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FEATURES

LTC3524 Adjustable TFT Bias Supply with WLED Driver DESCRIPTION
The LTC(R)3524 is an integrated BIAS and white LED power converter solution for small/medium-sized polysilicon thin film transistor (TFT) liquid crystal (LCD) display panels. The device operates from a single Lithium-Ion/polymer battery or any voltage source between 2.5V and 6V. A 1.5MHz synchronous boost converter generates a programmable low noise, high efficiency 25mA TFT supply of up to 6.0V. Regulated, low ripple charge pumps are used to generate up to +20V and -20V at 2mA. Output sequencing is internally controlled to insure proper initialization and rapid discharge of the LCD panel in shutdown. A second 1.5MHz boost converter powers one or two LED strings with up to five series elements each. LED current and display brightness can be controlled over a wide range using analog or digital means up to 25mA. The LTC3524 is offered in the 4mm x 4mm 24-pin QFN package, minimizing the total solution footprint.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. True Color PWM is a registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.
Generates Three Adjustable, Low Noise Rails for Small/Medium TFT Displays Drives Up to Ten White LEDs LED Dimming and Open-Circuit Protection Controlled Power-Up/Power-Down Sequencing 1.5MHz Fixed Frequency, Low Noise Operation VIN Range 2.5V to 6V, VOUT Range 3V to 6V TFT Supply Efficiency Up to 90% LED Supply Efficiency Up to 78% Two Independantly Enabled LED Strings 200 to 1 True Color PWMTM Dimming Tiny External Solution 24-Lead QFN Package (4mm x 4mm x 0.75mm)
APPLICATIONS

PDAs, Palmtop Computers Digital Still and Video Cameras Handheld GPS Portable Instrument Displays Portable Media Players
TYPICAL APPLICATION
+5V, -7.5V, +12.5V, 8 LED Power Supply + -
+5V 25mA 10F +10V 0.47F 0.1F +12.5V 2mA 0.47F 220k 2M FBH CH+ CH- GND CN+ 0.1F VN 324k 1M FBVO VNIN V2x C2+ C2- VH LED1 LTC3524 PROG ELED2 ELED1 ELCD FBN 1M -7.5V 2mA 0.47F 100k for 20mA 470k VIN VOUT 60 5 100 Li-Ion 2.2F SW1 10H VIN 3.3H SW2 VLED LED2 10F 90 EFFICIENCY (%)
LCD Bias and LED Efficiency VIN = 3.6V, VOUT = 5V, 8 LEDs
VIN = 3.6V
LCD 80 LED
VOUT
70
15 20 10 VOUT OR LED STRING CURRENT (mA)
25
3524 TA01b
0.1F
3524 TA01a
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LTC3524 ABSOLUTE MAXIMUM RATINGS
(Referred to GND)
PIN CONFIGURATION
TOP VIEW ELED1 ELED2 PROG VLED LED1 18 LED2 17 CH- 16 CH+ 25 15 VH 14 FBH 13 FBN 7 C2+ 8 V2x 9 10 11 12 VNIN CN+ NC
VN
VIN, SW1, VOUT, C2-....................................... -0.3 to 7V ELCD, ELED1, ELED2, PROG ......................... -0.3 to 7V FBN, FBH, FBVO ............................................. -0.3 to 7V V2x, C2+, CH- .............................................. -0.3 to 13V LED1, LED2, VLED, SW2 ............................. -0.3 to 22V VNIN, VH, CH+, CN+ ...................................... -0.3 to 21V VN .............................................................. -21 to +0.3V Operating Temperature Range (Note 2) ...-40C to 85C Storage Temperature Range...................-65C to 125C
24 23 22 21 20 19 ELCD 1 VIN 2 FBVO 3 VOUT 4 SW1 5 C2- 6
UF PACKAGE 24-LEAD (4mm x 4mm) PLASTIC QFN TJMAX = 125C, JA = 37C/W EXPOSED PAD (PIN 25) MUST BE SOLDERED TO PCB AND CONNECTED TO GND
ORDER INFORMATION
LEAD FREE FINISH LTC3524EUF#PBF LEAD BASED FINISH LTC3524EUF TAPE AND REEL LTC3524EUF#TRPBF TAPE AND REEL LTC3524EUF#TR PART MARKING 3524 PART MARKING 3524 PACKAGE DESCRIPTION 24-Lead (4mm x 4mm) Plastic QFN PACKAGE DESCRIPTION 24-Lead (4mm x 4mm) Plastic QFN TEMPERATURE RANGE -40C to 85C TEMPERATURE RANGE -40C to 85C
Consult LTC Marketing for parts specified with wider operating temperature ranges. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
PARAMETER Input Voltage Range VIN Quiescent Supply Current LCD VIN Quiescent Supply Current LED VOUT Quiescent Supply Current LCD VIN Quiescent Current Shutdown Switching Frequency Maximum Duty Cycle VOUT Boost Regulator FBVO Regulation Voltage VOUT Adjust Range See Note 3 CONDITIONS
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are TA = 25C. VIN = 3.6V, VOUT = 5.1V, TA = 25C, unless otherwise noted.
