AOZ1282CI [AOS]
EZBuck⢠1.2A Simple Buck Regulator; EZBuckâ ?? ¢简单1.2A降压稳压器
| 型号： | AOZ1282CI |
| 厂家： | 
ALPHA & OMEGA SEMICONDUCTORS |
| 描述： | EZBuck⢠1.2A Simple Buck Regulator |
| 文件： | 总13页 (文件大小：728K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
AOZ1282CI
EZBuck™ 1.2A Simple Buck Regulator
General Description
Features
The AOZ1282CI is a high efficiency, simple to use, 1.2A
buck regulator flexible enough to be optimized for a
variety of applications. The AOZ1282CI works from a
4.5V to 36V input voltage range, and provides up to 1.2A
of continuous output current. The output voltage is
adjustable down to 0.8V. The fixed switching frequency
of 450kHz PWM operation reduces inductor size.
4.5V to 36V operating input voltage range
240mΩ internal NMOS
Up to 95% efficiency
Internal compensation
1.2A continuous output current
Fixed 450kHz PWM operation
Internal soft start
Output voltage adjustable down to 0.8V
Cycle-by-cycle current limit
Short-circuit protection
Thermal shutdown
Small size SOT23-6L
Applications
Point of load DC/DC conversion
Set top boxes and cable modems
DVD drives and HDDs
LCD Monitors & TVs
Telecom/Networking/Datacom equipment
Typical Application
VIN
C3
C1
4.7µF
VIN
BS
EN
L1
VOUT
LX
FB
AOZ1282CI
22µH
R1
C2
10µF
GND
R2
Figure 1. 1.2A Buck Regulator
Rev. 0.5 September 2012
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Page 1 of 13
AOZ1282CI
Ordering Information
Part Number
Ambient Temperature Range
Package
Environmental
Green Product
AOZ1282CI
-40 °C to +85 °C
SOT23-6L
AOS Green Products use reduced levels of Halogens, and are also RoHS compliant.
Please visit www.aosmd.com/media/AOSGreenPolicy.pdf for additional information.
Pin Configuration
1
2
3
6
5
4
BST
GND
FB
LX
VIN
EN
SOT23-6L
(Top View)
Pin Description
Pin Number
Pin Name
Pin Function
1
BST
Bootstrap Voltage Input. High side driver supply. Connected to 100nF capacitor between
BST and LX.
2
3
GND
FB
Ground.
Feedback Input. It is regulated to 0.8V. The FB pin is used to determine the PWM output
voltage via a resistor divider between the output and GND.
4
5
6
EN
VIN
LX
Enable Pin. The enable pin is active high. Connect EN pin to VIN through current limiting
resistor. Do not leave the EN pin floating.
Supply Voltage Input. Range from 4.5V to 36V. When VIN rises above the UVLO
threshold the device starts up.
PWM Output. Connect to inductor.
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Page 2 of 13
AOZ1282CI
Absolute Maximum Ratings
Exceeding the Absolute Maximum Ratings may damage the
device.
Recommended Operating Conditions
The device is not guaranteed to operate beyond the
Recommended Operating Conditions.
Parameter
Supply Voltage (VIN)
Rating
Parameter
Supply Voltage (VIN)
Output Voltage (VOUT
Rating
40V
4.5V to 36V
0.8V to VVIN
LX to GND
-0.7V to VVIN+ 0.3V
-0.3V to 40V
-0.3V to 6V
VLX + 6V
)
EN to GND
Ambient Temperature (TA)
-40°C to +85°C
FB to GND
Package Thermal Resistance (JA)
SOT23-6L
55°C/W
BST to GND
Junction Temperature (TJ)
Storage Temperature (TS)
ESD Rating(1)
Note:
+150°C
-65°C to +150°C
2kV
1. Devices are inherently ESD sensitive, handling precautions are
required. Human body model rating: 1.5k
Ω in series with 100pF.
Electrical Characteristics
TA = 25 °C, VIN = VEN = 12V, unless otherwise specified. Specifications in BOLD indicate a temperature range of -40°C to +85°C.
These specifications are guaranteed by design.
Symbol
Parameter
Supply Voltage
Conditions
Min.
4.5
Typ.
Max.
