1
AAT1451
DATA SHEET
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202306A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 24, 2012
1. The maximum number of LEDs in each string is dependent upon the maximum VF of the diodes in that string. Under no event should the absolute maximum voltage at SW be
exceeded.
General Description
The AAT1451 is a highly integrated, high efficiency LED
backlight solution for notebook, netbook computers,
monitors and portable TVs. The device operates from DC
inputs, cigarette adapters and single to multi-cell Li-ion
batteries in the voltage range from 2.7V to 26V.
An integrated boost (step-up) converter provides a high
voltage output up to 50V for driving series LEDs. Four
precision current sinks are programmable up to 30mA
per string through one external RSET resistor, supporting
up to 481 white LEDs at 120mA total output current.
The boost output voltage is determined by the highest
total forward voltage of the LED strings, allowing for a
wide range of LED characteristics. Each string is PWM
dimmed with 90 degree phase shift to minimize ripple
currents, and filter capacitor sizes. The PWM input fre-
quency range is 100Hz to 10kHz with a dimming range of
256:1.
The integrated boost regulator switching frequency is
programmable from 600kHz to 1MHz by external resistor
for optimum efficiency and the smallest external L/C fil-
tering components.
Boost current mode control provides fast response to
line and load transients. Integrated light-load mode
ensures highest efficiency across the entire load range.
Fault tolerant circuitry extends system life by disabling
open and shorted LED(s) strings. The unique high volt-
age current sinks prevent damage resulting from shorted
LEDs. The FAULT pin indicates the presence of shorted
LEDs or over-temperature conditions.
The AAT1451 is available in a Pb-free, thermally
enhanced 16-pin 3x4 TDFN package.
Features
• VIN Range: 2.7V to 26V
•Integrated 50V Boost Converter
▪ Maximum IOUT: 120mA
▪ Programmable Switching Frequency
• 600kHz to 1MHz
▪ Up to 93% Efficiency
▪ High Efficiency Light-Load Mode
• Four White LED Strings
▪ Programmable Max Current Sink up to 30mA Each
▪ ±2% Accuracy (22mA)
▪ ±1.5% Matching (22mA)
•Direct PWM Dimming
▪ Automatic Phase Shifting
▪ Fast Turn-On/Off
•Integrated Fault Protection for
▪ Independent Disable of Open/Shorted LED(s)
String(s)
▪ Over-Voltage
▪ Over-Temperature
• FAULT Indication for Shorted LED(s) and Over-
Temperature
•Soft-Start Minimizes Inrush Current
•TDFN34-16 Low Profile Package
•-40°C to +85°C Temperature Range
Applications
• Tablets
• Notebook and Netbook Computers
• Portable Media Players
• Monitors
2
AAT1451
DATA SHEET
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202306A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 24, 2012
Typical Application Circuit (VIN = 2.7V)
SW
R2
OVP
D 1 Up to 48 WLEDs
AAT 1451
TDFN34-16
C
COMP
R1
FB1
FB2
FB3
FB4
COMP
ISET
VIN
C
VDD
2.2µF/6.3V
VDD
GND
FSLCT
C
IN
2.2µF/
50V
SHDN
PWM
V
OUT
: 45V at 120mA
R
FS
V
IN
= 2.7-26V
L1 4.7µH
R
SET
μC (GPIO)
R
COMP
FAULT
PGND
R3
VDD
5V
C
OUT
2.2µF/50V
Typical Application Circuit (VIN = 5.0V)
SW OVP
D1 Up to 48 WLED Backlight
AAT1451
TDFN34-16
CCOMP
FB1
FB2
FB3
FB4
COMP
COUT
2.2µF/
50V
ISET
VIN
CVDD
2.2µF/
6.3V
VDD
GND
FSLCT
CIN
2.2µF/
50V
SHDN
PWM
V
OUT
= 45V
at 120mA
RFS
VIN = 5V–26V L1 4.7µH
RSET
μC (GPIO)
RCOMP
FAULT
VDD
R3
R1
R2
3
AAT1451
DATA SHEET
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202306A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 24, 2012
Pin Descriptions
Pin # Symbol Function Description
1 PGND GND Power Ground. Connect to GND underneath the IC.
2 VIN I Input voltage to IC. Tied to input voltage source and input boost inductor.
3 FSLCT I Connect an external resistor to set boost switching frequency from 600kHz to 1MHz.
4 ISET I Connect resistor to ground to set maximum current up to 30mA through the LED strings.
5 FB1 O Output current sink 1. Connect to GND to disable channel 1.
6 FB2 O Output current sink 2. Connect to GND to disable channel 2.
7 FB3 O Output current sink 3. Connect to GND to disable channel 3.
8 FB4 O Output current sink 4. Connect to GND to disable channel 4.
9FAULT OOpen drain FAULT signal. Pull up to VDD with external resistor. Low indicates a shorted LED
condition.
10 VDD I/O
Internal regulated voltage when operating from input voltage range 5.0V to 26.0V. De-couple
with a 2.2µF capacitor to ground. Do not source current from this node. Connect to 5.0V rail
when operating from VIN less than 5.0V.
