202306A.pdf

DATA SHEET
AAT1451
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
General Description
Features
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.
•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 OverTemperature
• Soft-Start Minimizes Inrush Current
• TDFN34-16 Low Profile Package
• -40°C to +85°C Temperature Range
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 frequency 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 filtering 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 voltage current sinks prevent damage resulting from shorted
LEDs. The FAULT pin indicates the presence of shorted
LEDs or over-temperature conditions.
Applications
•Tablets
• Notebook and Netbook Computers
• Portable Media Players
•Monitors
The AAT1451 is available in a Pb-free, thermally
enhanced 16-pin 3x4 TDFN package.
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.
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
DATA SHEET
AAT1451
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Typical Application Circuit (VIN = 2.7V)
VIN = 2.7-26V
L1
4.7µH
D1
Up to 48 WLEDs
VOUT: 45V at 120mA
R1
CIN
2.2µF/
50V
VIN
SW
COUT
2.2µF/50V
OVP
R2
AAT1451
TDFN34-16
5V
VDD
CVDD
2.2µF/6.3V
μC (GPIO)
CCOMP
FB1
FB2
FB3
FB4
SHDN
PWM
COMP
VDD
ISET
RSET
RCOMP
R3
FSLCT
RFS
FAULT
PGND GND
Typical Application Circuit (VIN = 5.0V)
VIN = 5V–26V
L1 4.7µH
Up to 48 WLED Backlight
D1
VOUT = 45V
at 120mA
COUT
2.2µF/
50V
R1
CIN
2.2µF/
50V
CVDD
2.2µF/
6.3V
VIN
VDD
SW
AAT1451
TDFN34-16
SHDN
μC (GPIO)
FB1
FB2
FB3
FB4
PWM
COMP
CCOMP
RCOMP
2
OVP
ISET
RSET
FSLCT
RFS
R2
VDD
R3
FAULT
GND
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
DATA SHEET
AAT1451
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Pin Descriptions
Pin #
Symbol
Function
1
2
3
4
5
6
7
8
PGND
VIN
FSLCT
ISET
FB1
FB2
FB3
FB4
GND
I
I
I
O
O
O
O
9
FAULT
O
10
VDD
I/O
11
PWM
I
12
SHDN
I
13
COMP
I
14
15
AGND
OVP
AGND
I
16
SW
O
EP
PGND
Description
Power Ground. Connect to GND underneath the IC.
Input voltage to IC. Tied to input voltage source and input boost inductor.
Connect an external resistor to set boost switching frequency from 600kHz to 1MHz.
Connect resistor to ground to set maximum current up to 30mA through the LED strings.
Output current sink 1. Connect to GND to disable channel 1.
Output current sink 2. Connect to GND to disable channel 2.
Output current sink 3. Connect to GND to disable channel 3.
Output current sink 4. Connect to GND to disable channel 4.
Open drain FAULT signal. Pull up to VDD with external resistor. Low indicates a shorted LED
condition.
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.
PWM input pin. Connect logic level PWM input signal in the frequency range 100Hz-10kHz to
this pin to enable PWM dimming.
Logic high to enable the device. Logic low disables the device and minimizes quiescent current
and also disables the internal linear regulator.
Connect an external resistor in series with a capacitor to ground to compensate the boost
converter.
Connect to AGND
Over-voltage protection pin. Connect to output of boost converter through a resistor divider.
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.
Exposed paddle. Connect to PCB PGND plane. Input and output capacitor GND should connect
to EP.
Pin Configuration
TDFN34-16
(Top View)
PGND
VIN
FSLCT
ISET
FB1
FB2
FB3
FB4
1
16
2
15
3
14
4
13
5
12
6
11
7
10
8
9
SW
OVP
GND
COMP
SHDN
PWM
FAULT
VDD
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
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3
DATA SHEET
AAT1451
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Absolute Maximum Ratings1
Symbol
VSW
VIN
VFBx
VDD, VFAULT
SHDN, COMP, PWM,
ISET, FSLCT, OVP
IOUT
TJ
TLEAD
PD
ΘJA
Description
Value
Voltage to GND
Input Voltage to GND
Output Current Sinks FB1 – FB4 to GND
Low Voltage Pin to GND
Voltage to GND
Units
50
-0.3 to 30
-0.3 to 40
-0.3 to 7.0
V
-0.3 to VDD + 0.3
Maximum DC Output Current
Maximum Junction Operating Temperature
Maximum Soldering Temperature (at leads, 10 sec.)
