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High Efficiency White LED Drivers for Backlight and Keypad General Description
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. 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. A similar device is also available with a proprietary Advanced Simple Serial ControlTM (AS2CwireTM) 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 -40C to +85C temperature range.
AAT1236
Features
* * * * * *
SwitchRegTM
*
* * *
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
Applications
* * * * * * * Digital Still Cameras (DSCs) Keypad Backlight Large Panel Displays Mobile Handsets PDAs and Notebook PCs Personal Media Players White LED Backlight
Keypad or RGB LEDs
Typical Application
L=2.2H D1 Up to 24V max
C OUT 2.2F
Backlight LEDs Input : 2.7V~5.5V CIN 2.2F LIN VIN IN SW B1 B2 A1 A2 A3 OV GND AGND
R2 187k
AAT1236
EN SDA SCL RSET R1 22.6k
R3 12.1k
Enable I2C Interface SDA SCL
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High Efficiency White LED Drivers for Backlight and Keypad Pin Descriptions
Pin #
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 EP
AAT1236
Symbol
VIN OV EN B1 B2 RSET IN GND SW SDA SCL AGND A3 A2 A1 LIN
Function
Input supply for the converter. Connect a 2.2F 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 22.6k 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 2.2H 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
1 2 3 4 5 6 7 8
16 15 14 13 12 11 10 9
LIN A1 A2 A3 AGND SCL SDA SW
2
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High Efficiency White LED Drivers for Backlight and Keypad Absolute Maximum Ratings1
TA = 25C unless otherwise noted. Symbol
VIN, IN SW EN, SCL, SDA, Bx, Ax, RSET, OV, LIN TS TJ TLEAD
AAT1236
Description
Input Voltage Switching Node Maximum Rating Storage Temperature Range Operating Temperature Range Maximum Soldering Temperature (at leads, 10 sec)
Value
-0.3 to 6.0 28 VIN + 0.3 -65 to 150 -40 to 150 300
Units
V V V C C C
Thermal Information2
Symbol
JA PD
Description
Thermal Resistance Maximum Power Dissipation3
Value
50 2
Units
C/W W
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 20mWC above 40C ambient temperature. 1236.2007.02.1.1
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High Efficiency White LED Drivers for Backlight and Keypad Electrical Characteristics1
VIN = 3.6V; CIN = 2.2F;TA = -40C to +85C, unless otherwise noted. Typical values are at TA = 25C. Symbol Description Conditions Min
2.7 VIN Rising Hysteresis VIN Falling B1 = B2 = A1 = A2 = A3 = 1.2V, 2mA Setting, RSET = 226k EN = GND VO = 24V RSET = 22.6k RSET = 22.6k, A1 = A2 = A3 = B1 = B2 = 0.4V VOUT Rising IOUT = 100mA IOUT = 100mA From Enable to Output Regulation; VFB = 300mV ISINK/IRSET, VRSET = 0.6V 1.2 0.4 1.4 400 1.3 0.6 0.6 0.6 100 0 0.6 1.3 2.7 VIN 5.5 2.7 VIN 5.5 IPULLUP = 3mA 0.4 1.4 -1.0 1.0 0.4
AAT1236
Typ
Max Units
5.5 24 2.7 V V V mV V A A mA mA % V mV m m s A/A A V V kHz s s s s ns s s s V V A V
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
150 1.8 300 1.0 100 18 20 2 1.1 1.2 100 80 200 300 760 22 5 1.3
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)
0.9
1. The AAT1236 is guaranteed to meet performance specifications over the -40C to +85C operating temperature range and is assured by design, characterization, and correlation with statistical process controls.
