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19-3321; Rev 0; 6/04
KIT ATION EVALU ILABLE AVA
Low-Cost CCFL Backlight Controller
General Description Features
Synchronized to Resonant Frequency Longer Lamp Life Guaranteed Striking Capability High Power to Light Efficiency Wide Input Voltage Range (4.6V to 28V) Input-Voltage Feed-Forward for Excellent Line Rejection Accurate Dimming Control with Analog Interface 10:1 Dimming Range Adjustable Accurate DPWM Frequency with Sync Function Adjustable Lamp Current Rise and Fall Time Secondary Voltage Limit Reduces Transformer Stress Lamp-Out Protection with Adjustable Timeout Secondary Overcurrent Protection with Adjustable Timeout Low-Cost 24-Pin QSOP Package
MAX8722
The MAX8722 integrated backlight controller is optimized to drive cold-cathode fluorescent lamps (CCFLs) using a full-bridge resonant inverter architecture. Resonant operation maximizes striking capability and provides near-sinusoidal waveforms over the entire input range to improve CCFL lifetime. The controller operates over a wide input voltage range (4.6V to 28V) with high power to light efficiency. The device also includes safety features that effectively protect against many single-point fault conditions including lamp-out and short-circuit faults. The MAX8722 achieves 10:1 dimming range by "chopping" the lamp current on and off using a digital pulsewidth modulation (DPWM) method. The DPWM frequency can be accurately adjusted with a resistor or synchronized to an external signal. The brightness is controlled by an analog voltage on the CNTL pin. The device directly drives the four external n-channel power MOSFETs of the full-bridge inverter. An internal 5.3V linear regulator powers the MOSFET drivers, the DPWM oscillator, and most of the internal circuitry. The MAX8722 is available in a low-cost, 24-pin QSOP package and operates over a -40C to +85C temperature range.
Applications
Notebook Computer Displays LCD Monitors LCD TVs
PART MAX8722EEG
Ordering Information
TEMP RANGE -40C to +85C PIN-PACKAGE 24 QSOP
Minimal Operating Circuit Pin Configuration
VIN VCC BATT VDD
TOP VIEW
BATT 1 SHDN 2 ILIM 3 TFLT 4 CNTL 5 DPWM 6 SYNC 7 FREQ 8 COMP 9 IFB 10 VFB 11 ISEC 12 24 GND 23 VCC 22 VDD 21 PGND
VCC
GND VCC
BST2 BST1 GH1
ILIM
MAX8722 LX1
LX2 FREQ GL1
MAX8722
20 GL2 19 GL1 18 GH1
PGND
17 LX1 16 BST1 15 BST2 14 LX2 13 GH2
SHDN CNTL SYNC DPWM COMP TFLT
GL2 GH2 VFB ISEC IFB
QSOP
________________________________________________________________ Maxim Integrated Products
1
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim's website at www.maxim-ic.com.
Low-Cost CCFL Backlight Controller MAX8722
ABSOLUTE MAXIMUM RATINGS
BATT to GND..........................................................-0.3V to +30V BST1, BST2 to GND ...............................................-0.3V to +36V BST1 to LX1, BST2 to LX2 ........................................-0.3V to +6V CNTL, FREQ, SYNC, VCC, VDD to GND ...................-0.3V to +6V COMP, DPWM, ILIM, TFLT to GND ............-0.3V to (VCC + 0.3V) GH1 to LX1 ..............................................-0.3V to (VBST1 + 0.3V) GH2 to LX2 ..............................................-0.3V to (VBST2 + 0.3V) GL1, GL2 to GND .......................................-0.3V to (VDD + 0.3V) IFB, ISEC, VFB to GND................................................-3V to +6V SHDN to GND...........................................................-0.3V to +6V PGND to GND .......................................................-0.3V to +0.3V Continuous Power Dissipation (TA = +70C) 24-Pin QSOP (derate 9.5mW/C above +70C)........761.9mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) .................................+300C
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(Circuit of Figure 1. VBATT = 12V, VCC = VDD, VSHDN = 5.3V, TA = 0C to +85C. Typical values are at TA = +25C, unless otherwise noted.)
PARAMETER BATT Input Voltage Range BATT Quiescent Current BATT Quiescent Current, Shutdown VCC Output Voltage, Normal Operation VCC Output Voltage, Shutdown VCC Undervoltage-Lockout Threshold VCC Undervoltage-Lockout Hysteresis GH1, GH2, GL1, GL2 OnResistance, High GH1, GH2, GL1, GL2 OnResistance, Low GH1, GH2, GL1, GL2 Maximum Output Current BST1, BST2 Leakage Current Resonant Frequency Range Minimum Off-Time Maximum Off-Time Current-Limit Threshold LX1 to PGND, LX2 to PGND (Fixed) Current-Limit Threshold LX1 to PGND, LX2 to PGND (Adjustable) ILIM = VCC VBST_ = 12V, VLX_ = 7V Guaranteed by design 30 340 24 180 470 33 200 ITEST = 10mA, VCC = VDD = 5.3V ITEST = 10mA, VCC = VDD = 5.3V VCC = VDD = VBATT VCC = VDD = open VSHDN = VCC, VIFB = 1V SHDN = GND VSHDN = 5.5V, 6V < VBATT < 28V, 0 < ILOAD < 10mA SHDN = GND, no load VCC rising (leaving lockout) VCC falling (entering lockout) 4.0 200 20 10 0.3 5 80 600 43 220 37 20 5.25 3.5 VBATT = 28V VBATT = VCC = 5V 6 5.40 4.6 CONDITIONS MIN 4.6 5.5 1 TYP MAX 5.5 28.0 2 2 20 5.55 5.5 4.58 UNITS V mA A V V V mV A A kHz ns s mV
VILIM = 0.5V VILIM = 2.0V
80 370
100 400
120 mV 430
2
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Low-Cost CCFL Backlight Controller
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1. VBATT = 12V, VCC = VDD, VSHDN = 5.3V, TA = 0C to +85C. Typical values are at TA = +25C, unless otherwise noted.)