Maximum Power Dissipation3
Thermal Resistance3,4
2
134
-40 to +150
300
2
50
mA
O
O
C
W
C/W
Recommended Operating Conditions
Symbol
VIN
VOUT
FPWM
TA
TJ
Description
Input Voltage Range
Output Voltage Range
PWM Dimming Frequency Range
Operating Ambient Temperature
Operating Junction Temperature
Value
5 to 26
VIN + 3 to 45
0.1 to 10
-40 to 85
-40 to 130
Units
V
kHz
O
C
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.
4
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
DATA SHEET
AAT1451
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
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
Power Supply, Current Sinks
VIN
Input Voltage Range
VUVLO
Under-Voltage Threshold
VDD
VDD Output Voltage
VFBx
Current Sink Voltage
VFBx(SHORT)
IQ
FOSC
FPWMI(MAX)
FPWMO(MAX)
TSS
26.0
4.3
V
V
mV
V
V
500
3.2
4.0
4.5
6.0
0.3
V
5
V
FB1-FB4 = Open, SHDN= Logic low
FB1-FB4 = Open, SHDN = VPWM = Logic
low, does not include SW leakage current
IFBx = 22mA, TA = 25°C
3
mA
-5
IFBx= 22mA
VOUT Rising
VOUT Falling
VDD = 4.5V
Current Sink Accuracy
Current Matching Between
Any Sink Channel
Over Voltage Threshold
Over Voltage Hysteresis
Low Side Switch ON Resistance
Maximum Duty Cycle
Minimum On-Time
Voltage at ISET
Voltage at FSCLT
Current Set Ratio
Low Side Switch Current Limit
SW Pin Leakage
FBx Pin Leakage
Oscillator Frequency
Maximum Input PWM Frequency1
Maximum Output PWM Frequency
Soft-Start Time
ILEAK
Units
IFBx= 30mA
IFBx
RDS(ON)LO
DMAX
TMIN
VISET
VFSCLT
IFBx / IRSET
ILIMIT
5.0
VIN Rising
Hysteresis
VIN Falling
SHDN = Logic High, IDD(OUT) = 0mA
SHDN = Logic High, IFBx = 22mA
(RSET = 7.5kΩ)
Max
IN Quiescent Current
IN Pin Shutdown Current
VOVP
Typ
Shorted Diode(s) Detection Threshold
ISD
IFBx-Matching
Min
40.0
µA
±2
+5
%
-2
±1.5
+2
%
1.1
1.2
100
500
1.3
V
mV
mΩ
%
ns
V
V
A/A
A
µA
µA
kHz
Hz
Hz
ms
90
IFBx/IISET , VISET = 0.6V
VIN = 5.0V to 26.0V
SHDN = Logic Low, VSW = 45V
VFBx = 30V, VPWM = logic high
RFS = 20kΩ
100
0.6
0.6
264
3.0
850
100
6750
VOUT = 35V, CCOMP = 18nF, RCOMP = 10kΩ
1000
8000
1.5
5.0
1
10
1150
10000
9250
Logic Level Inputs: SHDN, PWM
VLSHDN
SHDN Threshold Low
VL
PWM Threshold Low
VH
PWM and SHDN Threshold High
ILK
DPWMI
SHDN, PWM Input Leakage Current
Input PWM Duty Cycle
0.4
V
0.8
V
99
µA
%
0.4
±1
V
µA
2.2
VPWM = VSHLD = VDD
V
10
0
FAULT Output
VFAULTLOW
FAULT Logic Output Low
IFAULT
FAULT Leakage Current
Thermal Protection
TJ(SD)
TJ Thermal Shutdown Threshold
TJ(SD-HYS)
TJ Thermal Shutdown Hysteresis
ISINK = 1mA
VFAULT = 3.3V, No Faults
Maintains previous dimming setting
150
15
°C
°C
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 correlation with statistical process controls.
