripple
50mV/div
Vlx
10V/div
IL
1A/div
IL
500mA/div
Io
1A/div

200μs/div 500ns/div

Short Circuit Protection Short Circuit Recovery

Vlx Vlx
10V/div 10V/div

Vo
1V/div Vo
1V/div

lL
lL 1A/div
1A/div

2ms/div 2ms/div

Start-up Through Enable with IOUT = 1A

Start-up Through Enable No Load Resistive Load

Ven Ven
5V/div 5V/div

Vo Vo
2V/div 2/div

Vlx
Vlx 10V/div
10V/div

IL
IL 1A/div
1A/div

1ms/div 1ms/div

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 AOZ1280

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.

Shut-down Through Enable with IOUT = 1A

Shut-down Through Enable No Load Resistive Load

Ven Ven
5V/div 5V/div

Vo Vo
2/div 2/div

Vlx
Vlx 10V/div
10V/div

IL
IL 1A/div
1A/div

1ms/div 1ms/div

Efficiency
Efficiency (VIN = 12V) vs. Load Current Efficiency (VIN = 24V) vs. Load Current
100 100

5.0V OUTPUT
90 90
5.0V OUTPUT
3.3V OUTPUT
80 80 3.3V OUTPUT
Efficieny (%)

Efficieny (%)

70 70

60 60

50 50

40 40
0 0.2 0.4 0.6 0.8 1.0 1.2 0 0.2 0.4 0.6 0.8 1.0 1.2
Load Current (A) Load Current (A)

Efficiency (VIN = 5V) vs. Load Current

100

5.0V OUTPUT
90
3.3V OUTPUT
80
Efficieny (%)

70

60

50

40
0 0.2 0.4 0.6 0.8 1.0 1.2
Load Current (A)

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 AOZ1280

Detailed Description
The AOZ1280 is a current-mode step down regulator Switching Frequency
with integrated high side NMOS switch. It operates from The AOZ1280 switching frequency is fixed and set by an
a 3 V to 26 V input voltage range and supplies up to 1.2 A internal oscillator. The switching frequency is set
of load current. Features include: enable control, under internally 1.5 MHz.
voltage lock-out, internal soft-start, output over-voltage
protection, over-current protection, and thermal shut Output Voltage Programming
down. Output voltage can be set by feeding back the output to
the FB pin with a resistor divider network. In the
The AOZ1280 is available in SOT23-6L package.
application circuit shown in Figure 1. The resistor divider
Enable and Soft Start network includes R1 and R2. Usually, a design is started
by picking a fixed R2 value and calculating the required
The AOZ1280 has an internal soft start feature to limit
R1 with equation below.
in-rush current and ensure the output voltage ramps up
smoothly to regulation voltage. A soft start process  R 1
begins when the input voltage rises to a voltage higher V O = 0.8   1 + -------
than UVLO and the voltage level on the EN pin is HIGH.  R 2
In the soft start process, the output voltage is typically
ramped to regulation voltage in 400 s. The 400 s Some standard values of R1 and R2 for the most
soft start time is set internally. commonly used output voltage values are listed in
Table 1.
The EN pin of the AOZ1280 is active high. Connect the
EN pin to VIN if the enable function is not used. Pulling Table 1.
EN to ground will disable the AOZ1280. Do not leave EN Vo (V) R1 (kΩ) R2 (kΩ)
open. The voltage on the EN pin must be above 1.2 V to
enable the AOZ1280. When voltage on the EN pin falls 1.8 80.6 64.2
below 0.4 V, the AOZ1280 is disabled. 2.5 49.9 23.4
3.3 49.9 15.8
Steady-State Operation
5.0 49.9 9.53
Under steady-state conditions, the converter operates
in fixed frequency and Continuous-Conduction Mode
The combination of R1 and R2 should be large enough to
(CCM).
avoid drawing excessive current from the output, which
The AOZ1280 integrates an internal NMOS as the will cause power loss.
high-side switch. Inductor current is sensed by amplifying
the voltage drop across the drain to the source of the Protection Features
high-side power MOSFET. Output voltage is divided The AOZ1280 has multiple protection features to prevent
down by the external voltage divider at the FB pin. system circuit damage under abnormal conditions.
The difference of the FB pin voltage and reference
voltage is amplified by the internal transconductance Over Current Protection (OCP)
error amplifier. The error voltage is compared against the The sensed inductor current signal is also used for over
current signal, which is sum of inductor current signal current protection.
plus ramp compensation signal, at the PWM comparator
input. If the current signal is less than the error voltage, The cycle-by-cycle current limit threshold is set normal
the internal high-side switch is on. The inductor current value of 2 A. When the load current reaches the current
flows from the input through the inductor to the output. limit threshold, the cycle-by-cycle current limit circuit
When the current signal exceeds the error voltage, the immediately turns off the high-side switch to terminate
high-side switch is off. The inductor current is the current duty cycle. The inductor current stop rising.
freewheeling through the external Schottky diode to The cycle-by-cycle current limit protection directly limits
output. 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.