4
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High Efficiency White LED Drivers for Backlight and Keypad Electrical Characteristics (continued)1
VIN = 3.6V; CIN = 2.2F;TA = -40C to +85C, unless otherwise noted. Typical values are at TA = 25C. Symbol Description Conditions Min Typ
140 15
AAT1236
Max Units
C C
Thermal Protection TJ-TH TJ Thermal Shutdown Threshold TJ-HYS TJ Thermal Shutdown Hysteresis
I2C Interface Timing Details
SDA TSU_DAT
TLOW SCL THD_STA
THD_STA
TBUF
THD_DAT
THIGH
TSU_STA
TSU_STO
1. The AAT1236 is guaranteed to meet performance specifications over the -40C to +85C operating temperature range and is assured by design, characterization, and correlation with statistical process controls. 1236.2007.02.1.1
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High Efficiency White LED Drivers for Backlight and Keypad Typical Characteristics
Efficiency vs. LED Current
(Group B On; Group A Off)
83 82 84
AAT1236
Efficiency vs. LED Current
(Group B Off; Group A On) VIN = 5V
VIN = 5V
Efficiency (%)
83 82 81 80 79 78 77
Efficiency (%)
81 80 79 78 77 76 1.6 3.9 6.2 8.5 10.8 13.1 15.4 17.7 20
VIN = 3.6V VIN = 4.2V
VIN = 3.6V VIN = 4.2V
1.6
3.9
6.2
8.5
10.8
13.1
15.4
17.7
20
LED Current (mA)
LED Current (mA)
Efficiency vs. LED Current
(Group A and B On)
86 85
LED Current Accuracy vs. Supply Voltage
3 2
Efficiency (%)
84 83 82 81 80 79 1.6
VIN = 5V VIN = 3.6V VIN = 4.2V
Accuracy (%)
IB1, B2, A1, A2, A3
1 0 -1 -2 -3
3.9
6.2
8.5
10.8
13.1
15.4
17.7
20
2.7
3.1
3.4
3.8
4.1
4.5
4.8
5.2
5.5
LED Current (mA)
Supply Voltage (V)
LED Current vs. Supply Voltage
20.0 19.8
Shutdown Current vs. Supply Voltage and Temperature
0.7
Shutdown Current (A)
I B1 I A1
LED Current (mA)
0.6 0.5 0.4 0.3 0.2 0.1 0.0 2.7
19.6 19.4 19.2 19.0 18.8 18.6 18.4 2.7 3.1 3.4
25C 85C
I A3 I B2 I A2
-40C
3.1 3.5 3.9 4.3 4.7 5.1 5.5
3.8
4.1
4.5
4.8
5.2
5.5
Supply Voltage (V)
Supply Voltage (V)
6
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High Efficiency White LED Drivers for Backlight and Keypad Typical Characteristics
LED Current vs. Temperature
(All Channels = 20mA)
21.0
AAT1236
LED Current Accuracy vs. Temperature
(All Channels = 20mA)
IA3
LED Current Accuracy (%)
21.2
4 3 2 1 0 -1 -2 -3 -4 -5 -6 -40 -15 10 35 60 85
LED Current (mA)
20.8 20.6 20.4 20.2 20.0 19.8 19.6 19.4 19.2 19.0 -40 -15 10 35
IB1
IB2
IA2
IB1 IA2 IB2, A1 IA3
IA1
60 85
Temperature (C)
Temperature (C)
Shutdown Operation
(All Channels) Enable (V) IGROUP_A (mA) IGROUP_B (mA)
0.5
Output Ripple
(All Channels = 20mA)
Inductor Current (bottom) (A) Output Voltage (top) (V) Switching Node (middle) (V)
14.5 14.0 13.5 0V 1.0 0.5 0.0 16V
5 0 50 0 50 0
IINDUCTOR (A)
0
Time (50s/div)
Time (200ns/div)
Output Ripple
(All Channels = 10mA)
Inductor Current (bottom) (A) Output Voltage (top) (V) Switching Node (middle) (V)
Switching Frequency vs. Supply Voltage and Temperature
Switching Frequency (MHz)
2.5 2.0 1.5 1.0 0.5 0.0 2.7
13.5 13.0 12.5 14V 0V
25C
-40C +85C
0.5 0.0
3.1
3.5
3.9
4.3
4.7
5.1
5.5
Time (200ns/div)
Supply Voltage (V)
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High Efficiency White LED Drivers for Backlight and Keypad Typical Characteristics
Line Transient
(All Channels = 20mA)
Enable Threshold Low (V)
4.5
1.1 1.0
AAT1236
Enable Threshold Low vs. Supply Voltage and Temperature
Output Voltage (bottom) (V)
Input Voltage (top) (V)
4.0 3.5 3.0 14.2 14.1 14.0 13.9 13.8
-40C
0.9 0.8 0.7
25C
+85C
0.6 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5
Time (50s/div)
