General Description
The MAX8709B integrated backlight controller is
optimized to drive cold-cathode fluorescent lamps
(CCFLs) using a resonant full-bridge inverter architecture.
The 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 of 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 MAX8709B achieves 10:1 dimming range by “chop-
ping” the lamp current on and off using a digital pulse-
width-modulation (DPWM) method. The minimum DPWM
duty cycle of the MAX8709B is 12.5%. The brightness is
controlled with a 2-wire SMBus™-compatible interface.
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
MAX8709B is available in a space-saving 28-pin
thin QFN package and operates over a -40°C to +85°C
temperature range.
Applications
Notebook Computer Displays LCD TVs
LCD Monitors Automotive Displays
Features
♦Synchronized to Resonant Frequency
Longer Lamp Life
Guaranteed Striking Capability
High Power-to-Light Efficiency
♦Wide Input Voltage Range (4.6V to 28V)
♦Feed Forward for Excellent Line Rejection
♦SMBus Dimming Control Interface
♦10:1 Dimming Range
♦Guaranteed 200Hz to 220Hz DPWM Frequency
♦Secondary Voltage Limit Reduces Transformer
Stress
♦Adjustable Lamp-Out Protection with 1s Timer
♦Secondary Current Limit Protects Against High-
Voltage Short Circuits to Ground
♦Small, 5mm x 5mm, Thin QFN Package
MAX8709B
High-Efficiency CCFL Backlight
Controller with SMBus Interface
________________________________________________________________ Maxim Integrated Products 1
Ordering Information
SCL
SDA
SUS
CCI
CCV
BATT
GND
LOT
REF
ILIM
VCC
VDD
BST2
BST1
GH1
LX1
LX2
GL1
PGND
GL2
GH2
VFB
ISEC
IFB
MAX8709B
VIN
Minimal Operating Circuit
19-0768; Rev 0; 3/07
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.
PART TEMP RANGE PIN-PACKAGE
MAX8709BETI -40°C to +85°C 28 Thin QFN 5mm x 5mm
SMBus is a trademark of Intel Corp.
Pin Configuration appears at end of data sheet.
MAX8709B
High-Efficiency CCFL Backlight
Controller with SMBus Interface
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(Circuit of Figure 1. VBATT = 12V, VLOT = VREF, VCC = VDD, VSUS = 5.3V, TA= 0°C to +85°C. Typical values are at TA= +25°C,
unless otherwise noted.)
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.
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
GH1 to LX1 ..............................................-0.3V to (VBST1 + 0.3V)
GH2 to LX2 ..............................................-0.3V to (VBST2 + 0.3V)
VCC, VDD to GND .....................................................-0.3V to +6V
REF, ILIM to GND .......................................-0.3V to (VCC + 0.3V)
GL1, GL2 to GND .......................................-0.3V to (VDD + 0.3V)
CCI, CCV, LOT to GND ............................................-0.3V to +6V
IFB, ISEC, VFB to GND................................................-6V to +6V
SDA, SCL, SUS to GND............................................-0.3V to +6V
PGND to GND .......................................................-0.3V to +0.3V
Continuous Power Dissipation (TA= +70°C)
28-Pin Thin QFN (derate 20.84mW/°C above +70°C) ..1667mW
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature......................................................+150°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
PARAMETER CONDITIONS MIN TYP MAX UNITS
VCC = VDD = VBATT 4.6 5.5
VBATT Input Voltage Range VCC = VDD = open 5.5 28.0 V
VBATT = 28V 1.5 3
VBATT Quiescent Current VSUS = 5.5V VBATT = VCC = 5V 3 mA
VBATT Quiescent Current, Shutdown SUS = GND 6 20 µA
VCC Output Voltage, Normal Operation VSUS = 5.5V, 6V < VBATT < 28V,
0 < ILOAD < 20mA 5.0 5.35 5.5 V
VCC Output Voltage, Shutdown SUS = GND, no load 3.5 4.6 5.5 V
VCC rising (leaving lockout) 4.5
VCC Undervoltage-Lockout Threshold VCC falling (entering lockout) 4.0 V
VCC Undervoltage-Lockout Hysteresis 200 mV
VCC Power-On Reset (POR) Threshold Rising edge 0.90 1.75 2.70 V
VCC POR Hysteresis 50 mV
REF Output Voltage, Normal Operation 4.5V < VCC < 5.5V, ILOAD = 40µA 1.96 2.00 2.04 V
GH1, GH2, GL1, GL2 On-Resistance ITEST = 100mA, VCC = VDD = 5.3V 9 18 Ω
GH1, GH2, GL1, GL2 Output Current 0.5 A
BST1, BST2 Leakage Current VBST_ = 12V, VLX_ = 7V 5 µA
Input Resonant Frequency Guaranteed by design 25 300 kHz
Minimum Off-Time 180 280 380 ns
Maximum Off-Time 18 28 38 µs
Current-Limit Threshold
LX1 - GND, LX2 - GND (Fixed) ILIM = VCC 180 200 220 mV
VILIM = 0.5V 80 100 120
Current-Limit Threshold
LX1 - GND, LX2 - GND (Adjustable) VILIM = 2.0V 370 400 430 mV
Minimum Current Threshold
LX1 - GND, LX2 - GND 6mV
MAX8709B
High-Efficiency CCFL Backlight
Controller with SMBus Interface
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1. VBATT = 12V, VLOT = VREF, VCC = VDD, VSUS = 5.3V, TA= 0°C to +85°C. Typical values are at TA= +25°C,
unless otherwise noted.)