2. Output voltage must result in a voltage lower than the SW maximum ratings under all operating conditions.
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
5
DATA SHEET
AAT1451
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Typical Characteristics
Boost Efficiency vs Input Voltage
Boost Efficiency vs Load Current
(L = 4.7µH; VDD = 5V; PWM = 5V; IOUT = 30mA/ch)
100
100
90
90
80
80
Efficiency (%)
Efficiency (%)
(L = 4.7µH; VDD = 5V; PWM = 5V;IOUT = 30mA/ch)
70
60
50
40
VOUT = 40V
30
70
60
50
40
VIN = 12 V; VOUT = 40 V
30
VIN = 12 V; VOUT = 28 V
VIN = 26 V; VOUT = 40 V
20
VOUT = 28V
10
20
6
8
10
12
14
16
18
20
22
24
26
Input Voltage (V)
0
150
180
210
240
270
300
(VIN= 12V; IOUT= 30mA/ch)
Output Current (mA)
90
Efficiency (%)
120
120
100
80
70
60
VIN = 12 V; VOUT = 40V
50
VIN = 26 V; VOUT = 40V
40
VIN = 26 V; VOUT = 28V
30
0
10
20
30
40
50
60
70
80
90
100
80
60
40
20
0
100
PWM Duty Cycle (%)
0
20
40
60
80
100
Duty Cycle (%)
UVLO vs Temperature
Current Limit vs Temperature
4.4
3.95
4.2
3.9
4
3.85
Current Limit (A)
UVLO (V)
90
Output Current vs PWM Duty Cycle
(L = 4.7µH; VDD = PWM = 5V; IOUT = 30mA/ch)
3.8
3.6
3.4
3.2
UVLO Rising
3
UVLO Falling
-40
-15
10
35
60
3.8
3.75
3.7
3.65
3.6
3.55
2.8
3.5
85
-40
Temperature (C°)
6
60
Output Current (mA)
Boost Efficiency vs PWM Duty Cycle
20
30
-15
10
35
60
Temperature (C°)
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
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85
DATA SHEET
AAT1451
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Typical Characteristics
Quiscent Current vs Input Voltage
VDD Line Regulation vs Input Voltage
(Non-Swtiching)
4.7
4.65
3.4
4.6
3.3
3.2
VDD (V)
Quiescent Current (mA)
3.5
3.1
3
2.9
4.55
4.5
4.45
85C
4.4
2.8
85C
2.7
25C
4.35
2.6
-40C
4.3
25C
-40C
6
2.5
6
8
10
12
14
16
18
20
22
24
Input Voltage (V)
Switching Frequency Error(%)
Shutdown Current (uA)
14
12
10
8
6
85C
4
25C
2
-40C
0
12
14
16
18
22
24
26
20
22
24
26
1
0
-1
2
-3
-4
-40
-15
Current Sink Matching (%)
2
1
0
-1
-2
-3
-4
Temperature (C°)
60
85
(VIN=12V; 30mA/ch)
3
60
35
Current Sink Matching vs Temperature
4
35
10
Temperature (C°)
(VIN = 12V; 30mA/ch)
Current Sink Accuracy (%)
20
2
Current Sink Accuracy vs Temperature
10
18
3
-15
16
(VIN = 12V)
4
Input Voltage (V)
-40
14
Frequency vs Temperature
16
10
12
18
8
10
Input Voltage (V)
Shutdown Current vs Input Voltage
6
8
26
2
1.75
1.5
1.25
1
0.75
0.5
0.25
0
-40
85
-15
10
35
60
85
Temperature (C°)
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
7
DATA SHEET
AAT1451
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Typical Characteristics
Low Side Switch On Resistance vs Temperature
Over Voltage Threshold vs Temperature
VIN=12V
1.30
600
500
1.26
1.24
RDS(ON) (mµs)
OVP Threshold (V)
1.28
1.22
1.20
1.18
1.16
1.14
400
300
200
100
1.12
1.10
-40
-15
10
35
60
0
85
Temperature (C°)
-40
10
1.4
1.3
1.3
Threshold Voltage (V)
1.4
1.2
1.1
1.0
0.9
-40C
0.8
25C
0.7
35
60
85C
0.6
PWM, FSET Logic Low Threshold Voltage
