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AAT2856
High Current Charge Pump with Dual LDO for BacklightApplications General Description
The AAT2856 is a highly integrated charge pump with dual linear regulators optimized for systems powered from lithium-ion/polymer batteries. The charge pump provides power for white LED backlight. Six backlight LEDs can be driven at up to 30mA. AnalogicTech's AS2CwireTM (Advanced Simple Serial ControlTM) single-wire interface is used to enable, disable, and set the current to one of 32 levels for the backlight. Backlight current matching is 1% for uniform display brightness. The AAT2856 offers two high-performance lownoise MicroPowerTM low dropout (LDO) linear regulators. Both regulators use individual enable inputs and each will supply up to 200mA load current. LDO ground pin current is only 80A, making the AAT2856 ideal for battery-operated applications. The AAT2856 is equipped with built-in short-circuit and over-temperature protection. The soft start circuitry prevents excessive inrush current at start-up and mode transitions. The AAT2856 is available in a Pb-free TQFN44-28 package and operates over the -40C to +85C ambient temperature range.
Features
* *
ChargePumpTM
*
* * * *
Input Voltage Range: 2.7V to 5.5V Tri-Mode Charge Pump: -- Drives up to Six Backlight LEDs -- 32 Programmable Backlight Current Settings Ranging from 115A to 30mA -- 2MHz Switching Frequency Two Linear Regulators: -- 200mA Output Current -- 200mV Dropout Voltage -- Output Voltage Adjustable from 1.2V to VBATTERY -- Output Auto-Discharge for Fast Shutdown -- Individual LDO Enable Inputs Built-In Thermal Protection Automatic Soft Start -40C to +85C Temperature Range TQFN44-28 Package
Applications
* * * Camera-Enabled Mobile Devices Digital Still Cameras Multimedia Mobile Phones
Typical Application
C1 1F C1+ C1IN CIN 4.7F C2 1F C2+ C2OUT COUT 2.2F IN BL1 BL2 BL3 BL4 BL5 BL6 OUTA CBYP 0.1F REF FBA R1A R2A COUTA 2.2F VOUTA VOUT
VBAT
WLEDs OSRAM LW M673 or equivalent
AAT2856
ENABLE/SET ENS
EN_LDOA EN_LDOB
ENA ENB AGND
OUTB R2B FBB PGND R1B
VOUTB COUTB 2.2F
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AAT2856
High Current Charge Pump with Dual LDO for BacklightApplications Pin Descriptions
Pin #
1 2 3 4, 5, 23, 24 6 7 8 9, 18 10 11 12 13 14, 21, 22
Symbol
BL3 BL2 BL1 AGND REF FBB OUTB IN FBA OUTA C1C1+ OUT
Description
Backlight LED 3 current sink. BL3 controls the current through backlight LED 3. Connect the cathode of backlight LED 3 to BL3. If not used, connect BL3 to OUT. Backlight LED 2 current sink. BL2 controls the current through backlight LED 2. Connect the cathode of backlight LED 2 to BL2. If not used, connect BL2 to OUT. Backlight LED 1 current sink. BL1 controls the current through backlight LED 1. Connect the cathode of backlight LED 1 to BL1. If not used, connect BL1 to OUT. Analog ground. Connect AGND to PGND at a single point as close to the AAT2856 as possible. Reference output. Bypass REF to AGND with a 0.1F or larger ceramic capacitor. Feedback input for LDO B. FBB measures the output voltage of LDO B. Connect a resistive voltage divider from the output of LDO B to FBB. FBB feedback regulation voltage is 1.2V. LDO B regulated voltage output. OUTB is the voltage output of LDO B. Bypass OUTB to AGND with a 2.2F or larger ceramic capacitor as close to the AAT2856 as possible. Power input. Connect IN to the input source voltage. Bypass IN to PGND with a 4.7F or larger ceramic capacitor as close to the AAT2856 as possible. Feedback input for LDO A. FBA measures the output voltage of LDO A. Connect a resistive voltage divider from the output of LDO A to FBA. FBA feedback regulation voltage is 1.2V. LDO A regulated voltage output. OUTA is the voltage output of LDO A. Bypass OUTA to AGND with a 2.2F or larger ceramic capacitor as close to the AAT2856 as possible. Negative node of charge pump capacitor 1. Connect the 1F charge pump capacitor 1 from C1+ to C1-. Positive node of charge pump capacitor 1. Connect the 1F charge pump capacitor 1 from C1+ to C1-. Charge pump output; supplies current to the backlight LEDs. Connect the backlight LED anodes to OUT. Bypass OUT to PGND with a 2.2F or larger ceramic capacitor as close to the AAT2856 as possible. LDO B enable input. ENB turns on or off low dropout regulator B (LDO B). Drive ENB high to turn on LDO B; drive it low to turn it off. Positive node of charge pump capacitor 2. Connect the 1F charge pump capacitor 2 from C2+ to C2-. Negative node of charge pump capacitor 2. Connect the 1F charge pump capacitor 2 from C2+ to C2-. Power ground. Connect AGND to PGND at a single point as close to the AAT2856 as possible. Backlight enable and serial control input. ENS turns the backlight on/off and is the AS2Cwire input to serially control the backlightLED brightness. Backlight LED 6 current sink. BL6 controls the current through backlight LED 6. Connect the cathode of backlight LED 6 to BL6. If not used, connect BL6 to OUT. Backlight LED 5 current sink. BL5 controls the current through backlight LED 5. Connect the cathode of backlight LED 5 to BL5. If not used, connect BL5 to OUT. LDO A enable input. ENA turns on or off low dropout regulator A (LDO A). Drive ENA high to turn on LDO A; drive low to turn it off. Backlight LED 4 current sink. BL4 controls the current through backlight LED 4. Connect the cathode of backlight LED 4 to BL4. If not used, connect BL4 to OUT. Exposed paddle (bottom); connect to ground as closely as possible to the device.
