AAT1236
High Efficiency White LED Drivers
for Backlight and Keypad
General Description
Features
The AAT1236 is a highly integrated, high efficiency
power solution for white LED and keypad backlights
in mobile/portable devices. It is based on a switching
boost converter which steps up the single cell lithium-ion/polymer battery voltage to drive 5 strings of
series-connected white LEDs with precision current
regulation. The AAT1236 is capable of driving a total
of four LEDs per channel.
•
•
•
•
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
Two-Wire, I2C Compliant Serial Interface
— Two 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, 400kHz Serial Transfer Rate
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
•
•
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.
•
A two-wire I2C serial digital interface 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 AAT1236 drives the LEDs with constant, nonpulsating current. The interface is fully compliant to
the Fast/Standard mode I2C specification, allowing a
transfer rate of up to 400kHz.
SwitchReg™
•
•
•
Applications
A similar device is also available with a proprietary
Advanced Simple Serial Control™ (AS2Cwire™) single wire interface; please see the AAT1235 datasheet.
•
•
•
•
•
•
•
The AAT1236 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.
Digital Still Cameras (DSCs)
Keypad Backlight
Large Panel Displays
Mobile Handsets
PDAs and Notebook PCs
Personal Media Players
White LED Backlight
Typical Application
L=2.2µH
D1
Keypad or
RGB LEDs
Up to 24V max
C OUT
2.2µF
R2
187kΩ
Backlight
LEDs
LIN
Input :
2.7V~5.5V
VIN
CIN
2.2µF
IN
Enable
I2C
Interface
1236.2007.02.1.1
B2
B1
SW
A1
RSET
A3
OV
AAT1236
EN SDA SCL
A2
GND
AGND
R3
12.1kΩ
R1
22.6kΩ
SDA
SCL
1
AAT1236
High Efficiency White LED Drivers
for Backlight and Keypad
Pin Descriptions
Pin #
Symbol
1
2
VIN
OV
3
4
5
6
EN
B1
B2
RSET
7
8
IN
GND
9
SW
10
11
12
13
14
15
16
EP
SDA
SCL
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.
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 AAT1236.
Power ground for the boost converter. Connect GND to AGND at a single point as close to
the AAT1236 as practical.
Boost converter switching node. A 2.2µH inductor, connected between SW and LIN, sets the
boost converter's switching frequency.
I2C interface serial data line.
I2C interface serial clock line.
Ground pin. Connect AGND to GND at a single point as close to the AAT1236 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
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
SCL
SDA
SW
1236.2007.02.1.1
AAT1236
High Efficiency White LED Drivers
for Backlight and Keypad
Absolute Maximum Ratings1
TA = 25°C unless otherwise noted.
Symbol
VIN, IN
SW
EN, SCL, SDA, 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.
1236.2007.02.1.1
3
AAT1236
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
Description
Conditions
Power Supply
VIN
Input Voltage Range
VOUT(MAX)
Maximum Output Voltage
VUVLO
ICC
ISHDN(MAX)
IOX
IDX
IDX-Matching
VOV
RDS(ON)N
RDS(ON)IN
TSS
UVLO Threshold
Operating Current (No Switching)
VIN Pin Shutdown Current
Maximum Continuous Output Current
Current Sink Accuracy
Current Matching Between Any
Sink Channels
OVP Threshold Voltage
OVP Threshold Hysteresis
Low Side Switch On Resistance
Input Disconnect Switch
Soft-Start Time
ISET
Current Set Ratio
ILIMIT
Input Switch Current Limit
Enable Input – EN
VEN(L)
Enable Threshold Low
VEN(H)
Enable Threshold High
I2C Serial Interface – SCL, SDA
FSCL
Clock Frequency
TLOW
Clock Low Period
THIGH
Clock High Period
THD_STA
Hold Time START Condition
TSU_STA
Setup Time for Repeat START
TSU_DAT
Data Setup Time
THD_DAT
Data Hold Low
TSU_STO
Setup Time for STOP Condition
Bus Free Time Between STOP
TBUF
and START Condition
VIL
Input Threshold Low
VIH
Input Threshold High
II
Input Current
VOL
Output Logic Low (SDA)
