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PRODUCT DATASHEET
AAT1231
SwitchRegTM Step-Up DC/DC Converters for White LED Backlight Applications
Features
* Input Voltage Range: 2.7V to 5.5V * Maximum Continuous Output 24V @ 50mA * Drives 6 LEDs in Series, 12 LEDs in Parallel / Series Configuration Constant LED Current with 6% Accuracy * Digital Control with S2Cwire Single Wire Interface 26 Discrete Steps No PWM Control Required No Additional Circuitry * Up to 82% Efficiency * Up to 2MHz Switching Frequency Allows Small External Chip Inductor and Capacitors * Hysteretic Control No External Compensation Components Excellent Load Transient Response High Efficiency at Light Loads * Integrated Soft Start with No External Capacitor * True Load Disconnect Guarantees <1.0A Shutdown Current * Selectable Feedback Voltage Ranges for High Resolution Control of Load Current * Short-Circuit, Over-Voltage, and Over-Temperature Protection * 12-Pin TSOPJW Package * -40C to +85C Temperature Range
General Description
The AAT1231/1231-1 are high frequency, high efficiency constant current boost converters capable of 24V maximum output voltage. Both devices are ideal power solutions for backlight applications with up to six white LEDs in series or up to twelve white LEDs in a parallel/series configuration. The input voltage is 2.7V to 5.5V for singlecell lithium-ion/polymer (Li-ion) based portable devices. The LED current is digitally controlled across a 6x operating range using AnalogicTech's Simple Serial ControlTM (S2CwireTM) interface. Programmability across 26 discrete current steps provides high resolution, low noise, flicker-free, constant LED outputs. In programming AAT1231 operation, LED brightness increases based on the data received at the EN/SET pin. In programming AAT1231-1 operation, LED brightness decreases based on the data received at the EN/SET pin. The SEL logic pin changes the feedback voltage between two programmable ranges. The AAT1231 and the AAT1231-1 feature high current limit and fast, stable transitions for stepped or pulsed current applications. The high switching frequency (up to 2MHz) provides fast response and allows the use of ultra-small external components, including chip inductors and capacitors. Fully integrated control circuitry simplifies design and reduces total solution size. The AAT1231 and the AAT1231-1 offer a true load disconnect feature which isolates the load from the power source while in the OFF or disabled state. This eliminates leakage current, making the devices ideally suited for battery-powered applications. The AAT1231 and the AAT1231-1 are available in Pb-free, thermally-enhanced 12-pin TSOPJW packages.
Applications
* * * * * Digital Still Cameras (DSCs) Mobile Handsets MP3 Players PDAs and Notebook PCs White LED Drivers
Typical Application
L = 2.2H DS1
Up to 24V/ 50mA max
OSRAM LW M678 C2 2.2F
PVIN
C1 2.2F Li-Ion: VIN = 2.7V to 4.2V
LIN SW OVP
R3 12k R2 226k
VIN PGND
AAT1231/ 1231-1
Enable/Set Select
EN/SET SEL FB AGND
R1 (RBALLAST) 30.1
Capable of Driving Six LEDs in Series (see Applications Section)
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PRODUCT DATASHEET
AAT1231
SwitchRegTM
Pin Descriptions
Pin #
1 2
Step-Up DC/DC Converters for White LED Backlight Applications
Symbol
PVIN EN/SET
Function
Input power pin; connected to the source of the P-channel MOSFET. Connect to the input capacitor(s). IC enable pin and S2Cwire input control to set output current. FB voltage range select. For the AAT1231, a logic LOW sets the FB voltage range from 0.1V to 0.4V; a logic HIGH sets the FB voltage range from 0.3V to 0.6V. For the AAT1231-1, a logic LOW sets the FB voltage range from 0.4V to 0.1V; a logic HIGH sets the FB voltage range from 0.6V to 0.3V. Input voltage for the converter. Connect directly to the PVIN pin. No connection. Boost converter switching node. Connect the power inductor between this pin and LIN. Power ground for the boost converter. Ground pin. Feedback pin. Connect a resistor to ground to set the maximum LED current. Feedback pin for over-voltage protection sense. Switched power input. Connect the power inductor between this pin and SW.
3
SEL
4 5 6, 7 8 9 10 11 12
VIN N/C SW PGND AGND FB OVP LIN
Pin Configuration
TSOPJW-12 (Top View)
PVIN EN/SET SEL VIN N/C SW
1 2 3 4 5 6
12 11 10 9 8 7
LIN OVP FB AGND PGND SW
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PRODUCT DATASHEET
AAT1231
SwitchRegTM Step-Up DC/DC Converters for White LED Backlight Applications
Part Number Descriptions
SEL Polarity Part Number
AAT1231ITP AAT1231ITP-1 HIGH 0.3V VFB 0.6V 0.6V VFB 0.3V LOW 0.1V VFB 0.4V 0.4V VFB 0.1V
S2Cwire Feedback Voltage Programming
See Table 2 See Table 3
Absolute Maximum Ratings1
TA = 25C unless otherwise noted. Symbol
PVIN, VIN SW LIN, EN/SET, SEL, FB TJ TS TLEAD
Description
Input Voltage Switching Node Maximum Rating Operating Temperature Range Storage Temperature Range Maximum Soldering Temperature (at leads, 10 sec)
Value
-0.3 to 6.0 28 VIN + 0.3 -40 to 150 -65 to 150 300
Units
V V V C C C
Thermal Information
Symbol
JA PD
Description
Thermal Resistance Maximum Power Dissipation
Value
160 625
Units
C/W mW
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.