MIN
SW2
TYP 200 4 250 .02
MAX 6.0
UNITS V A mA A
2.5
ELCD = 1.5V, ELED1,2 = GND ELCD = GND, ELED1,2 = 1.5V (LED1 and LED2 Open) ELCD = 1.5V, ELED1,2 = GND ELCD = ELED1,2 = GND LED and LCD Boosts LED and LCD Boosts
2 2
A MHz %
1 85 1.20 3.0
1.5 94 1.225
1.25 6.0
V V
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LTC3524 ELECTRICAL CHARACTERISTICS
PARAMETER Switch Current Limit Charge Pumps V2x Output Voltage Output Impedance V2x V2x Maximum Operating Voltage VH Output Voltage (Quadrupler) Output Impedance (2X + Quadrupler) FBH Regulation Voltage VH Maximum Operating Voltage VN Output Voltage FBN Regulation Voltage Output Impedance VN (2X + VN) VN Minimum Operating Voltage Switching Frequency Charge Pumps V2x to VN Delay VN to VH Delay LED Boost LED1,2 Current Accuracy SW2 Maximum Current Limit SW2 VCESAT Logic Inputs ELED1, ELED2 , ELCD Thresholds Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LTC3524E is guaranteed to meet specifications from 0C to 85C. Specifications over the -40C to 85C operating temperature range are assured by design, characterization, and statistical process controls.
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are TA = 25C. VIN = 3.6V, VOUT = 5.1V, TA = 25C, unless otherwise noted.
CONDITIONS MIN 100 Load on V2x = 250A Flying Capacitors = 0.1F (Note 3) Load = 250A (FBH = 1V) Flying Capacitors = 0.1F
TYP 150 10 250 12 20 1200
MAX
UNITS mA V V V
1.15
1.225 20 -9.7
1.30
V V
(Note 3) Load on VN = 250A, VNIN = 10.2V, External Schottkys
0.94
1 650 -20 94 2 2
1.06
V V KHz ms ms
Flying Capacitor = 0.1F (Note 3) (Note 4) (Note 4) RPROG = 100k ISW = 350mA 0.4 18 500
20 700 350 0.8
22
mA mA mV
1.2
V
Note 3: Specification is guaranteed by design and not 100% tested in production. Note 4: Measured from point at which VN crosses -VOUT to point at which CH+ starts switching.
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LTC3524 TYPICAL PERFORMANCE CHARACTERISTICS
LCD Boost Efficiency vs Load Current
100 L = 10H VOUT = 5V 85 L = 4.7H 80 80 EFFICIENCY (%) EFFICIENCY (%) 75 70 65 60 55 3 4 3.5 VIN (V) 4.5 5
3524 G02
TA = 25C, unless otherwise noted. 4 LEDs per String Efficiency vs VIN and LED Current
85 L = 4.7H
LED Efficiency vs VIN
90 EFFICIENCY (%)
80
75
70 VIN = 5 VIN = 4.2 VIN = 3.6 VIN = 3.1 VIN = 2.5 0 10 40 30 20 50 VOUT CURRENT (mA) 60 70
60
70
PER STRING: 5 LEDs 4 LEDs 3 LEDs 2 LEDs
VIN = 5 VIN = 4.2 VIN = 3.6 VIN = 3.1 VIN = 2.5 5 10 15 20 LED CURRENT (mA) 25
3524 G03
50
65 2.5
3524 G01
LED1 String Current vs VIN and Number of LEDs
22.0 21.5 21.0 CURRENT (mA) 20.5 20.0 19.5 19.0 18.5 18.0 2.5 3 3.5 VIN (V)
3524 G04
LED2 String Current vs VIN and Number of LEDs
22.0 21.5 21.0 V2X VOLTAGE (V) CURRENT (mA) 20.5 20.0 19.5 19.0 18.5 5 18.0 2.5 3 3.5 VIN (V)
3524 G05
V2X Output Voltage vs V2X Load Current
10.0
9.5
PER STRING: 5 LEDs 4 LEDs 3 LEDs 2 LEDs 4 4.5
PER STRING: 5 LEDs 4 LEDs 3 LEDs 2 LEDs 4 4.5 5
9.0
8.5
0
1
2 4 3 V2X LOAD CURRENT (mA)
5
3524 G06
VH Voltage vs VH and VN Load Current (FBH = 0V)
20 VN = 0mA 19 VH VOLTAGE (V) VN = 1mA -VN VOLTAGE (V) 9.0 8.5 8.0 VN = 2mA 10.0 9.5
VN Voltage vs VN and VH Load Current (FBN = 1.3V)
8.0 7.5 VH = 1mA VOLTAGE (V) 7.0 6.5 6.0 5.5 5.0 7.5 4.5
VOUT, |VN|, and VH/2 Regulation Overtemperature
|VN|
VH = 0mA
VH/2
18
VH = 2mA
17
VOUT
16
0
0.5 1.5 1 VH LOAD CURRENT (mA)
2
3524 G07
7.0 0 0.5 1 1.5 VN LOAD CURRENT (mA) 2
3524 G09
4.0 -40
-15
10 35 TEMPERATURE (C)
60
85
3524 G10
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LTC3524 TYPICAL PERFORMANCE CHARACTERISTICS
LCD Bias Sequencing
VH
TA = 25C, unless otherwise noted. SW1 Voltage and 10H Inductor Current at 25mA Load
LCD Bias Sequencing
VH VOUT SW1 2V/DIV
5V/DIV
V2X VN VOUT
5V/DIV VN ILCD BOOST INDUCTOR CURRENT 200mA/DIV ILCD BOOST INDUCTOR CURRENT 50mA/DIV 200ns/DIV
3524 G13
5ms/DIV
3524 G11
5ms/DIV
3524 G12
SW1 Voltage and 10H Inductor Current at 5mA Load
SW2 200mV/DIV SW1 2V/DIV ILCD BOOST INDUCTOR CURRENT 200mA/DIV SW2 5V/DIV
SW2 Voltage and 4.7H Inductor Current at 20mA
LED Initial Start-Up Waveforms
ILED BOOST INDUCTOR CURRENT VLED 5V/DIV LED1 LED2