PARAMETER Zero-Current Crossing Threshold LX1 to GND, LX2 to GND Current-Limit Leading Edge Blanking IFB Input Voltage Range IFB Regulation Point IFB Input Bias Current IFB Lamp-Out Threshold IFB to COMP Transconductance COMP Output Impedance COMP Discharge Current During Overvoltage or Overcurrent Fault COMP Discharge Current During DPWM Off-Time DPWM Rising to Falling Ratio ISEC Overcurrent Threshold ISEC Input Bias Current VFB Input Bias Current VFB Overvoltage Threshold RFREQ = 100k DPWM Chopping Frequency DPWM Input Low Voltage DPWM Input High Voltage DPWM Input Hysteresis DPWM Input Bias Current DPWM Output Low Resistance DPWM Output High Resistance SYNC Input Low Voltage SYNC Input High Voltage SYNC Input Hysteresis SYNC Input Bias Current SYNC Input Frequency Range CNTL Minimum Duty-Cycle Threshold CNTL Maximum Duty-Cycle Threshold CNTL Input Current DPWM ADC Resolution 0 < VCNTL < VCC Guaranteed monotonic VSYNC = 2V -0.3 10 0.20 1.9 -0.1 5 0.23 2.0 2.1 100 +0.3 50 0.26 2.1 +0.1 RFREQ = 169k RFREQ = 340k SYNC = VCC, RFREQ = 169k SYNC = VCC, RFREQ = 169k SYNC = VCC, RFREQ = 169k SYNC = VCC, RFREQ = 169k SYNC = GND, FREQ = VCC SYNC = VCC, FREQ = VCC -0.3 2.1 100 +0.3 2.4 2.4 0.8 204 0 < VISEC < 2V VVFB = 0.5V VIFB = 800mV, VISEC = 2V CNTL = GND, VCOMP = 2V VIFB = 0 1.15 -0.3 -0.3 2.2 2.3 343 209 106 0.8 V V mV A k k V V mV A kHz V V A Bits 214 Hz 0.5V < VCOMP < 4V 0 < VIFB < 2V -2V < VIFB < 0 CONDITIONS MIN 1 240 -2 770 -2 -150 560 60 7 600 100 10 400 100 2.5 1.21 1.28 +0.3 +0.3 2.4 V A A V 640 160 18 790 TYP 6 350 MAX 12 460 +2 810 +2 UNITS mV ns V mV A mV S M A A
MAX8722
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Low-Cost CCFL Backlight Controller MAX8722
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1. VBATT = 12V, VCC = VDD, VSHDN = 5.3V, TA = 0C to +85C. Typical values are at TA = +25C, unless otherwise noted.)
PARAMETER SHDN Input Low Voltage SHDN Input High Voltage SHDN Input Bias Current FREQ Dual Mode Input High Level FREQ Input Regulation Level FREQ Input Bias Current TFLT Charge Current TFLT Trip Threshold FREQ = VCC VISEC < 1.25V and VIFB < 600mV; VFLT = 2V VISEC < 1.25V and VIFB > 600mV; VFLT = 2V VISEC > 1.25V and VIFB < 600mV; VFLT = 2V 3.95 0.95 2.1 -1 VCC 0.35 VCC / 2 230 1.00 -1 116 4.10 4.20 V 1.05 A +1 CONDITIONS MIN TYP MAX 0.8 UNITS V V A V V A
Dual Mode is a trademark of Maxim Integrated Products, Inc.
ELECTRICAL CHARACTERISTIC
(Circuit of Figure 1. VBATT = 12V, VCC = VDD, V SHDN = 5.3V, TA = -40C to +85C, unless otherwise noted.) (Note 1)
PARAMETER BATT Input Voltage Range BATT Quiescent Current BATT Quiescent Current, Shutdown VCC Output Voltage, Normal Operation VCC Output Voltage, Shutdown VCC Undervoltage-Lockout Threshold GH1, GH2, GL1, GL2 OnResistance, High GH1, GH2, GL1, GL2 OnResistance, Low BST1, BST2 Leakage Current Resonant Frequency Range Minimum Off-Time Maximum Off-Time Current-Limit Threshold LX1 - PGND, LX2 - PGND (Fixed) ILIM = VCC VCC = VDD = VBATT VCC = VDD = open VSHDN = VCC, VIFB = 1V SHDN = GND VSHDN = 5.5V, 6V < VBATT < 28V 0 < ILOAD < 20mA SHDN = GND, no load VCC rising (leaving lockout) VCC falling (entering lockout) ITEST =10mA, VCC = VDD = 5.3V ITEST =10mA, VCC = VDD = 5.3V VBST_ = 12V, VLX_ = 7V Guaranteed by design 30 340 24 180 4.0 37 20 5 80 600 43 220 5.25 3.5 VBATT = 28V VBATT = VCC = 5V CONDITIONS MIN 4.6 5.5 TYP MAX 5.5 28.0 2 2 20 5.55 5.5 4.58 UNITS V mA A V V V A kHz ns s mV
4
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Low-Cost CCFL Backlight Controller
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1. VBATT = 12V, VCC = VDD, V SHDN = 5.3V, TA = -40C to +85C, unless otherwise noted.) (Note 1)
PARAMETER Current-Limit Threshold LX1 - PGND, LX2 - PGND (Adjustable) Zero-Current Crossing Threshold LX1 - GND, LX2 - GND Current-Limit Leading Edge Blanking IFB Input Voltage Range IFB Regulation Point IFB Input Bias Current IFB Lamp-Out Threshold IFB to COMP Transconductance COMP Output Impedance ISEC Overcurrent Threshold VFB Overvoltage Threshold DPWM Chopping Frequency DPWM Input Low Voltage DPWM Input High Voltage DPWM Output Low Resistance DPWM Output High Resistance SYNC Input Low Voltage SYNC Input High Voltage SYNC Input Bias Current SYNC Input Frequency Range CNTL Minimum Duty-Cycle Threshold CNTL Maximum Duty-Cycle Threshold SHDN Input Low Voltage SHDN Input High Voltage FREQ Dual-Mode Input High Level TFLT Trip Threshold 2.1 VCC 0.35 3.95 4.20 VSYNC = 2V 2.1 -0.3 10 0.20 1.9 +0.3 50 0.26 2.1 0.8 RFREQ = 169k SYNC = VCC, RFREQ = 169k SYNC = VCC, RFREQ = 169k SYNC = GND, FREQ = VCC SYNC = VCC, FREQ = VCC 2.1 2.4 2.4 0.8 0.5V < VCOMP < 4V 0 < VIFB < 2V -2V < VIFB < 0 VILIM = 0.5V VILIM = 2.0V CONDITIONS MIN 80 370 1 240 -2 770 -2 -150 560 60 7 1.15 2.2 200 640 160 18 1.28 2.4 218 0.8 TYP MAX 120 mV 430 12 460 +2 810 +2 mV ns V mV A mV S M V V Hz V V k k V V A kHz V V V V V V UNITS
MAX8722
Note 1: Specifications to -40C are guaranteed by design based on final characterization results.
