ANALOGICTECH AAT1235IRN-T1

AAT1235
High Efficiency White LED Drivers
for Backlight and Keypad
General Description
Features
The AAT1235 is a highly integrated, high efficiency
power solution for white LED backlight and keypad
backlights in mobile/portable devices. It is based on a
switching boost converter which steps up the singlecell lithium-ion/polymer battery voltage to drive 5
strings of series-connected white LEDs with precision current regulation. The AAT1235 is capable of
driving a total of four LEDs per channel.
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The boost converter can produce an output drive of
up to 24V at 100mA. The high switching frequency
(up to 2MHz) provides fast response to load transients and allows the use of small external components. A fully integrated control circuit simplifies the
design and reduces total solution size.
•
AnalogicTech's Advanced Simple Serial Control™
(AS2Cwire™) serial digital input is used to individually turn each output sink on/off and adjust the LED
current by group. Unlike conventional pulse width
modulation (PWM) control of LED brightness, the
AAT1235 drives the LEDs with constant, non-pulsating current.
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•
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A similar device is also available with an I2C twowire interface; please see the AAT1236 datasheet.
Input Supply Voltage Range: 2.7V to 5.5V
Maximum Boost Output Drive: Up to 24V at 100mA
Up to 85% Efficient Operation
Up to 2MHz Switching Frequency with Small
Inductor
User-Programmable Full-Scale LED Current, Up
to 30mA
Single-Wire AS2Cwire Serial Interface
— Five Addressable Registers
• Independent LED Current Control by Group
— Backlight Group B1-B2, 16 Settings
— Auxiliary Group A1-A3, 16 Settings
• Independent LED ON/OFF Control
— Fast, 1MHz Serial Interface
Non-Pulsating, High-Performance LED Current
Drive for Uniform Illumination
— 10% Absolute Accuracy
— 2% Channel-to-Channel Matching
Over-Voltage and Over-Temperature Protection
Automatic Soft-Start Minimizes Large Inrush
Current at Startup
Available in 3x4mm TDFN34-16 Package
Applications
The AAT1235 is available in a Pb-free, thermallyenhanced 16-pin 3x4mm TDFN package and is
specified for operation over the -40°C to +85°C temperature range.
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Typical Application
L = 2.2µH
SwitchReg™
D1
Digital Still Cameras (DSCs)
Keypad Backlight
Large Panel Displays
Mobile Handsets
Personal Media Players
PDAs and Notebook PCs
White LED Backlight
Keypad or
RGB LEDs
Up to 24V Max
C OUT
2.2µF
Backlight
LEDs
LIN
Input :
2.7V~5.5V
SW
VIN
CIN
2.2µF
IN
R2
187kΩ
B1
B2
A1
EN/SET
RSET
A2
A3
OV
AAT1235
GND
AGND
R3
12.1kΩ
R1
22.6kΩ
AS 2Cwire Control
1235.2007.02.1.1
1
AAT1235
High Efficiency White LED Drivers
for Backlight and Keypad
Pin Descriptions
Pin #
Symbol
1
VIN
2
OV
3
4
5
6
EN/SET
B1
B2
RSET
7
8
IN
GND
9
SW
10, 11
12
13
14
15
16
EP
N/C
AGND
A3
A2
A1
LIN
Function
Input supply for the converter. Connect a 2.2µF or larger ceramic capacitor from VIN to
GND.
Boost output over voltage detect pin. Use resistor divider to set the circuit's external overvoltage protection. See Applications Information for details.
AS2Cwire control or enable pin.
Backlight current sink 1. Connect the cathode of the last LED in the string to B1.
Backlight current sink 2. Connect the cathode of the last LED in the string to B2.
LED current set resistor. A 22.6kΩ resistor from RSET to AGND sets the maximum LED current in A1-A3 and B1-B2 to 20mA.
Input bias supply for the internal circuitry. Connect IN to VIN directly at the AAT1235.
Ground for the boost converter. Connect GND to AGND at a single point as close to the
AAT1235 as practical.
Boost converter switching node. A 2.2µH inductor, connected between SW and LIN, sets the
boost converter's switching frequency.
Not connected.
Ground pin. Connect AGND to GND at a single point as close to the AAT1235 as practical.
Auxiliary current sink 3. Connect the cathode of the last LED in the string to A3.
Auxiliary current sink 2. Connect the cathode of the last LED in the string to A2.
Auxiliary current sink 1. Connect the cathode of the last LED in the string to A1.
Switched power input. Connect LIN to the external power inductor.
Exposed paddle (bottom) Connected internally to SW. Connect to SW or leave floating.
Pin Configuration
TDFN34-16
(Top View)
VIN
OV
EN/SET
B1
B2
RSET
IN
GND
2
1
16
2
15
3
14
4
13
5
12
6
11
7
10
8
9
LIN
A1
A2
A3
AGND
N/C
N/C
SW
1235.2007.02.1.1
AAT1235
High Efficiency White LED Drivers
for Backlight and Keypad
Absolute Maximum Ratings1
TA = 25°C unless otherwise noted.
Symbol
VIN, IN
SW
EN/SET, Bx, Ax,
RSET, OV, LIN
TS
TJ
TLEAD
Description
Value
Units
Input Voltage
Switching Node
-0.3 to 6.0
28
V
V
Maximum Rating
VIN + 0.3
V
-65 to 150
-40 to 150
300
°C
°C
°C
Value
Units
50
2
°C/W
W
Storage Temperature Range
Operating Temperature Range
Maximum Soldering Temperature (at leads, 10 sec)
Thermal Information2
Symbol
θJA
PD
Description
Thermal Resistance
Maximum Power Dissipation3
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. Only one Absolute Maximum Rating should be applied at any one time.