500mV/DIV
ILCD BOOST INDUCTOR CURRENT 50mA/DIV 200ns/DIV
3524 G14
200ns/DIV
3524 G15
50s/DIV
3524 G16
LED Burst Dimming Waveforms LED1 and SW2
ELED1 AND ELED2 ILED BOOST INDUCTOR LED1 SW2 12V 10V/DIV 10V/DIV 5V/DIV 200mA/DIV ELED1 AND ELED2 ILED BOOST INDUCTOR VLED LED2
LED Burst Dimming Waveforms LED2 and VLED
5V/DIV 200mA/DIV 12.5V 12V 10V/DIV 10V/DIV
500s/DIV
3524 G17
500s/DIV
3524 G18
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LTC3524 PIN FUNCTIONS
VIN (Pin 2): Common Input Supply for LCD Bias and White LED Boost Converters. This pin must be locally bypassed with a minimum of 2.2F. GND/Exposed Pad (Pin 25): Signal and Power Ground for the LTC3524. Provide a short, direct PCB path between GND and the (-) side of the boost (VOUT, VLED) filter capacitors, and the (-) side of the charge pump outputs (V2x, VH, VN) filter capacitors. PCB ground must be soldered to the Exposed Pad for proper operation.
LCD BIAS PIN FUNCTIONS
ELCD (Pin 1): Enable Input for the LTC3524's LCD Circuits. LCD bias supplies are actively discharged to GND when ELCD is low through internal pull down devices. An optional RC network on ELCD provides a slower ramp-up of the LCD boost converter inductor current during startup (soft-start). Shutdown mode is activated by driving ELCD, ELED1, and ELED2 low. Shutdown disables all IC functions and reduces quiescent current from the battery to less than 2A. FBVO (Pin 3): Feedback Pin for the VOUT Switcher. Reference voltage is 1.225V. Connect resistive divider tap here with minimum trace area. R1 VOUT = 1 . 225 1 + (See Block Diagram) R2 VOUT (Pin 4): Main Output of the LCD Boost Regulator and Input to the Voltage Doubler (2X) Stage. Bypass VOUT with a low ESR, ESL ceramic capacitor (X5R type) between 4.7 and 22F. SW1 (Pin 5): Synchronous Boost Switch. Connect a 4.7H-15H inductor between SW1 and VIN. Keep PCB trace lengths as short and wide as possible to reduce EMI and voltage overshoot. If the inductor current falls to zero, the PMOS synchronous rectifier is turned off to prevent reverse charging of the inductor and an internal switch connects SW1 to VIN to reduce EMI. C2- (Pin 6): Charge pump doubler flying capacitor negative node. The charge pump doubler flying capacitor is connected between C2+ and C2-. The voltage on C2- will alternate between GND and VOUT at an approximate 50% duty cycle while the charge pump is operating. Use a 0.1F X5R type ceramic capacitor for best results. C2+ (Pin 7): Charge pump doubler flying capacitor positive node. The charge pump doubler flying capacitor is connected between C2+ and C2-. The voltage on C2+ will alternate between VOUT and V2x at an approximate 50% duty cycle while the charge pump is operating. Use a 0.1F X5R type ceramic capacitor for best results. V2x (Pin 8): Charge Pump Doubler Output and Input to the Charge Pump Quadrupler. This output generates 2X VOUT. V2x should be bypassed to GND with a 0.47F X5R type ceramic capacitor. C2+ and C2- should be left open and V2x connected to VOUT if the doubler is not needed to generate VH or VN. VNIN (Pin 9): Positive Voltage Input for the Charge Pump Inverter. The charge pump inverter can generate a regulated negative voltage up to the voltage applied to VNIN. Connect VNIN to VOUT, V2x, or VH. If VNIN is connected to VH, external diodes and a capacitor are required for sequencing (see the Applications Information section). CN+ (Pin 10): Charge Pump Inverter Flying Capacitor Positive Node. The charge pump inverter flying capacitor is connected between CN+ and external Schottky diodes (see Typical Application figures). The voltage on CN+ will alternate between GND and VNIN at an approximate 50% duty cycle while the inverting charge pump is operating. Use a 0.1F X5R type ceramic capacitor for best results. NC (PIN 11): No Connect. This pin should be connected to GND. VN (Pin 12): Negative Charge Pump Converter Output. VN can be regulated down to approximately -VNIN volts depending on where VNIN is connected. VN should be bypassed to GND with at 0.47F or larger X5R type ceramic capacitor.