15 16 17 19 20 25 26 27 28 EP
ENB C2+ C2PGND ENS BL6 BL5 ENA BL4
2
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AAT2856
High Current Charge Pump with Dual LDO for BacklightApplications Pin Configuration
TQFN44-28 (Top View)
OUT AGND AGND BL6 BL5 ENA BL4
28 27 26 25 24 23 22 21 20 19 18 17 16 15 8 9 10 11 12 13 14
BL3 BL2 BL1 AGND AGND REF FBB
1 2 3 4 5 6 7
OUT ENS PGND IN C2C2+ ENB
OUT C1+ C1OUTA FBA IN OUTB
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AAT2856
High Current Charge Pump with Dual LDO for BacklightApplications Absolute Maximum Ratings1
Symbol Description
IN, OUT, BL1, BL2, BL3, BL4, BL5, BL6 Voltage to AGND C1+, C1-, C2+, C2- Voltage to AGND REF, FBB, OUTA, FBA, OUTB, ENA, ENB, ENS Voltage to AGND PGND Voltage to AGND Operating Junction Temperature Range Maximum Soldering Temperature (at leads, 10 sec)
Value
-0.3 to 6.0 -0.3 to VOUT + 0.3 -0.3 to VIN + 0.3 -0.3 to 0.3 -40 to 150 300
Units
V V V V C C
TJ TLEAD
Thermal Information2
Symbol
PD JA
Description
Maximum Power Dissipation Maximum Thermal Resistance
3
Value
2 50
Units
W C/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 a FR4 circuit board. 3. Derate 6.25 mW/C above 25C ambient temperature.
4
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AAT2856
High Current Charge Pump with Dual LDO for BacklightApplications Electrical Characteristics1, 2
VIN = 3.6V; CIN = 4.7F; COUT = 2.2F; C1 = C2 = 1F; TA = -40C to +85C, unless otherwise noted. Typical values are TA = 25C. Symbol
VIN
Description
IN Operating Voltage Range
Conditions
1X Mode, 3.0V VIN 5.5V, Active, No Load; ENL = AGND, ENS = IN 1.5X Mode, 3.0V VIN 5.5V, Active, No Load; ENL = AGND, ENS = IN 2X Mode, 3.0V VIN 5.5V, Active, No Load; ENL = AGND, ENS = IN ENA = ENB = ENS = AGND
Min
2.7
Typ
Max Units
5.5 1 4 5 1.0 A C C mA V
IIN(Q)
IN Operating Current
IN Shutdown Current Over-Temperature Shutdown TSD Threshold Over-Temperature Shutdown TSD(HYS) Hysteresis Charge Pump Section IOUT OUT Maximum Output Current VIN(TH_H) Charge Pump Mode Hysteresis Charge Pump Oscillator fOSC Frequency BL1-BL6 Backlight LED Outputs IBL_(MAX) I(BL_) BL1-BL6 Maximum Current
IIN(SHDN)
140 15
Address 0, Data 1 TA = 25C Address 0, Data 1; VIN - VF = 1.5V Address 12, Data 2; VIN - VF = 1.5V Address 0, Data 1; VIN - VF = 1.5V 18
200 500 2
mA mV MHz
20 30
22 1.0
mA % mV
BL1-BL6 Current Matching2 BL1-BL6 Charge Pump Mode VBL_(TH) Transition Threshold ENS Logic Control VENS(L) ENS Input Low Threshold VENS(H) ENS Input High Threshold IENS ENS Input Leakage Current tENS(LOW) ENS Serial Interface Low Time tENS(HI_MIN), ENS Serial Interface Minimum tENS(HI_MIN) High Time tENS(HI_MAX), ENS Serial Interface Maximum tENS(HI_MAX) High Time tENS(OFF) ENS Off Timeout ENS Serial Interface Latch tENS(LAT) Timeout
150
0.4 VENS = VIN = 5V VIN 3.3V 1.4 -1.0 0.3 50 VIN 3.3V 75 500 500 1.0 75
V V A s ns s s s
1. The AAT2856 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. 2. Current matching is defined as the deviation of any sink current from the average of all active channels. 2856.2007.06.1.0
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AAT2856
High Current Charge Pump with Dual LDO for BacklightApplications Electrical Characteristics1
VIN = 3.6V; CIN = 4.7F; COUT = 2.2F; C1 = C2 = 1F; RFSET = 280k; TA = -40C to +85C, unless otherwise noted. Typical values are TA = 25C. Symbol Description Conditions
IOUT = 1mA to 200mA ENA = ENB = IN, ENS = AGND ENA = IN, ENB = AGND or ENA = AGND, ENB = IN, ENS = AGND
Min
1.17
Typ
1.2
Max
1.23 200 150
Units
V A
Linear Regulators VFBA, VFBB Output Voltage Tolerance IIN IOUTA(MAX), IOUTB(MAX) VOUTA(DO), VOUTB(DO) PSRRA, PSRRB VEN_(L) VEN_(H) tEN_(DLY) Ground Pin Current OUTA, OUTB Maximum Load Current OUTA, OUTB Dropout Voltage OUTA, OUTB Power Supply Rejection Ratio ENA, ENB Voltage Low Threshold ENA, ENB Voltage High Threshold ENA, ENB Enable Delay