vs. Input Voltage
1.2
1.1
1.0
0.9
0.8
-40C
25C
0.7
85C
0.6
6
8
10
12
14
16
18
20
22
Input Voltage (V)
24
26
6
8
10
12
14
16
12
20
Start UP
Switching Waveforms
VOUT
20V/div
VSHDN
VOUT
50mV/div
(AC)
8
24
26
(VIN = 12V; VOUT = 40V; IOUT = 65mV; Duty Cycle = 50%)
VIN
50mV/div
(AC)
5V/div
IOUT
50mA/div
IL
0
500mA/div
Time (400ns/div)
22
Input Voltage (V)
(VIN = 12V; VOUT = 37V; IOUT = 80mA)
IL 1A/div
85
Temperature (C°)
Enable High Threshold Voltage vs. Input Voltage
Threshold Voltage (V)
-15
Time (2ms/div)
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
DATA
SHEET
PRODUCT
DATASHEET
AAT1451
AAT1451
SwitchRegTM
Four LED Strings,
Strings–High
Efficiency White
White LED
LED Driver
Driver for
for LCD
LCD Backlighting
Backlighting
Four-LED
High Efficiency
Typical Characteristics
PWM Switching Waveforms
PWM Switching Waveforms
(VIN = 12V; VOUT = 40V; Duty Cycle = 30%)
(VIN = 12V; VOUT = 40V; Duty Cycle = 80%)
60
VOUT (AC)
200mV/div
40
VOUT (AC)
200mV/div
5V
VPWM
5V/div
20
VPWM
5V/div
0V
IOUT
50mA/div
0V
IOUT -20
50mA/div
50mA
IL
500mA/div
5V
0
100mA
0mA
-40
50mA
500mA
-60
500mA
IL
-80
500mA/div
0mA
0mA
-100
Time (200µs/div)
Time (200µs/div)
PWM Switching Waveforms
(VIN = 12V; VOUT = 40V; Duty Cycle = 50%)
VOUT (AC)
200mV/div
VPWM
5V/div
5V
0
0V
1.5
100mA
IOUT
50mA/div
-40
50mA
1.0
-60
500mA
0.5
IL
-80
500mA/div
0.0
0mA
-100
-0.5
Time (200µs/div)
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
9
DATA SHEET
AAT1451
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Functional Block Diagram
VIN
SW
Linear
Reg
VDD
FB1
SHDN
OVP
VREF
FB2
COMP
Logic
PWM
FB3
FB4
Osc
FSLCT
PGND
GND FAULT
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 efficiency, 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 shifting feature, makes it easy to filter the LED current
switching noise on the output when designing the system.
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.)
10
RSET
The boost switching frequency is programmable from
600kHz up to 1MHz by external resistor for optimum efficiency and the smallest external filter components.
Current mode control provides fast response to line and
load transients. Integrated light-load mode ensures highest 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 saturation and any resulting damage to the switching device
occurring during an overload fault condition.
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
DATA SHEET
AAT1451
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Boost Converter Switching Frequency
The RSET value can be calculated as follows:
The AAT1451’s boost converter frequency can be adjusted 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 values.
2 · 1010
FSW
RFS =
RSET =
CurrentSetRatio · VISET
IFB
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.
RSET =
RFS (kΩ)
Frequency (kHz)
20
22
24
26
28
30
33
1000
909
833
769
714
667
606
Switching Frequency (kHz)
Table 1: Examples of Standard 1% RFS Values for
Setting Switching Frequency.