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LTC3524 LCD BIAS PIN FUNCTIONS
FBN (Pin 13): Feedback Pin for the VN Charge-Pump Output. Reference voltage is 1.0V. Connect the resistive divider tap between VOUT and VN here with minimum trace area. VN = -R6 ( VOUT - 1) + 1 (See Block Diagram) R5 up to 2mA to a load. VH should be bypassed to GND with a 0.47F X5R type ceramic capacitor. Connect V2x to VOUT for applications requiring a regulated voltage less than 2X VOUT. CH+ (Pin 16): Charge Pump Quadrupler Flying Capacitor Positive Node. The charge pump quadrupler (4X) flying capacitor is connected between CH+ and CH-. The voltage on CH+ will alternate between V2x and VH at an approximate 50% duty cycle while the charge pump is operating. Use a 0.1F X5R type ceramic capacitor for best results. CH- (Pin 17): Charge Pump Quadrupler (4X) Flying Capacitor Negative Node. The voltage on CH- will alternate between GND and V2x at an approximate 50% duty cycle while the charge pump is operating. Use a 0.1F X5R type ceramic capacitor for best results.
FBH (Pin 14): Feedback Pin for the VH Charge-Pump Output. Reference voltage is 1.225V. Connect resistive divider tap here with minimum trace area. R3 VH = 1 . 225 1 + (See Block Diagram) R4 VH (Pin 15): Charge Pump Quadrupler Output. This output can be regulated to 4X VOUT and is capable of delivering
WHITE LED DRIVER PIN FUNCTIONS
LED2 (Pin 18): Output for Second LED String. Connect up to five white LEDs between LED2 (anode) and GND (cathode). For best current matching and efficiency use the same number of white LEDs in both strings. LED1 (Pin 19): Output for First LED String. VLED (Pin 20): Output of the LED Switcher. Bypass VLED with a low ESR, ESL ceramic capacitor (X5R type) of at least 1F. Keep PCB trace lengths as short and wide as possible to minimize EMI and voltage overshoot. SW2 (Pin 21): White LED Boost Switch. Connect a 3.315H inductor between SW2 and VIN. This is the collector of the internal NPN power switch. Connect an external Schottky diode between SW2 and VLED. Keep PCB trace lengths as short and wide as possible to minimize EMI and voltage overshoot. ELED2 (Pin 22): Enable and PWM Dimming Control Input for the LED2 String. The LED2 string is disabled when this pin is grounded. Digital dimming can be implemented by driving the ELED2 pin between 0V and >1.2V at low frequency (ie., 500Hz). Driving ELCD, ELED1, and ELED2 low initiates shutdown mode which disables all IC functions and reduces quiescent current from the battery to less than 2A. PROG (Pin 23): A single resistor (RPROG) between PROG and GND sets the current in the LED strings. LED current in mA is programmed by: 2 x 10 6 ILED1 = ILED2 = mA RPROG A 100K resistor programs 20mA in each string. Analog dimming can be implemented by connecting a second resistor between PROG and a control voltage. ELED1 (Pin 24): Enable and Pulse Dimming Control Input for the LED1 String. For applications with five or fewer LEDs, better efficiency is achieved by operating a single LED string. For example, ELED1 = 1, ELED2 = 0, LED2 left open circuit and the LED string connected to LED1.