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Low-Cost CCFL Backlight Controller MAX8722
Typical Operating Characteristics
(Circuit of Figure 1. VBATT = 12V, VCC = VDD, VSHDN = 5.3V, TA = +25C, unless otherwise noted.) HIGH INPUT-VOLTAGE OPERATION (VBATT = 20V)
MAX8722 toc02
LOW INPUT-VOLTAGE OPERATION (VBATT = 8V)
MAX8722 toc01
LINE-TRANSIENT RESPONSE
MAX8722 toc03
0V A
0V A
0V A
0V B
0V B
0V B
C C 0V D 0V 10s/div A: VIFB, 2V/div B: VVFB, 2V/div C: VLX1, 10V/div D: VLX2, 10V/div 10s/div A: VIFB, 2V/div B: VVFB, 2V/div C: VLX1, 10V/div D: VLX2, 10V/div 0V 20s/div A: VVFB, 2V/div B: VIFB, 2V/div C: VLX1, 10V/div D: VBATT, 10V/div 0V D
C 0V 20V 10V D 0V
LINE-TRANSIENT RESPONSE
MAX8722 toc04
MINIMUM BRIGHTNESS STARTUP WAVEFORM (VCNTL = 0V)
MAX8722 toc05
MINIMUM BRIGHTNESS DIGITAL PWM OPERATION (VCNTL = 0V)
MAX8722 toc06
0V A
0V A
0V A
0V B
0V B
0V B
C 0V 20V 10V D 0V 20s/div A: VVFB, 2V/div B: VIFB, 2V/div C: VLX1, 10V/div D: VBATT, 10V/div 2ms/div A: VIFB, 2V/div B: VVFB, 2V/div C: VCOMP, 1V/div D: VSHDN, 5V/div 0V 0V
C 0V D 0V 2ms/div A: VIFB, 2V/div B: VVFB, 2V/div C: VCOMP, 1V/div D: VDPWM, 5V/div
C
D
6
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Low-Cost CCFL Backlight Controller MAX8722
Typical Operating Characteristics (continued)
(Circuit of Figure 1. VBATT = 12V, VCC = VDD, VSHDN = 5.3V, TA = +25C, unless otherwise noted.)
50% BRIGHTNESS DIGITAL PWM OPERATION (VCNTL = 1V)
MAX8722 toc07
DIGITAL PWM SOFT-START
MAX8722 toc08
DIGITAL PWM SOFT-STOP
MAX8722 toc09
0V A
0V A
0V A
0V B
0V B
0V B
C 0V D 0V 2ms/div A: VIFB, 2V/div B: VVFB, 2V/div C: VCOMP, 1V/div D: VDPWM, 5V/div 40s/div A: VIFB, 2V/div B: VVFB, 2V/div C: VCOMP, 1V/div D: VDPWM, 5V/div 0V 0V
C 0V D 0V 40s/div A: VIFB, 2V/div B: VVFB, 2V/div C: VCOMP, 1V/div D: VDPWM, 5V/div
C
D
LAMP-OUT VOLTAGE LIMITING AND TIMEOUT
MAX8722 toc10
SECONDARY OVERCURRENT PROTECTION AND TIMEOUT
MAX8722 toc11
SWITCHING FREQUENCY vs. INPUT VOLTAGE
MAX8722 toc12
60 A
SWITCHING FREQUENCY (kHz)
A 0V
57
0V B 0V
54
51
B C 0V 200ms/div A: VIFB, 2V/div B: VVFB, 2V/div C: VTFLT, 5V/div A: VISEC, 500mV/div B: VTFLT, 2V/div 4ms/div 0V
48
45 8 12 16 INPUT VOLTAGE (V) 20 24
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Low-Cost CCFL Backlight Controller MAX8722
Typical Operating Characteristics (continued)
(Circuit of Figure 1. VBATT = 12V, VCC = VDD, VSHDN = 5.3V, TA = +25C, unless otherwise noted.)
DIGITAL PWM FREQUENCY vs. RFREQ
MAX8722 toc13
DIGITAL PWM FREQUENCY vs. INPUT VOLTAGE
MAX8722 toc14
RMS LAMP CURRENT vs. INPUT VOLTAGE
MAX8722 toc15
400 DIGITAL PWM FREQUENCY (Hz) 350 300 250 200 150 100 100 150 200 250 300
215 DIGITAL PWM FREQUENCY (Hz)
6.10 6.05 RMS LAMP CURRENT (mA) 6.00 5.95 5.90 5.85 5.80 NOMINAL CURRENT SET POINT
212
209
206
203
200 350 7 10 13 16 19 22 25 RFREQ () INPUT VOLTAGE (V)
8
12
16 INPUT VOLTAGE (V)
20
24
NORMALIZED BRIGHTNESS vs. CNTL VOLTAGE
MAX8722 toc16
VCC LINE REGULATION
MAX8722 toc17
100 NORMALIZED BRIGHTNESS (%)
0.4 0.2 VCC ACCURACY (%) 0 -0.2 -0.4 -0.6
80
60
40
20 -0.8 0 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 CNTL VOLTAGE (V) -1.0 4 8 12 16 20 24 INPUT VOLTAGE (V)
VCC LOAD REGULATION
MAX8722 toc18
VCC ACCURACY vs. TEMPERATURE
MAX8722 toc19
0
0.04 0.03 VCC ACCURACY (%) 0.02 0.01 0
-0.1 VCC ACCURACY (%)
-0.2
-0.3
-0.4
-0.01 -0.02 0 2 4 6 8 10 -40 -20 0 20 40 60 80 100 LOAD CURRENT (mA) TEMPERATURE (C)
-0.5
8
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Low-Cost CCFL Backlight Controller MAX8722
Pin Description
PIN 1 2 3 NAME BATT SHDN ILIM FUNCTION Supply Input. BATT is the input to the internal 5.4V linear regulator that powers the device. Bypass BATT to GND with a 0.1F ceramic capacitor. Shutdown Control Input. The device shuts down when SHDN is pulled to GND. Primary Current-Limit Adjustment Input. Connect a resistive voltage-divider between VCC and GND to set the primary current limit. The current-limit threshold is 1/5 of the voltage at ILIM. Connect it to VCC with a pullup resistor to select the default current-limit threshold of 0.2V. Fault Timer Adjustment Pin. Connect a capacitor from TFLT to GND to set the timeout periods for openlamp and secondary overcurrent faults. Brightness Control Input. Varying VCNTL between 0 and 2V varies the DPWM duty cycle (brightness) between 10% (minimum) and 100% (maximum). The brightness remains at maximum for VCNTL greater than 2V. Dual-Function DPWM Signal Pin. The DPWM pin can be used either as the DPWM signal output or as a low-frequency sync input. See the Digital PWM Dimming Control and Digital PWM Frequency Setting sections. DPWM High-Frequency Sync Input. The DPWM chopping frequency can be synchronized to an external high-frequency signal by connecting FREQ to VCC and SYNC to the external signal source. The DPWM chopping frequency is 