200 IOUT = 150mA IOUT = 10mA, CREF = 10nF, 1kHz 1.4 REF = Open 15 150 50 0.4 300
mA mV dB V V s
1. The AAT2856 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.
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AAT2856
High Current Charge Pump with Dual LDO for BacklightApplications Typical Characteristics
Backlight Efficiency vs. Input Voltage
100
Backlight Current Matching vs. Temperature
(20mA/Ch; Data 1)
21
Efficiency (%)
80 70 60 50 40 30 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5
LED Current (mA)
90
20mA/ch
20.5 20 19.5 19 18.5 -40
10.2mA/ch 1.6mA/ch
-15
10
35
60
85
Input Voltage (V)
Temperature (C)
Turn On to 1X Mode Backlight
(30mA/ch; Data 1; VIN = 4.2V) VEN (2V/div) VOUT (2V/div) VSINK (500mV/div) IIN (200mA/div) VEN (2V/div)
0V
Turn On to 1.5X Mode Backlight
(30mA/ch; Data 1; VIN = 3.4V)
0V
0V 0V
VOUT (2V/div) VSINK (500mV/div)
0A
0V
0V 0A
IIN (200mA/div) Time (200s/div)
Time (200s/div)
Turn On to 2X Mode Backlight
(30mA/ch; Data 1; VIN = 2.7V) VEN (2V/div) VOUT (2V/div) VSINK (500mV/div) IIN (200mA/div)
Turn Off from 1.5X Mode Backlight
(30mA/ch; Data 1)
0V 0V 0V 0A
VEN (2V/div) VOUT (2V/div) IIN (200mA/div)
0V
0V
0A
Time (200s/div)
Time (100s/div)
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AAT2856
High Current Charge Pump with Dual LDO for BacklightApplications Typical Characteristics
BENS, FENS High Threshold Voltage vs. Input Voltage
1.4 1.3
1.4 1.3
BENS, FENS Low Threshold Voltage vs. Input Voltage
VBENS(L), VFENS(L) (V)
VBENS(H), VFENS(H) (V)
1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 2.7 3.1 3.5 3.9
-40C
1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5
-40C
25C
85C
4.3 4.7 5.1 5.5
25C
2.7 3.1 3.5 3.9
85C
4.3 4.7 5.1 5.5
Input Voltage (V)
Input Voltage (V)
BENS, FENS Latch Timeout vs. Input Voltage
260
BENS, FENS Off Timeout vs. Input Voltage
300
TBENS(LAT), TFENS(LAT) (s)
240 220 200 180 160 140 120 100 80 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 100 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5
25C -40C 25C
VBENS(H), VFENS(H) (V)
260
-40C
220
25C 85C
180 140
Input Voltage (V)
Input Voltage (V)
LDOs A and B Turn On Characteristic
Output Voltage Error (%)
1.0
LDOs A and B Load Regulation
VEN (2V/div)
0.5
0V
OUTA
0.0
VOUT (500mV/div)
OUTB
-0.5
0V
-1.0 0.1 1 10 100 1000
Time (50s/div)
Load Current (mA)
8
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AAT2856
High Current Charge Pump with Dual LDO for BacklightApplications Typical Characteristics
LDOs A and B Line Regulation
Output Voltage Error (%)
1.0 1.5
LDOs A and B Output Voltage vs. Temperature
Output Voltage (%)
1 0.5 0 -0.5 -1 -1.5 -40
0.5
OUTA
0
OUTB
-0.5
-1.0 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5
-15
10
35
60
85
Input Voltage (V)
Temperature (C)
LDOs A and B Dropout Characteristics
3.2
LDOs A and B Line Transient Response
(10mA Load) VIN = 3.6V VIN (250mV/div)
Output Voltage (V)
3.0 2.8 2.6 2.4 2.2 2.0 2.7
IOUT = 100mA
IOUT = 200mA
VIN = 3.1V VOUT (AC Coupled) (20mV/div)
2.8
2.9
3.0
3.1
3.2
Input Voltage (V)
Time (50s/div)
LDOs A and B Load Transient Response
(10mA to 200mA Load Step) IOUT = 200mA
IOUT (100mA/div)
VOUT (AC Coupled) (100mV/div)
Time (50s/div)
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AAT2856
High Current Charge Pump with Dual LDO for BacklightApplications Functional Block Diagram
IN IN
C1+ C1C2+ C2-
OUTA
1X/1.5X/2X Tri-mode Charge Pump
FBA VREF OUTB FBB VREF REF OUT BL1 BL2 BL3 Control Logic
ENA ENB
To LDO A To LDO B
ENS
BL4 BL5 BL6
AGND PGND
Functional Description
The AAT2856 is a highly integrated backlight LED driver with two LDO linear regulators. The charge pump LED driver drives backlight LEDs from a 2.7V to 5.5V input voltage. The LDO regulators are operated from the same input voltage range and produce regulated output voltages as low as 1.2V.