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LTC3524 BLOCK DIAGRAM
2.5V TO 6V 10H 4.7H
5
SW1 VBEST
2
VIN
21
SW2 VLED 20 STRING ENABLE LED CURRENT SHARING OVP LED1 19 LED2 18 10F
+5V VOUT R1 1M R2 324k
4 SHDN HIGH VOLTAGE PWM BOOST CONVERTER
SYNCHRONOUS PWM BOOST CONVERTER
3
FBVO 1.225V
7 6 +10V 8
C2+ C2- V2x OUT SHDN CHARGE PUMP DOUBLER
IN
VOUT
PROG 23 ELED2 22
RPROG ANALOG DIMMING ENABLE/PULSE DIM LED2 STRING ENABLE/PULSE DIM LED1 STRING LCD BIAS ENABLE V2x
CHARGE PUMP SEQUENCER OSCILLATOR CONTROL
SHUTDOWN WHEN ELED1 ELCD = ELED1 = ELED2 = 0V 24 ELCD 1 VNIN 9 IN REGULATED CHARGE PUMP INVERTER OUT CN+ 10 NC 11 VN 12
16 17 +12.5V 15
CH+ CH - VH
IN REGULATED CHARGE PUMP QUADRUPLER OUT SHDN
-7.5V
10F R4 220k
R3 2M
SHDN
R6 1M R5 470k VOUT
14
FBH
1.225V GND, EXPOSED PAD 25 1V
FBN 13
3524 BD
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LTC3524 OPERATION
The LTC3524 is a highly integrated power converter intended for small to medium-sized TFT LCD display modules. The part generates the required bias voltages for the LCD panel as well as regulated current for one or two white LED backlight strings. The LCD bias and white LED boost converters are powered from a common input voltage between 2.5V and 6V and share a 1.5MHz oscillator, allowing tiny inductors and capacitors to be used. The LCD bias supply and each white LED string can be independently enabled and a low current shutdown mode (<2A) is activated when all outputs are disabled. The LCD bias includes a synchronous PWM boost converter that can be programmed between 3.0V and 6.0V. This output (VOUT) is used as the main LCD supply and to power three charge pump converters. The charge pump circuits operate at one-sixteenth the boost frequency (about 94kHz). The generated output voltages are internally sequenced to insure proper initialization of the LCD panel. A digital shutdown input (ELCD) rapidly discharges each generated output voltage to provide a near instantaneous turn-off of the LCD display. The white LED driver circuitry consists of a PWM boost converter with an internal low loss NPN power switch and external Schottky diode. The LED boost output (VLED) can power as many as ten white LEDs at up to 25mA. LED current is programmable and current in each string matched with an internal loop. PWM dimming can be implemented through the enable pins (ELED1 and ELED2) to extend the dimming range of the application. LCD Bias Boost Converter A synchronous boost converter is used to generate the main analog LCD bias supply for the TFT display. The converter utilizes current mode control and includes internally set control loop and slope compensation for optimized performance and a simple design. Only an inductor, output capacitor and VOUT programming resistors at FBVO are required to complete the design of the 25mA boost. The 1.5MHz operating frequency produces very low output ripple and allows the use of small low profile inductors and tiny external ceramic capacitors. The boost converter also disconnects its output from VIN during shutdown to avoid loading the input power source. Soft-start produces a controlled ramp of the converter input current during start-up, greatly reducing the burden on the input power source. Very low operating quiescent current and synchronous operation allow for greater than 90% conversion efficiency.
VIN RSS 1M ELCD CSS 6.8nF
3524 F01
Figure 1. 1ms Soft-Start with 3.6V VIN
Soft-start operation provides a gradual increase in the current drawn from the input power source during initial start-up of the LCD bias boost converter. The rate at which the input current will increase is set by two external components (RSS and CSS) connected to ELCD (refer to Figure 2). Upon initial application of power the voltage on ELCD will increase relative to the time constant RSS x CSS. After one time constant, ELCD will rise to approximately 63.2% of the voltage on VIN. From 0V to approximately 0.65V on ELCD, no switching will occur because the threshold is 0.65V (typ). From 0.65V to 1V the maximum switch pin current capability of the LTC3524 will gradually increase from near 0A to the maximum current limit. LCD Bias Charge Pumps The LTC3524 uses three internal charge pump circuits to generate low current, high voltage outputs typically used to bias the LCD gate drive. The three charge pumps include a doubler, quadrupler, and inverting configuration. Each charge pump requires two small external capacitors, one to transfer charge, and one for filtering. The charge pumps feature fixed frequency operation for high efficiency and lowest noise performance. The charge pump converters operate at one-sixteenth the boost converter frequency.
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LTC3524 OPERATION
The doubler is internally connected to VOUT and generates a voltage of approximately 2X VOUT at V2x. The quadrupler has its input connected to V2x and output to VH. The regulated VH voltage is programmed at FBH and can be set to produce a voltage up to 4X VOUT. The maximum voltage VH can source depends on charge pump loading and the output impedance of the doubler and quadrupler stages (see Typical Performance Characteristics). The inverting charge pump has its input at VNIN and output at VN. Regulated VN voltage is set at FBN and can be programmed to a minimum negative voltage of VNIN minus diode drops. VNIN can be connected to VOUT, V2x, or VH depending on the negative voltage value required for the application. Efficiency is improved by using the lowest voltage possible on VNIN. As with the other charge pump outputs, the maximum negative voltage that VN can maintain will depend on loading. Two Schottky diodes are required to complete the negative charge pump as shown on the front page and applications circuits. LCD BIAS Sequencing Referring to the following text and Figure 2, the LTC3524 power-up and discharge sequence is explained. When input power is applied and ELCD is active, the boost converter initializes and charges its output towards the final programmed value. When the boost converter output (VOUT) has reached approximately 90% of its final value, an internal signal is asserted which allows the charge pump doubler