1/128 of the frequency of the external signal. DPWM Frequency Dual-Mode Adjustment Pin. Connect a resistor from FREQ to GND to set the DPWM frequency. Connect FREQ to VCC to set DPWM frequency using SYNC. fDPWM = 209Hz x 169k / RFREQ Transconductance Error-Amplifier Output. A compensation capacitor connected between COMP and GND sets the rise and fall time of the lamp current in DPWM operation. Lamp-Current Feedback Input. The average voltage on IFB is regulated to 0.8V by controlling the on-time of high-side switches. If VIFB falls below 0.6V for a period longer than the timeout period set by TFLT, the MAX8722 activates the fault latch. Transformer Secondary Voltage Feedback Input. A capacitive voltage-divider between the high-voltage terminal of the CCFL tube and GND sets the maximum average lamp voltage during lamp strike and openlamp conditions. When the average voltage on VFB exceeds the internal overvoltage threshold, the controller turns on an internal current sink discharging the COMP capacitor. Transformer Secondary Current Feedback Input. A current-sense resistor connected between the lowvoltage end of the transformer secondary and ground sets the maximum secondary current during faults. When the average voltage on ISEC exceeds the internal overcurrent threshold, the controller turns on an internal current sink discharging the COMP capacitor. High-Side MOSFET NH2 Gate-Driver Output GH2 Gate-Driver Return. LX2 is the input to the current-limit and zero-crossing comparators. The device senses the voltage across the low-side MOSFET NL2 to detect primary current zero-crossing and primary overcurrent. GH2 Gate-Driver Supply Input. Connect a 0.1F capacitor from LX2 to BST2 and a diode from VDD to BST2 to form a bootstrap circuit. GH1 Gate-Driver Supply Input. Connect a 0.1F capacitor from LX1 to BST1 and a diode from VDD to BST1 to form a bootstrap circuit. GH1 Gate-Driver Return. LX1 is the input to the current-limit and zero-crossing comparators. The device senses the voltage across the low-side MOSFET NL1 to detect primary current zero-crossing and primary overcurrent.
4
TFLT
5
CNTL
6
DPWM
7
SYNC
8
FREQ
9
COMP
10
IFB
11
VFB
12
ISEC
13 14
GH2 LX2
15 16
BST2 BST1
17
LX1
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Low-Cost CCFL Backlight Controller MAX8722
Pin Description (continued)
PIN 18 19 20 21 22 23 24 NAME GH1 GL1 GL2 PGND VDD VCC GND High-Side MOSFET NH1 Gate-Driver Output Low-Side MOSFET NL1 Gate-Driver Output Low-Side MOSFET NL2 Gate-Driver Output Power Ground. PGND is the return for the GL1 and GL2 gate drivers. Low-Side Gate-Driver Supply Input. Connect VDD to the output of the internal linear regulator (VCC). Bypass VDD with a 0.1F capacitor to PGND. 5.3V/10mA Internal Linear-Regulator Output. VCC is the supply voltage for the device. Bypass VCC with a 1F ceramic capacitor to GND. Analog Ground. The ground return for VCC, REF, and other analog circuitry. Connect GND to PGND under the IC at the IC's backside exposed metal pad. FUNCTION
VIN
F1 2A VCC C1 4.7F 25V C7 0.47F D1 BST2 BST1 GH1 ILIM R5 200k LX1 LX2 FREQ R6 169k 1% GL1 PGND SHDN GL2 GH2 CNTL VFB ISEC SYNC IFB COMP DPWM TFLT C11 0.22F C10 0.01F R3 40.2 1% C4 15nF R1 150 1% NL1 NL2 C3 18pF 3kV C5 0.1F C6 0.1F NH1 NH2 C2 1F T1 CCFL
GND VDD C8 0.1F VCC C9 0.47F R4 100k VCC BATT GND
MAX8722
ON/OFF
BRIGHTNESS
SYNC
DPWM
Figure 1. Typical Operating Circuit of the MAX8722 10 ______________________________________________________________________________________
Low-Cost CCFL Backlight Controller MAX8722
BATT GND VCC OVERVOLTAGE COMPARATOR 2.3V VFB COMP OVERCURRENT
LINEAR REGULATOR
BIAS SUPPLY EN FLT UVLO 4.2V UVLO COMPARATOR
SHDN
MAX8722
BST1 GH1 LX1 BST2
RAMP
PWM COMPARATOR 400A PWM CONTROL LOGIC
100A
GATE-DRIVER CONTROL STATE MACHINE
GH2 LX2 VDD GL1 PGND
IFB
F.W. RECT 800mV ERROR AMPLIFIER
PRIMARY OVERCURRENT AND ZEROCROSSING
GL2
FREQ DPWM SYNC CNTL
DPWM OSCILLATOR AND DIMMING CONTROL LOGIC OPEN-LAMP COMPARATOR 600mV OVERCURRENT FAULT DELAY BLOCK FAULT LATCH S SHDN 1.25V SECONDARY OVERCURRENT COMPARATOR UVLO Q RESET R
MUX ILIM
ISEC
H.W. RECT
FLT
TFLT
Figure 2. MAX8722 Functional Diagram
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Low-Cost CCFL Backlight Controller MAX8722
Typical Operating Circuit
The typical operating circuit of the MAX8722 (Figure 1) is a complete CCFL backlight inverter for TFT-LCD panels. The input voltage range of the circuit is from 8V to 24V. The maximum RMS lamp current is set to 6mA, and the maximum RMS striking voltage is set to 1600V. Table 1 lists some important components, and Table 2 lists the component suppliers' contact information. duty cycle of the DPWM signal, which is set through an analog voltage on the CNTL pin. Figure 2 shows the functional diagram of the MAX8722.