current sink to AGND, allowing individual current control with high accuracy over a wide range of input voltages and LED forward voltages while maintaining high efficiency. The charge pump is controlled by the voltage across the LED current sinks. When any one of the active current sinks begins to dropout, the charge pump goes to the next higher mode (from 1X to 1.5X or from 1.5X to 2X mode) to maintain sufficient LED voltage for constant LED current. The AAT2856 continuously monitors the LED forward voltages and uses the input voltage to determine when to reduce the charge pump mode for better efficiency. There is also a 500mV mode-transition hysteresis that prevents the charge pump from oscillating between charge pump modes. The backlight LED current levels are dynamically controllable by the AS2Cwire single-wire interface. The backlight section has multiple current level
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LED Drivers
The LEDs are driven from an internal charge pump that, depending on the battery voltage and LED forward voltage, drives LEDs directly from the supply voltage (1X or bypass mode) or steps up the supply voltage by a factor of 1.5 (1.5X mode) or 2 (2X mode). The charge pump requires only two tiny 1F ceramic capacitors, providing a more compact solution than typical inductor-based step-up converter solutions. Each individual LED is driven by a 10
AAT2856
High Current Charge Pump with Dual LDO for BacklightApplications
scales and the maximum current level is fixed at 20mA or 30mA, depending on the scale chosen through programming. If any one of the backlight or flash current sinks is not used, connect that current sink to OUT. The current controller monitors the sink voltage and, if it is connected to OUT, then the controller determines that the current sink is not used or that the LED is shorted. In either case, the controller turns off the affected current sink.
AS2Cwire Serial Interface Addressing
ENS Rising Edges
17 18 19 20 29 32 19 20
Address
0 1 2 3 12 15 Sub-2 Sub-3
Function
Backlight Current BL1-BL6 Main Backlight Current BL1-BL5 Sub Backlight Current BL6 Low Current Backlight Maximum Backlight Current (Main and Sub) Backlight Independent Control BL3-BL6 On/Off Control BL1/BL2 On/Off Control
AS2Cwire Serial Interface
The AAT2856 is dynamically programmable by the AS2Cwire single-wire interface. AS2Cwire records rising edges detected at the ENS pin to address and load the data registers. AS2Cwire latches data or address after the ENS input has been held high for time tLAT (500s). Address or data is differentiated by the number of ENS rising edges. Since the data registers are 4 bits each, the differentiating number of pulses is 24 or 16, so that Address 0 is identified by 17 rising edges, Address 1 by 18 rising edges, Address 2 by 19 rising edges, etc. Data is set to any number of rising edges between 1 and 16. A typical write protocol is a burst of ENS rising edges identifying a particular address, followed by a pause with ENS held high for the tLAT timeout period, then a burst of rising edges signifying data, and another tLAT timeout after the data has been sent. Once an address is set, multiple writes to that address are allowed since the address is not reset after each write. Address edges are needed when changing the address, or writing to an address other than the default after shutdown. Address 0 is the default address after shutdown. If the part is enabled with only data edges and no address, then Address 0 will be programmed and backlight channels BL1-BL6 will turn-on according to the number of data edges applied. When ENS is held low for a time longer than tOFF (500s), the AAT2856 enters shutdown mode and draws less than 1A of current from IN. At shutdown, the data and address registers are reset to 0.