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 AOZ1280

The AOZ1280 has internal short circuit protection to The relationship between the input capacitor RMS
protect itself from catastrophic failure under output short current and voltage conversion ratio is calculated and
circuit conditions. The FB pin voltage is proportional to shown in Figure 2. It can be seen that when VO is half of
the output voltage. Whenever the FB pin voltage is below VIN, CIN is under the worst current stress. The worst
0.2 V, the short circuit protection circuit is triggered. As a current stress on CIN is 0.5 x IO.
result, the converter is shut down and hiccups. The
0.5
converter will start up via a soft start once the short circuit
condition is resolved. In the short circuit protection mode,
0.4
the inductor average current is greatly reduced.

Under Voltage Lock Out (UVLO) ICIN_RMS(m) 0.3

An UVLO circuit monitors the input voltage. When the IO
0.2
input voltage exceeds 2.9 V, the converter starts
operation. When input voltage falls below 2.3 V, the
0.1
converter will stop switching.

Thermal Protection 0
0 0.5 1
An internal temperature sensor monitors the junction m
temperature. The sensor shuts down the internal control
circuit and high side NMOS if the junction temperature Figure 2. ICIN vs. Voltage Conversion Ratio
exceeds 150 °C. The regulator will restart automatically
under the control of soft-start circuit when the junction For reliable operation and best performance, the input
temperature decreases to 100 °C. capacitors must have a current rating higher than
ICIN_RMS at the worst operating conditions. Ceramic
Application Information capacitors are preferred for use as input capacitors
because of their low ESR and high ripple current rating.
The basic AOZ1280 application circuit is shown in
Depending on the application circuits, other low ESR
Figure 1. Component selection is explained below.
tantalum capacitor or aluminum electrolytic capacitor
Input Capacitor may be used. When selecting ceramic capacitors,
X5R or X7R type dielectric ceramic capacitors are
The input capacitor must be connected to the VIN pin and preferred for their better temperature and voltage
the GND pin of the AOZ1280 to maintain steady input characteristics.
voltage and filter out the pulsing input current. The
voltage rating of the input capacitor must be greater than Note that the ripple current rating from capacitor
maximum input voltage plus ripple voltage. manufactures are based on a fixed life time. Further de-
rating may be necessary for practical design
The input ripple voltage can be approximated by requirement.
equation below:
Inductor
IO  VO  VO
V IN = -----------------   1 – ---------  --------- The inductor is used to supply constant current to output
f  C IN  V IN V IN when it is driven by a switching voltage. For given input
and output voltage, inductance and switching frequency
Since the input current is discontinuous in a buck together decide the inductor ripple current, which is:
converter, the current stress on the input capacitor is VO  VO 
another concern when selecting the capacitor. For a I L = -----------   1 – --------
-
buck circuit, the RMS value of input capacitor current can fL  V IN
be calculated by:
The peak inductor current is:
VO  VO 
-  1 – --------
I CIN_RMS = I O  -------- - I L
V IN  V IN I Lpeak = I O + --------
2
if we let m equal the conversion ratio:
High inductance provides a low inductor ripple current
VO but requires larger size inductor to avoid saturation.
--------
- = m
V IN Low ripple current reduces inductor core losses and also