PARAMETER CONDITIONS MIN TYP MAX UNITS
LOT Input Voltage Range 0.5 VREF V
LOT Input Bias Current -2 +2 µA
IFB Input Voltage Range -1.7 +1.7 V
IFB Regulation Point 380 400 420 mV
IFB Input Bias Current VIFB = 0.4V -2 +2 µA
IFB Lamp-Out Threshold LOT = REF 500 600 700 mV
IFB to CCI Transconductance 1V < VCCI < 2.5V 100 µS
CCI Output Impedance 20 MΩ
ISEC Input Voltage Range -2 +2 V
ISEC Regulation Threshold 1.20 1.25 1.30 V
ISEC Input Bias Current VISEC = 1.25V -2 +2 µA
VFB Input Voltage Range -2 +2 V
VFB Input Bias Current VVFB = 0.5V -0.5 +0.5 µA
VFB Regulation Point 490 510 530 mV
VFB to CCV Transconductance 1V < VCCV < 2.7V 40 µS
VFB Zero-Voltage Crossing Threshold -10 +10 mV
CCV Output Impedance 20 MΩ
DPWM Chopping Frequency 204 210 216 Hz
Lamp-Out Detection Timeout Timer VIFB < 0.1V (Note 1) 1.14 1.22 1.30 s
SDA, SCL, SUS Input Low Voltage 0.8 V
SDA, SCL, SUS Input High Voltage 2.1 V
SDA, SCL, SUS Input Hysteresis 300 mV
SDA, SCL, SUS Input Bias Current -1 +1 µA
SDA Output Low Sink Current VSDA = 0.4V 4 mA
SCL Serial Clock High Period THIGH 4µs
SCL Serial Clock Low Period TLOW 4.7 µs
START Condition Setup Time tSU:STA 4.7 µs
START Condition Hold Time tHD:STA 4µs
SDA Valid to SCL Rising-Edge Setup Time,
Slave Clocking-In Data tSU:DAT 250 ns
SCL Falling Edge to SDA Transition tHD:DAT 0ns
SCL Falling Edge to SDA Valid,
Reading Out Data TDV 700 ns
MAX8709B
High-Efficiency CCFL Backlight
Controller with SMBus Interface
4 _______________________________________________________________________________________
ELECTRICAL CHARACTERISTICS
(Circuit of Figure 1. VBATT = 12V, VLOT = VREF, VCC = VDD, VSUS = 5.3V, TA= -40°C to +85°C. Typical values are at TA= +25°C,
unless otherwise noted.) (Note 2)
PARAMETER CONDITIONS MIN TYP MAX UNITS
VCC = VDD = VBATT 4.6 5.5
VBATT Input Voltage Range VCC = VDD = open 5.5 28.0 V
VBATT = 28V 3
VBATT Quiescent Current VSUS = 5.5V VBATT = VCC = 5V 3 mA
VBATT Quiescent Current, Shutdown SUS = GND 20 µA
VCC Output Voltage, Normal Operation VSUS = 5.5V, 6V < VBATT < 28V,
0 < ILOAD < 20mA 5.0 5.5 V
VCC Output Voltage, Shutdown SUS = GND, no load 3.5 5.5 V
VCC rising (leaving lockout) 4.5
VCC Undervoltage-Lockout Threshold VCC falling (entering lockout) 4.0 V
VCC Power-On Reset (POR) Threshold Rising edge 0.90 2.70 V
REF Output Voltage, Normal Operation 4.5V < VCC < 5.5V, ILOAD = 40µA 1.95 2.05 V
GH1, GH2, GL1, GL2 On-Resistance ITEST = 100mA, VCC = VDD = 5.3V 18 Ω
BST1, BST2 Leakage Current VBST_ = 12V, VLX_ = 7V 5 µA
Input Resonant Frequency Guaranteed by design 25 300 kHz
Minimum Off-Time 180 380 ns
Maximum Off-Time 18 38 µs
Current-Limit Threshold
LX1 - GND, LX2 - GND (Fixed) ILIM = VCC 180 220 mV
VILIM = 0.5V 80 120