2. Mounted on an FR4 circuit board.
3. Derate 20mW°C above 40°C ambient temperature.
1235.2007.02.1.1
3
AAT1235
High Efficiency White LED Drivers
for Backlight and Keypad
Electrical Characteristics1
VIN = 3.6V; CIN = 2.2µF;TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = 25°C.
Symbol
Power Supply
VIN
VOUT(MAX)
VUVLO
ICC
ISHDN(MAX)
IOX
IDX
IDX-Matching
TSS
VOV
Description
Conditions
Input Voltage Range
Maximum Output Voltage
UVLO Threshold
Operating Current (No Switching)
IN Pin Shutdown Current
Maximum Continuous Output Current
Current Sink Accuracy
Current Matching Between Any
Sink Channels
Soft-Start Time
OVP Threshold Voltage
OVP Threshold Hysteresis
Low Side Switch On Resistance
Input Disconnect Switch
Current Set Ratio
Input Switch Current Limit
RDS(ON)N
RDS(ON)IN
ISET
ILIMIT
EN/SET Input
VEN(L)
Enable Threshold Low
VEN(H)
Enable Threshold High
TEN/SET LO
EN/SET Low Time
TEN/SET HI
EN/SET High Time
TOFF
EN/SET Off Timeout
TLAT
EN/SET Latch Timeout
IEN/SET
EN/SET Input Leakage
Thermal Protection
TJ-TH
TJ Thermal Shutdown Threshold
TJ-HYS
TJ Thermal Shutdown Hysteresis
Min
Typ
2.7
VIN Rising
Hysteresis
VIN Falling
B1 = B2 = A1 = A2 = A3 = 1.2V,
2mA Setting, RSET = 226kΩ
EN = GND
VO = 24V
RSET = 22.6kΩ
RSET = 22.6kΩ,
A1 = A2 = A3 = B1 = B2 = 0.4V
From Enable to Output
Regulation; VFB = 300mV
VOUT Rising
Max Units
5.5
24
2.7
V
V
V
mV
V
300
µA
1.0
µA
mA
mA
%
150
1.8
100
18
20
2
22
5
300
1.1
IOUT = 100mA
IOUT = 100mA
ISINK/IRSET, VRSET = 0.6V
1.2
100
80
200
760
µs
1.3
V
mV
mΩ
mΩ
A/A
A
0.4
V
V
µs
µs
µs
µs
µA
1.2
1.4
0.3
VEN/SET = VIN = 5V
75
75
500
500
1
-1
140
15
°C
°C
1. The AAT1235 is guaranteed to meet performance specifications over the -40°C to +85°C operating temperature range is assured by
design, characterization, and correlation with statistical process controls.
4
1235.2007.02.1.1
AAT1235
High Efficiency White LED Drivers
for Backlight and Keypad
Typical Characteristics
Efficiency vs. LED Current
Efficiency vs. LED Current
(Group B On; Group A Off)
(Group B Off; Group A On)
83
84
VIN = 5V
81
80
VIN = 3.6V
79
VIN = 4.2V
78
VIN = 5V
83
Efficiency (%)
Efficiency (%)
82
77
82
81
VIN = 3.6V
80
79
VIN = 4.2V
78
76
77
1.6
3.9
6.2
8.5
10.8
13.1
15.4
17.7
20
1.6
3.9
Output LED Current (mA)
6.2
8.5
10.8
13.1
15.4
17.7
20
Output LED Current (mA)
Efficiency vs. LED Current
LED Current Accuracy vs. Supply Voltage
86
3
85
2
84
Accuracy (%)
Efficiency (%)
(Group A and B On)
VIN = 5V
83
82
VIN = 3.6V
81
VIN = 4.2V
80
79
1.6
IB1, B2, A1, A2, A3
1
0
-1
-2
-3
3.9
6.2
8.5
10.8
13.1
15.4
17.7
2.7
20
3.1
3.4
Output LED Current (mA)
Shutdown Current (µA)
LED Current (mA)
I A1
19.4
19.2
19.0
I A3
I A2
I B2
18.6
18.4
2.7
3.1
3.4
3.8
4.1
4.5
Supply Voltage (V)
1235.2007.02.1.1
4.8
5.2
5.5
0.7
I B1
18.8
4.5
Shutdown Current vs.
Supply Voltage and Temperature
19.8
19.6
4.1
Supply Voltage (V)
LED Current vs. Supply Voltage
20.0
3.8
4.8
5.2
5.5
0.6
25°C
0.5
85°C
0.4
0.3
0.2
0.1
0.0
2.7
-40°C
3.1
3.5
3.9
4.3
4.7
5.1
5.5
Supply Voltage (V)
5
AAT1235
High Efficiency White LED Drivers
for Backlight and Keypad
Typical Characteristics
LED Current vs. Temperature
LED Current Accuracy vs. Temperature
21.2
LED Current (mA)
21.0
(All Channels = 20mA)
LED Current Accuracy (%)
(All Channels = 20mA)
IA3
20.8
20.6
20.4
20.2
IB1
IA2
IB2
20.0
19.8
19.6
19.4
IA1
19.2
19.0
-40
-15
10
35
60
4
3
2
1
0
IB1
-1
IA2
IA3
-2
-3
IB2, A1
-4
-5
-6
-40
85
-15
10
Temperature (°°C)
Shutdown Operation
Output Ripple
(All Channels)
(All Channels = 20mA)
Output Voltage (top) (V)
Switching Node (middle) (V)
0
50
IGROUP_B (mA)
IINDUCTOR (A)
0
0.5
0
14.5
14.0
13.5
16V
0V
1.0
0.5
0.0
Time (50µs/div)
Time (200ns/div)
Switching Frequency vs.