11 PWM I PWM input pin. Connect logic level PWM input signal in the frequency range 100Hz-10kHz to
this pin to enable PWM dimming.
12 SHDN I Logic high to enable the device. Logic low disables the device and minimizes quiescent current
and also disables the internal linear regulator.
13 COMP I Connect an external resistor in series with a capacitor to ground to compensate the boost
converter.
14 AGND AGND Connect to AGND
15 OVP I Over-voltage protection pin. Connect to output of boost converter through a resistor divider.
16 SW O Switching node of boost converter. Connect an inductor between this pin and input voltage
source. Connect the Schottky diode between this pin and boost output capacitor.
EP PGND Exposed paddle. Connect to PCB PGND plane. Input and output capacitor GND should connect
to EP.
Pin Configuration
TDFN34-16
(Top View)
FSLCT
ISET
FB1
PGND
VIN
3
FB2
FB3
FB4
GND
COMP
SHDN
SW
OVP
PWM
FAULT
VDD
4
5
1
2
6
7
8
14
13
12
16
15
11
10
9
4
AAT1451
DATA SHEET
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202306A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 24, 2012
1. Stresses above those listed in Absolute Maximum Ratings may cause permanent damage to the device. Functional operation at conditions other than the operating conditions
specified is not implied.
2. Based on long-term current density limitation.
3. Mounted on an FR4 board.
4. Derate 20mW/°C above 25°C.
Absolute Maximum Ratings1
Symbol Description Value Units
VSW Voltage to GND 50
V
VIN Input Voltage to GND -0.3 to 30
VFBx Output Current Sinks FB1 – FB4 to GND -0.3 to 40
VDD, VFAULT Low Voltage Pin to GND -0.3 to 7.0
SHDN, COMP, PWM,
ISET, FSLCT, OVP Voltage to GND -0.3 to VDD + 0.3
IOUT Maximum DC Output Current2134 mA
TJMaximum Junction Operating Temperature -40 to +150 OC
TLEAD Maximum Soldering Temperature (at leads, 10 sec.) 300
PDMaximum Power Dissipation32 W
ΘJA Thermal Resistance3,4 50 OC/W
Recommended Operating Conditions
Symbol Description Value Units
VIN Input Voltage Range 5 to 26 V
VOUT Output Voltage Range VIN + 3 to 45
FPWM PWM Dimming Frequency Range 0.1 to 10 kHz
TAOperating Ambient Temperature -40 to 85 OC
TJOperating Junction Temperature -40 to 130
5
AAT1451
DATA SHEET
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202306A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 24, 2012
1. The AAT1451 is guaranteed to meet performance specifications over the -40°C to +85°C operating temperature range and is assured by design, characterization, and correla-
tion with statistical process controls.
2. Output voltage must result in a voltage lower than the SW maximum ratings under all operating conditions.
Electrical Characteristics1
VIN = 12V; CIN = 2.2µF, COUT = 2.2µF; CVDD = 2.2µF; L1 = 4.7µH; RSET =7.5kΩ (IFBx = 22mA); RFSCLT = 20kΩ; TA = -40°C
to 85°C unless otherwise noted. Typical values are at TA = 25°C.
Symbol Description Conditions Min Typ Max Units
Power Supply, Current Sinks
VIN Input Voltage Range 5.0 26.0 V
VUVLO Under-Voltage Threshold
VIN Rising 4.3 V
Hysteresis 500 mV
VIN Falling 3.2 V
VDD VDD Output Voltage SHDN = Logic High, IDD(OUT) = 0mA 4.0 4.5 6.0 V
VFBx Current Sink Voltage SHDN = Logic High, IFBx = 22mA
(RSET = 7.5kΩ) 0.3 V
VFBx(SHORT) Shorted Diode(s) Detection Threshold IFBx= 30mA 5 V
IQIN Quiescent Current FB1-FB4 = Open, SHDN= Logic low 3 mA
ISD IN Pin Shutdown Current FB1-FB4 = Open, SHDN = VPWM = Logic
low, does not include SW leakage current 40.0 µA
IFBx Current Sink Accuracy IFBx = 22mA, TA = 25°C -5 ±2 +5 %
IFBx-Matching
Current Matching Between
Any Sink Channel IFBx= 22mA -2 ±1.5 +2 %
VOVP
Over Voltage Threshold VOUT Rising 1.1 1.2 1.3 V
Over Voltage Hysteresis VOUT Falling 100 mV
RDS(ON)LO Low Side Switch ON Resistance VDD = 4.5V 500 mΩ
DMAX Maximum Duty Cycle 90 %
TMIN Minimum On-Time 100 ns
VISET Voltage at ISET 0.6 V
VFSCLT Voltage at FSCLT 0.6 V
IFBx / IRSET Current Set Ratio IFBx/IISET , VISET = 0.6V 264 A/A
ILIMIT Low Side Switch Current Limit VIN = 5.0V to 26.0V 3.0 5.0 A
ILEAK
SW Pin Leakage SHDN = Logic Low, VSW = 45V 1 µA
FBx Pin Leakage VFBx = 30V, VPWM = logic high 10 µA
FOSC Oscillator Frequency RFS = 20kΩ 850 1000 1150 kHz