Resonant Operation
The MAX8722 drives the four n-channel power MOSFETs that make up the zero-voltage-switching (ZVS) full-bridge inverter as shown in Figure 3. Assume that NH1 and NL2 are turned on at the beginning of a switching cycle as shown in Figure 3(a). The primary current flows through MOSFET NH1, DC blocking capacitor C2, the primary side of transformer T1, and MOSFET NL2. During this interval, the primary current ramps up until the controller turns off NH1. When NH1 turns off, the primary current forward biases the body diode of NL1, which clamps the LX1 voltage just below ground as shown in Figure 3(b). When the controller turns on NL1, its drain-to-source voltage is near zero because its forward-biased body diode clamps the drain. Since NL2 is still on, the primary current flows through NL1, C2, the primary side of T1, and NL2. Once the primary current drops to the minimum current threshold (6mV/RDS(ON)), the controller turns off NL2. The remaining energy in T1 charges up the LX2 node until the body diode of NH2 is forward biased. When NH2 turns on, it does so with near-zero drain-to-source voltage. The primary current reverses polarity as shown in Figure 3(c), beginning a new cycle with the current flowing in the opposite direction, with NH2 and NL1 on. The primary current ramps up until the controller turns off NH2. When NH2 turns off, the primary current forward biases the body diode of NL2, which clamps the LX2 voltage just below ground as shown in Figure 3(d). After the LX2 node goes low, the controller losslessly turns on NL2. Once the primary current drops to the minimum current threshold, the controller turns off NL1. The remaining energy charges up the LX1 node until the body diode of NH1 is forward biased. Finally, NH1 losslessly turns on, beginning a new cycle as shown in Figure 3(a). Note that switching transitions on all four power MOSFETs occur under ZVS condition, which reduces transient power losses and EMI. A simplified CCFL inverter circuit is shown in Figure 4(a). The full-bridge power stage is simplified and represented as a square-wave AC source. The resonant tank circuit can be further simplified to Figure 4(b) by removing the transformer. CS is the primary series capacitor, C'S is the series capacitance reflected to the secondary, CP is the secondary parallel capacitor, N is the transformer turns ratio, L is the transformer secondary leakage inductance, and RL is an idealized resistance that models the CCFL in normal operation.
Detailed Description
The MAX8722 controls a full-bridge resonant inverter to convert an unregulated DC input into a near-sinusoidal, high-frequency AC output for powering CCFLs. The lamp brightness is adjusted by turning the lamp on and off with a digital pulse-width-modulation (DPWM) signal. The brightness of the lamp is proportional to the
Table 1. List of Important Components
DESIGNATION DESCRIPTION 4.7F 20%, 25V X5R ceramic capacitor Murata GRM32RR61E475K Taiyo Yuden TMK325BJ475MN TDK C3225X7R1E475M 1F 10%, 25V X7R ceramic capacitor 18pF 1pF, 3kV, high-voltage ceramic capacitor Murata GRM42D1X3F180J TDK C4520C0G3F180F Dual silicon switching diode, common anode, SOT-323 Central Semiconductor CMSD2836 Diodes, Inc. BAW56W Dual n-channel MOSFETs, 30V, 0.095, SOT23-6 Fairchild FDC6561AN CCFL transformer, 1:93 turns ratio TOKO T912MG-1018
C1
C2
C3
D1
NH1/2, NL1/2
T1
Table 2. Component Suppliers
SUPPLIER Central Semiconductor Diodes Inc. Fairchild Semiconductor Murata Sumida Taiyo Yuden TDK TOKO WEBSITE www.centralsemi.com www.diodes.com www.fairchildsemi.com www.murata.com www.sumida.com www.t-yuden.com www.components.tdk.com www.tokoam.com
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Low-Cost CCFL Backlight Controller MAX8722
VBATT VBATT
NH1 ON T1 C2 LX1 LX2
NH2 OFF
NH1 OFF T1 C2 LX1 LX2
NH2 ON
NL1 OFF
NL2 ON
NL1 ON
NL2 OFF
(a) VBATT
(c) VBATT
NH1 OFF T1 C2 LX1 LX2
NH2 OFF
NH1 OFF T1 C2 LX1 LX2
NH2 OFF
NL1 ON
NL2 ON
NL1 ON
NL2 ON
(BODY DIODE TURNS ON FIRST) (b) (d)
(BODY DIODE TURNS ON FIRST)
Figure 3. Resonant Operation
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13
Low-Cost CCFL Backlight Controller MAX8722
CS
1:N
L
4 AC SOURCE CP CCFL VOLTAGE GAIN (V/V) 3 RL INCREASING 2 fS fP
(a)
C C'S = S N2
L
1
AC SOURCE
0 CP RL 0 20 40 60 80 100 FREQUENCY (kHz)
(b)
Figure 4. Equivalent Resonant Tank Circuit
Figure 5. Frequency Response of the Resonant Tank
Figure 5 shows the frequency response of the resonant tank's voltage gain under different load conditions. The primary series capacitor is 1F, the secondary parallel capacitor is 15pF, the transformer turns ratio is 1:93, and the secondary leakage inductance is 260mH. Notice that there are two peaks, fS and fP, in the frequency response. The first peak, fS, is the series resonant peak determined by the secondary leakage inductance (L) and the series capacitor reflected to the secondary (C'S): fS = 1 2 LC'S The second peak, fP , is the parallel resonant peak determined by the secondary leakage inductance (L), the parallel capacitor (CP), and the series capacitor reflected to the secondary (C'S): fP = 2 L 1 C'S CP C'S + CP The inverter is designed to operate between these two resonant peaks. When the lamp is off, the operating point of the resonant tank is close to the parallel resonant peak due to the lamp's infinite impedance. The circuit
14
displays the characteristics of a parallel-loaded resonant converter. While in parallel-loaded resonant operation, the inverter behaves like a voltage source to generate the necessary striking voltage. Theoretically, the output voltage of the resonant converter will increase until the lamp is ionized or until it reaches the IC's secondary voltage limit, without regard to the transformer turns ratio or the input voltage level. Once the lamp is ionized, the equivalent load resistance decreases rapidly and the operating point moves toward the series resonant peak. While in series resonant operation, the inverter behaves like a current source.