Supply Voltage and Temperature
Output Ripple
13.0
12.5
14V
0V
0.5
0.0
6
Switching Frequency (MHz)
13.5
Inductor Current (bottom) (A)
Output Voltage (top) (V)
Switching Node (middle) (V)
(All Channels = 10mA)
Time (200ns/div)
85
Inductor Current (bottom) (A)
0
50
IGROUP_A (mA)
60
Temperature (°°C)
5
Enable (V)
35
2.5
25°C
2.0
1.5
-40°C
1.0
+85°C
0.5
0.0
2.7
3.1
3.5
3.9
4.3
4.7
5.1
5.5
Supply Voltage (V)
1235.2007.02.1.1
AAT1235
High Efficiency White LED Drivers
for Backlight and Keypad
Typical Characteristics
Enable Threshold Low vs.
Supply Voltage and Temperature
Line Transient
Input Voltage (top) (V)
4.0
3.5
14.2
3.0
14.1
14.0
13.9
13.8
Output Voltage (bottom) (V)
4.5
Enable Threshold Low (V)
(All Channels = 20mA)
1.1
1.0
-40°C
0.9
25°C
0.8
0.7
+85°C
0.6
2.7
3.1
3.5
4.3
4.7
5.1
5.5
Supply Voltage (V)
Time (50µs/div)
Enable Threshold High vs.
Supply Voltage and Temperature
Input Disconnect Switch Resistance vs.
Supply Voltage and Temperature
280
1.2
260
1.1
-40°C
25°C
RDS(ON)IN (mΩ
Ω)
Enable Threshold High (V)
3.9
1.0
0.9
+85°C
0.8
+120°C
+100°C
240
220
200
+85°C
180
+25°C
160
0.7
2.7
3.1
3.5
3.9
4.3
4.7
5.1
5.5
140
2.5
3.0
Supply Voltage (V)
3.5
4.0
4.5
5.0
5.5
6.0
Supply Voltage (V)
Low Side Switch On Resistance
vs. Supply Voltage and Temperature
Soft Start Operation
(All Channels = 20mA)
160
RDS(ON)N (mΩ
Ω)
2
EN/SET (V)
140
0
+120°C
120
20
+100°C
10
100
VOUT (V)
80
+25°C
60
40
2.5
0
0.5
+85°C
IINDUCTOR (A)
3.0
3.5
4.0
4.5
Supply Voltage (V)
1235.2007.02.1.1
5.0
5.5
0
6.0
Time (200µs/div)
7
AAT1235
High Efficiency White LED Drivers
for Backlight and Keypad
EN/SET Off Timeout (TOFF) (µs)
EN/SET Off Timeout vs.
Supply Voltage and Temperature
400
-40°C
350
300
250
+85°C
200
150
+25°C
100
50
2.7
3.1
3.5
3.9
4.3
4.7
5.1
5.5
EN/SET Latch Timeout (TLAT) (µs)
Typical Characteristics
EN/SET Latch Timeout vs.
Supply Voltage and Temperature
400
350
250
200
+85°C
150
+25°C
100
50
2.7
Supply Voltage (V)
3.1
3.5
3.9
4.3
4.7
5.1
5.5
Supply Voltage (V)
Transition of LED Current
(Group B = 1.8mA to 20mA; Group A = 20mA)
4
4
2
2
0.02
0
0
0.02
0
0.02
0
IA1 (middle) (A)
IB1 (bottom) (A)
0
EN/SET (top) (V)
Transition of LED Current
(Group B = 1.8mA; Group A = 20mA to 1.8mA)
IA1 (middle) (A)
IB1 (bottom) (A)
EN/SET (top) (V)
-40°C
300
0
Time (250µs/div)
Time (250µs/div)
Transition of LED Current
(Group B = 20mA; Group A = 20mA to 1.8mA)
4
0
0.02
0
0.02
IA1 (middle) (A)
IB1 (bottom) (A)
EN/SET (top) (V)
2
0
Time (250µs/div)
8
1235.2007.02.1.1
AAT1235
High Efficiency White LED Drivers
for Backlight and Keypad
Functional Block Diagram
LIN
SW
VIN
IN
OV
ROM
Boost
Converter
Control
V(A1, A2, A3)
VREF
V(B1, B2)
AS2Cwire
Control
D/A
A2
D/A
A3
D/A
B1
D/A
B2
Max Current
Adjustment
GND
AGND
Functional Description
The AAT1235 consists of a controller for the step-up
switching converter and its power switch, and five
regulated current sinks each programmable at 16
levels into two groups, which can be turned on/off
individually. An external Schottky diode, a power
inductor, an output capacitor, and a resistor divider
are required to complete the solution.
The AAT1235's boost controller is designed to
deliver 100mA up to 24V. The AAT1236 is capable
of driving a total of five channels divided into two
groups with four white LEDs connected in series at
each channel.