FPWMI(MAX) Maximum Input PWM Frequency1100 10000 Hz
FPWMO(MAX) Maximum Output PWM Frequency 6750 8000 9250 Hz
TSS Soft-Start Time VOUT = 35V, CCOMP = 18nF, RCOMP = 10kΩ 1.5 ms
Logic Level Inputs: SHDN, PWM
VLSHDN SHDN Threshold Low 0.4 V
VLPWM Threshold Low 0.8 V
VHPWM and SHDN Threshold High 2.2 V
ILK SHDN, PWM Input Leakage Current VPWM = VSHLD = VDD 10 µA
DPWMI Input PWM Duty Cycle 0 99 %
FAULT Output
VFAULTLOW FAULT Logic Output Low ISINK = 1mA 0.4 V
IFAU LT FAULT Leakage Current VFAU LT = 3.3V, No Faults ±1 µA
Thermal Protection
TJ(SD) TJ Thermal Shutdown Threshold 150 °C
TJ(SD-HYS) TJ Thermal Shutdown Hysteresis Maintains previous dimming setting 15 °C
6
AAT1451
DATA SHEET
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202306A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 24, 2012
Typical Characteristics
20
30
40
50
60
70
80
90
100
6810 12 14 16 18 20 22 24 26
VOUT = 40V
VOUT = 28V
Boost Efficiency vs Input Voltage
(L = 4.7µH; VDD = 5V; PWM = 5V;IOUT = 30mA/ch)
Input Voltage (V)
Efficiency (%)
10
20
30
40
50
60
70
80
90
100
0306090120 150180 210 240 270300
V
IN
= 12 V; V
OUT
= 40 V
V
IN
= 12 V; V
OUT
= 28 V
V
IN
= 26 V; V
OUT
= 40 V
Boost Efficiency vs Load Current
(L = 4.7µH; VDD = 5V; PWM = 5V; IOUT = 30mA/ch)
Output Current (mA)
Efficiency (%)
20
30
40
50
60
70
80
90
100
0102030405060708090100
VIN = 12 V; VOUT = 40V
VIN = 26 V; VOUT = 40V
VIN = 26 V; VOUT = 28V
Boost Efficiency vs PWM Duty Cycle
(L = 4.7µH; VDD = PWM = 5V; IOUT = 30mA/ch)
PWM Duty Cycle (%)
Efficiency (%)
0
20
40
60
80
100
120
020406080 100
Output Current vs PWM Duty Cycle
(VIN= 12V; IOUT= 30mA/ch)
Duty Cycle (%)
Output Current (mA)
2.8
3
3.2
3.4
3.6
3.8
4
4.2
4.4
-40-15 10 35 60 85
UVLO Rising
UVLO Falling
UVLO vs Temperature
Temperature (C°)
UVLO (V)
3.8
3.85
3.9
3.95
3.5
3.55
3.6
3.65
3.7
3.75
-40
-1510356
08
5
Current Limit vs Temperature
Temperature (C°)
Current Limit (A)
7
AAT1451
DATA SHEET
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202306A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 24, 2012
Typical Characteristics
2.5
2.6
2.7
2.8
2.9
3
3.1
3.2
3.3
3.4
3.5
6810 12 14 16 18 20 22 24 26
85C
25C
-40C
Quiscent Current vs Input Voltage
(Non-Swtiching)
Input Voltage (V)
Quiescent Current (mA)
4.3
4.35
4.4
4.45
4.5
4.55
4.6
4.65
4.7
6810 12 14 16 18 20 22 24 26
85C
25C
-40C
VDD Line Regulation vs Input Voltage
Input Voltage (V)
VDD (V)
6
8
10
12
14
16
18
0
2
4
6810 12 14 16 18 20 22 24 26
85C
25C
-40C
Shutdown Current vs Input Voltage
Input Voltage (V)
Shutdown Current (uA)
2
-1
0
1
2
3
4
-4
-3
2
-40
-1510356
08
5
Frequency vs Temperature
(VIN = 12V)
Temperature (C°)
Switching Frequency Error(%)
-4
-3
-2
-1
0
1
2
3
4
-40-15 10 35 60 85
Current Sink Accuracy vs Temperature
(VIN = 12V; 30mA/ch)
Temperature (C°)
Current Sink Accuracy (%)
0
0.25
0.5
0.75
1
1.25
1.5
1.75
2
-40-15 10 35 60 85
Current Sink Matching vs Temperature
(VIN=12V; 30mA/ch)
Temperature (C°)
Current Sink Matching (%)
8
AAT1451
DATA SHEET
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202306A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 24, 2012
Typical Characteristics
1.10
1.12
1.14
1.16
1.18
1.20
1.22
1.24
1.26
1.28
1.30
-40-15 10 35 60 85
Over Voltage Threshold vs Temperature
VIN=12V
Temperature (C°)
OVP Threshold (V)
0
100
200
300
400
500
600
-40-15 10 35 60 85
Low Side Switch On Resistance vs Temperature
Temperature (C°)
R
DS(ON)
(mµs)
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
68101214161820222426
-40C
25C
85C
Enable High Threshold Voltage vs. Input Voltag
e
Input Voltage (V)
Threshold Voltage (V)
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
6810 12 14 16 12 20 22 24 26
-40C
25C
85C
PWM, F
SET
Logic Low Threshold Voltage
vs. Input Voltage
Input Voltage (V)
Threshold Voltage (V)
0
Switching Waveforms
(VIN = 12V; VOUT = 37V; IOUT = 80mA)
Time (400ns/div)
VOUT
50mV/div
(AC)
IL 1A/div
VIN
50mV/div
(AC)
VSHDN
VOUT
20V/div
5V/div
I
OUT
50mA/div
I
L
500mA/div
Start UP
(VIN = 12V; VOUT = 40V; IOUT = 65mV; Duty Cycle = 50%)
Time (2ms/div)
9
AAT1451
DATA SHEET
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202306A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 24, 2012