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PRODUCT DATASHEET
AAT1231
SwitchRegTM Step-Up DC/DC Converters for White LED Backlight Applications
Electrical Characteristics1
TA = -40C to +85C unless otherwise noted. Typical values are at 25C, VIN = 3.6V. Symbol
Power Supply PVIN, VIN VOUT(MAX) IQ ISHDN IOUT VLINEREG(FB)/ VIN RDS(ON) L TSS VOVP ILIMIT TSD THYS SEL, EN/SET VSEL(L) VSEL(H) VEN/SET(L) VEN/SET(H) TEN/SET (LO) TEN/SET(HI) TOFF TLAT IEN/SET AAT1231 FB AAT1231-1 FB FB Pin Regulation VIN = 2.7V to 5.5V, SEL = GND, EN/SET = DATA16 VIN = 2.7V to 5.5V, SEL = HIGH, EN/SET = HIGH 0.09 0.564 0.1 0.6 0.11 V 0.636
Description
Input Voltage Range Maximum Output Voltage Operating Current Shutdown Current Maximum Continuous Output Current2 Line Regulation Low Side Switch On Resistance Soft-Start Time Over-Voltage Protection Threshold Over-Voltage Hysteresis N-Channel Current Limit TJ Thermal Shutdown Threshold TJ Thermal Shutdown Hysteresis SEL Threshold Low SEL Threshold High Enable Threshold Low Enable Threshold High EN/SET Low Time EN/SET High Time EN/SET Off Timeout EN/SET Latch Timeout EN/SET Input Leakage
Conditions
Min
2.7
Typ
Max
5.5 24 70 1.0 50
Units
V V A A mA %/V m s
SEL = GND, FB = 0.1V EN/SET = GND 2.7V < VIN < 5.5V, VOUT = 24V VIN = 2.7V to 5.5V, VFB = 0.6V From Enable to Output Regulation; VFB = 300mV VOUT Rising VOUT Falling
40
0.7 80 300 1.1 1.2 100 2.5 140 15 1.3
V mV A C C V V V V s s s s A
0.4 1.4 0.4 VEN/SET VEN/SET VEN/SET VEN/SET VEN/SET < > < > = 0.6V 1.4V 0.6V 1.4V 5V, VIN = 5V 1.4 0.3 75 75 500 500 1 0.1 0.6 0.11
-1 0.09 0.564
FB Pin Regulation
VIN = 2.7V to 5.5V, SEL = GND, EN/SET = HIGH VIN = 2.7V to 5.5V, SEL = HIGH, EN/SET = DATA16
V 0.636
1. Specification over the -40C to +85C operating temperature range is assured by design, characterization, and correlation with statistical process controls. 2. Maximum continuous output current increases with reduced output voltage, but may vary depending on operating efficiency and thermal limitations.
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PRODUCT DATASHEET
AAT1231
SwitchRegTM Step-Up DC/DC Converters for White LED Backlight Applications
Typical Characteristics
Efficiency vs. LED Current
(4 White LEDs; RBALLAST = 30.1)
85 84
83
Efficiency vs. LED Current
(5 White LEDs; RBALLAST = 30.1)
VIN = 5V Efficiency (%)
82 81 80 79 78 77 76 75
Efficiency (%)
83 82 81 80 79 78 77 2 4 6 8 10 12 14 16 18 20
VIN = 5V VIN = 4.2V VIN = 3.6V
VIN = 3.6V
VIN = 4.2V
2
4
6
8
10
12
14
16
18
20
LED Current (mA)
LED Current (mA)
Efficiency vs. LED Current
(6 White LEDs; RBALLAST = 30.1)
81 80 84
(12 White LEDs; RBALLAST = 30.1)
83 82
Efficiency vs. LED Current
Efficiency (%)
Efficiency (%)
79 78 77 76 75 74 73 2
VIN = 5V
81 80 79 78 77 76 75 74
VIN = 5V
VIN = 4.2V VIN = 3.6V
VIN = 4.2V VIN = 3.6V
4
6
8
10
12
14
16
18
20
2
4
6
8
10
12
14
16
18
20
LED Current (mA)
LED Current (mA)
Shutdown Current vs. Input Voltage
(EN = GND)
1.0 700
Feedback Voltage vs. Temperature
(RBALLAST = 30.1)
Feedback Voltage (mV)
Shutdown Current (A)
0.8 0.6 0.4 0.2 0.0 2.7
600 500 400 300 200 100 0 -40 -15 10 35 60 85
85C 25C -40C
3.1 3.5 3.9 4.3 4.7 5.1 5.5
Input Voltage (V)
Temperature (C)
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PRODUCT DATASHEET
AAT1231
SwitchRegTM Step-Up DC/DC Converters for White LED Backlight Applications
Typical Characteristics
Accuracy ILED vs. Temperature
(VFB = 0.6V; RBALLAST = 30.1)
1.0 0.8 2.0 1.5
Accuracy ILED vs. Input Voltage
(VFB = 0.6V; RBALLAST = 30.1)
Accuracy ILED (%)
0.5 0.3 0.0 -0.3 -0.5 -0.8 -1.0 -40 -15 10 35 60 85
Accuracy ILED (%)
1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 2.7 3.2 3.7 4.2
-40C
25C
85C
4.7
5.2
5.7
Temperature (C)
Input Voltage (V)
Line Transient
(6 White LEDs; RBALLAST = 30.1) Feedback Voltage (bottom) (V) Enable Voltage (V) (top) Feedback Voltage (V) (middle) Input Voltage (top) (V) Output Voltage (middle) (V)
2.5V 0V 0.6 0.4 0.2 0
Shutdown
(VFB = 0.6V; ILED = 20mA) Inductor Current (A) (bottom)
4.2V 20.8 20.6 20.4 20.2 0.8 0.6 0.4 3.6V
0.5 0.0
Time (50s/div)
Time (50s/div)
Output Ripple
(6 White LEDs; ILED = 13mA) VOUT (DC Offset 19.8V) (50mV/div)
20
Output Ripple
(6 White LEDs; ILED = 20mA) VOUT (DC Offset 20.7V) (20mV/div)
VLX (V)
0
VLX (V)
20 0
0.5
0.5
IL (A)
0
IL (A)
0
Time (400ns/div)
Time (200ns/div)
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PRODUCT DATASHEET
AAT1231
SwitchRegTM Step-Up DC/DC Converters for White LED Backlight Applications
Typical Characteristics
AAT1231 Soft Start with S2Cwire
(6 White LEDs; VFB = 0.6V) Enable Voltage (top) (V) Feedback Voltage (middle) (V) Enable Voltage (top) (V) Feedback Voltage (middle) (V) Inductor Current (bottom) (A)
2.5V 0V 0.4 0.2 0 2 1 0 2.5V 0V 0.2 0 2 1 0
AAT1231 Soft Start
(6 White LEDs; VFB = 0.3V) Inductor Current (bottom) (A)
Time (100s/div)
Time (50s/div)
AAT1231-1 Soft Start with S2Cwire
(6 White LEDs; VFB = 0.3V) Enable Voltage (top) (V) Feedback Voltage (middle) (V) Enable Voltage (top) (V) Feedback Voltage (middle) (V) Inductor Current (bottom) (A)
2.5V 0.6 0.4 0.2 0 1 0 0V
AAT1231-1 Soft Start
(6 White LEDs; VFB = 0.6V) Inductor Current (bottom) (A)
2.5V 0.6 0.4 0.2 0 1 0 0V
Time (100s/div)
Time (50s/div)
Transition of LED Current
(6 White LEDs; SEL = Low; ILED = 13.3mA to 6.6mA) Feedback Voltage (bottom) (V)
Transition of LED Current
(6 White LEDs; SEL = Low; ILED = 3.3mA to 13.3mA) Feedback Voltage (bottom) (V)
Output Voltage (top) (V)
22 20 18 0.4 0.3 0.2 0.1 0.0
Output Voltage (top) (V)
22 20 18 0.4 0.3 0.2 0.1 0.0
Time (20s/div)
Time (20s/div)
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PRODUCT DATASHEET
AAT1231
SwitchRegTM Step-Up DC/DC Converters for White LED Backlight Applications
Typical Characteristics
EN/SET Latch Timeout vs. Input Voltage
EN/SET Latch Timeout (s)
350 300
EN/SET Off Timeout vs. Input Voltage
EN/SET Off Timeout (s)
300 250 200 150 100 2.7 3.1 3.5 3.9 4.3
25C -40C 85C
250 200 150 100 50
-40C
25C
85C
4.7
5.1
5.5
2.7
3.1
3.5
3.9
4.3
4.7
5.1
5.5
Input Voltage (V)