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
VOUT Rising
Max Units
5.5
24
2.7
V
V
V
mV
V
300
µA
1.0
150
1.8
100
18
1.1
IOUT = 100mA
IOUT = 100mA
From Enable to Output
Regulation; VFB = 300mV
ISINK/IRSET, VRSET = 0.6V
20
22
µA
mA
mA
2
5
%
1.2
100
80
200
1.3
V
mV
mΩ
mΩ
300
µs
760
A/A
A
1.2
0.4
V
V
400
kHz
µs
µs
µs
µs
ns
µs
µs
1.4
1.3
0.6
0.6
0.6
100
0
0.6
0.9
1.3
2.7 ≤ VIN ≤ 5.5
2.7 ≤ VIN ≤ 5.5
IPULLUP = 3mA
µs
0.4
1.4
-1.0
1.0
0.4
V
V
µA
V
1. The AAT1236 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.
4
1236.2007.02.1.1
AAT1236
High Efficiency White LED Drivers
for Backlight and Keypad
Electrical Characteristics (continued)1
VIN = 3.6V; CIN = 2.2µF;TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = 25°C.
Symbol
Description
Conditions
Min
Thermal Protection
TJ-TH
TJ Thermal Shutdown Threshold
TJ-HYS
TJ Thermal Shutdown Hysteresis
Typ
Max Units
140
15
°C
°C
I2C Interface Timing Details
SDA
TSU_DAT
TLOW
THD_STA
TBUF
SCL
THD_STA
THD_DAT
THIGH
TSU_STA
TSU_STO
1. The AAT1236 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.
1236.2007.02.1.1
5
AAT1236
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
6.2
LED Current (mA)
8.5
10.8
13.1
15.4
17.7
20
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
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)
6
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)
1236.2007.02.1.1
AAT1236
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)
0
0.5
IINDUCTOR (A)
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
1236.2007.02.1.1
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)
7
AAT1236
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
4.7
5.1
5.5
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)
Enable Threshold High vs.
Supply Voltage and Temperature
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)
5
160
SCL (V)
140
RDS(ON)N (mΩ
Ω)
4.3
Supply Voltage (V)
Time (50µs/div)
0
5
+120°C
SDA (V)
120
0
+100°C
14V
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)
8
3.9
5.0
5.5
0
6.0
Time (200µs/div)
1236.2007.02.1.1
AAT1236
High Efficiency White LED Drivers
for Backlight and Keypad
Typical Characteristics
Transition of LED Current
Transition of LED Current
(All Channels = 20mA to 1.8mA)
(Group A= 20mA to 1.8mA; Group B = 20mA)
5
SCL (V)
SDA (V)
SCL (V)
0
5
0
5
SDA (V)
0
5
0
0.05
IB1 (A)
0
0.05
IB1 (A)
0
0.05
IA1 (A)
0
Time (100µs/div)
1236.2007.02.1.1
0.05
IA1 (A)
0
Time (100µs/div)
9
AAT1236
High Efficiency White LED Drivers
for Backlight and Keypad
Functional Block Diagram
LIN
SW
VIN
IN
ROM
Boost
Converter
Control
OV
V(A1, A2, A3)
VREF
V(B1, B2)
D/A
A1
D/A
A2
D/A
A3
D/A
B1
D/A
B2
ROM
VREF
EN
SCL
Max Current
Adjustment
I2C Interface
SDA
GND
AGND
Functional Description
The AAT1236 consists of a controller for the step-up
switching converter and its power switch, and five
regulated current sinks programmable over 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 AAT1236'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. I2C interface programming allows independent control of two groups of
10
RSET
current sinks (A1 to A3 and B1 to B2) and control
on/off with a different configuration on each channel. Unused sink channel(s) must be connected to
AGND to ensure proper function of the AAT1236.