Current-Limit Threshold
LX1 - GND, LX2 - GND (Adjustable) VILIM = 2.0V 370 430 mV
Current-Limit Leading-Edge Blanking 250 450 ns
LOT Input Voltage Range 0.5 VREF V
LOT Input Bias Current -2 +2 µA
IFB Input Voltage Range -1.7 +1.7 V
IFB Regulation Point 380 420 mV
IFB Input Bias Current VIFB = 0.4V -2 +2 µA
IFB Lamp-Out Threshold LOT = REF 500 700 mV
ISEC Input Voltage Range -2 +2 V
ISEC Regulation Point 1.20 1.30 V
ISEC Input Bias Current VISEC = 1.25V -2 +2 µA
VFB Input Voltage Range -2 +2 V
VFB Input Bias Current VVFB = 0.5V -0.5 +0.5 µA
VFB Regulation Point 490 530 mV
VFB Zero-Voltage Crossing Threshold -10 +10 mV
DPWM Chopping Frequency 204 216 Hz
MAX8709B
High-Efficiency CCFL Backlight
Controller with SMBus Interface
_______________________________________________________________________________________ 5
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1. VBATT = 12V, VLOT = VREF, VCC = VDD, VSUS = 5.3V, TA= -40°C to +85°C. Typical values are at TA= +25°C,
unless otherwise noted.) (Note 2)
PARAMETER CONDITIONS MIN TYP MAX UNITS
Lamp-Out Detection Timeout Timer VIFB < 0.1V (Note 1) 1.14 1.30 s
SDA, SCL, SUS Input Low Voltage 0.8 V
SDA, SCL, SUS Input High Voltage 2.1 V
SDA, SCL, SUS Input Bias Current -1 +1 µA
SDA Output Low Sink Current VSDA = 0.4V 4 mA
SCL Serial Clock High Period THIGH 4µs
SCL Serial Clock Low Period TLOW 4.7 µs
START Condition Setup Time tSU:STA 4.7 µs
START Condition Hold Time tHD:STA 4µs
SDA Valid to SCL Rising-Edge Setup Time,
Slave Clocking-In Data tSU:DAT 250 ns
SCL Falling Edge to SDA Transition tHD:DAT 0ns
Note 1: Corresponds to 256 DPWM cycles.
Note 2: Specifications to -40°C are guaranteed by design based on final characterization results.
Typical Operating Characteristics
(Circuit of Figure 1. VBATT = 12V, VLOT = VREF, VCC = VDD, VSUS = 5.3V, TA= +25°C, unless otherwise noted.)
LOW INPUT-VOLTAGE
OPERATION (VBATT = 8V)
MAX8709 toc01
A: VIFB, 2V/div
B: VVFB, 2V/div
C: VLX1, 10V/div
D: VLX2, 10V/div
10μs/div
0V
0V
0V A
B
C
D
0V
HIGH INPUT-VOLTAGE
OPERATION (VBATT = 20V)
MAX8709 toc02
A: VIFB, 2V/div
B: VVFB, 2V/div
C: VLX1, 10V/div
D: VLX2, 10V/div
10μs/div
0V
0V
0V A
B
C
D
0V
LINE-TRANSIENT RESPONSE
MAX8709 toc03
A: VBATT, 5V/div
B: VIFB, 2V/div
C: VVFB, 2V/div
D: VLX1, 10V/div
40μs/div
0V
0V
8V
A
B
C
D
0V
LAMP-OUT VOLTAGE
LIMITING AND TIMEOUT
MAX8709 toc09
A: VVFB, 1V/div
B: VIFB, 1V/div
0V
0V
A
B
200ms/div
MAX8709B
High-Efficiency CCFL Backlight
Controller with SMBus Interface
6 _______________________________________________________________________________________
Typical Operating Characteristics (continued)
(Circuit of Figure 1. VBATT = 12V, VLOT = VREF, VCC = VDD, VSUS = 5.3V, TA= +25°C, unless otherwise noted.)