Typical Characteristics
AAT1451
Four LED Strings–High Efficiency White LED Driver for LCD BacklightingSwitchRegTM
PRODUCT DATASHEET
AAT1451
Four LED Strings–High Efficiency White LED Driver for LCD BacklightingSwitchRegTM
PRODUCT DATASHEET
IOUT
50mA/div
IL
500mA/div
V
PWM
5V/div
0mA
V
OUT (AC)
200mV/div
50mA
0V
5V
0mA
500mA
PWM Switching Waveforms
(VIN = 12V; VOUT = 40V; Duty Cycle = 30%)
Time (200µs/div)
-100
-80
-60
-40
-20
0
20
40
60
IOUT
50mA/div
IL
500mA/div
V
PWM
5V/div
V
OUT (AC)
200mV/div
5V
0V
100mA
50mA
500mA
0mA
PWM Switching Waveforms
(VIN = 12V; VOUT = 40V; Duty Cycle = 80%)
Time (200µs/div)
-0.5
0.0
0.5
1.0
1.5
500mA
0mA
5V
0V
100mA
50mA
500mA
0mA
-100
-80
-60
-40
0
IOUT
50mA/div
IL
500mA/div
V
PWM
5V/div
V
OUT (AC)
200mV/div
PWM Switching Waveforms
(VIN = 12V; VOUT = 40V; Duty Cycle = 50%)
Time (200µs/div)
10
AAT1451
DATA SHEET
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202306A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 24, 2012
Functional Description
The AAT1451 adopts a synchronous peak current detect
step-up structure to drive up to 48 white LEDs with up
to 30mA each (120mA total) for backlight solutions. The
controller derives output feedback from the lowest sink
voltage of the four LED sink channels while maintaining
the programmed current accuracy and matching. This
ensures the lowest possible output voltage, highest effi-
ciency, and continuous operation with mismatched LED
strings. LED dimming is controlled by an external 100Hz
to 10kHz PWM signal. The LED current is on/off with
fixed frequency of 8kHz with the same duty cycle as the
PWM signal. This feature, together with the phase shift-
ing feature, makes it easy to filter the LED current
switching noise on the output when designing the sys-
tem.
The AAT1451 is designed for maximum flexibility allowing
unused current sinks to be disabled by connecting them
to ground. The unique high voltage current sinks support
non-matching LED strings (LED quantity, type, etc.)
The boost switching frequency is programmable from
600kHz up to 1MHz by external resistor for optimum effi-
ciency and the smallest external filter components.
Current mode control provides fast response to line and
load transients. Integrated light-load mode ensures high-
est efficiency across the entire input voltage and load
range.
The AAT1451 integrates several fault protection features
to deal with LED opens/shorts and thermal faults. Fault
tolerant circuitry extends system life by disabling current
sinks with open LEDs. The high voltage current sinks
maintain normal operation with non-matched strings
while also preventing damage due to shorted LEDs.
When all LED sinks are open, the over voltage protection
is active to prevent the boost output voltage from
becoming too high by disabling power MOSFET switching
when the OVP voltage threshold is exceeded.
Boost switching is re-enabled when OVP hysteresis is
satisfied. Over-current protection prevents inductor sat-
uration and any resulting damage to the switching device
occurring during an overload fault condition.
Functional Block Diagram
Logic
FB1
VDD Linear
Reg
SHDN
RSET
GND
OVP VREF
COMP
PWM
Osc
FSLCT
FB2
FB4
FB3
PGND FAULT
VINSW
11
AAT1451
DATA SHEET
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202306A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 24, 2012
Boost Converter Switching Frequency
The AAT1451’s boost converter frequency can be adjust-
ed between 600kHz to 1MHz using an external resistor
(RFS). For maximum accuracy, a 1% tolerance resistor is
recommended.
Please refer to Table 1 and Figure 1 for RFS resistor val-
ues.
2 · 1010
FSW
R
FS
=
RFS (kΩ) Frequency (kHz)
20 1000
22 909
24 833
26 769
28 714
30 667
33 606
Table 1: Examples of Standard 1% RFS Values for
Setting Switching Frequency.
R
FS
(kΩ)
18 20 22 24 26 28 30 32 34
Figure 1: Switch Frequency vs. RFS
Maximum LED Current Selection
The current sink is controlled by the internal reference
voltage (VISET) and the external resistor (RSET) at the ISET
pin. The maximum LED current programmable range is
from 15mA to 30mA by RSET
. For maximum accuracy, a 1%
tolerance resistor is recommended.