The output load current can be programmed by the
current sink magnitudes. AS2Cwire programming
allows independent control of two current sink
groups (A1 to A3 and B1 to B2) and control on/off
1235.2007.02.1.1
A1
ROM
VREF
EN/SET
D/A
RSET
with a different configuration on each channel.
Unused sink channel(s) must be connected to
AGND to ensure proper function of the AAT1235.
Control Loop
The AAT1235 provides the benefits of current
mode control with a simple hysteretic output current loop providing exceptional stability and fast
response with minimal design effort. The device
maintains exceptional constant current regulation,
transient response, and cycle-by-cycle current limit
without additional compensation components.
The AAT1235 modulates the power MOSFET
switching current to maintain the programmed sink
current through each channel. The sink voltage at
each channel is monitored and the controller provides direct feedback in order to maintain the
desired LED current.
9
AAT1235
High Efficiency White LED Drivers
for Backlight and Keypad
The switching cycle initiates when the N-channel
MOSFET is turned ON and current ramps up in the
inductor. The ON interval is terminated when the
inductor current reaches the programmed peak current level. During the OFF interval, the input current
decays until the lower threshold, or zero inductor
current, is reached. The lower current is equal to the
peak current minus a preset hysteresis threshold,
which determines the inductor ripple current. Peak
current is adjusted by the controller until the
desired LED output current level is met.
The magnitude of the feedback error signal determines the average input current. Therefore, the
AAT1235 controller implements a programmed current source connected to the output capacitor, parallel with the LED channels. There is no right-half
plane zero, and loop stability is achieved with no
additional compensation components. The controller responds by increasing the peak inductor
current, resulting in higher average current in the
inductor and LED channels.
Under light load conditions, the inductor OFF interval current goes below zero and the boost converter enters discontinuous mode operation. Further
reduction in the load current results in a corresponding reduction in the switching frequency. The
AAT1235 provides a pulsed frequency operation
which reduces switching losses and maintains high
efficiency under light load conditions.
Operating frequency varies with changes in the
input voltage, output voltage, and inductor size.
Once the boost converter has reached continuous
mode, further increases in the LED current will not
significantly change the operating frequency. A
small 2.2µH (±20%) inductor is selected to maintain high frequency switching (up to 2MHz) and
high efficiency operation for outputs up to 24V.
Soft Start / Enable
The input disconnect switch is activated when a
valid supply voltage is present and the EN/SET pin
is strobed high. Slew rate control on the input disconnect switch ensures minimal inrush current as
10
the output voltage is charged to the input voltage,
prior to switching of the N-channel power MOSFET.
A monotonic turn-on is guaranteed by the built-in
soft-start circuitry, which eliminates output current
overshoot across the full input voltage range and
all load conditions.
Current Limit and Over-Temperature
Protection
The switching of the N-channel MOSFET terminates when a current limit of 1.5A (typical) is
exceeded. This minimizes power dissipation and
component stresses under overload and short-circuit conditions. Switching resumes when the current decays below the current limit.
Thermal protection disables the AAT1235 when
internal power dissipation becomes excessive, as it
disables both MOSFETs. The junction over-temperature threshold is 140°C with 15°C of temperature hysteresis. The output voltage automatically
recovers when the over-temperature fault condition
is removed.
Over-Voltage Protection
Over-voltage protection prevents damage to the
AAT1235 during open-circuit causing high output
voltage conditions. An over-voltage event is defined
as a condition where the voltage on the OV pin
exceeds the over-voltage threshold limit (VOV =
1.2V typical). When the voltage on the OV pin has
reached the threshold limit, the converter stops
switching and the output voltage decays. Switching
resumes when the voltage on the OV pin drops
below the lower hysteresis limit, maintaining an
average output voltage between the upper and
lower OV thresholds multiplied by the resistor
divider scaling factor.
Under-Voltage Lockout
Internal bias of all circuits is controlled via the VIN
input. Under-voltage lockout (UVLO) guarantees
sufficient VIN bias and proper operation of all internal circuitry prior to soft start.
1235.2007.02.1.1
AAT1235
High Efficiency White LED Drivers
for Backlight and Keypad
Constant Current Output Level Settings
and AS2Cwire Serial Interface
The LED current sink level of each group and the
on/off status of each channel is controlled by
AnalogicTech's AS2Cwire serial digital input. Since
each current sink is programmable, no PWM or
additional control circuitry is needed to control LED
brightness. This feature greatly reduces the burden
on a microcontroller or system IC to manage LED
or display brightness, allowing the user to "set it
and forget it." With its high-speed serial interface
(1MHz data rate), the input sink current can be
changed quickly and easily. Also the non-pulsating
LED current reduces system noise and improves
LED reliability.
AS2Cwire relies on the number of rising edges of
the EN/SET pin to address and load the registers.
AS2Cwire latches data or address after the EN/SET
pin has been held high for time TLAT (500µs).
Address or data is differentiated by the number of
EN/SET rising edges. Since the data registers are
4 bits each, the differentiating number of pulses is
24 or 16, so that Address 0 is signified by 17 rising
edges, Address 1 by 18 rising edges, and so forth.
Data is set to any number of rising edges between
1 and including 16. A typical write protocol is a
burst of EN/SET rising edges, signifying a particular address, followed by a pause with EN/SET held
high for the TLAT timeout period, a burst of rising
edges signifying data, and a TLAT timeout for the
data registers. Once an address is set, then multiple writes to that address are allowed where only
data is issued. When EN/SET is held low for an
amount of time longer than TOFF (500µs), the
AAT1235 enters into shutdown mode and draws
less than 1µA from the input. Data and Address
registers are cleared (reset to 0) during shutdown.
