Rev. 1.0 June 2015 www.aosmd.com Page 1 of 12
AOZ1084
1.2A Buck LED Driver
General Description
The AOZ1084 is a high efficiency, simple to use, 1.2A
buck HB LED driver optimized for general lighting
applications. The AOZ1084 works from a 4.5V to 36V
input voltage range, and provides up to 1.2A of
continuous LED current. The 160mV LED current
feedback voltage minimizes the power dissipation of the
external sense resistor. The fixed switching frequency of
450kHz PWM operation reduces inductor and capacitor
sizes.
The AOZ1084 is available in a tiny DFN2x2-8L package.
Features
Up to 36V operating input voltage range
420m internal NMOS
Up to 95% efficiency
Internal compensation
1.2A continuous output current
Fixed 450kHz PWM operation
Internal soft start
160mV LED current feedback voltage with ±8%
accuracy
Cycle-by-cycle current limit
Short-circuit protection
Thermal shutdown
Small size DFN2x2-8L
Applications
General LED lighting
Architectural lighting
Signage lighting
Typical Application
Figure 1. 1.2A Buck HB LED Driver
LX
VIN BS
VIN
VOUT
FB
DIM
GND
C2
10µF
C3
C1
4.7µF
L1
2.2µH
AOZ1084
RS
LED1
AOZ1084
Rev. 1.0 June 2015 www.aosmd.com Page 2 of 12
Ordering Information
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
Pin Description
Part Number Ambient Temperatu r e Ra nge Package Environmental
AOZ1084DI -40 °C to +85 °C DFN2x2-8L Green Product
Pin Number Pin Name Pin Function
1 LX PWM output connection to inductor.
2VIN
Supply voltage input. Input range from 4.5V to 36V. When VIN rises above the UVLO
threshold the device starts up.
3VIN
Supply voltage input. Input range from 4.5V to 36V. When VIN rises above the UVLO
threshold the device starts up.
4 DIM PWM dimming pin. This pin is active high.
5FB
LED current feedback. The FB pin regulation voltage is 160mV. Connect an external
sense resistor between the cathode of the LED string and GND to set LED current.
6 GND Ground.
7 GND Ground.
8 BST Bootstrap voltage input. High side driver supply. Connected to 10nF capacitor between
BST and LX.
Exposed Pad EPAD Thermal exposed pad. Pad cam be connected to GND if necessary for improved thermal
performance.
BST
GND
GND
FB
LX
VIN
VIN
DIM
DFN2x2-8
(Top View)
1
2
3
4
8
7
6
5
AOZ1084
Rev. 1.0 June 2015 www.aosmd.com Page 3 of 12
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: 1.5kΩ in series with 100pF.
Recommended Operating Conditions
The device is not guaranteed to operate beyon d the
Recommended Operating Conditions.
Electrical Characteristics
TA = 25°C, VVIN = 12V, VEN = 12V, unless otherwise specified. Specifications in BOLD indicate a temperature range of -40°C to
+85°C. These specifications are guaranteed by design.
Parameter Rating
Supply Voltage (VVIN) 40V
LX to GND -0.7V to VVIN+ 2V
DIM to GND -0.3V to 36V
FB to GND -0.3V to 6V
BST to GND VLX + 6V
Junction Temperature (TJ) +150°C
Storage Temperature (TS) -65°C to +150°C
ESD Rating(1) 2kV
Parameter Rating
Supply Voltage (VVIN) 4.5V to 36V
Output Voltage Range Up to 0.85 * VVIN
Ambient Temperature (TA) -40°C to +85°C
Package Thermal Resistance (ΘJA)
DFN2x2-8L 55°C/W
Symbol Parameter Conditions Min. Typ. Max. Units
VVIN Supply Voltage 4.5 36 V
VUVLO Input Under-Voltage Lockout Threshold VVIN Rising
VVIN Falling 2.2
2.9 V
V
UVLO Hysteresis 200 mV
IVIN Supply Current (Quiescent) IOUT = 0, VFB = 1V, VEN > 1.2V 11.5mA
IOFF Shutdown Supply Current VEN = 0V 8 μA
VFB Feedback Voltage TA = 25ºC 147 160 173 mV
VFB_LOAD Load Regulation 120mA < Load < 1.08A 0.5 %
VFB_LINE Line Regulation Load = 600mA 0.03 %/V
IFB Feedback Voltage Input Current VFB = 160mV 100 nA
PWM DIMMING
VDim_OFF
VDim_ON
Dimming Input Threshold Off Threshold
On Threshold 1.2 0.4 V
V
VDim_HYS Dimming Input Hysteresis 200 mV
IEN Dimming Input Current 3μA
MODULATOR
fOFrequency 360 450 540 kHz
DMAX Maximum Duty Cycle 87 %
TON_MIN Minimum On Time 150 ns
ILIM Current Limit 1.5 1.9 2.3 A
Over-Temperature Shutdown Limit TJ Rising
TJ Falling
150
110
°C
°C
TSS Soft Start Interval 600 μs
POWER STATE OUTPUT
RDS(ON) NMOS On-Resistance VIN = 12V 420 m
ILEAKAGE NMOS Leakage VEN = 0V, VLX = 0V 10 μA
AOZ1084
Rev. 1.0 June 2015 www.aosmd.com Page 4 of 12
Block Diagram
GND
VIN
DIM
FB
Low Voltage
Regulator
DIM
Detection
OC
Detect
Short
Detect
OTP
Detect
Softstart
PWM
Logic
Error
Amplifier
–
+
0.25V
+
–PWM
Comparator
OSC
CLK
Current
Sense
Driver
BST
LDO
LX
BST
AOZ1084
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Typical Performance Characteristics
VIN = 12 V, Load = 1 White LED unless otherwise specified.