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 AOZ1280

reduces RMS current through inductor and switches. For lower output ripple voltage across the entire
This results in less conduction loss. operating temperature range, X5R or X7R dielectric type
of ceramic, or other low ESR tantalum capacitor or
When selecting the inductor, make sure it is able to aluminum electrolytic capacitor may also be used as
handle the peak current without saturation at the highest output capacitors.
operating temperature.
In a buck converter, output capacitor current is
The inductor takes the highest current in a buck circuit. continuous. The RMS current of output capacitor is
The conduction loss on inductor needs to be checked for decided by the peak to peak inductor ripple current.
thermal and efficiency requirements. It can be calculated by:

Surface mount inductors in different shape and styles are I L

I CO_RMS = ----------
available from Coilcraft, Elytone and Murata. Shielded 12
inductors are small and radiate less EMI noise but cost
more than unshielded inductors. The choice depends on
Usually, the ripple current rating of the output capacitor is
EMI requirement, price and size.
a smaller issue because of the low current stress. When
Output Capacitor the buck inductor is selected to be very small and
inductor ripple current is high, output capacitor could be
The output capacitor is selected based on the DC output overstressed.
voltage rating, output ripple voltage specification and
ripple current rating. Schottky Diode Selection
The selected output capacitor must have a higher rated The external freewheeling diode supplies the current
voltage specification than the maximum desired output to the inductor when the high side NMOS switch is off.
voltage including ripple. De-rating needs to be To reduce the losses due to the forward voltage drop and
considered for long term reliability. recovery of diode, a Schottky diode is recommended.
The maximum reverse voltage rating of the Schottky
Output ripple voltage specification is another important diode should be greater than the maximum input voltage,
factor for selecting the output capacitor. In a buck and the current rating should be greater than the
converter circuit, output ripple voltage is determined by maximum load current.
inductor value, switching frequency, output capacitor
value and ESR. It can be calculated by the equation Thermal Management and Layout
below: Consideration
1 In the AOZ1280 buck regulator circuit, high pulsing
V O = I L   ESR CO + ------------------------- current flows through two circuit loops. The first loop
 8fC  O starts from the input capacitors, to the VIN pin, to the
where, LX pin, to the filter inductor, to the output capacitor and
CO is output capacitor value, and load, and then returns to the input capacitor through
ground. Current flows in the first loop when the high side
ESRCO is the equivalent series resistance of the output switch is on. The second loop starts from inductor, to the
capacitor. output capacitors and load, to the anode of Schottky
diode, to the cathode of Schottky diode. Current flows in
When low ESR ceramic capacitor is used as output
the second loop when the low side diode is on.
capacitor, the impedance of the capacitor at the switching
frequency dominates. Output ripple is mainly caused by In PCB layout, minimizing the area of the two loops will
capacitor value and inductor ripple current. The output reduce the noise of this circuit and improves efficiency.
ripple voltage calculation can be simplified to: A ground plane is strongly recommended to connect the
1 input capacitor, the output capacitor, and the GND pin of
V O = I L   ------------------------- the AOZ1280.
8  f  C 
O
In the AOZ1280 buck regulator circuit, the major power
If the impedance of ESR at switching frequency dissipating components are the AOZ1280, the Schottky
dominates, the output ripple voltage is mainly decided by diode and the output inductor. The total power dissipation
capacitor ESR and inductor ripple current. The output of converter circuit can be measured by input power
ripple voltage calculation can be further simplified to: minus output power.

V O = I L  ESR CO P total_loss =  V IN  I IN  –  V O  V IN 

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 AOZ1280

The power dissipation in Schottky can be approximated Several layout tips are listed below for the best electric
as: and thermal performance.

P diode_loss = I O   1 – D   V FW_Schottky 1. The input capacitor should be connected as close as

possible to the VIN pin and the GND pin.
where, 2. The inductor should be placed as close as possible
VFW_Schottky is the Schottky diode forward voltage drop. to the LX pin and the output capacitor.
3. Keep the connection of the schottky diode between
The power dissipation of inductor can be approximately
the LX pin and the GND pin as short and wide
calculated by output current and DCR of inductor.
as possible.
P inductor_loss = IO2  R inductor  1.1 4. Place the feedback resistors and compensation
components as close to the chip as possible.
The actual junction temperature can be calculated with 5. Keep sensitive signal traces away from the LX pin.
power dissipation in the AOZ1280 and thermal 6. Pour a maximized copper area to the VIN pin, the
impedance from junction to ambient. LX pin and especially the GND pin to help thermal
T junction dissipation.
=  P total_loss – P inductor_loss    JA + T amb 7. Pour a copper plane on all unused board area and
connect the plane to stable DC nodes, like VIN,
GND or VOUT.
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.