Address
EN/SET
Rising Edges
Data
Register
0
1
2
3
4
17
18
19
20
21
B1-B2 current
A1-A3 current
B1-B2 on/off
A1-A3 on/off
A1-A3, B1-B2 on
Address 0 and 1: LED Brightness Control
Outputs A1, A2, A3, B1, B2 are each capable of
sinking up to 30mA. The maximum current can be
programmed by an external resistor at the RSET
pin. It is suggested to connect up to four white
LEDs in series for each channel and to keep the
same number of LEDs in each channel.
Outputs B1 and B2 are intended to drive the white
LEDs for the backlight in a phone, while A1:A3 can
supply the keypad LEDs. For large displays, all five
outputs can be used.
Data
All Outputs (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
100
84
71
60
51
43
35
31
26
21
18
15
13.5
12.0
10.5
9.0
Table 2: Address 0 and 1 for Current Setting as
Percentage of the Maximum Level Set by RSET.
Address 2: LED Group B ON/OFF
Control
Data
B1
B2
1
2
3
4
Off
Off
On
On
Off
On
Off
On
Table 3: Address 2 for Group B ON/OFF State.
Table 1: AS2Cwire Serial Interface Addressing.
1235.2007.02.1.1
11
AAT1235
High Efficiency White LED Drivers
for Backlight and Keypad
Address 3: LED Group A ON/OFF
Control
Address 4: LED Groups A & B ON
Control
Data
A1
A2
A3
B2
B1
A1
A2
A3
1
2
3
4
5
6
7
8
Off
Off
Off
Off
On
On
On
On
Off
Off
On
On
Off
Off
On
On
Off
On
Off
On
Off
On
Off
On
On
On
On
On
On
Table 5: Address 4 for Turning Group A and B
All On.
Address 4 turns on all channels with the current
programmed by Addresses 0 and 1. No DATA
needs to be provided.
Table 4: Address 3 for Group A ON/OFF State.
AS2Cwire Serial Interface Timing
Address
Data
THI
TLO
TLAT
TLAT
EN/SET
1
Address
12
2
17
18
1
0
2...
n ≤ 16
1
Data Reg 1
0
Data Reg 2
0
n
1235.2007.02.1.1
AAT1235
High Efficiency White LED Drivers
for Backlight and Keypad
Application Information
Channel Disable
Tie all unused channels to AGND. On start-up,
these channels will be automatically disabled.
LED Selection
LED Current (mA)
35
30
25
20
15
10
5
0
Although the AAT1235 is specifically designed to
drive white LEDs, the device can also be used to
drive most types of LEDs with forward voltages
ranging between 2.0V and 4.7V. Since the A1, A2,
A3, and B1, B2 input current sinks are matched
with low voltage dependence, the LED-to-LED
brightness will be matched regardless of the individual LED forward voltage (VF) levels. In some
instances, it may be necessary to drive high-VF
type LEDs. The low dropout (~0.1V @ 20mA ILED)
current sinks in the AAT1235 make it capable of
driving LEDs with forward voltages as high as 4.7V
from an input supply as low as 3.0V. LED outputs
A1-A3 and B1-B2 can be combined to drive highcurrent LEDs without complication, making the
AAT1235 a perfect application for large LCD display backlighting and keypad LED applications.
Constant Current Setting
The LED current is controlled by the RSET resistor.
For maximum accuracy, a 1% tolerance resistor is
recommended. Table 6 shows the RSET resistor
value for AAT1235 for various LED full-scale current levels.
ILED (mA)
Ω)
RSET (kΩ
30
25
20
15
10
5
14.7
17.4
22.6
29.4
44.2
93.1
Table 6: Maximum LED Current and RSET
Resistor Values (1% Resistor Tolerance).
10
36
62
88
114
140
166
192
218
244
270
RSET (kΩ
Ω)
Figure 1: LED Current vs. RSET Values.
Over-Voltage Protection
The over-voltage protection circuit consists of a
resistor network connected from the output voltage
to the OV pin (see Figure 2). This over voltage protection circuit prevents damage to the device when
one of the five channels has an open LED circuit.
The AAT1235 continues to operate; however, the
LED current in the remaining channels is no longer
regulated and the actual LED current will be determined by the externally programmed over-voltage
protection threshold, the inductor value, and the
switching frequency.
The resistor divider can be selected such that the
over-voltage threshold occurs prior to the output
reaching 24V (VOUT(MAX)). The value of R3 should
be selected from 10kΩ to 20kΩ to minimize switching losses without degrading noise immunity:
R2 = R 3 ·
⎛ VOUT(PROTECTION) ⎞
-1
VOV
⎝
⎠
VOUT
AAT1235
R2
COUT
OV
GND
R3
Figure 2: Over-Voltage Protection Circuit.
Maximum LED current per channel versus RSET
value is shown in Figure 1.
1235.2007.02.1.1
13
AAT1235
High Efficiency White LED Drivers
for Backlight and Keypad
If four LEDs are connected in series on one channel, the total VF from the WLEDs could be as high
as 18.8V. Therefore, using R3 = 12.1kΩ and setting
VOUT(PROTECTION) = 20V is recommended. Selecting
a 1% resistor, this results in R2 = 187kΩ (rounded to
the nearest standard 1% value).