VH
(V2x) to begin operation toward its final goal of 2X VOUT. Approximately 2ms later, the charge pump inverter (VN) begins operation toward its programmed value. When the VN has reached approximately 50% of its final value, a 2ms (nominal) timeout period begins. At the conclusion of the 2ms timeout period, the charge pump quadrupler (VH) is allowed to begin operation. During the initial power-up sequence, the charge pumps run at half speed. If VNIN is connected to VH, a diode-OR circuit is needed between V2x, VH, and VNIN (see the Typical Applications) to ensure proper sequencing. When ELCD is brought low, internal transistors discharge the outputs in an orderly fashion. As shown in Figure 2, VN and V2x are initially discharged, followed by VH, followed by VOUT. VOUT must be discharged before the part can enter low current shutdown mode (ELCD, ELED1, ELED2 must be low, as well). White LED Boost Driver The white LED driver portion of the LTC3524 consists of a nonsynchronous, fixed frequency, current mode boost converter that generates the voltage required for one or two LED strings. The converter has an internal feedback loop and slope compensation circuitry, reducing external components and simplifying the design. As with the LCD bias boost converter, the 1.5MHz operation allows tiny external components to be used. The boost converter
VH
V2x
V2x
VOUT ELCD ELCD
VOUT
TIME
VN
VN
3524 F02
Figure 2. LCD Power-Up and Power-Down Timing Diagram
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LTC3524 OPERATION
output voltage is not set to a fixed voltage, but rather controlled to produce the programmed current in the LED strings. The output (VLED) is rated for a maximum of 21V which will support two strings of up to five series LED in most cases. The boost output is used to power one or two white LED strings with a common ground. If only one string is enabled (ELED1 or ELED2) the voltage on that string (LED1 or LED2) will be controlled to regulate the LED current set at the PROG pin. The voltage on VLED will be slightly greater due to the overhead needed for the internal sense element and share circuitry. For example, a single string application with four white LEDs programmed at 20mA would require 14.4V on LED1 if the forward drop on each LED is 3.6V. The voltage on VLED may need to be 15V to support the drops on the internal share circuitry. For applications with five or fewer LED elements, a single-string operation will provide better efficiency. If both strings are enabled, the boost output (VLED) will generate the voltage required to regulate current in the higher voltage string. Voltage on the lower string is controlled by the internal share circuit to provide the programmed current. The LTC3524 achieves current matching between the strings while minimizing the voltage drop between VLED and the higher voltage string (to maintain high efficiency). For example, an application with four LEDs on LED1 and five LEDs on LED2 is programmed for 20mA (RPROG = 100k). In this instance, assuming a 3.6V forward drop, LED1 is 14.4V, LED2 is 18V, and VLED is 18.6V. The drop between VLED and LED1 is 4V at 20mA, resulting in lower efficiency. For this reason, it is recommended when possible to keep the number of LEDs in each string matched. Analog Dimming: The LTC3524's white LED driver allows both analog and PWM dimming to be implemented. Analog dimming provides a lower noise solution but a reduced dynamic range. Analog dimming can be implemented by resistively summing a current into the PROG pin. The LED string currents with RPROG, VSUM, and RSUM will be: 1 . 225V 1 . 225V - VSUM ILED = 1625 * + R SUM RPROG A 0V to 3V VSUM with RSUM = 300k and RPROG = 150k will produce LED currents between 3mA and 20mA.
VSUM RSUM 300k PROG RPROG 150k
3524 F03
0V - 3V
Figure 3. Analog Dimming Circuit Using VSUM
True Color PWM Dimming: PWM dimming can be implemented by enabling and disabling the LED strings with ELED1 and ELED2. A PWM frequency between 100Hz and 500Hz is generally recommended to get wide dimming range while operating at a frequency faster than the eye can detect. For best results, the LCD bias portion of the device should be enabled (to keep the device out of shutdown) and ELED1 and ELED2 should be driven with a common low frequency PWM signal. PWM dimming waveforms are shown in the Typical Performance Characteristics section of this datasheet. The achievable dimming range is dependant on the PWM dimming frequency (FPWM) and the settling time of the LED strings when enabled (TSETTLE). The minimum duty cycle (or light output) that the strings can be controlled to is given by: MinDuty = FPWM * TSETTLE For example, if the settling time is 50S and the PWM frequency is 100Hz, the minimum duty cycle is 0.5% which corresponds to a 200:1 dimming range. Open LED: The LTC3524 has internal over voltage protection in the event that one of the white LED strings becomes open circuited. If VLED reaches 24V (nominal) due to an open circuit on either string, the boost converter will regulate at 24V while current in the remaining string (if enabled) is controlled to the programmed value.