264 · 0.6V
= 5.23kΩ
30mA
Maximum LED Current (mA)
RSET (kΩ)
30
25
22
20
15
5.23
6.34
7.5
7.87
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.
40
35
ICSx (mA)
30
25
20
15
18
20
22
24
26
28
30
32
34
RFS (kΩ)
Figure 1: Switch Frequency vs. RFS
Maximum LED Current Selection
10
5
4
5
6
7
8
9
10
11
12
13
14
RSET (kΩ)
Figure 2: Choosing an RSET Value
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.
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11
DATA SHEET
AAT1451
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
PWM Dimming
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 maximum 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%
95%
90%
85%
80%
75%
70%
65%
60%
55%
50%
45%
40%
35%
30%
25%
20%
15%
10%
5%
22
20
19
18
17
16
15
14
13
12
11
9
8
7
6
5
4
3
2
1
PWM Duty vs WLED Current
AAT1451
35
30
Current per Channel (mA)
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 electrical characteristic table for proper operation. After initial 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 typically 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.
25
20
15
10
5
0
0
10
20
30
40
50
60
70
80
90
100
PWM Duty Ratio(%)
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 automatically 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:
Θn =
360 · (n - 1)
N
TD =
TPWM
N
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.
Table 3: AAT1451 PWM Duty Cycle vs. LED
Current at Maximum 22mA Setting
12
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DATA SHEET
AAT1451
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Duty Cycle = 50%
PWM
Input
ICS1
0°
TD
ICS2
90°
2TD
ICS3
180°
3TD
ICS4
270°
Figure 4: AAT1451 Automatic Phase Shift PWM Timing Diagram
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 controller at startup. The low voltage is detected by the controller 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 dissipation. The AAT1451 automatically monitors the current sink voltage for two or more shorted LEDs. To prevent thermal shutdown, the shorted LED string is disabled 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 output.
OVP Protection
Under all conditions, the over-voltage protection circuitry 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 hysteresis 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 protection is provided. Over-current prevents inductor saturation and any resulting damage to the switching device
occurring during an overload fault condition.
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13
DATA SHEET
AAT1451
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
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 specifications 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 performance 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.
DMAX =
VOUT + VD - VIN(MIN)
VOUT + VD
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 calculated by:
The ESR zero of the output capacitor can be calculated
by:
fZ_ESR =
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:
fZ_ESR =
14
IOUT
D
· VIN(MIN)
+ MAX
1 - DMAX
2 · FS · L
VIN2
2π · L1 · IOUT · VOUT
It is recommended to design the bandwidth to one
decade lower than the frequency of RHP zero to guarantee 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:
fP_COM =
VOUT is the boost converter output voltage;
VD is the forward voltage of Schottky diode;
VIN(MIN) is the minimum input voltage.
IPEAK =
1
2π · RESR · COUT
Where:
Where:
The output inductor (L) is selected to avoid saturation at
minimum input voltage, maximum output load conditions. Peak current may be calculated from the following
equation, again assuming continuous conduction mode.
Worst-case peak current occurs at minimum input voltage (maximum duty cycle) and maximum load. Switching
frequency is estimated at 600kHz with a 4.7µH inductor.
1
2π · RO · COUT
fP =
fZ_COM =
1
2π · REA · CCOMP
1
2π · RCOMP · CCOMP
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 chosen for optimum phase margin and fast transient
response.
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DATA SHEET
AAT1451
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Capacitor Selection
AAT1451 applications.
Careful selection of the external capacitor CIN is important
because it will affect turn-on time and transient performance. 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 levels then the input capacitor size may be decreased.
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.
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 counterparts. A ceramic capacitor typically has very low ESR, is
lower cost, has a smaller PCB footprint, and is non-polarized. 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 connections, size or area, material composition, and ambient
temperature. Capacitor ESR is typically measured in milliohms 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 generally have tight tolerance and are very stable over temperature. 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 capacitors larger than 4.7µF are not typically required for
PCB Layout Considerations
When designing a PCB for the AAT1451, the key requirements 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 sufficient 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 connections 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 function 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.