Supply Voltage (V)
Enable Threshold High vs. Supply Voltage and Temperature
Enable Threshold High (V)
1.2 1.1 1.0 0.9 0.8 0.7 2.7
280 260
Input Disconnect Switch Resistance vs. Supply Voltage and Temperature
+120C +100C
RDS(ON)IN (m)
-40C
25C
240 220 200
+85C
180 160 140 2.5
+85C
+25C
3.1
3.5
3.9
4.3
4.7
5.1
5.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
Supply Voltage (V)
Supply Voltage (V)
Low Side Switch On Resistance vs. Supply Voltage and Temperature
160 140 5
Soft Start Operation
(All Channels = 20mA) SCL (V)
0 5
RDS(ON)N (m)
+120C
120
SDA (V) +100C
0 14V
100 80
VOUT (V) +25C +85C IINDUCTOR (A)
3.0 3.5 4.0 4.5 5.0 5.5 6.0
0 0.5 0
60 40 2.5
Supply Voltage (V)
Time (200s/div)
8
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High Efficiency White LED Drivers for Backlight and Keypad Typical Characteristics
Transition of LED Current
(All Channels = 20mA to 1.8mA)
5
AAT1236
Transition of LED Current
(Group A= 20mA to 1.8mA; Group B = 20mA) SCL (V) SDA (V)
0.05 0 0.05
SCL (V) SDA (V) IB1 (A) IA1 (A)
5 0 5 0
0 5 0
IB1 (A) IA1 (A)
0.05 0 0.05 0
0
Time (100s/div)
Time (100s/div)
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High Efficiency White LED Drivers for Backlight and Keypad Functional Block Diagram
LIN SW
AAT1236
VIN IN D/A A1
OV
Boost Converter Control
ROM
D/A
A2
D/A V(A1, A2, A3) VREF V(B1, B2) VREF ROM D/A
A3
D/A
B1
B2
EN SCL SDA I2C Interface Max Current Adjustment
GND
AGND
RSET
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
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-
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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 2.2H (20%) inductor is selected to maintain high frequency switching (up to 2MHz) and high efficiency operation for outputs up to 24V. 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.
AAT1236
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 140C with 15C 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.
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
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High Efficiency White LED Drivers for Backlight and Keypad I2C Serial Interface and Programmability
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." BR_CTRL [BX3:BX0], [AX3:AX0]
0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111
AAT1236
All Outputs (%)
100 84 71 60 51 43 35 31 26 21 18 15 13.5 12.0 10.5 9.0
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 D0 AX0
Table 1: LED Current Setting as Percentage of the Maximum Level Set by RSET. CH_CRTL - Channel ON/OFF Control Register (Address: 01h)
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]
00 01 10 11
B1
OFF OFF ON ON
B2
OFF ON OFF ON
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.
CH_CTRL [AY1:AY3]
000 001 010 011 100 101 110 111
A1
OFF OFF OFF OFF ON ON ON ON
A2
OFF OFF ON ON OFF OFF ON ON
A3
OFF ON OFF ON OFF ON OFF ON
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1236.2007.02.1.1
High Efficiency White LED Drivers for Backlight and Keypad Application Information
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.
AAT1236
I2C Address Bit Map
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 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.
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.
start
Device Address
w
ACK
Register Address
ACK
DATA
ACK
stop
SCL
SDA start AAT1236 Device Addr = 36h w ACK Address = 00h ACK Data = 06h ACK stop
Figure 1: I2C Serial Interface Diagram.