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LTC3524 APPLICATIONS INFORMATION
Inductor Selection 3.3H to 15H inductors are recommended for use with the LTC3524's two boost converters. The synchronous LCD bias boost inductor should have a saturation current (ISAT) rating of at least 150mA, where the nonsynchronous white LED boost inductor should have a rating of at least 600mA. In most applications, the inductor value for the LCD bias will be larger (10H to 15H) to prevent operation in deep discontinuous mode. The inductor value for the white LED can be smaller (3.3H to 6.8H), since it operates at higher currents. Ferrite core materials are strongly recommended for their superior high frequency performance characteristics. Inductors meeting these requirements are listed in Table 1. The maximum current and DCR ranges in the table correspond to the respective Inductance range (for example, the 3.3H inductor will have the highest maximum current and lowest DCR). Shielded inductor series parts are in bold text. The VIN input capacitor should be an X5R type of at least 2.2F using a low impedance connection to the battery. The VLED output capacitor should be X5R type and at least 1F for analog dimming and 4.7F for PWM dimming. The VOUT capacitor should also be an X5R type between 2.2F and 10F. A larger capacitor (10F) should be used if lower output ripple is desired or the output load required is close to the 25mA maximum. The charge pumps require flying capacitors (C2+ to C2-, CN+, and CH+ to CH-) that should be at least 0.1F to obtain specified performance. Ceramic X5R types are strongly recommended for their low ESR and ESL and capacitance vs bias voltage stability. The filter capacitors on V2x, VN,
Table 1. Recommended Inductors
L (H) 3.3-15 3.3-10 3.3-15 3.3-15 3-10 4.7-15 4.7-15 3.3-15 3.3-12 3.3-15 3.3-15 MAXIMUM CURRENT (mA) 1300-700 950-570 1100-440 970-405 1000-200 650-300 300-200 1100-520 820-420 750-400 1200-560 DIMENSIONS (mm) (L x W x H) 3.2 x 2.5 x 2.0 3.0 x 3.0 x 1.0 4.0 x 4.0 x 2.0 3.1 x 3.1 x 1.2 3.2 x 2.6 x 1.0 3.2 x 2.5 x 1.5 2 x 1.6 x 0.9 3.8 x 3.8 x 1.8 3.2 x 3.2 x 1.5 3.0 x 3.0 x 1.0 3.0 x 3.0 x 1.5
PART ME3220 LP03010 MSS4020 SD3112 MIP3226D LQH32CN LQH2MC CDRH3D16 CDRH2D14 NR3010 NR3015
DCR () 0.14-0.52 0.2-0.52 0.09-0.33 0.16-0.65 0.1-0.16 0.15-0.58 0.8-1.6 0.09-0.41 0.12-0.32 0.16-0.74 0.1-0.36
MANUFACTURER Coil Craft www.coilcraft.com Cooper www.cooperet.com FDK www.fdk.com Murata www.murata.com Sumida www.sumida.com Taiyo Yuden www.t-yuden.com
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12
LTC3524 APPLICATIONS INFORMATION
and VH should be at least 0.47F. Please be certain that the capacitors used are rated for the maximum voltage with adequate safety margin. Refer to Table 2 for a listing of capacitor vendors.
Table 2. Capacitor Vendor Information
Supplier AVX Murata Samsung Taiyo Yuden TDK Phone (803) 448-9411 (714) 852-2001 (408) 544-5200 (800) 368-2496 (847) 803-6100 Website www.avxcorp.com www.murata.com www.sem.samsung.com www.t-yuden.com www.component.tdk.com
Printed Circuit Board Layout Guidelines High-speed operation of the LTC3524 demands careful attention to PCB layout. You will not get advertised performance with a careless layout. Figure 4 shows the recommended component placement for a double layer PCB. The bottom layer is used as a common ground plane except for the VN trace.
ELCD ELED1 ELED2
SCHOTTKY DIODE LAYOUT NOTES: LIGHT GREY TOP LAYER VIA TO BOTTOM GROUND PLANE. GROUND PLANE FILLS BOTTOM *KEEP RPROG AWAY FROM SW2 TRACES WHITE LEDs L2
*
ELED1 24 ELCD 1 VIN 2 FBVO 3 VOUT L1 VOUT 4 SW1 5 C2- 6 C2+ 7
PROG 23
ELED2 22
SW2 21
VLED 20
LED1 19 LED2 18 - CH 17 + CH 16 VH 15 FBH 14 FBN 13
WHITE LEDs
GND
TOP VIEW COMPONENT AND IC SIZES NOT TO SCALE GND
VIN R1
GND
GND
VH
GND
V2x 8
VNIN 9
CN+ 10
NC 11
VN 12
SCHOTTKY DIODE
VN
3524 F04
Figure 4. Suggested Layout Two Layer Board (Not to Scale)
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13
LTC3524 TYPICAL APPLICATIONS