Units
VIN
36
V
VUVLO
Input Under-Voltage Lockout Threshold
VIN rising
VIN falling
2.9
V
V
2.3
UVLO Hysteresis
260
mV
mA
A
IIN
Supply Current (Quiescent)
Shutdown Supply Current
Feedback Voltage
IOUT = 0, VFB = 1V, VEN > 1.2V
VEN = 0V
1
1.5
8
IOFF
VFB
TA = 25ºC
784
800
0.5
816
mV
%
VFB_LOAD Load Regulation
VFB_LINE Line Regulation
120mA < Load < 1.08A
Load = 600mA
0.03
500
%/V
nA
IFB
Feedback Voltage Input Current
VFB = 800mV
ENABLE
VEN_OFF EN Input Threshold
VEN_ON
Off threshold
On threshold
0.4
V
V
1.2
VEN_HYS EN Input Hysteresis
200
mV
IEN
Enable Input Current
3
A
MODULATOR
fO
Frequency
Maximum Duty Cycle
360
1.5
450
87
540
kHz
%
DMAX
TON_MIN Minimum On Time
150
1.9
ns
A
ILIM
Current Limit
Over-Temperature Shutdown Limit
TJ rising
TJ falling
150
110
°C
°C
TSS
Soft Start Interval
1.5
ms
POWER STATE OUTPUT
ILEAKAGE NMOS Leakage
RDS(ON) NMOS On-Resistance
VEN = 0V, VLX = 0V
VIN = 12V
10
A
420
mꢀ
Rev. 0.5 September 2012
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Page 3 of 13
AOZ1282CI
Block Diagram
VIN
Regulator
Current
Sense
BST
LDO
Enable
Detect
BST
SoftStart
EN
Ramp
Generator
OC
CLK
OSC
Driver
FB
PWM
Logic
LX
0.8V
Error
Amplifier
PWM
Comparator
GND
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Page 4 of 13
AOZ1282CI
Typical Performance Characteristics
Circuit of Figure 1. TA = 25°C, VIN = VEN = 12V, VOUT = 3.3 V, unless otherwise specified.
Full Load Operation
Light Load Operation
IN
IN
Voltage
(1V/div)
Voltage
(500mV/div)
OUT
Voltage
(100mV/div)
OUT
Voltage
(100mV/div
LX
Voltage
(10V/div)
LX
Voltage
(10V/div)
LOAD
Current
(1A/div)
LOAD
Current
(1A/div)
2µs/div
2µs/div
Start Up to Full Load
Load Transient
IN
Voltage
(5V/div)
OUT
Voltage
(100mV/div
OUT
Current
(1A/div)
OUT
Voltage
(2V/div)
OUT
Current
(1A/div)
200µs/div
5ms/div
Short Circuit Protection
Short Circuit Recovery
LX
Voltage
(10V/div)
LX
Voltage
(10V/div)
OUT
Voltage
(2V/div)
OUT
Voltage
(2V/div)
LOAD
Current
(1A/div)
LOAD
Current
(1A/div)
2ms/div
2ms/div
Rev. 0.5 September 2012
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Page 5 of 13
AOZ1282CI
Typical Performance Characteristics (continued)
Efficiency (Vo=5V)
vs. Load Current
Efficiency (Vo=3.3V)
vs. Load Current
95
95
90
85
80
75
70
65
60
55
12V–5V
5V–3.3V
90
85
12V–3.3V
24V–5V
80
18V–5V
75
24V–3.3V
70
65
60
55
18V–3.3V
50
50
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)
Current Limit vs. Input Voltage
(Vo=3.3V)
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
5
9
13
17
21
25
29
33
37
Input Voltage (V)
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AOZ1282CI
Detailed Description
The AOZ1282CI is a current-mode step down regulator
with integrated high side NMOS switch. It operates from
a 4.5V to 36V input voltage range and supplies up to
1.2A 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.
Switching Frequency
The AOZ1282CI switching frequency is fixed and set by
an internal oscillator. The switching frequency is set
internally 450kHz.
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.
The AOZ1282CI is available in SOT23-6L package.
Enable and Soft Start
The AOZ1282CI 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 the voltage higher
than UVLO and voltage on EN pin is HIGH. In soft start
process, the output voltage is ramped to regulation
voltage in typically 400µs. The 400µs soft start time is set
internally.
R
1
------
V
= 0.8 1 +
O
R
2
Some standard values of R1 and R2 for the most
commonly used output voltage values are listed in
Table 1.
The EN pin of the AOZ1282CI is active high. Connect the
EN pin to VIN if enable function is not used. Pull it to
ground will disable the AOZ1282CI. Do not leave it open.
The voltage on EN pin must be above 1.2 V to enable the
AOZ1282CI. When voltage on EN pin falls below 0.4V,
the AOZ1282CI is disabled.