Lamp-Current Regulation
The MAX8722 uses a lamp-current control loop to regulate the current delivered to the CCFL. The heart of the control loop is a transconductance error amplifier. The AC lamp current is sensed with a resistor connected in series with the low-voltage terminal of the lamp. The voltage across this resistor is fed to the IFB input and is internally full-wave rectified. The transconductance error amplifier compares the rectified IFB voltage with a 790mV (typ) internal threshold to generate an error current. The error current charges and discharges a capacitor connected between COMP and ground to create an error voltage (VCOMP). VCOMP is then compared with an internal ramp signal to set the high-side MOSFET switch on-time (tON).
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Low-Cost CCFL Backlight Controller
Transformer Secondary Voltage Limiting
The MAX8722 reduces the voltage stress on the transformer's secondary winding by limiting the secondary voltage during startup and open-lamp faults. The AC voltage across the transformer secondary winding is sensed through a capacitive voltage-divider. The small voltage across the larger capacitor of the divider is fed to the VFB input and is internally half-wave rectified. An overvoltage comparator compares the VFB voltage with a 2.3V (typ) internal threshold. Once the sense voltage exceeds the overvoltage threshold, the MAX8722 turns on a 400A current source that discharges the COMP capacitor. The high-side MOSFET on-time shortens as the COMP voltage decreases, reducing the transformer secondary's peak voltage below the threshold set by the capacitive voltage-divider. regulation for short on-times and makes startup transients less dependent on the input voltage. Feed-forward control is implemented by increasing the internal voltage ramp rate for higher VBATT. This has the effect of varying t ON as a function of the input voltage while maintaining approximately the same signal levels at V COMP . Since the required voltage change across the compensation capacitor is minimal, the controller's response to input voltage changes is essentially instantaneous.
MAX8722
Digital PWM Dimming Control
The MAX8722 controls the brightness of the CCFL by chopping the lamp current on and off using a low-frequency (between 100Hz and 350Hz) digital PWM signal either from the internal oscillator or from an external signal source. The CCFL brightness is proportional to the digital PWM duty cycle, which can be adjusted from 9.375% to 100% by the CNTL pin. CNTL is an analog input with a usable input voltage range between 0 and 2000mV, which is digitized to select one of 128 brightness levels. As shown in Figure 6, the MAX8722 ignores the first twelve steps, so the first twelve steps all represent the same brightness. When V CNTL is between 0 and 187.5mV, the digital PWM duty cycle is always 9.375%. When V CNTL is above 187.5mV, a 15.625mV change on CNTL results in a 0.78125% change in the digital PWM duty cycle. When VCNTL is equal to or above 2000mV, the digital PWM duty cycle is always 100%.
Lamp Startup
A CCFL is a gas discharge lamp that is normally driven in the avalanche mode. To start ionization in a nonionized lamp, the applied voltage (striking voltage) must be increased to the level required for the start of avalanche. At low temperatures, the striking voltage can be several times the typical operating voltage. Because of the MAX8722's resonant topology, the striking voltage is guaranteed. Before the lamp is ionized, the lamp impedance is infinite. The transformer secondary leakage inductance and the high-voltage parallel capacitor determine the unloaded resonant frequency. Since the unloaded resonant circuit has a high Q, it can generate very high voltages across the lamp. Upon power-up, two soft-start features acting together smooth the startup behavior. First, VCOMP slowly rises, increasing the duty cycle of the high-side MOSFET switches and providing a measure of soft-start. Second, the MAX8722 charges VFB to the overvoltage threshold (2.3V typ) immediately after the device is enabled. The DC voltage on VFB is gradually discharged through an internal 300k (typ) resistor during startup. This feature is equivalent to slowly raising the overvoltage threshold during startup, so it further improves the soft-start behavior.
100 90 80 BRIGHTNESS (%) 60 70 50 40 30 20 10 0 0 400 800 1200 1600 2000 CONTROL VOLTAGE (mV)
Feed-Forward Control and Dropout Operation
The MAX8722 is designed to maintain tight control of the lamp current under all transient conditions. The feed-forward control instantaneously adjusts the ontime for changes in input voltage (VBATT). This feature provides immunity to input-voltage variations and simplifies loop compensation over wide input voltage ranges. The feed-forward control also improves the line
Figure 6. Theoretical Brightness vs. Control Voltage ______________________________________________________________________________________ 15
Low-Cost CCFL Backlight Controller MAX8722
In digital PWM operation, COMP controls the rise and fall time of the lamp-current envelope. At the beginning of the digital PWM on-cycle, VCOMP rises linearly, gradually increasing tON, which provides soft-start. At the end of the digital PWM on-cycle, the COMP capacitor discharges linearly, gradually decreasing tON and providing soft-stop. 3) The digital PWM frequency can be synchronized to an external low-frequency signal. To enable this mode, connect SYNC to V CC , connect FREQ to GND through a 100k resistor, and connect DPWM to the external low-frequency signal. The digital PWM frequency and duty cycle are equal to those of the external signal. The frequency range of the external signal is between 100Hz and 350Hz. In this mode, the brightness control input CNTL is disabled, and the brightness is proportional to the duty cycle of the external signal. Table 3 summarizes the three ways of setting the digital PWM frequency.
Digital PWM Frequency Setting
There are three ways to set the digital PWM frequency. 1) The digital PWM frequency can be set with an external resistor. Connect SYNC to GND and connect a resistor between FREQ and GND. The digital PWM frequency is given by the following equation: fDPWM = 209Hz x 169k / RFREQ The adjustable range of the digital PWM frequency is between 100Hz and 350Hz (RFREQ is between 353k and 101k). CNTL controls the digital PWM duty cycle. 2) The digital PWM frequency can be clocked by an external high-frequency signal. Connect FREQ to VCC and connect SYNC to the external high-frequency signal. The digital PWM frequency is 1/128 of the frequency of the external signal: fDPWM = fEXT 128
UVLO
The MAX8722 includes an undervoltage-lockout (UVLO) circuit. The UVLO circuit monitors the VCC voltage. When VCC is below 4.2V (typ), the MAX8722 disables both high-side and low-side MOSFET drivers and resets the fault latch.
Low-Power Shutdown
When the MAX8722 is placed in shutdown, all functions of the IC are turned off except for the 5.4V linear regulator. In shutdown, the linear-regulator output voltage drops to about 4.5V and the supply current is 6A (typ). While in shutdown, the fault latch is reset. The device can be placed into shutdown by pulling SHDN to its logic-low level.