Control Loop
The AAT1236 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 AAT1236 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 pro-
1236.2007.02.1.1
AAT1236
High Efficiency White LED Drivers
for Backlight and Keypad
vides direct feedback in order to maintain the
desired LED currents.
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
AAT1236 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
AAT1236 provides 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
1236.2007.02.1.1
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
over 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 AAT1236 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
AAT1236 during open-circuit on any LED channel
causing high output voltage conditions. An overvoltage 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.
11
AAT1236
High Efficiency White LED Drivers
for Backlight and Keypad
I2C Serial Interface and
Programmability
BR_CTRL
[BX3:BX0], [AX3:AX0]
All Outputs (%)
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
100
84
71
60
51
43
35
31
26
21
18
15
13.5
12.0
10.5
9.0
The current sink magnitude of each group and the
on/off status of each channel is controlled via an
I2C serial interface. I2C is a widely used interface
which requires a master to initiate all the communications with the device. I2C protocol consists of
two active wire SDA (serial data line) and SCL
(serial clock line). Both wires are open drain and
require an external pull-up resistor to VCC. The
SDA pin serves the I/O function, and the SCL pin
controls and references the I2C bus. The I2C protocol is a bidirectional bus which allows both read
and write actions to take place; the AAT1236 supports the write protocol only. Since the protocol
has a dedicated bit for Read or Write (R/W), when
communicating with the AAT1236, this bit must be
set to "0."
Table 1: LED Current Setting as Percentage of
the Maximum Level Set by RSET.
I2C Programming Register Address
and Register Data
After sending the device address, the I2C master
should send an 8-bit register address and 8-bit data
for programming. The AAT1236 has two registers;
The Brightness Control Register determines the
percentage of the maximum current set by RSET
applied to each channel and the Channel Control
Register determines which channels are enabled or
disabled. The programming is as follows:
BR_CRTL – LED Brightness Control Register
(Address: 00h)
BR_CTRL
Bit name
D7
BX3
D6
BX2
D5
BX1
D4
BX0
D3
AX3
D2
AX2
D1
AX1
CH_CTRL
Bit name
D7
–
D6
–
D5
–
D4
BY1
D3
BY2
D2
AY1
D1
AY2
D0
AY3
Control register CH_CRTL can be used to disable
(OFF) or enable (ON) individual channels.
CH_CTRL [BY1:BY2]
B1
B2
00
01
10
11
OFF
OFF
ON
ON
OFF
ON
OFF
ON
D0
AX0
Control register BR_CRTL can be used to control
LED brightness for each group. Control bits BX3,
BX2, BX1, BX0 set the percentage of the maximum
LED level in Group B. Control bits AX3, AX2, AX1,
AX0 set the percentage of the maximum LED level
in Group A.
12
CH_CRTL – Channel ON/OFF Control Register
(Address: 01h)
CH_CTRL
[AY1:AY3]
A1
A2
A3
000
001
010
011
100
101
110
111
OFF
OFF
OFF
OFF
ON
ON
ON
ON
OFF
OFF
ON
ON
OFF
OFF
ON
ON
OFF
ON
OFF
ON
OFF
ON
OFF
ON
1236.2007.02.1.1
AAT1236
High Efficiency White LED Drivers
for Backlight and Keypad
Application Information
I2C Address Bit Map
I2C Serial Interface
The AAT1236 is fully compliant with the industrystandard I2C interface. The I2C two-wire communications bus consists of SDA and SCL lines. SDA provides data, while SCL provides clock synchronization. SDA data transfers device address followed by
a register address and data bits sequence. When
using the I2C interface, EN/SET is pulled high to
enable the device or low to disable the device. The
I2C serial interface requires a master to initiate all the
communications with target devices. The AAT1236
is a target device and only supports the write protocol. The AAT1236 is manufactured with a target
device address of 0x36 (Hex). See Figure 1 for the
I2C interface diagram.