Vo (V)
R1 (kΩ)
R2 (kΩ)
1.8
2.5
3.3
5.0
80.6
49.9
49.9
49.9
64.2
23.4
15.8
9.53
Steady-State Operation
Under steady-state conditions, the converter operates in
fixed frequency and Continuous-Conduction Mode
(CCM).
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.
The AOZ1282CI integrates an internal NMOS 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 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 external
Schottky diode to output.
Protection Features
The AOZ1282CI 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 1.9A. When the load current reaches the current
limit threshold, the cycle by cycle current limit circuit turns
off the high side switch immediately 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.
Rev. 0.5 September 2012
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Page 7 of 13
AOZ1282CI
The AOZ1282CI 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 FB pin voltage is below
0.2V, 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 disappears. In short circuit protection mode, the
inductor average current is greatly reduced.
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 V is half of
O
V , C is under the worst current stress. The worst
IN
IN
current stress on C is 0.5 x I .
IN
O
0.5
0.4
0.3
0.2
0.1
0
Under Voltage Lock Out (UVLO)
ICIN_RMS(m)
IO
An UVLO circuit monitors the input voltage. When the
input voltage exceeds 2.9V, the converter starts
operation. When input voltage falls below 2.3V, the
converter will stop switching.
Thermal Protection
0
0.5
m
1
An internal temperature sensor monitors the junction
temperature. It shuts down the internal control circuit and
high side NMOS if the junction temperature exceeds
150ºC. The regulator will restart automatically under the
control of soft-start circuit when the junction temperature
decreases to 110°C.
Figure 2. ICIN vs. Voltage Conversion Ratio
For reliable operation and best performance, the input
capacitors must have current rating higher than I
CIN-RMS
at worst operating conditions. Ceramic capacitors are
preferred for 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 also 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 is
based on certain amount of life time. Further de-rating
may be necessary for practical design requirement.
Application Information
The basic AOZ1282CI 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 PGND pin of the AOZ1282CI to maintain steady
input voltage and filter out the pulsing input current. The
voltage rating of input capacitor must be greater than
maximum input voltage plus ripple voltage.
Inductor
The input ripple voltage can be approximated by
equation below:
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:
I
V
V
O
O
O
-----------------
--------
--------
V
=
1 –
IN
f C
V
V
IN
IN
IN
V
V
O
O
----------
--------
I
=
1 –
L
f L
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:
V
IN
The peak inductor current is:
I
L
--------
I
= I +
Lpeak
O
2
V
V
O
O
--------
--------
I
= I
1 –
CIN_RMS
O
V
V
High inductance gives low inductor ripple current but
requires larger size inductor to avoid saturation. Low
ripple current reduces inductor core losses. It also
reduces RMS current through inductor and switches,
which results in less conduction loss.
IN
IN
if we let m equal the conversion ratio:
V
O
--------
= m
V
IN
Rev. 0.5 September 2012
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Page 8 of 13
AOZ1282CI
When selecting the inductor, make sure it is able to
handle the peak current without saturation even at the
highest operating temperature.
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.
The inductor takes the highest current in a buck circuit.
The conduction loss on inductor needs to be checked for
thermal and efficiency requirements.
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:
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 they
cost more than unshielded inductors. The choice
depends on EMI requirement, price and size.
I
L
----------
I
=
CO_RMS
12
Output Capacitor
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.
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.
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, Schottky diode is recommended to
use. The maximum reverse voltage rating of the chosen
Schottky diode should be greater than the maximum
input voltage, and the current rating should be greater
than the maximum load current.
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:
1
Thermal Management and Layout
Consideration
-------------------------
V = I ESR
+
O
L
CO
8 f C
O
In the AOZ1282CI 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
pins, to the filter inductor, to the output capacitor and
load, and then return 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.
where,
C is output capacitor value, and
O
ESR is the equivalent series resistance of the output
CO
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:
In PCB layout, minimizing the two loops area reduces the
noise of this circuit and improves efficiency. A ground
plane is strongly recommended to connect input
capacitor, output capacitor, and PGND pin of the
AOZ1282CI.
1
-------------------------
V = I
O
L
8 f C
O
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:
In the AOZ1282CI buck regulator circuit, the major power
dissipating components are the AOZ1282CI, the
Schottky diode and output inductor. The total power
dissipation of converter circuit can be measured by input
power minus output power.