Table 1: AS2Cwire Serial Interface Addressing.
Backlight Current Control (Address 0-3)
Use Addresses 0-3 to program all six backlight LED channels. All six backlight channels are programmed to the same current level by writing Address 0 followed by any Data between 1 and 16. To program only the main channels BL1 through BL5, use Address 1. Similarly, use Address 2 to program only the sub channel BL6 independently. Data
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
30mA Max (mA)
30.0 27.9 26.1 24.2 21.0 19.2 17.3 15.0 12.7 10.9 8.1 6.2 4.4 3.5 2.6 0
20mA Max (mA)
20.0 19.0 17.8 16.5 14.3 13.0 11.8 10.2 8.5 7.3 5.4 4.1 2.9 2.2 1.6 0
Table 2: Data for the Backlight Current Level (Address 0-3).
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AAT2856
High Current Charge Pump with Dual LDO for BacklightApplications
Address
THI TLO TLAT TLAT
Data
EN/SET
1 2 17 18 1 2... n <= 16
Address
0
1
Data Reg 1
0
n
Data Reg 2
0
Figure 1: AS2Cwire Serial Interface Timing.
35.0 30.0 25.0
Data
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Main Current On
No No No No No No No No Yes Yes Yes Yes Yes Yes Yes Yes
Sub Current On
No No No No Yes Yes Yes Yes No No No No Yes Yes Yes Yes
Current (A)
0 0 0 0 95 500 950 1900 95 500 950 1900 95 500 950 1900
IBLED (mA)
20.0 15.0 10.0 5.0 0.0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 20mA (Full Scale) 30mA (Full Scale)
Data Code
Figure 2: Data Code for Address 0-3 vs. Backlight Current Level. The AAT2856 incorporates additional circuitry that optimizes performance for exceptionally low backlight current settings. A separate address is used to activate this circuitry. To program the low current settings with improved performance and efficiency, write to Address 3. Unlike Addresses 0-2, which have current level settings according to Table 2 and Figure 2, Address 3 possesses a separate set of current levels described by the Low Current Backlight settings found in Table 3.
Table 3: Low-Level Backlight Current, Address 3, FS = 20mA range.
Maximum Backlight Current (Address 12)
There are two separate current level scales that apply to Addresses 0-2: 20mA and 30mA. According to the Maximum Backlight Current setting at Address 12, only one of the two scales can be active at any given time and never both. By default, the 20mA scale is active on startup. To change to the 30mA scale, or go back to the 20mA scale, write to Address 12.
12
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AAT2856
High Current Charge Pump with Dual LDO for BacklightApplications
Since only one of the scales can be active at any given time, the 20mA and 30mA scales cannot be mixed between main and sub. When setting Address 12 to the 30mA scale, only current levels from that scale can be mixed between main and sub. Data
1 2
Data
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
BL6
Off Off Off Off Off Off Off Off On On On On On On On On
BL5
Off Off Off Off On On On On Off Off Off Off On On On On
BL4
Off Off On On Off Off On On Off Off On On Off Off On On
BL3
Off On Off On Off On Off On Off On Off On Off On Off On
Maximum Current
20mA 30mA
Table 6: Address 12 Maximum Current Settings.
Backlight Independent Channel Control (Address 15)
The AAT2856 has a unique independent channel control mode whereby individual backlight LED channels can be enabled and disabled to form a custom arrangement of active channels. To enable independent channel control mode, write Data 8 to Address 15. To exit individual mode control, the AAT2856 state machine can be reset by strobing ENS low and holding ENS low longer than the A2SCwire's tOFF latch time. Data
8
Table 8: Sub-Address 2: BL3-BL6 On/Off Control. Data
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
BL2
Off Off Off Off Off Off Off Off On On On On On On On On
BL1
Off Off Off Off On On On On Off Off Off Off On On On On
Individual Backlight Control
On
Table 7: Address 15, Independent Backlight Control. With independent channel control mode enabled, Addresses 2 and 3 are re-mapped according to Tables 8 and 9. As indicated by the possible settings listed in the tables, any combination of backlighting channels can be enabled and disabled.