Input Voltage (V)
EN/SET Low Threshold vs. Input Voltage
1.2 1.1 1.0
EN/SET High Threshold vs. Input Voltage
1.2
-40C VIH (V)
1.1 1.0 0.9 0.8 0.7 0.6 0.5
-40C 25C 85C
VIL (V)
0.9 0.8 0.7 0.6 0.5 0.4 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5
25C
85C
0.4 2.7
3.1
3.5
3.9
4.3
4.7
5.1
5.5
Input Voltage (V)
Input Voltage (V)
Low Side Switch On Resistance vs. Input Voltage
160 140 300 280
Input Disconnect Switch Resistance vs. Input Voltage
RDS(ON)IN (m)
RDS(ON)L (m)
120 100 80 60 40 2.5 3 3.5
120C
260 240 220 200 180 160
120C
100C
100C
25C
85C
4 4.5 5 5.5 6
25C
3
85C
3.5 4 4.5 5 5.5 6
140 2.5
Input Voltage (V)
Input Voltage (V)
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PRODUCT DATASHEET
AAT1231
SwitchRegTM Step-Up DC/DC Converters for White LED Backlight Applications
Functional Block Diagram
PVIN
LIN
VIN
OVP
EN/SET
SW
Control
FB Reference Output Select
SEL
AGND
PGND
Functional Description
The AAT1231 and the AAT1231-1 consist of a DC/DC boost controller, an integrated slew rate controlled input disconnect MOSFET switch, and a high voltage MOSFET power switch. A high voltage rectifier, power inductor, output capacitor, and sense resistors are required to implement a DC/DC constant current boost converter. The input disconnect switch is activated when a valid input voltage is present and the EN/SET pin is pulled high. The slew rate control on the P-channel MOSFET ensures minimal inrush current as the output voltage is charged to the input voltage, prior to the switching of the N-channel power MOSFET. Monotonic turn-on is guaranteed by the integrated soft-start circuitry. Softstart eliminates output voltage overshoot across the full input voltage range and all loading conditions. The maximum current through the LED string is set by the ballast resistor and the feedback voltage of the IC. The output current may be programmed by adjusting
the level of the feedback reference voltage which is programmed through the S2Cwire interface. The SEL pin selects one of two feedback voltage ranges. For the AAT1231 and with a LOW logic level applied to the SEL pin, the FB pin voltage can be programmed from 0.1V to 0.4V. With a logic HIGH applied to the SEL pin, the FB pin voltage can be programmed from 0.3V to 0.6V. In the AAT1231-1, the SEL function is inverted in that the FB pin voltage can be programmed from 0.4V to 0.1V with a logic LOW applied to the SEL pin and 0.6V to 0.3V with a logic HIGH applied to the SEL pin. Regardless of which device is chosen, the feedback voltage can be set to any one of 16 current levels within each FB range, providing high-resolution control of the LED current, using the single-wire S2Cwire control. For torch and flash applications where a short duration, pulsed load is desired, applying a low-to-high transition on the AAT1231's SEL pin produces a 1.5x to 3.0x LED current step. In the AAT1231-1 on the other hand, the LED current step for a low-to-high transition on the SEL
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PRODUCT DATASHEET
AAT1231
SwitchRegTM Step-Up DC/DC Converters for White LED Backlight Applications
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 AAT1231/1231-1 provide 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. pin can be programmed from 3.0x to 1.5x. In both products, the step size is determined by the programmed voltage at the FB pin where the internal default setting is 3.0x in the AAT1231 and 1.5x in the AAT1231-1.
Control Loop
The AAT1231/1231-1 provide 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 AAT1231/1231-1 modulate the power MOSFET switching current to maintain the programmed FB voltage. This allows the FB voltage loop to directly program the required inductor current in order to maintain the desired LED current. 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. The peak current is adjusted by the controller until the LED output current requirement is met. The magnitude of the feedback error signal determines the average input current. Therefore, the AAT1231/1231-1 controller implements a programmed current source connected to the output capacitor, parallel with the LED string and ballast resistor. There is no right-half plane zero, and loop stability is achieved with no additional compensation components. An increase in the feedback voltage (VFB) results in an increased error signal sensed across the ballast resistor (R1). The controller responds by increasing the peak inductor current, resulting in higher average current in the inductor and LED string(s). Alternatively, when the VFB is reduced, the controller responds by decreasing the peak inductor current, resulting in lower average current in the inductor and LED string(s).
Soft Start / Enable
The input disconnect switch is activated when a valid input voltage is present and the EN/SET pin is pulled high. The slew rate control on the P-channel MOSFET ensures minimal inrush current as the output voltage is charged to the input voltage, prior to switching of the N-channel power MOSFET. Monotonic turn-on is guaranteed by the built-in soft-start circuitry. Soft start eliminates output current overshoot across the full input voltage range and all loading conditions. After the soft start sequence has terminated, the initial LED current is determined by the internal, default FB voltage across the external ballast resistor at the FB pin. Additionally, the AAT1231 and the AAT1231-1 have been designed to offer the system designer two choices for the default FB voltage based on the state of the SEL pin. Changing the LED current from its initial default setting is easy by using the S2Cwire single wire serial interface; the FB voltage can be increased (as in the AAT1231; see Table 2) or decreased (as in the AAT1231-1; see Table 3) relative to the default FB voltage. Some applications may require the output to be active when a valid input voltage is present. In these cases, add a 10k resistor between the VIN, VP, and EN/SET pins to avoid startup issues.