LED Short Test (36V/1 LED) LED Short Recovery (36V/1 LED)
LED Open to Normal (36V/3 LED)Normal to LED Open (36V/3 LED)
500μs/div500μs/div
50μs/div 500μs/div
VO
5V/div
VLX
10V/div
ILX
500mA/div
VO
5V/div
VLX
10V/div
DIM
5V/div
ILX
500mA/div
VO
5V/div
DIM
5V/div
VLX
10V/div
ILX
500mA/div
VLX
10V/div
VO
5V/div
ILX
500mA/div
200Hz Dimming Test (12V/3 LED)
2ms/div
DIM
5V/div
VO
10V/div
VLX
10V/div
ILX
500mA/div
AOZ1084
Rev. 1.0 June 2015 www.aosmd.com Page 6 of 12
Detailed Description
The AOZ1084 is a high efficiency, simple to use, 1.2A
buck HB LED driver optimized for general lighting
applications. Features include enable control, under
voltage lock-out, internal soft-start, output over-voltage
protection, over-current protection and thermal shut
down.
The AOZ1084 is available in a DFN2x2-8L package.
Soft Start and PWM Dimming
The AOZ1084 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 Dim pin is HIGH. In soft start
process, the output voltage is ramped to regulation
voltage in typically 600μs. The 600μs soft start time is set
internally.
The DIM pin of the AOZ1084 is active high. Connect the
DIM pin to VIN if enable function is not used. Pull it to
ground will disable the AOZ1084. Do not leave it open.
The voltage on DIM pin must be above 1.2V to enable
the AOZ1084. When voltage on DIM pin falls below 0.4V,
the AOZ1084 is disabled.
Steady-State Operation
Under steady-state conditions, the converter operates in
fixed frequency and Continuous-Conduction Mode
(CCM).
The AOZ1084 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.
Switching Frequency
The AOZ1084 switching frequency is fixed and set by an
internal oscillator. The switching frequency is set
internally 450kHz.
LED Current Programming
LED current can be set by feeding back the output to the
FB pin with the sense resistor RS shown in Figure 1.
The LED current can be programmed as:
Protection Features
The AOZ1084 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 2A. 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.
The AOZ1084 has internal short circuit protection to
protect itself from catastrophic failure under output short
circuit conditions. 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.
UVLO
An UVLO circuit monitors the input voltage. When the
input voltage exceeds 2.9V, the converter starts
operation. When input voltage falls below 2.2V, the
converter will stop switching.
Thermal Protection
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.
ILED 0.16
RS
-----------
=
AOZ1084
Rev. 1.0 June 2015 www.aosmd.com Page 7 of 12
Application Information
The basic AOZ1084 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 AOZ1084 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.
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 at 0.5 x IO.
Figure 2. ICIN vs. Voltage Conversion Ratio
For reliable operation and best performance, the input
capacitors must have current rating higher than ICIN-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 are
based on certain amount of 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 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.
When selecting the inductor, make sure it is able to
handle the peak current without saturation even 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 they
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.
ΔVIN 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
ΔILVO
fL×
-----------1VO
VIN
---------
–
×=
ILpeak IO
ΔIL
2
--------
+=
AOZ1084
Rev. 1.0 June 2015 www.aosmd.com Page 8 of 12
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.
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.
Thermal Management and Layout
Considerations
In the AOZ1084 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.
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
AOZ1084.
In the AOZ1084 buck regulator circuit, the major power
dissipating components are the AOZ1084, the Schottky
diode and output inductor. The total power dissipation of
converter circuit can be measured by input power minus
output power:
The power dissipation in the Schottky diode can be
approximated as:
where,
VFW_Schottky is the Schottky diode forward voltage
drop.
The power dissipation of the inductor can be
approximately calculated by output current and DCR of
the inductor:
ΔVOΔILESRCO 1
8fC
O
××
-------------------------
+
×=
ΔVOΔIL1
8fC
O
××
-------------------------
×=
ΔVOΔILESRCO
×=
ICO_RMS
ΔIL
12
----------
=
Ptotal_loss VIN IIN
×()VOIO
×()–=
Pdiode_loss IO1D–()VFW_Schottky
××=
Pinductor_loss IO2Rinductor 1.1××=
AOZ1084
Rev. 1.0 June 2015 www.aosmd.com Page 9 of 12
The actual junction temperature can be calculated with
power dissipation in the AOZ1084 and thermal
impedance from junction to ambient.