Table 9: Sub-Address 3: BL1 and BL2 On/Off Control. Because Addresses 2 and 3 are re-mapped when independent channel control mode is enabled, the functions originally assigned to Addresses 2 and 3 are no longer available.
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AAT2856
High Current Charge Pump with Dual LDO for BacklightApplications
It is also important to note that Address 0 is disabled when independent channel control mode is enabled. Additionally, Address 3 is disabled and the very low current settings cannot be used in this mode. Other addresses are unmodified by this mode so that the 30mA backlight scale can still be used. The LDO enables are always independent of AS2Cwire programming.
Applications Information
LDO Output Voltage Programming
The output voltages for LDOA and LDOB are programmed by an external resistor divider network. As shown below, the selection of R1 and R2 is a straight forward matter. R1 is chosen by considering the tradeoff between the feedback network bias current and resistor value. Higher resistor values allow stray capacitance to become a larger factor in circuit performance whereas lower resistor values increase bias current and decrease efficiency.
Low Dropout Regulators
The AAT2856 includes two independent LDO linear regulators. The regulators operate from a 2.7V to 5.5V input voltage at IN. The AAT2856 supplies separate LDO enable inputs (ENA and ENB) to control individually the operation of the LDOs. The LDO output voltages are set through resistive voltage dividers from the output (OUTA or OUTB) to the feedback input (FBA or FBB). The regulator controls the output voltage such that the voltage divider output is at the 1.2V feedback threshold. The low 200mV dropout voltage at 200mA load current allows the regulator to maintain output voltage regulation. Each LDO regulator can supply up to 200mA continuous current to the load. They include current limiting and thermal overload protection to prevent damage to the load or to the LDOs.
OUT(A/B) R2(A/B) FB(A/B) VREF(A/B) = 1.2V R1(A/B)
VOUT(A/B)
To select appropriate resistor values, first choose R1 such that the feedback network bias current is less than 10A. Then, according to the desired VOUT, calculate R2 according to the equation below. An example calculation follows. An R1 value of 120K is chosen, resulting in a small feedback network bias current of 1.2V/120K = 10A. The desired output voltage is 1.8V. From this information, R2 is calculated from the equation below.
R2 =
R1(VOUT - 1.2V) 1.2V
The result is R2 = 60K. Since 60K is not a standard 1%-value, 60.4K is selected. From this example calculation, for VOUT = 1.8V, use R1 = 120K and R2 = 60.4K. Example output voltages and corresponding resistor values are provided in Table 13.
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AAT2856
High Current Charge Pump with Dual LDO for BacklightApplications
mode when the input voltage drops too low in relation to the LED forward voltages. In 1.5X mode, the output voltage can be boosted to 3/2 the input voltage. The 3/2 conversion ratio introduces a corresponding 1/2 increase in input current. For ideal conversion, the 1.5X mode efficiency is given by:
VF * ILED VF = VIN * 1.5IIN 1.5 * VIN
R2 Standard 1% Values (R1 = 120K) VOUT (V)
2.8 2.5 2.0 1.8 1.5
R2 ()
160K 130K 79.6K 60.4K 30.1K
Table 13: Example Output Voltages and Corresponding Resistor Values Selection of set resistor values outside of the typical application must be carefully evaluated to ensure that the application's performance requirements can still be met.
=
Device Power Efficiency
The AAT2856 power conversion efficiency depends on the charge pump mode. By definition, device efficiency is expressed as the output power delivered to the LEDs divided by the total input power consumed.
Similarly, when the input falls further, such that 1.5X mode can no longer sustain LED drive current, the device will automatically switch to 2X mode. In 2X mode, the output voltage can be boosted to twice the input voltage. The doubling conversion ratio introduces a corresponding doubling of the input current. For ideal conversion, the 2X mode efficiency is given by:
VF * ILED VF = VIN * 2IIN 2 * VIN
=
=
POUT PIN
LED Selection
When the input voltage is sufficiently greater than the LED forward voltages, the device optimizes efficiency by operating in 1X mode. In 1X mode, the device is working as a bypass switch and passing the input supply directly to the output. By simplifying the conditions such that the LEDs have uniform VF, the power conversion efficiency can be approximated by:
VF * ILED VF VIN * IIN VIN
The AAT2856 is designed to drive high-intensity white LEDs. It is particularly suitable for LEDs with an operating forward voltage in the range of 1.5V to 4.2V. The charge pump can also drive other loads that have similar characteristics to white LEDs. For various load types, the AAT2856 provides a high-current, programmable ideal constant current source.