V = I ESR
CO
O
L
P
= V I – V V
IN IN O IN
total_loss
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Rev. 0.5 September 2012
Page 9 of 13
AOZ1282CI
The power dissipation in Schottky can be approximated
as:
Several layout tips are listed below for the best electric
and thermal performance.
P
= I 1 – D V
O FW_Schottky
1. The input capacitor should be connected as close as
possible to the VIN pin and the GND pin.
diode_loss
2. The inductor should be placed as close as possible
to the LX pin and the output capacitor.
where,
V
is the Schottky diode forward voltage drop.
FW_Schottky
3. Keep the connection of the schottky diode between
the LX pin and the GND pin as short and wide
as possible.
The power dissipation of inductor can be approximately
calculated by output current and DCR of inductor.
2
4. Place the feedback resistors and compensation
components as close to the chip as possible.
P
= I R
1.1
inductor
inductor_loss
O
5. Keep sensitive signal traces away from the LX pin.
The actual junction temperature can be calculated with
power dissipation in the AOZ1282CI and thermal
impedance from junction to ambient.
6. Pour a maximized copper area to the VIN pin, the
LX pin and especially the GND pin to help thermal
dissipation.
P
–P
total_loss diode_loss inductor_loss
--------------------------------------------------------------------------------------------------------------------------
=
–P
T
7. Pour a copper plane on all unused board area and
connect the plane to stable DC nodes, like VIN,
GND or VOUT.
junction
+ T
JA
ambient
The maximum junction temperature of AOZ1282CI is
150ºC, which limits the maximum load current capability.
The thermal performance of the AOZ1282CI 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.
Rev. 0.5 September 2012
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Page 10 of 13
AOZ1282CI
Package Dimensions, SOT23-6
Gauge Plane
c
Seating Plane
0.25mm
D
e1
L
E
E1
θ1
b
e
A2
A
.010mm
A1
Dimensions in millimeters
Dimensions in inches
Symbols Min.
Nom. Max.
Symbols Min.
Nom. Max.
RECOMMENDED LAND PATTERN
A
A1
A2
b
c
D
E
E1
e
0.90
0.00
0.70
0.30
0.08
2.70
2.50
1.50
—
—
1.10
0.40
0.13
2.90
2.80
1.60
1.25
0.15
1.20
0.50
0.20
3.10
3.10
1.70
A
A1
A2
b
c
D
E
E1
e
0.035
0.00
—
—
0.049
0.006
1.20
0.028 0.043 0.047
0.012 0.016 0.020
0.003 0.005 0.008
0.106 0.114 0.122
0.098 0.110 0.122
0.059 0.063 0.067
0.037 BSC
2.40
0.80
0.95
0.63
0.95 BSC
1.90 BSC
—
UNIT: mm
e1
L
θ1
e1
L
θ1
0.075 BSC
0.30
0°
0.60
8°
0.012
0°
—
—
0.024
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 otherꢀise specified.
4. Folloꢀed from JEDEC MO-178C & MO-193C.
5. Controlling dimension is millimeter. Converted inch dimensions are not necessarily exact.
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Page 11 of 13
AOZ1282CI
Tape and Reel Dimensions, SOT23-6
Tape
P1
D1
P2
T
E1
E2
E
B0
D0
K0
A0
P0
Feeding Direction
Unit: mm
Package
SOT-23
A0
B0
K0
D0
D1
E
E1
E2
P0
P1
P2
T
3.15
0.10
3.27
0.10
1.34
0.10
1.10
0.01
1.50
0.10
8.00
0.20
1.75
0.10
3.50
0.05
4.00
0.10
4.00
0.10
2.00
0.10
0.25
0.05
Reel
W1
S
G
N
K
M
V
R
H
W
Unit: mm
Tape Size
8 mm
Reel Size
M
N
W
W1
H
K
S
G
R
V
ø180
ø180.00
0.50
ø60.50
Min.
9.00
0.30
11.40
ø13.00
10.60 2.00 ø9.00 5.00
0.50
18.00
1.0 +0.50 / -0.20
Leader/Trailer and Orientation
Leader Tape
500mm min. or
125 Empty Pockets
Trailer Tape
300mm min. or
75 Empty Pockets
Components Tape
Orientation in Pocket
Rev. 0.5 September 2012
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Page 12 of 13
AOZ1282CI
Part Marking
AOZ1282CI
(SOT23-6)
Assembly Lot Code
AX 2D
Week & Year Code
Assembly Location Code
Part Number Code
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.
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.
Rev. 0.5 September 2012
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