It is always recommended to use the same number
of WLEDs on each channel and set the appropriate
over-voltage protection. Failure to do so may cause
any one of the (5) sink pins to exceed the absolute
maximum rating voltage and permanently damage
the device in case the channel is disconnected
(open circuit failure). Examples of over voltage settings for various strings of series-connected LEDs
are shown in Table 7.
Number of WLEDs
on Each Channel
Total Maximum VF (V)
VOUT(PROTECTION) (V)
Ω
R3 = 12.1kΩ
Ω)
R2 (kΩ
4
3
2
18.8
14.1
9.4
20
15
10
187
140
88.7
Table 7: Over-Voltage Protection Settings.
VOUT
JP1
0
LED1
R2
187k
VIN
LED7
LED3
LED8
LED4
LED9
3
2
1
Enable/Set
C1
2.2µF
R3
12.1k
R1
22.6k
JP3
0
LED6
LED2
JP6
0
JP6
JP2
0
D1
MBR0530T1
SW_Node
U1
AAT1235 TDFN3X4
JP7
0
1
2
3
4
5
6
7
8
VIN
LIN
OV
A1
EN/SET
A2
B1
A3
B2
AGND
RSET
NC
IN
NC
GND
SW
JP4
0
JP5
0
LED11
LED16
LED21
LED12
LED17
LED22
LED13
LED18
LED23
LED14
LED19
LED24
C2
2.2µF
25V
L1
2.2µH
16
15
14
13
12
11
10
9
JP8
0
JP9
0
JP10
0
RTN
L1: 2.2uH Taiyo Yuden NR4018T2R2M
C1: 0805 10V 2.2µF X7R GRM21BR71A225KA01
C2: 0805 25V 2.2µF X7R GRM21BR71E225KA73L
LED1-24: OSRAM LW M673 or equivalent
Figure 3: A AAT1235-based High Efficiency White LED Driver Schematic.
14
1235.2007.02.1.1
AAT1235
High Efficiency White LED Drivers
for Backlight and Keypad
LED Brightness Control
The AAT1235 uses AS Cwire programming to control LED brightness. The output current of the
AAT1235 can be changed successively to brighten
or dim the LEDs in smooth transitions (i.e., to fade
in or fade out) or in discrete steps, giving the user
complete programmability and real-time control of
LED brightness.
2
Selecting the Schottky Diode
To ensure minimum forward voltage drop and no
recovery, high voltage Schottky diodes are recommended for the AAT1235 boost converter. The output diode is selected to maintain acceptable efficiency and reasonable operating junction temperature under full load operating conditions. Forward
voltage (VF) and package thermal resistance (θJA)
are the dominant factors in selecting a diode. The
diode’s non-repetitive peak forward surge current
rating (IFSM) should be considered for high pulsed
load applications, such as camera flash. IFSM rating
drops with increasing conduction period.
Manufacturers’ datasheets should be reviewed
carefully to verify reliability under peak loading conditions. The diode's published current rating may
not reflect actual operating conditions and should
be used only as a comparative measure between
similarly rated devices.
20V rated Schottky diodes are recommended for
output voltages less than 15V, while 30V rated
Schottky diodes are recommended for output voltages higher than 15V.
Estimating Schottky Diode Power
Dissipation
The switching period is divided between ON and
OFF time intervals:
1
= TON + TOFF
FS
tor. During the OFF time, the N-channel power
MOSFET is not conducting. Stored energy is transferred from the input battery and boost inductor to
the output load through the output diode.
Duty cycle is defined as the ON time divided by the
total switching interval:
D=
TON
TON + TOFF
= TON ⋅ FS
The maximum duty cycle can be estimated from
the relationship for a continuous mode boost converter. Maximum duty cycle (DMAX) is the duty cycle
at minimum input voltage (VIN(MIN)):
DMAX =
VOUT - VIN(MIN)
VOUT
The average diode current during the OFF time can
be estimated:
IAVG(OFF) =
IOUT
1 - DMAX
The VF of the Schottky diode can be estimated from
the average current during the off time. The average diode current is equal to the output current:
IAVG(TOT) = IOUT
The average output current multiplied by the forward diode voltage determines the loss of the output diode:
PLOSS(DIODE) = IAVG(TOT) · VF
During the ON time, the N-channel power MOSFET
is conducting and storing energy in the boost induc-
1235.2007.02.1.1
= IOUT · VF
15
AAT1235
High Efficiency White LED Drivers
for Backlight and Keypad
For continuous LED currents, the diode junction
temperature can be estimated:
TJ(DIODE) = TAMB + θJA · PLOSS(DIODE)
External Schottky diode junction temperature
should be below 110ºC, and may vary depending
Manufacturer
Part Number
Rated IF(AV)
Current (A)1
Diodes, Inc.
ON Semi
ON Semi
B0520WS
MBR130LSFT
MBR0530T
0.50
1.00
0.50
on application and/or system guidelines. The diode
θJA can be minimized with additional metal PCB
area on the cathode. However, adding additional
heat-sinking metal around the anode may degrade
EMI performance. The reverse leakage current of
the rectifier must be considered to maintain low
quiescent (input) current and high efficiency under
light load. The rectifier reverse current increases
dramatically at elevated temperatures.
Rated
Voltage (V)
Thermal
Resistance
θJA, °C/W)1
(θ
Case
20
30
30
426
325
206
SOD-323
SOD-123
SOD-123
Table 8: Typical Surface Mount Schottky Rectifiers for Various Output Loads.
(select TJ < 110°C in application circuit).