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15
DATA SHEET
AAT1451
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
User Interface Functionality
Button(s) Pushed
UP
DOWN
CYCLE
Description
[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.
[Push/Release once] Channels FB1 to FB4 are turned on with 22mA per channel. With every push/release the current is decreased according to Table 3.
[Push/Release once] Auto cycling up and down.
Table 4: AAT1451 Evaluation Board User Interface.
SW
DS1
SS16L
L1 4.7µH
U1
AAT1451
J1
VIN
5V-26V
DC+
1
2
2
VIN
9
VDD
3
CIN
2.2µF/50V
GND
CVDD
2.2µF
6.3V
EN
SHDN
11
R4
10k
C2
0.1µF
PWM
VCC
2.2V-5V
VCC
RCOMP
20k
DOWN
S2
CYCLE
S3
ISET
3
FSLCT
PGND
RFS
20k
COUT
2.2µF
50V
R2
1.2k
15
1
R3
10k
10
FB1
5
FB1
1
FB2
6
FB2
2
FB3
7
FB3
3
FB4
8
FB4
0
4
VCC
5
J3
EP GND
14
MCU
VMCU
R8 R9 R10
1k 1k 1k
UP
S1
COMP
4
RSET
7.5k
FAULT
PWM
13
CCOMP
15nF
OVP
VOUT
FLT
12
R5
10k
R1
42.7k
16
SW
VOUT
U2 PIC12F675
1
S1
2
3
4
S2
VDD
GP5
GP4
GP3
VSS
GP0
GP1
GP2
C1
0.1µF
8
R7
1k
7
6
5
R6
330
LED1
Red
LED2
Green
S3
Figure 5: AAT1451 Evaluation Board Schematic.
16
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DATA SHEET
AAT1451
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
a: Top Side
b: Bottom Side
Figure 6: AAT1451 Evaluation Board Layout.
Component
Part Number
Description
Manufacturer
U1
U2
S1 – S3
RCOMP, RFS
RSET
R1
R2
R3, R4, R5
R6
R7, R8, R9, R10
CIN, COUT
CVDD
CCOMP
C1, C2
L1
DS1
LED1
LED2
AAT1451
PIC12F675
PTS645TL50
Chip Resistor
Chip Resistor
Chip Resistor
Chip Resistor
Chip Resistor
Chip Resistor
Chip Resistor
GRM31CR71H225KA88
GCM188R70J225KE22
GRM188R71H153KA01
GRM188R71H104KA93
SD53-4R7-R
SS16L
CMD15-21SRC/TR8
CMD15-21UGC/TR8
High Efficiency White Backlight LED Driver
8-bit CMOS, FLASH-based μC; 8-pin PDIP package
Switch Tact, SPST, 5mm
20kΩ, 1%, 1/4W; 0603
7.5kΩ, 1%, 1/4W; 0603
42.7kΩ, 1%, 1/4W; 0603
1.2kΩ, 1%, 1/4W; 0603
10kΩ, 1%, 1/4W; 0603
330Ω, 1%, 1/4W; 0603
1kΩ, 1%, 1/4W; 0603
2.2μF, 50V, X7R, 1206
2.2μF, 6.3V, X7R, 0603
15nF, 50V, X7R, 0603
0.1μF, 50V, X7R, 0603
4.7µH, 45mΩ, 2.01A, 20%
1.0A, 60V Surface Mount Schottky Barrier Rectifier
Red LED; 1206
Green LED; 1206
Skyworks
Microchip
ITT Industries
Vishay
Vishay
Vishay
Vishay
Vishay
Vishay
Vishay
Murata
Murata
Murata
Murata
Coiltronics
TSC
Chicago Miniature Lamp
Chicago Miniature Lamp
Table 5: AAT1451 Evaluation Board BOM List.