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PRODUCT DATASHEET
AAT1231
SwitchRegTM Step-Up DC/DC Converters for White LED Backlight Applications
Application Information
Over-Voltage Protection
OVP Protection with Open Circuit Failure
The OVP protection circuit consists of a resistor network tied from the output voltage to the OVP pin (see Figure 1). To protect the device from open circuit failure, 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 losses without degrading noise immunity.
Current Limit and Over-Temperature Protection
The switching of the N-channel MOSFET terminates when a current limit of 2.5A (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 AAT1231/1231-1 when internal dissipation becomes excessive. Thermal protection 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 AAT1231/1231-1 during open-circuit or high output voltage conditions. An over-voltage event is defined as a condition where the voltage on the OVP pin exceeds the Over-Voltage Threshold Limit (VOVP = 1.2V typical). When the voltage on the OVP pin has reached the threshold limit, the converter stops switching and the output voltage decays. Switching resumes when the voltage on the OVP pin drops below the lower hysteresis limit, maintaining an average output voltage between the upper and lower OVP thresholds multiplied by the resistor divider scaling factor.
R2 = R3 *
VOUT(MAX) -1 VOVP
VOUT
AAT1231/1231-1 R2 OVP GND R3 COUT
Figure 1: Over-Voltage Protection Circuit.
Over Voltage Protection Pin (top) (V) Inductor Current (bottom (A)
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.
Output Voltage (middle) (V)
1.224V 1.168V
26 24
2 1 0
22
Time (5ms/div)
Figure 2: Over-Voltage Protection Open Circuit Response (No LED).
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PRODUCT DATASHEET
AAT1231
SwitchRegTM Step-Up DC/DC Converters for White LED Backlight Applications
OVP Constant Voltage Operation
Cold Temperature Applied Self-Recovery
Assume R3 = 12k and VOUT(MAX) = 24V. Selecting 1% resistor for high accuracy, this results in R2 = 226k (rounded to the nearest standard value). The minimum OVP threshold can be calculated:
VOUT (5V/div)
VOUT(OVP_MIN) = VOVP(MIN) * = 21.8V
R2 +1 R3
ILED (10mA/div) ILED
To avoid OVP detection and subsequent reduction in the programmed output current (see following section), the maximum operating voltage should not exceed the minimum OVP set point.
Time (1s/div)
VOUT(MAX) < VOUT(OVP_MIN)
In some cases, this may disallow configurations with high LED forward voltage (VFLED) and/or greater than five series white LEDs. VFLED unit-to-unit tolerance can be as high as +15% of nominal for white LED devices.
Figure 3: Over-Voltage Protection Constant Voltage Operation (6 White LEDs; ILED = 13mA; R2 = 182k; R3 = 12k). While OVP is active, the maximum LED current programming error (ILED) is proportional to voltage error across an individual LED (VFLED).
OVP Constant Voltage Operation
Under closed loop constant current conditions, the output voltage is determined by the operating current, LED forward voltage characteristics (VFLED), quantity of series connected LEDs (N), and the feedback pin voltage (VFB).
VFLED =
(N * VFLED(MAX) - VOUT(OVP_MIN) - VFB) N
VOUT = VFB + N * VFLED
When the rising OVP threshold is exceeded, switching is stopped and the output voltage decays. Switching automatically restarts when the output drops below the lower OVP hysteresis voltage (100mV typical) and, as a result, the output voltage increases. The cycle repeats, maintaining an average DC output voltage proportional to the average of the rising and falling OVP levels (multiplied by the resistor divider scaling factor). High operating frequency and small output voltage ripple ensure DC current and negligible flicker in the LED string(s). The waveform in Figure 3 shows the output voltage and LED current at cold temperature with a six series white LED string and VOVP = 19.4V. As shown, the output voltage rises as a result of the increased VFLED which triggers the OVP constant voltage operation. Self heating of the LEDs triggers a smooth transition back to constant current control.
To minimize the ILED error, the minimum OVP voltage (VOUT(OVP_MIN)) may be increased, yielding a corresponding increase in the maximum OVP voltage (VOUT(OVP_MAX)). Measurements should confirm that the maximum switching node voltage (VSW(MAX)) is less than 28V under worstcase operating conditions.
VSW(MAX) = VOVP(MAX) *
R3 + 1 + VF + VRING R2
VF = -Schottky Diode DS1 forward voltage at turn-OFF VRING = Voltage ring occurring at turn-OFF
LED Selection and Current Setting
The AAT1231/1231-1 are well suited for driving white LEDs with constant current. Applications include main and sub-LCD display backlighting, and color LEDs. The LED current is controlled by the FB voltage and the ballast resistor. For maximum accuracy, a 1% tolerance resistor is recommended.
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PRODUCT DATASHEET
AAT1231
SwitchRegTM Step-Up DC/DC Converters for White LED Backlight Applications
Typical white LEDs are driven at maximum continuous currents of 15mA to 20mA. For maximum output, two parallel strings of six series LEDs are used. A total output current of 30mA or 40mA is required (15mA to 20mA in each string). The maximum quantity of series connected LEDs is determined by the minimum OVP voltage of the boost converter (VOUT(OVP_MIN)), minus the maximum feedback voltage (VFB(MAX)) divided by the maximum LED forward voltage (VFLED(MAX)). VFLED(MAX) can be estimated from the manufacturers' datasheet at the maximum LED operating current. The ballast resistor (RBALLAST) value can be calculated as follows:
RBALLAST =
where:
VFB(MAX) ILED(MAX)
VFB(MAX) = 0.4V when SEL = Low VFB(MAX) = 0.6V when SEL = High i.e., for a maximum LED current of 20mA (SEL = High):
RBALLAST =
ILED(MAX)
VFB
0.6 = = 30 30.1 0.020
RBALLAST ()
SEL = High 12.1 15.0 16.9 20.0 24.3 30.1 40.2 60.4 121.0 SEL = Low 8.06 10.0 11.3 13.3 16.2 20.0 26.7 40.2 80.6
VOUT(OVP_MIN) = VOVP(MIN) *
R2 +1 R3
N=
Maximum ILED Current (mA)
50 40 35 30 25 20 15 10 5
(VOUT(OVP_MIN) - VFB(MAX)) VFLED(MAX)
Figure 4 shows the schematic of using six LEDs in series. Assume VFLED @ 20mA = 3.5V (typical) from LW M673 (OSRAM) datasheet.
VOUT(OVP_MIN) = 1.1V *
226k + 1 = 21.82V 12k
N=
21.82V - 0.6V 3.5V
Table 1: Maximum LED Current and RBALLAST Resistor Values (1% Resistor Tolerance).