Lamp-Out Protection
For safety, the MAX8722 monitors the lamp-current feedback (IFB) to detect faulty or open CCFL tubes and secondary short circuits in the lamp and IFB sense resistor. As described in the Lamp-Current Regulation section, the voltage on IFB is internally full-wave rectified. If the rectified IFB voltage is below 600mV, the MAX8722 charges the TFLT capacitor with 1A. The
where fEXT is the frequency of the external signal. The frequency range of the external signal should be between 13kHz and 45kHz, resulting in a digital PWM frequency range between 100Hz and 350Hz. CNTL controls the DPWM duty cycle.
Table 3. Digital PWM Frequency Setting
FREQ Connect FREQ to GND through an external resistor. Connect FREQ to VCC. Connect FREQ to GND through a 100k resistor. SYNC Connect SYNC to GND. Connect SYNC to an external high-frequency signal. Connect SYNC to VCC. DPWM DPWM is used as the digital PWM signal output. DPWM is used as the digital PWM signal output. Connect DPWM to an external low-frequency signal. DIGITAL PWM FREQUENCY/DUTY CYCLE The resistor value sets the frequency. CNTL controls the duty cycle. The frequency is 1/128 of the frequency of the external signal. CNTL controls the duty cycle. The frequency and duty cycle are equal to those of the external signal.
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Low-Cost CCFL Backlight Controller
MAX8722 latches off if the voltage on TFLT exceeds 4V. Unlike the normal shutdown mode, the linear-regulator output (VCC) remains at 5.4V. Toggling SHDN or cycling the input power reactivates the device. During the delay period, the current control loop tries to maintain lamp-current regulation by increasing the high-side MOSFET on-time. Because the open-circuit lamp impedance is very high, the transformer secondary voltage rises as a result of the high Q-factor of the resonant tank. Once the secondary voltage exceeds the overvoltage threshold, the MAX8722 turns on a 400A current source that discharges the COMP capacitor. The on-time of the high-side MOSFET is reduced, lowering the secondary voltage, as the COMP voltage decreases. Therefore, the peak voltage of the transformer secondary winding never exceeds the limit set by a capacitive voltage-divider during the lamp-out delay period. Toggling SHDN or cycling the input power reactivates the device.
MAX8722
Linear-Regulator Output (VCC)
The internal linear regulator steps down the DC input voltage to 5.4V (typ). The linear regulator supplies power to the internal control circuitry of the MAX8722 and is also used to power the MOSFET drivers by connecting VCC to VDD. The VCC voltage drops to 4.5V in shutdown.
Applications Information
MOSFETs
The MAX8722 requires four external n-channel power MOSFETs NL1, NL2, NH1, and NH2 to form a fullbridge inverter circuit to drive the transformer primary. The regulator senses the on-state drain-to-source voltage of the two low-side MOSFETs NL1 and NL2 to detect the transformer primary current, so the RDS(ON) of NL1 and NL2 should be matched. For instance, if dual MOSFETs are used to form the full bridge, NL1 and NL2 should be in one package. Since the MAX8722 uses the low-side MOSFET RDS(ON) for primary overcurrent protection, the lower the MOSFET RDS(ON), the higher the current limit. Therefore, the user should select a dual, logic-level n-channel MOSFET with low RDS(ON) to minimize conduction loss, and keep the primary current limit at a reasonable level. The regulator uses zero-voltage switching (ZVS) to softly turn on each of the four switches in the full bridge. ZVS occurs when the external power MOSFETs are turned on when their respective drain-to-source voltages are near 0V (see the Resonant Operation section). ZVS effectively eliminates the instantaneous turn-on loss of MOSFETs caused by C OSS (drain-to-source capacitance) and parasitic capacitance discharge, and improves efficiency and reduces switching-related EMI.
Primary Overcurrent Protection (ILIM)
The MAX8722 senses transformer primary current in each switching cycle. When the regulator turns on the low-side MOSFET, a comparator monitors the voltage drop from LX_ to GND. If the voltage exceeds the current-limit threshold, the regulator turns off the high-side switch at the opposite side of the primary to prevent further increasing the transformer primary current. The current-limit threshold can be adjusted using the ILIM input. Connect a resistive voltage-divider between VCC and GND with the midpoint connected to ILIM. The current-limit threshold measured between LX_ and GND is 1/5 of the voltage at ILIM. The ILIM adjustment range is 0 to 3V. Connect ILIM to VCC to select the default current-limit threshold of 0.2V.
Secondary Current Limit (ISEC)
The secondary current limit provides fail-safe current limiting in case a failure, such as a short circuit or leakage from the lamp high-voltage terminal to ground, prevents the current control loop from functioning properly. ISEC monitors the voltage across a sense resistor placed between the transformer's low-voltage secondary terminal and ground. The ISEC voltage is internally half-wave rectified and continuously compared to the ISEC regulation threshold (1.25V typ). Any time the ISEC voltage exceeds the threshold, a controlled current is drawn from COMP to reduce the on-time of the bridge's high-side switches. At the same time, the MAX8722 charges the TFLT capacitor with a 116A current source. The MAX8722 latches off when the voltage on TFLT exceeds 4V. Unlike the normal shutdown mode, the linear-regulator output (VCC) remains at 5.3V.
Setting the Lamp Current
The MAX8722 senses the lamp current flowing through resistor R1 (Figure 1) connected between the low-voltage terminal of the lamp and ground. The voltage across R1 is fed to IFB and is internally full-wave rectified. The MAX8722 controls the desired lamp current by regulating the average of the rectified IFB voltage. To set the RMS lamp current, determine R1 as follows: R1 = x 790mV 2 2 x ILAMP(RMS)
where ILAMP(RMS) is the desired RMS lamp current and 790mV is the typical value of the IFB regulation point
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17
Low-Cost CCFL Backlight Controller MAX8722
specified in the Electrical Characteristics table. To set the RMS lamp current to 6mA, the value of R1 should be 148. The closest standard 1% resistors are 147 and 150. The precise shape of the lamp-current waveform, which is dependent on lamp parasitics, influences the actual RMS lamp current. Use a true RMS current meter connected between the R1/IFB junction and the low-voltage side of the lamp to make final adjustments to R1.