Manufacturer
Murata
Coiltronics
Part Number
L (µH)
Max DCR (mΩ)
Saturation Current (A)
LQH6PPN4R7M43
LQH6PPN6R8M43
SD53-4R7-R
SD53-6R8-R
4.7
6.8
4.7
6.8
20
28
45
68
3.2
2.8
2.01
1.65
Size WxLxH (mm)
6.0x6.0x4.3
5.2x5.2x3.0
Table 6: Surface Mount Inductors.
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17
DATA SHEET
AAT1451
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Manufacturer
Murata
AVX
KEMET
Part Number
Value (µF)
Voltage (V)
Tolerance
Temp. Co.
Case
GCM188R70J225KE22
GRM188R71H153KA01
GRM188R71H104KA93
GRM31CR71H225KA88
06036C225KAT
06035C163KAT
06035C104KAT
12065C225KAT
C0603C225K9RAC
C0603C153K5RAC
C0603C104K5RAC
C1206C225K5RAC
2.2
0.015
0.1
2.2
2.2
0.015
0.1
2.2
2.2
0.015
0.1
2.2
6.3
50
50
50
6.3
50
50
50
6.3
50
50
50
10%
10%
10%
10%
10%
10%
10%
10%
10%
10%
10%
10%
X7R
X7R
X7R
X7R
X7R
X7R
X7R
X7R
X7R
X7R
X7R
X7R
0603
0603
0603
1206
0603
0603
0603
1206
0603
0603
0603
1206
Table 7: Surface Mount Capacitors.
18
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DATA SHEET
AAT1451
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Single Li-ion Cell Powered Application:
plus AAT3110 solution is adopted to drive 6 series-4 parallel (6S4P) to 8 series-4 parallel (8S4P) typical of 13" and
smaller sized displays. Figure 8 shows the efficiency.
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
DS1
SS16L
L1
4.7µH
Battery
2.7-4.2V
CIN
4.7µF
2
CIN1
2.2µF
U2
AAT3110IJS-5.0-T1
1
COUT2
10µF
VIN
GND
C+
3
SHDN
6
VDD
12
SHDN
11
CFLY
1µF
13
CCOMP
15nF
CIN2
10µF
C- 4
VIN
9
R1
42.7k
16
SW OVP 15
1.2k
R2
CVDD
2.2µF
5
4
3
RSET
7.5k
RCOMP
20k
FAULT
PWM
FB1
COMP
FB2
ISET
FB3
FSLCT
FB4
PGND EP
1
0
RFS
20k
10
LED
6S4P~8S4P
10k VCC
R3
5
6
7
8
GND
14
Figure 7: Schematic of AAT1451 plus AAT3110
Efficiency
100
95
90
Efficiency (%)
ON/OFF
signal
2
VOUT
U1
AAT1451
VOUT
COUT1
2.2µF/50V
85
80
75
70
VIN = 3V
65
VIN = 3.6V
60
VIN = 4.2V
55
50
0
5
10
15
20
25
ILED (mA)
Figure 8: Efficiency vs ILED for driving 8series - 4parallel (8S4P) LEDs
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19
DATA SHEET
AAT1451
Four-LED Strings, High Efficiency White LED Driver for LCD Backlighting
Ordering Information
Package
Part Marking1
Part 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
0.450 ± 0.050
1.600 ± 0.050
Detail "A"
0.230 ± 0.050
0.450 ± 0.050
3.300 ± 0.050
4.000 ± 0.050
Index Area
Top View
Bottom View
0.750 ± 0.050
Detail "A"
0.000
+ 0.100
-0.000
0.203 REF
Side View
All dimensions in millimeters.
1. XYY = assembly and date code.
2. Sample stock is generally held on part numbers listed in BOLD.
Copyright © 2012 Skyworks Solutions, Inc. All Rights Reserved.
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Customers are responsible for their products and applications using Skyworks products, which may deviate from published specifications as a result of design defects, errors, or operation of products outside of published parameters or design specifications. Customers should include design and operating safeguards to minimize these and other risks. Skyworks assumes no liability for applications assistance, customer product
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