STARTUP
MAX8709 toc04
A: VSUS, 5V/div
B: VIFB, 2V/div
C: VVFB, 2V/div
D: VLX1, 10V/div
2ms/div
0V
0V
0V
A
B
C
D
0V
DPWM OPERATION (10%)
MAX8709 toc05
A: VCCV, 200mV/div
B: VIFB, 1V/div
C: VVFB, 1V/div
1ms/div
0V
0V
1.2V
A
B
C
DPWM OPERATION (50%)
MAX8709 toc06
A: VCCV, 200mV/div
B: VIFB, 1V/div
C: VVFB, 1V/div
1ms/div
0V
0V
1.2V
A
B
C
DPWM SOFT-START
MAX8709 toc07
A: VIFB, 1V/div
B: VVFB, 1V/div
40μs/div
0V
0V
1.2V
A
B
CCI
CCV
DPWM SOFT-STOP
MAX8709 toc08
A: VIFB, 1V/div
B: VVFB, 1V/div
40μs/div
0V
0V
A
B
CCI
CCV
MAX8709B
High-Efficiency CCFL Backlight
Controller with SMBus Interface
_______________________________________________________________________________________ 7
SWITCHING FREQUENCY
vs. INPUT VOLTAGE
MAX8709 toc10
INPUT VOLTAGE (V)
SWITCHING FREQUENCY (kHz)
2219161310
50
54
58
62
46
725
DPWM FREQUENCY
vs. INPUT VOLTAGE
MAX8709 toc11
INPUT VOLTAGE (V)
DPWM FREQUENCY (Hz)
2219161310
205
210
215
220
200
725
ELECTRICAL EFFICIENCY
vs. INPUT VOLTAGE
MAX8709 toc12
INPUT VOLTAGE (V)
ELECTRICAL EFFICIENCY (%)
2219161310
60
70
80
90
100
50
725
NORMALIZED RMS LAMP CURRENT
vs. INPUT VOLTAGE
MAX8709 toc13
INPUT VOLTAGE (V)
RMS LAMP-CURRENT ERROR (%)
221910 13 16
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
-0.8
725
-0.15
-0.10
0
-0.05
0.05
0.10
04020 60 80 100
REF LOAD REGULATION
MAX8709 toc14
REF LOAD CURRENT (μA)
REF VOLTAGE ERROR (%)
0
20
60
40
80
100
08124 1620242832
NORMALIZED BRIGHTNESS
vs. BRIGHTNESS CODE
MAX8709 toc15
BRIGHTNESS CODE
NORMALIZED BRIGHTNESS (%)
NORMALIZED VCC LINE REGULATION
MAX8709 toc16
INPUT VOLTAGE (V)
VCC VOLTAGE ERROR (%)
201510
-0.8
-0.6
-0.4
-0.2
0
0.2
-1.0
525
VCC = 5.3V
VCC LOAD REGULATION
MAX8709 toc17
VCC VOLTAGE ERROR (%)
161284
-1.2
-0.9
-0.6
-0.3
0
-1.5
020
EXTERNAL LOAD CURRENT (mA)
Typical Operating Characteristics (continued)
(Circuit of Figure 1. VBATT = 12V, VLOT = VREF, VCC = VDD, VSUS = 5.3V, TA= +25°C, unless otherwise noted.)
REF OUTPUT vs. TEMPERATURE
MAX8709 toc18
TEMPERATURE (°C)
REF VOLTAGE ERROR (%)
806040200-20
-0.20
-0.15
-0.10
-0.05
0
0.05
-0.25
-40 100
MAX8709B
High-Efficiency CCFL Backlight
Controller with SMBus Interface
8 _______________________________________________________________________________________
Pin Description
PIN NAME FUNCTION
1 ILIM
Current-Limit Threshold Adjustment. Connect a resistive voltage-divider between REF or VCC and GND.