6.1
Therefore, under typical operating conditions, six LEDs can be used in series.
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PRODUCT DATASHEET
AAT1231
SwitchRegTM
VIN = 2.7V to 5.5V
JP1
1 2 3
Step-Up DC/DC Converters for White LED Backlight Applications
L1 R4 10K U1 AAT1231/1231-1
1 2 3 4 5 6
DS1 R2 226K R3 12K
VOUT = 24V/20mA
D1 LED D2 LED D3 LED D4 LED C2 2.2F
2.2H
12 11 10 9 8 7
C1 2.2F
Enable JP2
1 2 3
VIN EN SEL VP N/C SW
LIN OVP FB GND PGND SW
TSOP12JW R1 30.1 D6 LED D5 LED
Select
U1 AAT1231/1231-1 TSOPJW-12 L1 2.2H SD3814-2R2 C1 2.2F 10V 0603 C2 2.2F 25V 0805 D1-D6 LW M673 White LED DS1 30V 0.2A BAT42W SOD-123 R1 30.1 0603 R2 226K 0603 R3 12K 0603 R4 10K 0603
Figure 4: AAT1231/1231-1 White LED Boost Converter Schematic.
LED Brightness Control
The AAT1231 and the AAT1231-1 use S2Cwire programming to control LED brightness and does not require PWM (pulse width modulation) or additional control circuitry. 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 output current of the AAT1231 and the AAT1231-1 can be changed successively to brighten or dim the LEDs in smooth transitions (i.e., to fade out) or in abrupt steps, giving the user complete programmability and real-time control of LED brightness.
25
25
LED Current (mA)
20 SEL=HIGH 15 10 SEL=LOW 5 0 1 4 7 10 13 16 (Default)
S2Cwire Data Register
Figure 6: Programming AAT1231-1 LED Current with RBALLAST = 30.1. Alternatively, toggling the SEL logic pin from low to high implements stepped or pulsed LED currents by increasing the FB pin voltage. Figures 7 and 8 illustrate the SELECT pin scaling factor, defined as the LED current with SEL=HIGH divided by the LED current with SEL=LOW. For the AAT1231, scaling factors from 1.5x to 3.0x are possible, depending on the S2Cwire data register (default = 3.0x). In the AAT1231-1, the possible scaling factors are 3.0x to 1.5x with the internal default setting of 1.5x.
LED Current (mA)
20
SEL = HIGH
15
10
(Default)
5
SEL = LOW
0 1 4 7 10 13 16
S2Cwire Data Register
Figure 5: Programming AAT1231 LED Current with RBALLAST = 30.1.
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PRODUCT DATASHEET
AAT1231
SwitchRegTM
3.5
Step-Up DC/DC Converters for White LED Backlight Applications
16 individual states. Each state corresponds to a reference feedback voltage setting on the FB pin, as shown in Table 2.
Select Pin Scaling Factor (High to Low)
(Default) 3.0
2.5 2.0
S2Cwire Serial Interface Timing
The S2Cwire single wire serial interface data can be clocked-in at speeds up to 1MHz. After data has been submitted, EN/SET is held high to latch the data for a period TLAT. The FB pin voltage is subsequently changed to the level as defined by the state of the SEL logic pin. When EN/SET is set low for a time greater than TOFF, the AAT1231/1231-1 is disabled. When either the AAT1231 or the AAT1231-1 is disabled, the register is reset to its default value. In the AAT1231, the default register value sets the FB pin voltage to 0.6V if the EN/SET pin is subsequently pulled HIGH. In the AAT1231-1, the FB pin voltage is set to 0.3V under the same condition.
1.5 1.0 1 4 7 10 13 16
S2Cwire Data Register
Figure 7: AAT1231 SEL Pin Scaling Factor: ILED (SEL = High) Divided by ILED (SEL = Low).
3. 5
Select Pin Scaling Factor (Low to High)
3. 0 2. 5 2. 0 (Default) 1. 5 1. 0 1 4 7 10 13 16
S2Cwire Feedback Voltage Programming
The FB pin voltage is set to the default level at initial powerup. The AAT1231 and the AAT1231-1 are programmed through the S2Cwire interface. Table 2 illustrates FB pin voltage programming for the AAT1231 and Table 3 illustrates FB pin voltage programming for the AAT1231-1. The rising clock edges applied at the EN/SET pin determine the FB pin voltage. If a logic LOW is applied at the SEL pin, the default feedback voltage range for the AAT1231 is 0.1V to 0.4V; for a logic HIGH condition at the SEL pin, the default feedback voltage range is 0.3V to 0.6V. Conversely, if a logic LOW is applied at the SEL pin of the AAT1231-1, the default feedback voltage range becomes 0.4V to 0.1V and 0.6V to 0.3V for a logic HIGH condition at the SEL pin.
S2Cwire Data Register
Figure 8: AAT1231-1 SEL Pin Scaling Factor: ILED (SEL = High) Divided by ILED (SEL = Low).
S2Cwire Serial Interface
AnalogicTech's S2Cwire single wire serial interface is a proprietary high-speed single-wire interface available only from AnalogicTech. The S2Cwire interface records rising edges of the EN/SET input and decodes them into
THI TLO T LAT TOFF
EN/SET
1 2 n-1 n 16
Data Reg
0
n-1
0
Figure 9: AAT1231/1231-1 S2Cwire Timing Diagram to Program the Output Voltage.
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PRODUCT DATASHEET
AAT1231
SwitchRegTM
Rising Clock Edges/Data Register
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Step-Up DC/DC Converters for White LED Backlight Applications
SEL = Low Reference Voltage (V)
0.1 (default) 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 0.28 0.30 0.32 0.34 0.36 0.38 0.40
SEL = High Reference Voltage (V)
0.3 (default) 0.32 0.34 0.36 0.38 0.40 0.42 0.44 0.46 0.48 0.50 0.52 0.54 0.56 0.58 0.60
LED Current (mA); RBALLAST = 30.1
3.32 3.99 4.65 5.32 5.98 6.64 7.31 7.97 8.64 9.30 9.97 10.63 11.30 11.96 12.62 13.29
LED Current (mA); RBALLAST = 30.1
9.97 10.63 11.30 11.96 12.62 13.29 13.95 14.62 15.28 15.95 16.61 17.28 17.94 18.60 19.27 19.93
Table 2: AAT1231 S2Cwire Reference Feedback Voltage Control Settings with RBALLAST = 30.1 (Assume Nominal Values). SEL = Low Reference Voltage (V)
0.4 (default) 0.38 0.36 0.34 0.32 0.30 0.28 0.26 0.24 0.22 0.20 0.18 0.16 0.14 0.12 0.10
Rising Clock Edges/Data Register
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
SEL = High Reference Voltage (V)
0.6 (default) 0.58 0.56 0.54 0.52 0.50 0.48 0.46 0.44 0.42 0.40 0.38 0.36 0.34 0.32 0.30
LED Current (mA); RBALLAST = 30.1
13.29 12.62 11.96 11.30 10.63 9.97 9.30 8.64 7.97 7.31 6.64 5.98 5.32 4.65 3.99 3.32
LED Current (mA); RBALLAST = 30.1
19.93 19.27 18.60 17.94 17.28 16.61 15.95 15.28 14.62 13.95 13.29 12.62 11.96 11.30 10.63 9.97
Table 3: AAT1231-1 S2Cwire Reference Feedback Voltage Control Settings With RBALLAST = 30.1 (Assumes Nominal Values).