The maximum junction temperature of AOZ1084 is
150ºC, which limits the maximum load current capability.
The thermal performance of the AOZ1084 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. Input capacitor should be connected to the VIN pin
and the GND pin as close as possible.
2. The inductor should be placed as close as possible
the LX pin and the output capacitor.
3. Keep the connection of 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 trace far 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 copper plane on all unused board area and
connect it to stable DC nodes, like VIN,GND or
VOUT.
Tjunction
Ptotal_loss Pdiode_loss Pinductor_loss
––()Θ×JA
Tambient
+
=
AOZ1084
Rev. 1.0 June 2015 www.aosmd.com Page 10 of 12
Package Dimensions, DFN2x2-8L
TOP VIEW BOTTOM VIEW
SIDE VIEW
RECOMMENDED LAND PATTERN
Notes:
1. Dimensions and tolerances conform to ASME Y14.5M-1994.
2. Controlling dimension is millimeter, converted inch dimensions are not necessarily exact.
3. Dimension b applied to metallized terminal and is measured between 0.10mm and 0.30mm from the terminal tip. If the terminal
has the optional radius on the other end of the terminal, dimension b should not be measured in that radius area.
4. Coplanarity ddd applies to the terminals and all other bottom surface metallization.
Symbols
A
A1
b
c
D
D1
E
E1
e
L
R
aaa
bbb
ccc
ddd
Dimensions in millimeters
Min.
0.70
0.00
0.18
1.35
0.75
0.20
Nom.
0.75
0.02
0.25
0.20 REF.
2.00 BSC
1.50
2.00 BSC
0.90
0.50 BSC
0.30
0.20
0.15
0.10
0.10
0.08
Max.
0.80
0.05
0.30
1.60
1.00
0.40
Symbols
A
A1
b
c
D
D1
E
E1
e
L
R
aaa
bbb
ccc
ddd
Dimensions in inches
Min.
0.028
0.000
0.007
0.053
0.030
0.008
Nom.
0.030
0.001
0.010
0.008 REF.
0.079 BSC
0.059
0.079 BSC
0.035
0.020 BSC
0.012
0.008
0.006
0.004
0.004
0.003
Max.
0.031
0.002
0.012
0.063
0.039
0.016
UNIT: mm
E
DbE
L
D1
Chamfer
0.2 x 45
A1
Seating
Plane
A
0.50 0.25
1.70
1.50
0.30
0.90
0.85
c
E1
R
BOTTOM VIEW
Pin #1 ID
Option 2
Pin #1 ID
Option 1
AOZ1084
Rev. 1.0 June 2015 www.aosmd.com Page 11 of 12
Tape and Reel Dimensions, DFN2x2
Carrier Tape
Reel
Leader / Trailer
& Orientation
Tape Size
8mm
Reel Size
ø180
M
ø180.0
±0.50
N
60.0
±0.50
Trailer Tape
300mm Min.
Components Tape
Orientation in Pocket
Leader Tape
500mm Min.
Package
DFN 2x2
DFN 2x2A
DFN 2x2B
DFN2x2C
Option
1
2
A0
2.25
±0.05
B0
2.25
±0.05
K0
1.00
±0.05
D0
1.50
+0.10/-0
D1
1.00
+0.25/-0
E
8.00
+0.30/-0.10
E1
1.75
±0.10
E2
3.50
±0.05
P0
4.00
±0.10
P1
4.00
±0.10
P2
2.00
±0.05
T
0.254
±0.02
2.30
±0.20
2.30
±0.20
1.00
±0.20
1.50
+0.10/-0
1.50
MIN.
8.00
+0.30/-0.10
1.75
±0.10
3.50
±0.05
4.00
±0.20
4.00
±0.20
2.00
±0.05
0.30
±0.05
UNIT: mm
UNIT: mm
W1
8.4
+1.5/-0
H
13.0
±0.20
S
1.5
MIN.
K
13.5
MIN.
R
3.0
±0.50
Feeding Direction
R
S
H
K
K0
D1
T
B0
A0
D0
P0
P1
P2
E1
E2
E
A
A
A - A
M
W1
N
AOZ1084
Rev. 1.0 June 2015 www.aosmd.com Page 12 of 12
Part Marking
AOZ1084DI
(DFN2x2-8)
129B
Assembly Lot Code
Part Number Code
Option Code
Assembly Location Code
AU0A
Week & Year 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.
LEGAL DISCLA IM ER
Alpha and Omega Semiconductor makes no representations or warranties with respect to the accuracy or
completeness of the information provided herein and takes no liabilities for the consequences of use of such
information or any product described herein. Alpha and Omega Semiconductor reserves the right to make changes
to such information at any time without further notice. This document does not constitute the grant of any intellectual
property rights or representation of non-infringement of any third party’s intellectual property rights.
LIFE SUPPORT POLICY
ALPHA AND OMEGA SEMICONDUCTOR PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL
COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS.