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High Efficiency White LED Drivers for Backlight and Keypad
START SDA SCL THD_STA TSU_STO STOP SDA SCL
AAT1236
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 MSB
2
3
4
5
6
7
8
9
LSB A5 A4 A3 A2 A1 A0 R/W ACK
SDA
A6
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 MSB
2
3
4
5
6
7
8
9
LSB D6 D5 D4 D3 D2 D1 D0 ACK
SDA
D7
Register Address / Data
Figure 4: I2C Register Address and Data Bit Map; 8-bit Data (D7-D0), 1-bit Acknowledge (ACK).
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High Efficiency White LED Drivers for Backlight and Keypad
I2C Software Protocol Example
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
AAT1236
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. 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 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
0 1
4. 5. 6. 7. 8. 9. 10. 11. 12.
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
Channel Disable
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.
Device Address (Write)
1 01 1 0 0
A C K
Register Address
00 0 0 00 0 0
A C K
01 0
Data
01 01 1
A C K
Register Address
00 0 0 00 0 1
A C K
00 0
Data
11 11 1
S AT CO KP
Figure 5: I2C Transfer Protocol for Example 1.
S T A R T SDA
0 1
Device Address (Write)
1 01 1 0 0
A C K
Register Address
00 0 0 00 0 0
A C K
00 0
Data
00 10 1
A C K
Register Address
00 0 0 00 0 1
A C K
00 0
Data
11 10 1
S AT CO KP
Figure 6: I2C Transfer Protocol for Example 2.
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High Efficiency White LED Drivers for Backlight and Keypad
Constant Current Setting
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. ILED (mA)
30 25 20 15 10 5
AAT1236
Maximum LED current per channel versus RSET value is shown in Figure 7.
35
LED Current (mA)
30 25 20 15 10 5 0 10 36 62 88 114 140 166 192 218 244 270
RSET (k)
14.7 17.4 22.6 29.4 44.2 93.1
RSET (k)
Table 2: Maximum LED Current and RSET Resistor Values (1% Resistor Tolerance).
Figure 7: LED Current vs. RSET Values.
VOUT
JP1 0 LED1 JP2 0 LED6 JP3 0 LED11 JP4 0 LED16 JP5 0 LED21
LED2
LED7 D1 MBR0530T1 SW_Node
LED12
LED17
LED22
LED3
LED8
LED13
LED18
LED23
R2 187k
LED4
LED9
LED14
LED19
LED24
VIN
JP6 0 JP6
3 2 1
JP7 0 U1
1 2 3 4 5 6 7 8
L1 2.2H
JP8 0
JP9 0
JP10 0
Enable/Set
JP7 SDA SCL
2 1
R4 4.7k (optional)
R5 4.7k (optional)
C1 2.2F R3 12.1k R1 22.6k
VIN LIN OV A1 EN A2 B1 A3 B2 AGND RSET SCL IN SDA GND SW
AAT1236_TDFN3X4
16 15 14 13 12 11 10 9
C2 2.2F 25V RTN
L1: 2.2H Taiyo Yuden NR4018T2R2M C1: 0805 10V 2.2F X7R GRM21BR71A225KA01 C2: 0805 25V 2.2F X7R GRM21BR71E225KA73L LED1-24: OSRAM LW M673 or equivalent
Figure 8: A AAT1236-based High Efficiency White LED Driver Schematic.
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1236.2007.02.1.1
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 10k to 20k to minimize switching losses without degrading noise immunity.
VOUT(PROTECTION) -1 VOV
VOUT AAT1236 R2 OV GND R3 COUT
AAT1236
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.
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.
R 2 = R3 *
Selecting the Schottky Diode
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 (JA) 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.
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 = 12.1k and setting VOUT(PROTECTION) = 20V is recommended. Selecting a 1% resistor, this results in R2 = 187k (rounded to the nearest standard 1% value). Number of WLEDs on Each Channel
4 3 2
Total Maximum VF (V)
18.8 14.1 9.4
VOUT(PROTECTION) (V)
20 15 10
R3 = 12.1k R2 (k)
187 140 88.7
Table 3: Over-Voltage Protection Settings.