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PRODUCT DATASHEET
AAT1231
SwitchRegTM Step-Up DC/DC Converters for White LED Backlight Applications
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)).
Selecting the Schottky Diode
To ensure minimum forward voltage drop and no recovery, high voltage Schottky diodes are considered the best choice for the AAT1231/1231-1 boost converters. The output diode is sized 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 to consider 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 consulted 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 outputs less than 15V, while 30V rated Schottky diodes are recommended for outputs greater than 15V. The switching period is divided between ON and OFF time intervals.
DMAX =
VOUT - VIN(MIN) VOUT
The average diode current during the OFF time can be estimated.
IAVG(OFF) =
IOUT 1 - DMAX
The following curves show the VF characteristics for different Schottky diodes (100C case). The VF of the Schottky diode can be estimated from the average current during the off time.
10000
Forward Current (mA)
B340LA 1000 MBR0530T ZHCS350 100
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.
BAT42W
10 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70
Forward Voltage (V)
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.
D=
TON TON + TOFF
PLOSS(DIODE) = IAVG(TOT) * VF = IOUT * VF
For continuous LED currents, the diode junction temperature can be estimated.
= TON FS
TJ(DIODE) = TAMB + JA * PLOSS(DIODE)
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PRODUCT DATASHEET
AAT1231
SwitchRegTM Step-Up DC/DC Converters for White LED Backlight Applications
Rated Forward Current (A)
3 0.2 0.5 0.35 0.2
Manufacturer
Diodes, Inc. Diodes, Inc. ON Semi Zetex Central Semi
Part Number
B340LA BAT42W MBR0530T ZHCS350 CMDSH2-3
Non-Repetitive Peak Surge Current (A)
70.0 4.0 5.5 4.2 1.0
Rated Voltage (V)
40 30 30 40 30
Thermal Resistance (JA, C/W)
25 500 206 330 500
Case
SMA SOD-123 SOD-123 SOD-523 SOD-323
Table 4: Typical Surface Mount Schottky Rectifiers for Various Output Levels.
Output diode junction temperature should be maintained below 110C, but may vary depending on application and/or system guidelines. The diode JA can be minimized with additional PCB area on the cathode. PCB heat-sinking 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.
The output inductor (L) is selected to avoid saturation at minimum input voltage, 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 from the curves and assumes a 2.2H inductor.
Switching Frequency (MHz)
2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 40 VIN = 2.7V VOUT = 18V 50 VIN = 2.7V VOUT = 15V 60 70 80 90 100 VIN = 3.0V VOUT = 18V VIN = 3.0V VOUT = 15V VIN = 3.6V VOUT = 18V VIN = 3.6V VOUT = 15V
Selecting the Boost Inductor
The AAT1231 and the AAT1231-1 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 sized from 1.5H to 2.7H is recommended. A better estimate of DMAX is possible once VF is known.
Output Current (mA)
Switching Frequency (MHz)
2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 40 VIN = 2.7V VOUT = 10V 50 VIN = 2.7V VOUT = 12V 60 70 80 90 100 VIN = 3.0V VOUT = 12V VIN = 3.0V VOUT = 10V VIN = 3.6V VOUT = 12V VIN = 3.6V VOUT = 10V
DMAX =
(VOUT + VF - VIN(MIN)) (VOUT + VF)
Where VF is the Schottky diode forward voltage. If not known, it can be estimated at 0.5V. 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 completed to ensure that the inductor does not saturate or exhibit excessive temperature rise.
Output Current (mA)
IPEAK =
IOUT D * VIN(MIN) + MAX (1 - DMAX) (2 * FS * L)
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PRODUCT DATASHEET
AAT1231
SwitchRegTM Step-Up DC/DC Converters for White LED Backlight Applications
ing applied to the LIN node (non-switching) can improve the inductor's thermal capability. PCB heatsinking may degrade EMI performance when applied to the SW node (switching) of the AAT1231/1231-1. 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, chip-type inductors have increased winding resistance (DCR) when compared to shielded, wound varieties. 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 from 0.5A to 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 ensure that the inductor does not saturate at maximum LED current and minimum input 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 manufacturer's temperature rise, or thermal derating, guidelines.
Inductor Efficiency Considerations
The efficiency for different inductors is shown in Figure 7 for six white LEDs in series. Smaller inductors yield increased DCR and reduced operating efficiency.
80
Efficiency (%)
IRMS =
IPEAK
3
77 74 71 68 65
Cooper SD3814-2R2 (77m)
Cooper SD3110-2R2 (161m)
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.
2
5
8
11
14
17
20
LED Current (mA)
PLOSS(INDUCTOR) = IRMS2 * DCR
To ensure high reliability, the inductor case temperature should not exceed 100C. In some cases, PCB heatsink-
Figure 10: AAT1231/1231-1 Efficiency for Different Inductor Types (VIN = 3.6V; Six White LEDs in Series).
Manufacturer
Sumida www.sumida.com Cooper Electronics www.cooperet.com Murata www.murata.com Taiyo Yuden www.t-yuden.com
Part Number
CDRH2D11-2R2 SD3814-2R2 SD3110-2R2 LQH3NPN2R2NG0 LQM2HPN2R2MG0 NR3010T-2R2M CBC2016T2R2M CBC2518T2R2M
Inductance (H)
2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2
Maximum DC ISAT Current (mA)
780 1900 910 1250 1300 1100 750 510
DCR (m)
78 77 161 164 80 95 200 90
Size (mm) LxWxH
3.2x3.2x1.2 4.0x4.0x1.4 3.1x3.1x1.0 3.0x3.0x1.0 2.5x2.0x1.0 3.0x3.0x1.0 2.0x1.6x1.6 2.5x1.8x1.8
Type
Shielded Shielded Shielded Chip Coil Shield Chip Coil Shield Shielded Chip Non-Shielded Shielded
Table 5: Recommended Inductors for Various Output Levels (Select IPEAK < ISAT).