=
Capacitor Selection
Careful selection of the four external capacitors CIN, C1, C2, and COUT is important because they will affect turn-on time, output ripple, and transient performance. Optimum performance will be obtained when low equivalent series resistance (ESR) ceramic capacitors are used. In general, low ESR may be defined as less than 100m. Ceramic composition capacitors are highly recommended over all other types of capacitors for use with the AAT2856. Ceramic capacitors offer many advantages over their tantalum and aluminum elec15
Due to the very low 1X mode quiescent current, the input current nearly equals the total output current delivered to the LEDs. Further, the low-resistance bypass switch introduces negligible voltage drop from input to output. The AAT2856 further maintains optimized performance and efficiency by detecting when the input voltage is not sufficient to sustain LED drive current. The device automatically switches to 1.5X
2856.2007.06.1.0
AAT2856
High Current Charge Pump with Dual LDO for BacklightApplications
trolytic counterparts. A ceramic capacitor typically has very low ESR, is lowest cost, has a smaller PCB footprint, and is non-polarized. Low ESR ceramic capacitors help maximize charge pump transient response. Since ceramic capacitors are non-polarized, they are not prone to incorrect connection damage. Figure 6 illustrates an bottom of the package paddle. The exposed transfer heat from the ground connection. example PCB layout. The features an exposed metal paddle acts, thermally, to chip and, electrically, as a
The junction-to-ambient thermal resistance (JA) for the connection can be significantly reduced by following a couple of important PCB design guidelines. The PCB area directly underneath the package should be plated so that the exposed paddle can be mated to the top layer PCB copper during the reflow process. Multiple copper plated thru-holes should be used to electrically and thermally connect the top surface paddle area to additional ground plane(s) and/or the bottom layer ground pour. The chip ground is internally connected to both the paddle and to the AGND and PGND pins. It is good practice to connect the GND pins to the exposed paddle area with traces as shown in the example. The flying capacitors C1 and C2 should be connected close to the IC. Trace length should be kept short to minimize path resistance and potential coupling. The input and output capacitors should also be placed as close to the chip as possible.
Equivalent Series Resistance
ESR is an important characteristic to consider when selecting a capacitor. ESR is a resistance internal to a capacitor that is caused by the leads, internal connections, size or area, material composition, and ambient temperature. Capacitor ESR is typically measured in milliohms for ceramic capacitors and can range to more than several ohms for tantalum or aluminum electrolytic capacitors.
Ceramic Capacitor Materials
Ceramic capacitors less than 0.1F are typically made from NPO or C0G materials. NPO and C0G materials generally have tight tolerance and are very stable over temperature. Larger capacitor values are usually composed of X7R, X5R, Z5U, or Y5V dielectric materials. Large ceramic capacitors are often available in lower-cost dielectrics, but capacitors greater than 10F are not typically required for AAT2856 applications. Capacitor area is another contributor to ESR. Capacitors that are physically larger will have a lower ESR when compared to an equivalent material smaller capacitor. These larger devices can improve circuit performance when compared to an equal value capacitor in a smaller package size.
PCB Layout
To achieve adequate electrical and thermal performance, careful attention must be given to the PCB layout. In the worst-case operating condition, the chip must dissipate considerable power at full load. Adequate heat-sinking must be achieved to ensure intended operation.
Figure 6: Example PCB Layout.
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AAT2856
High Current Charge Pump with Dual LDO for BacklightApplications
Evaluation Board Layout
Figure 7: AAT2856 Evaluation Board Layout Top Side.
Figure 8: AAT2856 Evaluation Board Layout Bottom Side.
Evaluation Board User Interface
The user interface for the AAT2856 evaluation board is provided through 4 buttons and a number of connection terminals. The board is operated by supplying external power and pressing individual buttons or button combinations. The table below indicates the function of each button or button combination. To power-on the board, connect a power supply or battery to the DC- and DC+ terminals. Make the board's supply connection by positioning the J1 jumper to the ON position. A red LED indicates that power is applied.
The Enables of both LDOs are connected with jumpers J3 and J4. These terminals must be connected to the external source to turn on/off the LDOs. When applying external enable signals, consideration must be given to the voltage levels. The externally applied voltages cannot exceed the supply voltage that is applied to the IN pins of the device (DC+). The LDO loads can be connected directly to the evaluation board. For adequate performance, be sure to connect the load between OUTA/OUTB and DC- as opposed to some other GND in the system.
Button(s) Pushed
SW1 SW2 SW3
Description
[Push/Release once] Increment the number of EN/SET edges, but the backlight current is decreased (dimmer). If held down, auto-cycle through the settings. [Push/Release once] Decrement the number of EN/SET edges, but the backlight current is increased (brighter). If held down, auto-cycle through the settings. [Push/Release once] Toggle between 20mA and 30mA maximum current.
Table 14: AAT2856 Evaluation Board User Interface1.