I2C START and STOP Conditions
START and STOP conditions are always generated by the master. Prior to initiating a START, both
the SDA and SCL pins are in idle mode (idle mode
is when there is no activity on the bus and both
SDA and SCL are pulled high by the external pullup resistors). A START condition occurs when the
master pulls the SDA line low and, after a short
period, pulls the SCL line low. A START condition
acts as a signal to all ICs that transmission activity
is about to occur on the BUS. A STOP condition,
as shown in Figure 2, is when the master releases
the bus and SCL changes from low to high followed
by SDA low-to-high transition.
start
Figure 3 illustrates the address bit transfer. The 7bit address is transferred with the Most Significant
Bit (MSB) first and is valid when SCL is high. This
is followed by the R/W bit in the Least Significant
Bit (LSB) location. The R/W bit determines the
direction of the transfer ('1' for read, '0' for write).
The AAT1236 is a write-only device and this bit
must be set low. The Acknowledge bit (ACK) is set
to low by the AAT1236 to acknowledge receipt of
the address.
I2C Register Address / Data Bit Map
Figure 4 illustrates the Register Address or the data
bit transfer. The 8-bit data is always transferred
with the most significant bit first and is valid when
SCL is high. The Acknowledge bit (ACK) is set low
by the AAT1236 to acknowledge receipt of the register address or the data.
I2C Acknowledge Bit (ACK)
The Acknowledge bit is the ninth bit of each transfer on the SDA line. It is used to send back a confirmation to the master that the data has been
received properly by the target device. For each
ACK to take place, the master must first release the
SDA line, then the target device will pull the SDA
line low, as shown in Figures 1, 3 and 4.
Device Address
w
ACK
Register Address
ACK
DATA
ACK
AAT1236 Device Addr = 36h
w
ACK
Address = 00h
ACK
Data = 06h
ACK
stop
SCL
SDA
start
stop
Figure 1: I2C Serial Interface Diagram.
1236.2007.02.1.1
13
AAT1236
High Efficiency White LED Drivers
for Backlight and Keypad
START
STOP
SDA
SDA
SCL
SCL
THD_STA
TSU_STO
Figure 2: I2C Start and Stop Conditions;
START: A High "1" to Low "0" Transition on the SDA Line While SCL is High "1"
STOP: A Low "0" to High "1" Transition on the SDA Line While SCL is High "1."
SCL
1
2
3
4
5
6
MSB
SDA
A6
8
7
9
LSB
A5
A4
A3
A2
A1
A0
R/W
ACK
Device Address
Figure 3: I2C Device Address Bit;
7-bit Slave Address (A6-A0), 1-bit Read/Write (R/W), 1-bit Acknowledge (ACK).
SCL
1
2
3
4
5
6
7
D6
D5
D4
D3
D2
D1
MSB
SDA
D7
8
9
LSB
D0
ACK
Register Address /
Data
Figure 4: I2C Register Address and Data Bit Map;
8-bit Data (D7-D0), 1-bit Acknowledge (ACK).
14
1236.2007.02.1.1
AAT1236
High Efficiency White LED Drivers
for Backlight and Keypad
I2C Software Protocol Example
4.
5.
6.
7.
8.
9.
10.
11.
12.
The AAT1236 I C programming protocol is shown
in the following two examples, detailing the device
address, register address and data bits. Figure 5
shows the I2C transfer protocol.
2
Example 1:
Turn on Group A with 15% from the max current
setting and turn on Group B with 51% from the max
current setting.
Channel Disable
1. Send a start condition
2. Send the AAT1236's I2C device address (0x36)
with the R/W bit set low
3. Wait for the acknowledge (ACK) bit within the
clock cycle
4. Send the BR_CTRL register address (0x00)
5. Wait for the ACK bit within the clock cycle
6. Send the BR_CTRL Data (0x4B)
7. Wait for the ACK bit within the clock cycle
8. Send the CH_CTRL register address (0x01)
9. Wait for the ACK bit within the clock cycle
10. Send the CH_CTRL Data (0x1F)
11. Wait for the ACK bit within the clock cycle
12. Send the stop condition
Tie all unused channels to AGND. On start-up
these channels will be automatically disabled.
LED Selection
Although the AAT1236 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 AAT1236 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 high-current LEDs without
complication, making the AAT1236 a perfect application for large LCD display backlighting and keypad LED applications.