The RSET value can be calculated as follows:
CurrentSetRatio · V
ISET
I
FB
R
SET
=
Where CurrentSetRatio = 264 and VISET = 0.6V.
For example, if the maximum current for each string
LEDs is 30mA, this corresponds to a minimum resistor of
5.23 kΩ.
264 · 0.6V
30mA
R
SET
=
=
5.23kΩ
Maximum LED Current (mA) RSET (kΩ)
30 5.23
25 6.34
22 7.5
20 7.87
15 10.5
Table 2: Examples of Standard 1% RSET Values for
Setting Maximum LED Current Levels.
Please also refer to Figure 2 for quickly choosing a RSET
value.
R
SET
(kΩ)
I
CSx
(mA)
45678910 11 12 13 14
5
10
15
20
25
30
35
40
Figure 2: Choosing an RSET Value
12
AAT1451
DATA SHEET
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202306A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 24, 2012
PWM Dimming
The AAT1451 uses a simple PWM interface to control the
effective LED current (RMS) at the current sinks. The
PWM signal should fit the requirements listed in the elec-
trical characteristic table for proper operation. After ini-
tial power-up and SHDN is pulled to high together with
PWM high, the device is enabled with 100% brightness
as determined by the RSET resistor value. For example,
when the PWM pin is constantly pulled high, which
means 100% duty ratio, the current per channel is typi-
cally 30mA with RSET =5.3kΩ. By feeding the PWM pin
with a proper PWM signal, the RMS current of each sink
is proportional to the duty ratio of the PWM signal. Table
3 shows the average LED current of each channel at
maximum 22mA as the PWM duty cycle change.
The AAT1451 integrates a clock hunting circuitry to
derive the external PWM signal duty cycle to generate
same duty cycle LED current on/off between the maxi-
mum current value and 0mA with fixed 8kHz frequency.
It can work bi-directionally when the PWM signal
increases or decreases to determine the duty cycle.
PWM Duty Cycle
FB1 - FB4 Current (mA)
(RSET = 7.5KΩ)
100% 22
95% 20
90% 19
85% 18
80% 17
75% 16
70% 15
65% 14
60% 13
55% 12
50% 11
45% 9
40% 8
35% 7
30% 6
25% 5
20% 4
15% 3
10% 2
5% 1
Table 3: AAT1451 PWM Duty Cycle vs. LED
Current at Maximum 22mA Setting
0
5
10
15
20
25
30
35
0102030405060708090100
Current per Channel (mA)
PWM Duty Ratio(%)
PWM Duty vs WLED Current
AAT1451
Figure 3: PWM Duty Cycle vs. LED Current at
Maximum 30mA Setting.
Automatic Phase Shift PWM
The AAT1451 has implemented an automatic phase shift
PWM mechanism for FB1-FB4 current sources. It will auto-
matically detect the number of operating channels and
phase shift each channel, "n", by Θn relative to the PWM
input.
The phase shift Θ and delay time TD are defined as:
360 · (n - 1)
N
Θ
n
=
T
PWM
N
T
D
=
Where N is the number of operating channels, and n is
the target channel.
The FB1-FB4 timing diagram is shown in Figure 4 to
elaborate the automatic phase shift working waveform.
13
AAT1451
DATA SHEET
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202306A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 24, 2012
Open LED Protection and
FAULT Indication for Shorted LEDs
The AAT1451 device is protected from faults arising from
LED opens and shorts.
An open LED(s) condition will be detected by the control-
ler at startup. The low voltage is detected by the control-
ler which disables the given current sink. The remaining
LED strings continue to operate normally. The controller
re-enables the disabled current sink in the event that
the LED open condition is removed during a power cycle
or SHDN cycle. This feature extends backlight life and
reliability.
Under the condition that PWM duty cycle is less than
100%, shorted LEDs condition results in a higher voltage
appearing on the affected channels’ current-sink pin.
The affected current sink automatically compensates for
the additional voltage. This current sink can withstand a
high voltage indefinitely. However, the increased voltage
across the current sink causes an increase in power dis-
sipation. The AAT1451 automatically monitors the cur-
rent sink voltage for two or more shorted LEDs. To pre-
vent thermal shutdown, the shorted LED string is dis-
abled while the remaining strings continue to operate.
The shorted LED string remains disabled until a power
cycle or SHDN cycle. The open drain FAULT output is
driven low to indicate thermal shutdown and shorted
LED condition(s). The FAULT output is latched low during
shorted LED fault, and is reset after a power cycle,
SHDN cycle or thermal shutdown. To prevent damage,
the backlight can be shutdown based on the FAULT out-
put.
OVP Protection
Under all conditions, the over-voltage protection cir-
cuitry prevents the switching node (SW) from exceeding
the maximum operating voltage prior to disabling the
current sink. Over-voltage protection (OVP) disables
boost switching while maintaining the programmed LED
current. Boost switching is re-enabled when OVP hyster-
esis is satisfied.