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PRODUCT DATASHEET
AAT1231
SwitchRegTM Step-Up DC/DC Converters for White LED Backlight Applications
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.
Selecting the Boost Capacitors
The high output ripple inherent in the boost converter necessitates 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 AAT1231/1231-1 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 sized 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 5). Typically, 25V rated capacitors are required for the 24V maximum boost output. Ceramic capacitors sized 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 acceptable. 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).
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 AAT1231/1231-1 boost converter is shown in Figures 10 and 11. The following PCB layout guidelines should be considered: Minimize the distance from Capacitor C1 and C2 negative terminal to the PGND pins. This is especially true with output capacitor C2, which conducts high ripple current from the output diode back to the PGND pins. 2. Minimize the distance between L1 to DS1 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 PGND pin(s) as well as the GND terminals of C1 and C2. 4. Consider additional PCB area on DS1 cathode to maximize heatsinking capability. This may be necessary when using a diode with a high VF and/or thermal resistance. 5. To avoid problems at startup, add a 10k resistor between the VIN, VP and EN/SET pins (R4). This is critical in applications requiring immunity from input noise during "hot plug" events, e.g. when plugged into an active USB port. 1.
COUT =
IOUT * DMAX FS * VOUT
To maintain stable operation at full load, the output capacitor should be sized to maintain VOUT between 100mV and 200mV.
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PRODUCT DATASHEET
AAT1231
SwitchRegTM
Manufacturer
Murata Murata Murata Murata Murata
Step-Up DC/DC Converters for White LED Backlight Applications
Part Number
GRM188R60J225KE19 GRM188R61A225KE34 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 X5R X5R X7R X5R
Case Size
0603 0603 0805 0805 0805
Table 6: Recommended Ceramic Capacitors.
AAT1231/1231-1 White LED Driver
S2Cwire Microcontroller
Figure 11: AAT1231/1231-1 Evaluation Board Top Side Layout (with six LEDs and microcontroller).
Figure 12: AAT1231/1231-1 Evaluation Board Bottom Side Layout (with six LEDs and microcontroller).
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PRODUCT DATASHEET
AAT1231
SwitchRegTM Step-Up DC/DC Converters for White LED Backlight Applications
VCC
S2Cwire Microcontroller
R7 1K R8 330 Up D7 RED Down Select
02 4
R6 1K
R5 1K
1 2 3 4
U2
C3 1F
SW3
1 3 5
02 4
SW2
1 3 5
VDD GP5 GP4 GP3
VSS GP0 GP1 GP2
8 7 6 5
R9 330 D8 GREEN (Select indicator)
PIC12F675 SW1
02 4
1 3 5
R4 10K J2 J3 DS1 Schottky R2 226K R3 12K
DC-
DC+
L1 J1
1 2 3
VOUT
D1 LED D2 LED D3 LED D4 LED C2 2.2F
VCC JP1 C1 2.2F
U1
1 2 3 4 5 6
2.2H
AAT1231/1231-1 White LED Driver
VIN EN SEL VP N/C SW
LIN OVP FB GND PGND SW
12 11 10 9 8 7
AAT1231/1231-1 R1 30.1 D6 LED D5 LED
U1 AnalogicTech AAT1231/1231-1 TSOPJW-12 package U2 PIC12F675 C1 GRM188R60J225KE01 C2 GRM21BR71E225KA73 C3 GRM216R61A105KA01 R1 30.1, 1%, 1/4W; 0603 R2 226k, 1%, 1/4W; 0603 R3 12.1k, 1%, 1/4W; 0603 R4 10k, 5%, 1/4W; 0603 R5, R6, R7 1K, 5%, 1/4W; 0805 R8, R9 330, 5%, 1/4W; 0805 JP1 0, 5%; 0805 DS1 BAT42W L1 Cooper Electronics 2.2H SD3814-2R2 D1-D6 White Hyper-Bright LED LW M673 D7 Red LED 1206 D8 Green LEC 1206 SW1 - SW3 SPST, 5mm J1, J2, J3 Conn. Header, 2mm
Figure 13: AAT1231/1231-1 Evaluation Board Schematic (with six LEDs and microcontroller).
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PRODUCT DATASHEET
AAT1231
SwitchRegTM Step-Up DC/DC Converters for White LED Backlight Applications
Additional Applications
Efficiency vs. LED Current
L = 2.2H PVIN Li-Ion VIN = 2.7V to 5.5V C1 2.2F VIN LIN SW R2 187k R3 12k ENSET SEL AGND 30.1 20mA FB DS1 Up to 24V/ 50mA max
(4 White LEDs; RBALLAST = 30.1)
85 84
VIN = 5V
Efficiency (%)
AAT1231/ 1231-1
PGND OVP
C2 2.2F
83 82 81 80 79 78 77 2 4 6 8 10 12 14 16 18 20
VIN = 3.6V
VIN = 4.2V
LED Current (mA)
Figure 14: Four LEDs In Series Configuration.
Efficiency vs. LED Current
L = 2.2H PVIN Li-Ion VIN = 2.7V to 5.5V C1 2.2F VIN LIN SW R2 196k R3 12k DS1 Up to 24V/ 50mA max
(5 White LEDs; RBALLAST = 30.1)
83 82
Efficiency (%)
AAT1231/ 1231-1
PGND OVP
C2 2.2F
81 80 79 78 77 76
VIN = 5V VIN = 4.2V VIN = 3.6V
ENSET SEL AGND 30.1 20mA FB
75 2 4 6 8 10 12 14 16 18 20
LED Current (mA)
Figure 15: Five LEDs In Series Configuration.
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PRODUCT DATASHEET
AAT1231
SwitchRegTM Step-Up DC/DC Converters for White LED Backlight Applications
Efficiency vs. LED Current
(6 White LEDs; RBALLAST = 30.1)
L = 2.2H PVIN LIN SW R2 226k R3 12k C2 2.2F DS1 Up to 24V/ 50mA max
81 80
AAT1231/ 1231-1
PGND OVP
Efficiency (%)
Li-Ion VIN = 2.7V to 5.5V
C1 2.2F
VIN
79 78 77 76 75 74 73 2
VIN = 5V
VIN = 4.2V VIN = 3.6V
EN/SET SEL AGND FB 30.1 20mA
4
6
8
10
12
14
16
18
20
LED Current (mA)
Figure 16: Six LEDs In Series Configuration.