Li-Ion to +5V, 25mA, +16V, 1mA, -13V, 1mA TFT LCD Power Supply + 10 White LEDs
+ -
Li-Ion
2.2F
10H VIN
4.7H SW2 VLED LED2 LED1 100k FOR 20mA PROG ELED2 ELED1 ELCD VNIN 287k FBN VOUT 1M -13V, 1mA 0.47F 10F
SW1 +5V, 25mA 1M 324k FBVO +10V 0.47F 0.1F C2- +16V, 1mA 0.47F 165k 2M FBH CH+ VH V2x C2+
VOUT
10F
LTC3524
0.47F VIN VH V2x REQUIRED FOR LCD BIAS SEQUENCING WHEN |VN| > V2X
CH- GND CN+ 0.1F
VN
0.1F
3524 TA02a
100
90 EFFICIENCY (%) 4.2 80 3.6
LCD (VIN )
LED (VIN ) 3.1 3.6
70 4.2 3.1 60 5
10 15 20 VOUT OR LED STRING CURRENT (mA)
25
3524 TA02b
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14
LTC3524 PACKAGE DESCRIPTION
UF Package 24-Lead Plastic QFN (4mm x 4mm) (Reference LTC DWG # 05-08-1697)
0.70 0.05
4.50 0.05 2.45 0.05 3.10 0.05 (4 SIDES)
PACKAGE OUTLINE 0.25 0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 0.75 0.05 BOTTOM VIEW--EXPOSED PAD R = 0.115 TYP PIN 1 NOTCH R = 0.20 TYP OR 0.35 x 45 CHAMFER
4.00 0.10 (4 SIDES) PIN 1 TOP MARK (NOTE 6)
23 24 0.40 0.10 1 2
2.45 0.10 (4-SIDES)
(UF24) QFN 0105
0.200 REF 0.00 - 0.05 NOTE: 1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGD-X)--TO BE APPROVED 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE, IF PRESENT 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE
0.25 0.05 0.50 BSC
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Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
15
LTC3524 TYPICAL APPLICATION
3NiMH or NiCD to +3.3V, 25mA, +10V, 1mA, -5V, 1mA TFT LCD Power Supply + 6 White LEDs
COILCRAFT MSS4020 SERIES
+
3 NiMH OR NiCD 2.2F SW1
10H VIN
4.7H SW2 VLED LED2 LED1 10F
100
-
+3.3V, 25mA 510k 301k
90 3.1V EFFICIENCY (%)
LCD (VIN )
VOUT
80 2.5V 3.1V 70 3.6V 3.6V LCD (VIN ) 60 LED (VIN ) 2.5V
10F
FBVO +6.6V 0.47F 0.1F C2- +10V, 1mA 0.47F 165k 2M FBH CH+ CH- GND CN+ VH V2x C2+ LTC3524
100k FOR 20mA PROG ELED2 ELED1 ELCD VNIN 232k FBN VN 0.1F 0.47F 604k - 5V, 1mA PHILIPS PMEG3005 VOUT VIN V2x
50 5 10 15 20 VOUT OR LED STRING CURRENT (mA) 25
3524 TA03b
0.1F
3524 TA03a
RELATED PARTS
PART NUMBER LT1942 LT1947 LTC3450 LT3465/LT3465A LT3466/LT3466-1 LT3471 LT3491 LT3494/LT3494A LT3497 LT3591 DESCRIPTION Quad DC/DC Converter for Triple Output TFT Supply Plus LED Driver 3MHz, 30V Adjustable Output TFT-LCD Triple Switching Regulator Constant-Current, 1.2MHz/2.7MHz High Efficiency White LED Boost Regulator with Integrated Schottky Diode Dual Constant-Current, 2MHz, High Efficiency White LED Boost Regulator with Integrated Schottky Diode Dual Output, Boost/Inverter, 1.3A ISW, 1.2MHZ, High Efficiency Boost-Inverting DC/DC Converter Constant-Current, 2.3MHz, High Efficiency White LED Boost Regulator with Integrated Schottky Diode 40V, 180mA/350mA Micropower Low Noise Boost Converter with Output Disconnect Constant-Current, 2.3MHz, Dual High Efficiency White LED Boost Regulator with Integrated Schottky Diode for 12 LEDs Constant-Current, 1MHz, High Efficiency White LED Boost Regulator with Integrated Schottky Diode COMMENTS VIN : 2.6V to 16V, VOUT(MAX) = 36V, IQ = 7mA, ISD = < 1A, 4mm x 4mm QFN-24 Package VIN : 2.7V to 8V, VOUT(MAX) = 30V, IQ = 9.5mA, ISD = < 1A, MSOP-10 Package VIN : 1.5V to 4.6V, VOUT(MAX) = 15V, IQ = 75A, ISD = < 1A, 3mm x 3mm QFN-16 Package VIN : 2.7V to 16V, VOUT(MAX) = 34V, IQ = 1.9mA, ISD = < 1A, ThinSOTTM Package VIN : 2.7V to 24V, VOUT(MAX) = 40V, IQ = 5mA, ISD = < 16A, 3mm x 3mm DFN-10 Package VIN : 2.4V to 16V, VOUT(MAX) = 40V, IQ = 2.5mA, ISD = < 1A, 3mm x 3mm DFN-10 Package VIN : 2.5V to 12V, VOUT(MAX) = 27V, IQ = 2.6mA, ISD = < 8A, 2mm x 2mm DFN-6 SC70 Package VIN : 2.3V to 16V, VOUT(MAX) = 40V, IQ = 65A, ISD = < 1A, 3mm x 2mm DFN-8 Package VIN : 2.5V to 10V, VOUT(MAX) = 32V, IQ = 6mA, ISD = < 12A, 3mm x 2mm DFN-10 Package VIN : 2.5V to 12V, VOUT(MAX) = 40V, IQ = 4mA, ISD = < 9A, 3mm x 2mm DFN-8 Package
ThinSOT is a trademark of Linear Technology Corporation.
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16 Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
LT 0208 * PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2008

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