Thermal Protection for
Over-Current and Short-Circuit
The AAT1451 has a built-in thermal protection circuit that
goes into shutdown when the die temperature rises
above the thermal limit, as is the case during a LED
short-circuit condition. Integrated over-current limit pro-
tection is provided. Over-current prevents inductor satu-
ration and any resulting damage to the switching device
occurring during an overload fault condition.
Duty Cycle = 50%
PWM
Input
ICS1
0°
ICS2
90°
ICS3
180°
ICS4
270°
TD
2TD
3TD
Figure 4: AAT1451 Automatic Phase Shift PWM Timing Diagram
14
AAT1451
DATA SHEET
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
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Application Information
LED Selection
The AAT1451 is specifically intended for driving white
LEDs. However, the device design will allow the AAT1451
to drive most types of LEDs with forward voltage speci-
fications typically ranging from 2.2V to 4.7V depending
upon supply voltage. LED applications may include
mixed arrangements for display backlighting, keypad
display, and any other application that needs a constant
current sink generated from a varying input voltage.
Since the FB1 to FB4 constant current sinks are matched
within 2% with negligible supply voltage dependence,
the constant current channels will be matched regardless
of the specific LED forward voltage (VF) levels. The low
dropout current sinks in the AAT1451 maximize perfor-
mance and make it capable of driving LEDs with high
forward voltages.
Shutdown
To activate the shutdown operation, the SHDN input for
the AAT1451 should be strobed low. In this case, the
AAT1451 typically draws less than 40µA from the input.
Inductor Selection
The white LED boost (step-up) converter is designed to
operate with a 4.7µH inductor for all input and output
voltage combinations. The inductor saturation current
rating should be greater than the NMOS current limit.
V
OUT
+ V
D
- V
IN(MIN)
V
OUT
+ V
D
D
MAX
=
Where:
VOUT is the boost converter output voltage;
VD is the forward voltage of Schottky diode;
VIN(MIN) is the minimum input voltage.
The output inductor (L) is selected to avoid saturation at
minimum input voltage, maximum output load condi-
tions. Peak current may be calculated from the following
equation, again assuming continuous conduction mode.
Worst-case peak current occurs at minimum input volt-
age (maximum duty cycle) and maximum load. Switching
frequency is estimated at 600kHz with a 4.7µH inductor.
I
OUT
1 - D
MAX
D
MAX
· V
IN(MIN)
2 · F
S
· L
I
PEAK
= +
Compensation Component Selection
The AAT1451 Main Boost architecture uses peak current
mode control to eliminate the double pole effect of the
output L&C filter and simplifies the compensation loop
design. The current mode control architecture simplifies
the transfer function of the control loop to be a one-pole,
one left plane zero and one right half plane (RHP) system
in frequency domain. The dominant pole can be calcu-
lated by:
1
2π · R
O
· C
OUT
f
P
=
The ESR zero of the output capacitor can be calculated
by:
1
2π · R
ESR
· C
OUT
f
Z_ESR
=
Where:
COUT is the output filter capacitor;
RO is the equivalent load resistor value;
RESR is the equivalent series resistance of the output
capacitor.
The right half plane (RHP) zero can be determined by:
V
IN2
2π · L
1
· I
OUT
· V
OUT
f
Z_ESR
=
It is recommended to design the bandwidth to one
decade lower than the frequency of RHP zero to guaran-
tee the loop stability. A series capacitor and resistor
network (RCOMP and CCOMP) connected to the COMP pin
sets the pole and zero which are given by:
1
2π · R
EA
· C
COMP
f
P_COM
=
1
2π · R
COMP
· C
COMP
f
Z_COM
=
Where:
CCOMP is the compensation capacitor;
RCOMP is the compensation resistor;
REA is the output resistance of the error amplifier (MΩ).
A 15nF capacitor and a 20kΩ resistor in series are cho-
sen for optimum phase margin and fast transient
response.
15
AAT1451
DATA SHEET
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202306A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 24, 2012
Capacitor Selection
Careful selection of the external capacitor CIN is important
because it will affect turn-on time and transient perfor-
mance. Optimum performance will be obtained when low
equivalent series resistance (ESR) ceramic capacitor is
used; in general, low ESR may be defined as less than
100mΩ. A value of 2.2µF for the input capacitor is a good
starting point when choosing a capacitor. If the constant
current sinks are only programmed for light current lev-
els then the input capacitor size may be decreased.
Capacitor Characteristics
Ceramic composition capacitor is highly recommended
over all other types of capacitors for use with the
AAT1451. Ceramic capacitors offer many advantages
over their tantalum and aluminum electrolytic counter-
parts. A ceramic capacitor typically has very low ESR, is
lower cost, has a smaller PCB footprint, and is non-po-
larized. Since ceramic capacitors are non-polarized, they
are not prone to incorrect connection damage.
Equivalent Series Resistance
ESR is an important characteristic to consider when
selecting a capacitor. ESR is a resistance internal to a
capacitor that is caused by the leads, internal connec-
tions, size or area, material composition, and ambient
temperature. Capacitor ESR is typically measured in mil-
liohms for ceramic capacitors and can range to more
than several ohms for tantalum or aluminum electrolytic
capacitors.