L = 2.2H PVIN LIN SW
DS1
Up to 24V/ 50mA max
(12 White LEDs; RBALLAST = 30.1)
84 83
Efficiency vs. LED Current
Efficiency (%)
Li-Ion VIN = 2.7V to 5.5V
C1 2.2F
VIN
AAT1231/ 1231-1
PGND OVP
R2 226k R3 12k
C2 2.2F
82 81 80 79 78 77 76 75
30.1 20mA
VIN = 5V
EN/SET SEL AGND FB 30.1 20mA
VIN = 4.2V VIN = 3.6V
74 2 4 6 8 10 12 14 16 18 20
LED Current (mA)
Figure 17: Twelve LEDs In Series/Parallel Configuration.
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PRODUCT DATASHEET
AAT1231
SwitchRegTM Step-Up DC/DC Converters for White LED Backlight Applications
(3S7P: 3 in series per string with 7 strings in parallel) is shown in Figure 18. A scalable schematic and PCB layout are illustrated in Figures 19 through 21.
Multi-String White LED Configurations for Digital Photo Frame Applications
The AAT1231 and AAT1231-1 can be configured to light up as many as thirty-two white LEDs (WLED). This solution is scalable, flexible and good for digital photo frame applications with multi-strings of WLEDs. The multi-string WLED configuration can be composed of many different parallel/series combinations, such as 6S2P, 6S3P, 5S4P, 5S5P, 5S6P, 4S7P, 4S8P, 3S9P, and 3S10P. `S' is defined as the number of WLEDs in a series per string. `P' is defined as the number of strings of WLEDs that are connected from the output voltage (VOUT) to the ballast resistor, or in parallel. To match the "brightness" of each separate string of WLEDs, each string must have the same number of WLEDs in them. The over-voltage protection (OVP) should also be adjusted according to the maximum feedback voltage plus the maximum forward voltage (VF) of each WLED multiplied by the total number of WLEDs in any of the parallel strings of WLEDs. The efficiency of one configuration
VCC R8 1k D12 RED Up SW2 Down SW1 Select
Efficiency vs. Total LED Current
(21 White LEDs [3 in Series, 7 in Parallel]; RBALLAST = 4.32)
90
85
Efficiency (%)
VIN = 5V VIN = 4.2V VIN = 3.6V
80
75
70
65 20
40
60
80
100
140
160
Total LED Current (mA)
Figure 18: Efficiency of the 3S7P Multi-String Configuration.
R6 1k SW3
R5 1k
R4 1k
1 2 3 4
U2 VDD VSS GP5 GP0 GP4 GP1 GP3 GP2 PIC12F675
C3 1F
8 7 6 5
R7 1k D11 GREEN (Select indicator)
R9 10k
VOUT
JP2 D1a LED D1b LED D1c LED D1d LED D1e LED D2a LED D2b LED D2c LED D2d LED D2e LED D2f LED D3a LED D3b LED D3c LED D3d LED D3e LED D3f LED D4a LED D4b LED D4c LED D4d LED D4e LED D5a LED D5b LED D5c LED D5d LED D5e LED D6a LED D6b LED D6c LED D6d LED D6e LED D7a LED D7b LED D7c LED D7d LED D8a LED D8b LED D8c LED D8d LED D9a LED D9b LED D9c LED D10a LED D10b LED D10c LED
DC-
DC+
J2
J3 L1 2.2H LIN OVP FB GND PGND SW
12 11 10 9 8 7
DS1 D Schottky
J1
VCC JP1
1
2
3 C1 2.2F
1 2 3 4 5 6
U1 VIN EN SEL VP N/C SW
R2 220K C2 2.2F R3 121K
AAT1231 TSOPJW-12 R1
D1f LED
C1 10V 0603 X5R 2.2F GRM188R60J225KE01 C2 25V 0805 X7R 2.2F GRM21BR71E225KA73 DS1 B340LA L1 2.2H SD10-2R2, SD12-2R2, SD18-2R2 D1a-D10c White LED
Figure 19: Multi-String WLED Application Schematic.
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PRODUCT DATASHEET
AAT1231
SwitchRegTM Step-Up DC/DC Converters for White LED Backlight Applications
Figure 20: Top Layer of the Multi-String WLED Application.
Figure 21: Bottom Layer of the Multi-String WLED Application.
The ballast resistor R1 is calculated based on the total number of WLED strings and the total WLED current required from the AAT1231 or AAT1231-1. The value of the ballast resistor for each application is listed in Table 7.
Number of Parallel Strings
10 9 8 7 6 5 4 3 2 1
Total LED Current (A)
0.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02
R1 () 1% Tolerance
3.01 3.32 3.74 4.32 4.99 6.04 7.50 10.0 15.0 30.1
Table 7: Ballast Resistor Values for Multi-String WLED Applications.
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1231.2008.06.1.5
PRODUCT DATASHEET
AAT1231
SwitchRegTM Step-Up DC/DC Converters for White LED Backlight Applications
Ordering Information
Package
TSOPJW-12 TSOPJW-12
LED Current Control
Increasing Decreasing
Marking1
SDXYY TUXYY
Part Number (Tape and Reel)2
AAT1231ITP-T1 AAT1231ITP-1-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/about/quality.aspx.
Package Information
TSOPJW-12
0.20 + 0.10 - 0.05
2.40 0.10
0.50 BSC 0.50 BSC 0.50 BSC 0.50 BSC 0.50 BSC
2.85 0.20
7 NOM 3.00 0.10
0.9625 0.0375 + 0.10 1.00 - 0.065
0.04 REF
0.15 0.05
0.055 0.045
4 4
0.010
0.45 0.15 2.75 0.25
All dimensions in millimeters. 1. XYY = assembly and date code. 2. Sample stock is generally held on part numbers listed in BOLD.
Advanced Analogic Technologies, Inc. 3230 Scott Boulevard, Santa Clara, CA 95054 Phone (408) 737-4600 Fax (408) 737-4611
(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. Except as provided in AnalogicTech's terms and conditions of sale, AnalogicTech assumes no liability whatsoever, and AnalogicTech disclaims any express or implied warranty relating to the sale and/or use of AnalogicTech products including liability or warranties relating to fitness for a particular purpose, merchantability, or infringement of any patent, copyright or other intellectual property right. In order to minimize risks associated with the customer's applications, adequate design and operating safeguards must be provided by the customer to minimize inherent or procedural hazards. 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.
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