1. The enable for LDOA and LDOB are manually set externally.
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AAT2856
High Current Charge Pump with Dual LDO for BacklightApplications Evaluation Board Schematics
DC+
1 2 3
VIN J1 C12 100F 100F (optional) lab supply bypass C3 2.2F
VOUT
D1
D2
D3
D4
D5
D6
J2 0 ENA
28
27
26
25
24
23
22
U1 AAT2856
AGND
AGND
1 2 3 4 5 6
OUT
BL4
BL5
ENA
BL6
BL3 BL2 BL1 AGND AGND REF FBB
OUTB OUTA OUT FBA C1+ C1IN
OUT ENS PGND IN C2C2+ ENB
21 20 19 18 17 16 15
ENS
C2 1.0F
C4 4.7F
C8 0.1F
7
ENB
8
9
10
11
12
13
14
VOUT OUTB R2 78.7k Programmed for 2.8V output by default R1 59k C5 2.2F C6 2.2F R4 29.4k R4 (), R3 = 59k R2 (), R1 = 59k R4 short, R5 open (R2 short, R1 open) 29.4K 78.7K 14.7K 63.4K 105K R3 59k C7 2.2F Programmed for 1.8V output by default OUTA C1 1.0F
VOUT A/B(V) 1.2 1.8 2.8 1.5 2.5 3.3
Figure 9: AAT2856 Section Schematic
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AAT2856
High Current Charge Pump with Dual LDO for BacklightApplications
U3 AAT4296
1 2
VIN
C11 0.1F
3 4
IN OUT3 OUT2 OUT4 OUT1 OUT5 EN/SET GND
8 7 6 5
J3 J4
ENA ENA ENB ENB
R6 100K (Opt) VIN VIN R8 R9 R10 1K 1K 1K U2
1 2
R5 100K (Opt)
SW1 SW2 SW3
3 4
VDD GP5 GP4 GP3 PIC12F675
VSS GP0 GP1 GP2
8 7 6 5
C10 1F
R7 330 LED7 RED
ENS
DC-
Figure 10: MCU and I/O Expander Section Schematic
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AAT2856
High Current Charge Pump with Dual LDO for BacklightApplications Evaluation Board Component Listing
Component
U1 U2 U3 D1-D6 C1, C2, C10 C3, C5, C6, C7 C4 C8, C11 C12 R8-R10 R7 R5, R6 R4 R2 R1, R3 J1-J4 LED7 SW1-SW3
Part#
AAT2856INJ-EE-T1 PIC12F675 AAT4296IJS-1-T1 LW M673 GRM18x GRM18x GRM18x GRM18x TAJBx Chip Resistor Chip Resistor Chip Resistor Chip Resistor Chip Resistor Chip Resistor PRPN401PAEN CMD15-21SRC/TR8 PTS645TL50
Description
High Eff. 1X/1.5X/2X CP for White LED, Dual LDO 8-bit CMOS, FLASH MCU; 8-pin PDIP I/O Expander Mini TOPLED White LED; SMT 1.0F, 10V, X5R, 0603, Ceramic 2.2F, 10V, X5R, 0603, Ceramic 4.7F, 10V, X5R, 0603, Ceramic 0.1F, 16V, X7R, 0603, Ceramic 100F, 10V, 10A, Tantalum 1K, 5%, 1/4W; 1206 330, 5%, 1/4W; 1206 100K, 5%, 1/10W; 0603 29.4K, 1%, 1/10W; 0603 78.7K, 1%, 1/10W; 0603 59K, 1%, 1/10W; 0603 Conn. Header, 2mm Zip Red LED; 1206 Switch Tact, SPST, 5mm
Manufacturer
AnalogicTech Microchip AnalogicTech OSRAM Murata Murata Murata Murata AVX Vishay Vishay Vishay Vishay Vishay Vishay Sullins Electronics Chicago Miniature Lamp ITT Industries
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2856.2007.06.1.0
AAT2856
High Current Charge Pump with Dual LDO for BacklightApplications Ordering Information
Package
TQFN44-28-0.4
Marking1
XVXYY
Part Number (Tape and Reel)2
AAT2856INJ-EE-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
TQFN44-28-0.4
2.400 REF
Detail "A" C0.3
Pin 1 Dot by Marking
4.000 0.050
4.000 0.050
2.600 0.050
2.600 0.050
Top View
Bottom View
0.400 0.050
0.430 0.050
0.750 0.050
0.200 0.005 0.230 0.050
0.203 REF 0.050 0.050
Side View
Pin 1 Indicator
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. 2856.2007.06.1.0
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AAT2856
High Current Charge Pump with Dual LDO for BacklightApplications
(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.
Advanced Analogic Technologies, Inc.
830 E. Arques Avenue, Sunnyvale, CA 94085 Phone (408) 737- 4600 Fax (408) 737- 4611 22
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