Transformer Design and Resonant Component Selection
The transformer is the most important component of the resonant tank circuit. The first step in designing the transformer is to determine the turns ratio (N). The ratio must be high enough to support the CCFL operating voltage at the minimum supply voltage. N can be calculated as follows: N VLAMP(RMS) 0.9 x VIN(MIN)
Setting the Secondary Voltage Limit
The MAX8722 limits the transformer secondary voltage during startup and lamp-out faults. The secondary voltage is sensed through the capacitive voltage-divider formed by C3 and C4 (Figure 1). The voltage on VFB is proportional to the CCFL voltage. The selection of the parallel resonant capacitor C3 is described in the Transformer Design and Resonant Component Selection section. C3 is usually between 10pF and 22pF. After the value of C3 is determined, select C4 using the following equation to set the desired maximum RMS secondary voltage VLAMP(RMS)_MAX: C4 = 2 x VLAMP(RMS)_ MAX 2.3V x C3
where 2.3V is the typical value of the VFB overvoltage threshold specified in the Electrical Characteristics table. To set the maximum RMS secondary voltage to 1600V using 18pF for C3, use approximately 15nF for C4.
Setting the Secondary Current Limit
The MAX8722 limits the secondary current even if the IFB sense resistor (R1) is shorted or transformer secondary current finds its way to ground without passing through R1. ISEC monitors the voltage across the sense resistor R3, connected between the low-voltage terminal of the transformer secondary winding and ground. Determine the value of R3 using the following equation: R3 = 1.217V 2 x ISEC(RMS)_ MAX
where ISEC(RMS)_MAX is the desired maximum RMS transformer secondary current during fault conditions, and 1.217V is the typical value of the ISEC regulation point specified in the Electrical Characteristics table. To set the maximum RMS secondary current in the circuit of Figure 1 to 22mA, use approximately 40.2 for R3.
where VLAMP(RMS) is the maximum RMS lamp voltage in normal operation, and VIN(MIN) is the minimum DC input voltage. If the maximum RMS lamp voltage in normal operation is 650V and the minimum DC input voltage is 8V, the turns ratio should be greater than 90. The turns ratio of the transformer used in the circuit of Figure 1 is 93. The next step in the design procedure is to determine the desired operating frequency range. The MAX8722 is synchronized to the natural resonant frequency of the resonant tank. The resonant frequency changes with operating conditions, such as the input voltage, lamp impedance, etc.; therefore, the switching frequency varies over a certain range. To ensure reliable operation, the resonant frequency range must be within the operating frequency range specified by the CCFL transformer manufacturer. As discussed in the Resonant Operation section, the resonant frequency range is determined by the transformer secondary leakage inductance L, the primary series DC blocking capacitor C2, and the secondary parallel resonant capacitor C3. Since it is difficult to control the transformer leakage inductance, the resonant tank design should be based on the existing secondary leakage inductance of the selected CCFL transformer. Leakageinductance values can have large tolerance and significant variations among different batches, so it is best to work directly with transformer vendors on leakageinductance requirements. The MAX8722 works best when the secondary leakage inductance is between 250mH and 350mH. The series capacitor C2 sets the minimum operating frequency, which is approximately two times the series resonant peak frequency. Choose: C2 N2 2 x fMIN
2
xL
18
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Low-Cost CCFL Backlight Controller
where fMIN is the minimum operating frequency range. In the circuit of Figure 1, the transformer's turns ratio is 93 and its secondary leakage inductance is approximately 300mH. To set the minimum operating frequency to 45kHz, use 1F for C2. The parallel capacitor C3 sets the maximum operating frequency, which is also the parallel resonant peak frequency. Choose C3 with the following equation: C3 C2 (4 2 x fMAX 2 x L x C2) - N2 high-voltage sections of the layout need to be well separated from the control circuit. Most layouts for singlelamp notebook displays are constrained to long and narrow form factors, so this separation occurs naturally. Follow these guidelines for good PC board layout: 1) Keep the high-current paths short and wide, especially at the ground terminals. This is essential for stable, jitter-free operation and high efficiency. 2) Use a star-ground configuration for power and analog grounds. The power and analog grounds should be completely isolated--meeting only at the center of the star. The center should be placed at the analog ground pin (GND). Using separate copper islands for these grounds may simplify this task. Quiet analog ground is used for V CC , COMP, FREQ, TFLT, and ILIM (if a resistive voltage-divider is used). 3) Route high-speed switching nodes away from sensitive analog areas (VCC, COMP, FREQ, TFLT, and ILIM). Make all pin-strap control input connections (ILIM, etc.) to analog ground or V CC rather than power ground or VDD. 4) Mount the decoupling capacitor from VCC to GND as close as possible to the IC with dedicated traces that are not shared with other signal paths. 5) The current-sense paths for LX1 and LX2 to GND must be made using Kelvin sense connections to guarantee the current-limit accuracy. 6) Ensure the feedback connections are short and direct. To the extent possible, IFB, VFB, and ISEC connections should be far away from the high-voltage traces and the transformer. 7) To the extent possible, high-voltage trace clearance on the transformer's secondary should be widely separated. The high-voltage traces should also be separated from adjacent ground planes to prevent lossy capacitive coupling. 8) The traces to the capacitive voltage-divider on the transformer's secondary need to be widely separated to prevent arcing. Moving these traces to opposite sides of the board can be beneficial in some cases.
MAX8722
In the circuit of Figure 1, to set the maximum operating frequency to 65kHz, use 18pF for C3. The transformer core saturation should also be considered when selecting the operating frequency. The primary winding should have enough turns to prevent transformer saturation under all operating conditions. Use the following expression to calculate the minimum number of turns N1 of the primary winding: N1 > DMAX x VIN(MAX) BS x S x fMIN
where DMAX is the maximum duty cycle (approximately 0.8) of the high-side switches, VIN(MAX) is the maximum DC input voltage, BS is the saturation flux density of the core, and S is the minimal cross-section area of the core.
COMP Capacitor Selection
The COMP capacitor sets the speed of the current loop that is used during startup, while maintaining lamp current regulation, and during transients caused by changing the input voltage. The typical COMP capacitor value is 0.01F. Larger values increase the transient-response delays. Smaller values speed up transient response, but extremely small values can cause loop instability.
Other Components
The external bootstrap circuits formed by D1 and C5/C6 in Figure 1 power the high-side MOSFET drivers. Connect VDD to BST1/BST2 through dual-diode D1 and couple BST1/BST2 to LX1/LX2 through C5 and C6. C5 = C6 = 0.1F or greater.
Layout Guidelines
Careful PC board layout is important to achieve stable operation. The high-voltage section and the switching section of the circuit require particular attention. The
Chip Information
TRANSISTOR COUNT: 2985 PROCESS: BiCMOS
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19
Low-Cost CCFL Backlight Controller MAX8722
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.maxim-ic.com/packages.)
PACKAGE OUTLINE, QSOP .150", .025" LEAD PITCH
21-0055
E
1 1
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
20 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
QSOP.EPS

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