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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. The average diode current during the OFF time can be estimated:
AAT1236
IAVG(OFF) =
IOUT 1 - DMAX
Estimating Schottky Diode Power Dissipation
The switching period is divided between ON and OFF time intervals:
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:
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 + JA * PLOSS(DIODE)
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
External Schottky diode junction temperature should be below 110C, and may vary depending on application and/or system guidelines. The diode JA 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.
Manufacturer
Diodes, Inc. ON Semi ON Semi
Part Number
B0520WS MBR130LSFT MBR0530T
Rated IF(AV) Current (A)1
0.50 1.00 0.50
Rated Voltage (V)
20 30 30
Thermal Resistance (JA, C/W)1
426 325 206
Case
SOD-323 SOD-123 SOD-123
Table 4: Typical Surface Mount Schottky Rectifiers for Various Output Loads (select TJ < 110C in application circuit).
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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 1.5H and 2.7H is recommended. A better estimate of DMAX is possible once VF is known: 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:
AAT1236
IRMS =
IPEAK
3
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 2.2H inductor:
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 100C. 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.
IPEAK =
IOUT D * VIN(MIN) + MAX (1 - DMAX) (2 * FS * L)
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
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
High Efficiency White LED Drivers for Backlight and Keypad
Inductance (H)
2.2 2.4 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2
AAT1236
Manufacturer
Sumida Sumida Sumida Murata Murata Taiyo Yuden Taiyo Yuden Coiltronics Coiltronics Coiltronics
Part Number
CDRH4D22/HP-2R2 CDR4D11/HP-2R4 CDRH4D18-2R2 LQH662N2R2M03 LQH55DN2R2M03 NR4018T2R2 NR3015T2R2 SD3814-2R2 SD3114-2R2 SD3112-2R2
Max DC ISAT Current (A)
2.50 1.70 1.32 3.30 3.20 2.70 1.48 1.90 1.48 1.12
DCR ()
35 105 75 19 29 60 60 77 86 140
Size (mm) LxWxH
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
Type
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
GRM188R60J225KE19 GRM21BR71A225KA01 GRM219R61E225KA12 GRM21BR71E225KA73L GRM21BR61E475KA12
Value (F)
2.2 2.2 2.2 2.2 4.7
Voltage Rating
6.3 10 25 25 25
Temp Co
X5R X7R X5R X7R X5R
Case Size
0603 0805 0805 0805 0805
Table 6: Recommended Ceramic Capacitors.
The output capacitor is selected to maintain the output load without significant voltage droop (VOUT) during the power switch ON interval, when the output diode is not conducting. A ceramic output capacitor from 2.2F to 4.7F is recommended (see Table 6). 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.
Output capacitor size can be estimated at a switching frequency (FS) of 500kHz (worst case):
COUT =
IOUT * DMAX FS * VOUT
To maintain stable operation at full load, the output capacitor should be selected to maintain VOUT 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.
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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. 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.
AAT1236
Figure 10: AAT1236 Evaluation Board Top Side Layout.
Figure 11: AAT1236 Evaluation Board Bottom Side Layout.
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High Efficiency White LED Drivers for Backlight and Keypad
AAT1236
Figure 12: Exploded View of AAT1236 Evaluation Board Top Side Layout Detailing Plated Through Vias.
22
1236.2007.02.1.1
High Efficiency White LED Drivers for Backlight and Keypad Ordering Information
Package
TDFN34-16
AAT1236
Marking1
UDXYY
Part Number (Tape and Reel)2
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"
Index Area
4.000 0.050
3.300 0.050
0.350 0.100
Top View
Bottom View
C0.3
0.230 0.050
(4x)
0.850 MAX
0.050 0.050
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
0.450 0.050
Pin 1 Indicator (optional)
23
High Efficiency White LED Drivers for Backlight and Keypad
AAT1236
(c) 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 24
1236.2007.02.1.1

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