Ceramic Capacitor Materials
Ceramic capacitor less than 0.1µF are typically made
from NPO or C0G materials. NPO and C0G materials gen-
erally have tight tolerance and are very stable over tem-
perature. Larger capacitor values are usually composed
of X7R, X5R, Z5U or Y5V dielectric materials. Large
ceramic capacitors (i.e. larger than 4.7µF) are often
available in low cost Y5V and Z5U dielectrics, but capac-
itors larger than 4.7µF are not typically required for
AAT1451 applications.
Capacitor area is another contributor to ESR. Capacitors
that are physically large will have a lower ESR when
compared to an equivalent material smaller capacitor.
These larger devices can improve circuit transient
response when compared to an equal value capacitor in
a smaller package size.
PCB Layout Considerations
When designing a PCB for the AAT1451, the key require-
ments are:
1. Place the input and output decoupling capacitors CIN
and COUT as close to the chip as possible to reduce
switching noise and output ripple.
2. Place the bypass capacitor CVDD as close to the chip as
possible.
3. Keep the power traces (GND, SW, and VIN) short,
direct, and wide to allow large current flow. Place suf-
ficient multiple-layer pads when needed to change the
trace layer.
4. Connect the output capacitor COUT
, output inductor L1
and Schottky diode DS1 as close as possible. Use con-
nections as short as possible for L1 to the SW pins and
place no signal lines under the inductor.
5. Place the peripheral components like RCOMP
, CCOMP
, RSET
and RFS as close to the chip as possible.
Evaluation Board User Interface
The user interface for the AAT1451 evaluation board is
provided by three buttons and two connection terminals.
The board is operated by supplying external power and
pressing individual buttons. Table 4 indicates the func-
tion of each button or button combination. To power-on
the evaluation board, connect a power supply or battery
to both the VIN (with 5 to 26V) and the VCC (with 2.2 to
5V) terminals.
A red LED indicates that VCC power is applied which is
necessary to enable the AAT1451. Once one button is
pressed, the green LED will flash once to indicate that
the related action is processed.
16
AAT1451
DATA SHEET
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202306A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 24, 2012
User Interface Functionality
Button(s) Pushed Description
UP [Push/Release once] Channels FB1 to FB4 are turned on with 1mA per channel. With every push/release
the current is increased according to Table 3.
DOWN [Push/Release once] Channels FB1 to FB4 are turned on with 22mA per channel. With every push/re-
lease the current is decreased according to Table 3.
CYCLE [Push/Release once] Auto cycling up and down.
Table 4: AAT1451 Evaluation Board User Interface.
GND
VIN
5V-26V
VCC
2.2V-5V
12 3
J1
DC+
VDD
9
VIN
2
GND
PWM
11
FAULT 10
FB1 5
COMP
13
EP
014
FB4 8
PGND
1
SW
16
OVP 15
FSLCT
3
ISET
4
FB2 6
FB3 7
SHDN
12
U1
AAT1451
CIN
2.2µF/50V
C2
0.1µF
CVDD
2.2µF
6.3V
CCOMP
15nF
COUT
2.2µF
50V
RCOMP
20k
RSET
7.5k
RFS
20k
L1 4.7µH DS1
SS16L
R1
42.7k R2
1.2k
R3
10k VCC
R4
10k
R5
10k
EN
0.1µF
C1R8
1k 1k
R7
DOWN
S2
UP
S1
CYCLE
S3
S1
S2
S3
R9
1k
R10
1k VDD
1
GP5
2
GP4
3
GP3
4GP2 5
GP1 6
GP0 7
VSS 8
U2 PIC12F675
LED1
Red
PWM
VCC
1
2
3
45
J3
R6
330
LED2
Green
VOUT VOUT
FLT
FB1
FB2
FB3
FB4
SW
MCU VMCU
Figure 5: AAT1451 Evaluation Board Schematic.
17
AAT1451
DATA SHEET
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202306A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 24, 2012
a: Top Side b: Bottom Side
Figure 6: AAT1451 Evaluation Board Layout.
Component Part Number Description Manufacturer
U1 AAT1451 High Efciency White Backlight LED Driver Skyworks
U2 PIC12F675 8-bit CMOS, FLASH-based μC; 8-pin PDIP package Microchip
S1 – S3 PTS645TL50 Switch Tact, SPST, 5mm ITT Industries
RCOMP
, RFS Chip Resistor 20kΩ, 1%, 1/4W; 0603 Vishay
RSET Chip Resistor 7.5kΩ, 1%, 1/4W; 0603 Vishay
R1 Chip Resistor 42.7kΩ, 1%, 1/4W; 0603 Vishay
R2 Chip Resistor 1.2kΩ, 1%, 1/4W; 0603 Vishay
R3, R4, R5 Chip Resistor 10kΩ, 1%, 1/4W; 0603 Vishay
R6 Chip Resistor 330Ω, 1%, 1/4W; 0603 Vishay
R7, R8, R9, R10 Chip Resistor 1kΩ, 1%, 1/4W; 0603 Vishay
CIN, COUT GRM31CR71H225KA88 2.2μF, 50V, X7R, 1206 Murata
CVDD GCM188R70J225KE22 2.2μF, 6.3V, X7R, 0603 Murata
CCOMP GRM188R71H153KA01 15nF, 50V, X7R, 0603 Murata
C1, C2 GRM188R71H104KA93 0.1μF, 50V, X7R, 0603 Murata
L1 SD53-4R7-R 4.7µH, 45mΩ, 2.01A, 20% Coiltronics
DS1 SS16L 1.0A, 60V Surface Mount Schottky Barrier Rectier TSC
LED1 CMD15-21SRC/TR8 Red LED; 1206 Chicago Miniature Lamp
LED2 CMD15-21UGC/TR8 Green LED; 1206 Chicago Miniature Lamp
Table 5: AAT1451 Evaluation Board BOM List.