Example 2:
Turn on A1 and A3 with 43% for the max LED current setting and turn on Group B with 100% for the
max LED current setting. Figure 6 shows the I2C
transfer protocol.
1. Send a start condition
2. Send the AAT1236's I2C device address (0x36)
with the R/W bit set low
3. Wait for the acknowledge (ACK) bit within the
clock cycle
S
T
A
R
T
SDA
A
C
K
Device Address
(Write)
0
1
1
0 1
1
0
0
A
C
K
Register Address
0 0
0
0
0 0
0
0
Send the BR_CTRL register address (0x00)
Wait for the ACK bit within the clock cycle
Send the BR_CTRL Data (0x05)
Wait for the ACK bit within the clock cycle
Send the CH_CTRL register address (0x01)
Wait for the ACK bit within the clock cycle
Send the CH_CTRL Data (0x1D)
Wait for the ACK bit within the clock cycle
Send the stop condition
A
C
K
Data
0 1
0
0 1
0 1
1
A
C
K
Register Address
0 0
0
0
0 0
0
1
S
A T
C O
K P
Data
0 0
0
1 1
1 1
1
Figure 5: I2C Transfer Protocol for Example 1.
S
T
A
R
T
SDA
A
C
K
Device Address
(Write)
0
1
1
0 1
1
0
0
A
C
K
Register Address
0 0
0
0
0 0
0
0
A
C
K
Data
0 0
0
0 0
1 0
1
A
C
K
Register Address
0 0
0
0
0 0
0
1
S
A T
C O
K P
Data
0 0
0
1 1
1 0
1
Figure 6: I2C Transfer Protocol for Example 2.
1236.2007.02.1.1
15
AAT1236
High Efficiency White LED Drivers
for Backlight and Keypad
Constant Current Setting
Maximum LED current per channel versus RSET
value is shown in Figure 7.
The LED current is controlled by the RSET resistor.
For maximum accuracy, a 1% tolerance resistor is
recommended. Table 2 shows the RSET resistor
value for AAT1236 for various LED full-scale current levels.
Ω)
RSET (kΩ
30
25
20
15
10
5
14.7
17.4
22.6
29.4
44.2
93.1
30
LED Current (mA)
ILED (mA)
35
25
20
15
10
5
0
10
36
62
88
114
140
166
192
218
244
270
RSET (kΩ
Ω)
Figure 7: LED Current vs. RSET Values.
Table 2: Maximum LED Current and RSET
Resistor Values (1% Resistor Tolerance).
VOUT
JP1
0
JP2
0
JP3
0
JP4
0
JP5
0
LED1
LED6
LED11
LED16
LED21
LED2
LED7
LED12
LED17
LED22
LED3
LED8
LED13
LED18
LED23
LED14
LED19
LED24
JP8
0
JP9
0
JP10
0
D1
MBR0530T1
SW_Node
R2
187k
LED4
LED9
JP6
0
JP7
0 U1
VIN
JP6
3
2
1
Enable/Set
JP7
SDA
SCL
R4
4.7k
(optional)
R5
4.7k
(optional)
C1
2.2µF
R3
12.1k
R1
22.6k
1
2
3
4
5
6
7
8
L1
2.2µH
VIN
LIN
OV
A1
EN
A2
B1
A3
B2
AGND
RSET
SCL
IN
SDA
GND
SW
16
15
14
13
12
11
10
9
C2
2.2µF
25V
RTN
AAT1236_TDFN3X4
2
1
L1: 2.2µH 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 8: A AAT1236-based High Efficiency White LED Driver Schematic.
16
1236.2007.02.1.1
AAT1236
High Efficiency White LED Drivers
for Backlight and Keypad
Over-Voltage Protection
The over-voltage protection circuit consists of a
resistor network connected from the output voltage
to the OV pin (see Figure 9). This over voltage protection circuit prevents damage to the device when
one of the five channels has an open LED circuit.
The AAT1236 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.