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 AOZ1280

Package Dimensions, SOT23-6

Gauge Plane Seating Plane
D
0.25mm
e1 c

E E1

θ1
e b

A A2
.010mm

A1

Dimensions in millimeters Dimensions in inches

Symbols Min. Nom. Max. Symbols Min. Nom. Max.
RECOMMENDED LAND PATTERN
A 0.90 — 1.25 A 0.035 — 0.049
1.20 A1 0.00 — 0.15 A1 0.00 — 0.006
A2 0.70 1.10 1.20 A2 0.028 0.043 0.047
b 0.30 0.40 0.50 b 0.012 0.016 0.020
2.40
c 0.08 0.13 0.20 c 0.003 0.005 0.008
0.80 D 2.70 2.90 3.10 D 0.106 0.114 0.122
E 2.50 2.80 3.10 E 0.098 0.110 0.122
0.95 E1 1.50 1.60 1.70 E1 0.059 0.063 0.067
0.63
e 0.95 BSC e 0.037 BSC
UNIT: mm e1 1.90 BSC e1 0.075 BSC
L 0.30 — 0.60 L 0.012 — 0.024
θ1 0° — 8° θ1 0° — 8°

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.

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 AOZ1280

Tape and Reel Dimensions, SOT23-6

Tape
P1

T D1 P2

E1

E2 E

B0
K0 A0 D0 P0
Feeding Direction
Unit: mm

Package A0 B0 K0 D0 D1 E E1 E2 P0 P1 P2 T

SOT-23 3.15 3.27 1.34 1.10 1.50 8.00 1.75 3.50 4.00 4.00 2.00 0.25
±0.10 ±0.10 ±0.10 ±0.01 ±0.10 ±0.20 ±0.10 ±0.05 ±0.10 ±0.10 ±0.10 ±0.05

Reel
W1

S
G

N K
M
V

R
H

Unit: mm W

Tape Size Reel Size M N W W1 H K S G R V

8 mm ø180 ø180.00 ø60.50 9.00 11.40 ø13.00 10.60 2.00 ø9.00 5.00 18.00
±0.50 Min. ±0.30 ±1.0 +0.50 / -0.20 ±0.50

Leader/Trailer and Orientation

Trailer Tape Components Tape Leader Tape

300mm min. or Orientation in Pocket 500mm min. or
75 Empty Pockets 125 Empty Pockets

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 AOZ1280

Part Marking
AOZ1280CI
(SOT23-6)

11
AX 2D Assembly Lot Code

Part Number Code Week & Year Code

Assembly Location Code

LEGAL DISCLAIMER

Applications or uses as critical components in life support devices or systems are not authorized. AOS does not
assume any liability arising out of such applications or uses of its products. AOS reserves the right to make
changes to product specifications without notice. It is the responsibility of the customer to evaluate suitability of the
product for their intended application. Customer shall comply with applicable legal requirements, including all
applicable export control rules, regulations and limitations.

AOS' products are provided subject to AOS' terms and conditions of sale which are set forth at:
http://www.aosmd.com/terms_and_conditions_of_sale

LIFE SUPPORT POLICY

ALPHA & OMEGA SEMICONDUCTOR PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL
COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS.

As used herein:

1. Life support devices or systems are devices or 2. A critical component in any component of a life
systems which, (a) are intended for surgical implant into support, device, or system whose failure to perform can
the body or (b) support or sustain life, and (c) whose be reasonably expected to cause the failure of the life
failure to perform when properly used in accordance support device or system, or to affect its safety or
with instructions for use provided in the labeling, can be effectiveness.
reasonably expected to result in a significant injury of
the user.

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