Selecting the Boost Inductor
The AAT1235 controller utilizes hysteretic control
and the switching frequency varies with output load
and input voltage. The value of the inductor determines the maximum switching frequency of the
boost converter. Increased output inductance
decreases the switching frequency, resulting in higher peak currents and increased output voltage ripple. To maintain 2MHz maximum switching frequency and stable operation, an output inductor selected
between 1.5µH and 2.7µH is recommended.
A better estimate of DMAX is possible once VF is
known:
DMAX =
(VOUT + VF - VIN(MIN))
(VOUT + VF)
Where VF is the Schottky diode forward voltage. If
not known or not provided by the manufacturer, a
starting value of 0.5V can be used.
Manufacturer’s specifications list both the inductor
DC current rating, which is a thermal limitation, and
peak inductor current rating, which is determined
by the saturation characteristics. Measurements at
full load and high ambient temperature should be
performed to ensure that the inductor does not saturate or exhibit excessive temperature rise.
16
The output inductor (L) is selected to avoid saturation
at minimum input voltage and maximum output load
conditions. Peak current may be estimated using the
following equation, assuming continuous conduction
mode. Worst-case peak current occurs at minimum
input voltage (maximum duty cycle) and maximum
load. Switching frequency (FS) can be estimated at
500kHz with a 2.2µH inductor:
IPEAK =
IOUT
D
· VIN(MIN)
+ MAX
(1 - DMAX)
(2 · FS · L)
At light load and low output voltage, the controller
reduces the operating frequency to maintain maximum operating efficiency. As a result, further
reduction in output load does not reduce the peak
current. Minimum peak current can be estimated
between 0.5A and 0.75A.
At high load and high output voltages, the switching frequency is somewhat diminished, resulting in
higher IPEAK. Bench measurements are recommended to confirm actual IPEAK and to ensure that
the inductor does not saturate at maximum LED
current and minimum input supply voltage.
The RMS current flowing through the boost inductor is equal to the DC plus AC ripple components.
Under worst-case RMS conditions, the current
1235.2007.02.1.1
AAT1235
High Efficiency White LED Drivers
for Backlight and Keypad
waveform is critically continuous. The resulting
RMS calculation yields worst-case inductor loss.
The RMS current value should be compared
against the inductor manufacturer's temperature
rise, or thermal derating, guidelines:
IRMS =
IPEAK
3
For a given inductor type, smaller inductor size leads
to an increase in DCR winding resistance and, in
most cases, increased thermal impedance. Winding
resistance degrades boost converter efficiency and
increases the inductor’s operating temperature:
To ensure high reliability, the inductor case temperature should not exceed 100ºC. In some cases,
PCB heatsinking applied to the LIN node (nonswitching) can improve the inductor's thermal capability. However, as in the case of adding extra metal
around the Schottky's anode, adding extra PCB
metal around the AAT1235's SW pin for heatsinking
may degrade EMI performance.
Shielded inductors provide decreased EMI and may
be required in noise sensitive applications.
Unshielded chip inductors provide significant space
savings at a reduced cost compared to shielded
(wound and gapped) inductors. In general, chiptype inductors have increased winding resistance
(DCR) when compared to shielded, wound varieties.
PLOSS(INDUCTOR) = IRMS2 · DCR
Manufacturer
Sumida
Sumida
Sumida
Murata
Murata
Taiyo Yuden
Taiyo Yuden
Coiltronics
Coiltronics
Coiltronics
Part Number
Inductance
(µH)
Max DC ISAT
Current (A)
DCR
Ω)
(Ω
Size (mm)
LxWxH
Type
CDRH4D22/HP-2R2
CDR4D11/HP-2R4
CDRH4D18-2R2
LQH662N2R2M03
LQH55DN2R2M03
NR4018T2R2
NR3015T2R2
SD3814-2R2
SD3114-2R2
SD3112-2R2
2.2
2.4
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.50
1.70
1.32
3.30
3.20
2.70
1.48
1.90
1.48
1.12
35
105
75
19
29
60
60
77
86
140
5.0x5.0x2.4
4.8x4.8x1.2
5.0x5.0x2.0
6.3x6.3x4.7
5.0x5.7x4.7
4.0x4.0x1.8
3.0x3.0x1.5
3.8x3.8x1.4
3.1x3.1x1.4
3.1x3.1x1.2
Shielded
Shielded
Shielded
Shielded
Non-Shielded
Shielded
Shielded
Shielded
Shielded
Shielded
Table 9: Typical Surface Mount Inductors for Various Output Loads
(select IPEAK < ISAT).
Selecting the Boost Capacitors
The high output ripple inherent in the boost converter
necessitates the use of low impedance output filtering.
Multi-layer ceramic (MLC) capacitors provide small
size and adequate capacitance, low parasitic
equivalent series resistance (ESR) and equivalent
series inductance (ESL), and are well suited for
use with the AAT1235 boost regulator. MLC capacitors of type X7R or X5R are recommended to
ensure good capacitance stability over the full
operating temperature range.
The output capacitor is selected to maintain the output load without significant voltage droop (ΔVOUT)
1235.2007.02.1.1
during the power switch ON interval, when the output diode is not conducting. A ceramic output
capacitor between 2.2µF and 4.7µF is recommended (see Table 8). Typically, 25V rated capacitors are
required for the 24V maximum boost output.
Ceramic capacitors selected as small as 0805 are
available which meet these requirements.