R 2 = R3 ·
⎛ VOUT(PROTECTION) ⎞
-1
VOV
⎝
⎠
AAT1236
R2
GND
LED Brightness Control
The AAT1236 uses the I2C interface to program
and control LED brightness. The output current of
the AAT1236 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.
Selecting the Schottky Diode
VOUT
COUT
OV
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 3.
R3
Figure 9: Over-Voltage Protection Circuit.
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).
To ensure minimum forward voltage drop and no
recovery, high voltage Schottky diodes are recommended for the AAT1236 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 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.
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 3: Over-Voltage Protection Settings.
1236.2007.02.1.1
17
AAT1236
High Efficiency White LED Drivers
for Backlight and Keypad
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:
The average diode current during the OFF time can
be estimated:
IAVG(OFF) =
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
1
= TON + TOFF
FS
During the ON time, the N-channel power MOSFET
is conducting and storing energy in the boost inductor. 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
The average output current multiplied by the forward diode voltage determines the loss of the output diode:
PLOSS(DIODE) = IAVG(TOT) · VF
= IOUT · VF
For continuous LED currents, the diode junction
temperature can then be estimated:
TJ(DIODE) = TAMB + θJA · PLOSS(DIODE)
= 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 =
IOUT
1 - DMAX
VOUT - VIN(MIN)
VOUT
Manufacturer
Part Number
Rated IF(AV)
Current (A)1
Diodes, Inc.
ON Semi
ON Semi
B0520WS
MBR130LSFT
MBR0530T
0.50
1.00
0.50
External Schottky diode junction temperature
should be below 110ºC, and may vary depending
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 4: Typical Surface Mount Schottky Rectifiers for Various Output Loads
(select TJ < 110°C in application circuit).
18
1236.2007.02.1.1
AAT1236
High Efficiency White LED Drivers
for Backlight and Keypad
Selecting the Boost Inductor
The AAT1236 controllers utilize 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.
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
1236.2007.02.1.1
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
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:
PLOSS(INDUCTOR) = IRMS2 · DCR
To ensure high reliability, the inductor case temperature should not exceed 100ºC. In some cases, PCB
heatsinking applied to the LIN node (non-switching)
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 AAT1236'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.
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 AAT1236 boost regulator. MLC capacitors of type X7R or X5R are recommended to
ensure good capacitance stability over the full
operating temperature range.
19
AAT1236
High Efficiency White LED Drivers
for Backlight and Keypad
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 5: Typical Surface Mount Inductors for Various Output Loads (select IPEAK < ISAT).
Manufacturer
Murata
Murata
Murata
Murata
Murata
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 6: Recommended Ceramic Capacitors.
The output capacitor is selected to maintain the
output load without significant voltage droop
(ΔVOUT) during the power switch ON interval, when
the output diode is not conducting. A ceramic output capacitor from 2.2µF to 4.7µF is recommended
(see Table 6).
Output capacitor size can be estimated at a switching frequency (FS) of 500kHz (worst case):
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.
To maintain stable operation at full load, the output
capacitor should be selected to maintain ΔVOUT
between 100mV and 200mV.
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.
20
COUT =
IOUT · DMAX
FS · ΔVOUT
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.
1236.2007.02.1.1
AAT1236
High Efficiency White LED Drivers
for Backlight and Keypad
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 AAT1236 boost converter is shown in Figures 10 and 11. 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.
Figure 10: AAT1236 Evaluation Board
Top Side Layout.
1236.2007.02.1.1
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 area 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.
Figure 11: AAT1236 Evaluation Board
Bottom Side Layout.
21
AAT1236
High Efficiency White LED Drivers
for Backlight and Keypad
Figure 12: Exploded View of AAT1236 Evaluation Board
Top Side Layout Detailing Plated Through Vias.
22
1236.2007.02.1.1
AAT1236
High Efficiency White LED Drivers
for Backlight and Keypad
Ordering Information
Package
Marking1
Part Number (Tape and Reel)2
TDFN34-16
UDXYY
AAT1236IRN-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.
1236.2007.02.1.1
23
AAT1236
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
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