Manufacturer Part Number L (µH) Max DCR (mΩ) Saturation Current (A) Size WxLxH (mm)
Murata LQH6PPN4R7M43 4.7 20 3.2 6.0x6.0x4.3
LQH6PPN6R8M43 6.8 28 2.8
Coiltronics SD53-4R7-R 4.7 45 2.01 5.2x5.2x3.0
SD53-6R8-R 6.8 68 1.65
Table 6: Surface Mount Inductors.
18
AAT1451
DATA SHEET
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
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202306A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 24, 2012
Manufacturer Part Number Value (µF) Voltage (V) Tolerance Temp. Co. Case
Murata
GCM188R70J225KE22 2.2 6.3 10% X7R 0603
GRM188R71H153KA01 0.015 50 10% X7R 0603
GRM188R71H104KA93 0.1 50 10% X7R 0603
GRM31CR71H225KA88 2.2 50 10% X7R 1206
AVX
06036C225KAT 2.2 6.3 10% X7R 0603
06035C163KAT 0.015 50 10% X7R 0603
06035C104KAT 0.1 50 10% X7R 0603
12065C225KAT 2.2 50 10% X7R 1206
KEMET
C0603C225K9RAC 2.2 6.3 10% X7R 0603
C0603C153K5RAC 0.015 50 10% X7R 0603
C0603C104K5RAC 0.1 50 10% X7R 0603
C1206C225K5RAC 2.2 50 10% X7R 1206
Table 7: Surface Mount Capacitors.
19
AAT1451
DATA SHEET
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
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202306A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 24, 2012
2.2µF
2.2µF
C
VDD
15nF
C
COMP
2.2µF/50V
C
OUT1
20k
R
COMP
7.5k
R
SET
20k
R
FS
4.7µH
L1 SS16L
DS1
VOUT
Battery
2.7-4.2V
SHDN
VOUT VIN
GNDC+
C-
AAT3110IJS-5.0-T1
U2
10µF
C
OUT2
10µF
C
IN2
1µF
C
FLY
4.7µF
C
IN
ON/OFF
signal
LED
6S4P~8S4P
1
2
3
5
4
6
VDD
9
VIN
2
GND
14
PWM
11 FB 15
COMP
13
EP
0
FB 4
7
SW
16
OVP15
FSLCT
3
ISET
4
FB 2
FB 3
6
SHDN
12
AAT1451
U1 42.7k
R1
1.2k
R2
FAULT10 10k
R3
PGND
1
8
VCC
C
IN1
Figure 7: Schematic of AAT1451 plus AAT3110
50
55
60
65
70
75
80
85
90
95
100
0510 15 20 25
Efficiency (%)
I
LED
(mA)
Efficiency
V
IN
= 3V
V
IN
= 3.6V
V
IN
= 4.2V
Figure 8: Efficiency vs ILED for driving 8series - 4parallel (8S4P) LEDs
Single Li-ion Cell Powered Application:
Figure 7 demonstrates a backlight solution for single cell
Li-ion battery powered application using the AAT1451 to
drive the WLEDs and the AAT3110 regulated charge pump
to supply the internal regulator of AAT1451. The AAT1451
plus AAT3110 solution is adopted to drive 6 series-4 paral-
lel (6S4P) to 8 series-4 parallel (8S4P) typical of 13" and
smaller sized displays. Figure 8 shows the efficiency.
20
AAT1451
DATA SHEET
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
202306A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 24, 2012
Copyright © 2012 Skyworks Solutions, Inc. All Rights Reserved.
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works may change its documentation, products, services, specications or product descriptions at any time, without notice. Skyworks makes no commitment to update the materials or information and shall have no
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Ordering Information
Package Part Marking1Part Number (Tape and Reel)2
TDFN34-16 N5XYY AAT1451IRN-T1
Skyworks Green™ products are compliant with
all applicable legislation and are halogen-free.
For additional information, refer to Skyworks
Definition of Green™, document number
SQ04-0074.
Package Information
TDFN34-16
3.000 ± 0.050 1.600 ± 0.050
0.000 + 0.100
-0.000 0.203 REF
0.750 ± 0.050
4.000 ± 0.050
3.300 ± 0.050
Index Area
Detail "A"
Top View Bottom View
Side View
0.450 ± 0.050
0.230 ± 0.0500.450 ± 0.050
Detail "A"
All dimensions in millimeters.
1. XYY = assembly and date code.
2. Sample stock is generally held on part numbers listed in BOLD.