MLC capacitors exhibit significant capacitance reduction with applied voltage. Output ripple measurements should confirm that output voltage droop and
operating stability are within acceptable limits. Voltage
derating can minimize this factor, but results may vary
with package size and among specific manufacturers.
17
AAT1235
High Efficiency White LED Drivers
for Backlight and Keypad
Output capacitor size can be estimated at a switching frequency (FS) of 500kHz (worst case):
COUT =
Manufacturer
Murata
Murata
Murata
Murata
Murata
To maintain stable operation at full load, the output
capacitor should be selected to maintain ΔVOUT
between 100mV and 200mV.
The boost converter input current flows during both
ON and OFF switching intervals. The input ripple
current is less than the output ripple and, as a
result, less input capacitance is required.
IOUT · DMAX
FS · ΔVOUT
Part Number
Value (µF)
Voltage Rating
Temp Co
Case Size
GRM188R60J225KE19
GRM21BR71A225KA01
GRM219R61E225KA12
GRM21BR71E225KA73L
GRM21BR61E475KA12
2.2
2.2
2.2
2.2
4.7
6.3
10
25
25
25
X5R
X7R
X5R
X7R
X5R
0603
0805
0805
0805
0805
Table 10: Recommended Ceramic Capacitors.
PCB Layout Guidelines
Boost converter performance can be adversely
affected by poor layout. Possible impact includes
high input and output voltage ripple, poor EMI performance, and reduced operating efficiency. Every
attempt should be made to optimize the layout in
order to minimize parasitic PCB effects (stray resistance, capacitance, and inductance) and EMI coupling from the high frequency SW node. A suggested PCB layout for the AAT1235 boost converter is
shown in Figures 4 and 5. The following PCB layout
guidelines should be considered:
1. Minimize the distance from Capacitor C1 and
C2’s negative terminals to the GND pins. This
is especially true with output capacitor C2,
which conducts high ripple current from the
output diode back to the GND pins.
18
2. Minimize the distance between L1 to D1 and
switching pin SW; minimize the size of the PCB
area connected to the SW pin.
3. Maintain a ground plane and connect to the IC
GND pin(s) as well as the GND connections of
C1 and C2.
4. Consider additional PCB metal on D1’s cathode to maximize heatsinking capability. This
may be necessary when using a diode with a
high VF and/or thermal resistance.
5. Do not connect the exposed paddle (bottom of
the die) to either AGND or GND because it is
connected internally to SW. Connect the
exposed paddle to the SW pin or leave floating.
1235.2007.02.1.1
AAT1235
High Efficiency White LED Drivers
for Backlight and Keypad
Figure 4: AAT1235 Evaluation Board
Top Side Layout.
Figure 5: AAT1235 Evaluation Board
Bottom Side Layout.
Figure 6: Exploded View of AAT1235 Evaluation Board
Top Side Layout Detailing Plated Through Vias.
1235.2007.02.1.1
19
AAT1235
High Efficiency White LED Drivers
for Backlight and Keypad
Ordering Information
Package
Marking1
Part Number (Tape and Reel)2
TDFN34-16
TKXYY
AAT1235IRN-T1
All AnalogicTech products are offered in Pb-free packaging. The term “Pb-free” means
semiconductor products that are in compliance with current RoHS standards, including
the requirement that lead not exceed 0.1% by weight in homogeneous materials. For more
information, please visit our website at http://www.analogictech.com/pbfree.
Package Information3
TDFN34-16
3.000 ± 0.050
1.600 ± 0.050
Detail "A"
3.300 ± 0.050
4.000 ± 0.050
Index Area
0.350 ± 0.100
Top View
0.230 ± 0.050
Bottom View
C0.3
(4x)
0.050 ± 0.050
0.450 ± 0.050
0.850 MAX
Pin 1 Indicator
(optional)
0.229 ± 0.051
Side View
Detail "A"
All dimensions in millimeters.
1. XYY = assembly and date code.
2. Sample stock is generally held on part numbers listed in BOLD.
3. The leadless package family, which includes QFN, TQFN, DFN, TDFN and STDFN, has exposed copper (unplated) at the end of the
lead terminals due to the manufacturing process. A solder fillet at the exposed copper edge cannot be guaranteed and is not required
to ensure a proper bottom solder connection.
20
1235.2007.02.1.1
AAT1235
High Efficiency White LED Drivers
for Backlight and Keypad
© Advanced Analogic Technologies, Inc.
AnalogicTech cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in an AnalogicTech product. No circuit patent licenses, copyrights, mask work rights,
or other intellectual property rights are implied. AnalogicTech reserves the right to make changes to their products or specifications or to discontinue any product or service without notice.
Customers are advised to obtain the latest version of relevant information to verify, before placing orders, that information being relied on is current and complete. All products are sold subject to the terms and conditions of sale supplied at the time of order acknowledgement, including those pertaining to warranty, patent infringement, and limitation of liability. AnalogicTech
warrants performance of its semiconductor products to the specifications applicable at the time of sale in accordance with AnalogicTech’s standard warranty. Testing and other quality control techniques are utilized to the extent AnalogicTech deems necessary to support this warranty. Specific testing of all parameters of each device is not necessarily performed.
AnalogicTech and the AnalogicTech logo are trademarks of Advanced Analogic Technologies Incorporated. All other brand and product names appearing in this document are registered trademarks or trademarks of their respective holders.
Advanced Analogic Technologies, Inc.
830 E. Arques Avenue, Sunnyvale, CA 94085
Phone (408) 737- 4600
Fax (408) 737- 4611
1235.2007.02.1.1
21