The current-limit threshold measured between LX_ and GND is 1/5 the voltage forced at ILIM. The ILIM
adjustment range is 0 to 3V. Connect ILIM to VCC to select the default current-limit threshold of 0.2V.
2 REF 2V Reference Output. Bypass REF to GND with a 0.1µF ceramic capacitor. REF is discharged to GND
during shutdown.
3 LOT Lamp-Out Threshold Adjustment. The lamp-out threshold is 30% of the voltage at LOT. The LOT
adjustment range is from 0.5V to VREF.
4 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.
5 ISEC Secondary Current-Limit Sense Input. The secondary current limit controls the transformer secondary
current even if the IFB sense resistor is shorted. See the Secondary Current Limit (ISEC) section.
6 SDA SMBus Serial Data Input
7 SCL SMBus Serial Clock Input
8 SUS SMBus Suspend Input
9, 10, 11, 23 N.C. No Connection. Not internally connected.
12 VDD Gate-Driver Supply Input. Connect VDD to VCC, the output of the linear regulator. Bypass VDD with a
0.1µF capacitor to PGND.
13 PGND Power Ground. Gate-driver current flows through this pin.
14 GL2 Low-Side MOSFET NL2 Gate-Driver Output
15 GL1 Low-Side MOSFET NL1 Gate-Driver Output
16 GH1 High-Side MOSFET NH1 Gate-Driver Output
17 LX1 Switching Node Connection. LX1 is the internal gate driver’s (GH1’s) source connection for the high-side
MOSFET NH1. LX1 is also the sense input to the current comparators.
18 BST1 Driver Bootstrap Input for High-Side MOSFET NH1. Connect BST1 through a diode to VDD and through a
0.1µF capacitor to LX1 (Figure 1).
19 BST2 Driver Bootstrap Input for High-Side MOSFET NH2. Connect BST2 through a diode to VDD and through a
0.1µF capacitor to LX2 (Figure 1).
20 LX2 Switching Node Connection. LX2 is the internal gate driver’s (GH2’s) source connection for the high-side
MOSFET NH2. LX2 is also the sense input to the current comparators.
21 GH2 High-Side MOSFET NH2 Gate-Driver Output
22 VFB
Lamp Output Feedback Sense Input. The average value on VFB is regulated during startup and open-
lamp conditions to 0.5V by controlling the on-time of high-side switches. A capacitive voltage-divider
between the CCFL lamp output and GND is sensed to set the maximum average lamp output voltage.
24 IFB Lamp Current-Sense Input. The voltage on IFB is used to regulate the lamp current. If the IFB input falls
below 30% of the LOT voltage for 1.22s, then the MAX8709B activates the lamp-out fault latch.
25 CCI
Current-Loop Compensation Pin. CCI is the output of the current-loop transconductance amplifier (GMI)
that regulates the CCFL current. The CCI voltage controls the time interval during which the full bridge
applies the input voltage (BATT) to the transformer primary. Connect CCI to GND through a 0.1µF
capacitor. CCI is internally discharged to GND in shutdown.
MAX8709B
High-Efficiency CCFL Backlight
Controller with SMBus Interface
_______________________________________________________________________________________ 9
Pin Description (continued)
PIN NAME FUNCTION
26 CCV
Voltage-Loop Compensation Pin. CCV is the output of the voltage-loop transconductance amplifier (GMV)
that regulates the maximum average secondary transformer voltage. The CCV voltage controls the time
interval during which the full bridge applies the input voltage (BATT) to the transformer primary. The CCV
capacitor also sets the rise time and fall time of the lamp current in DPWM. Connect CCV to GND with a
6.8nF capacitor. CCV is internally discharged to GND in shutdown.
27 BATT MAX8709B Supply Input. Input to the internal 5.3V linear regulator (VCC) that provides power to the
device. Bypass BATT to GND with a 0.1µF capacitor.
28 VCC 5.3V Linear-Regulator Output. VCC is the supply voltage for the MAX8709B. Bypass VCC to GND with a
0.47µF ceramic capacitor. VCC can also be connected to BATT if VBATT < 5.5V.
VIN
7V TO 24V
C8
0.1μF
R4
100kΩ
C9
0.1μF
R5
100kΩ
C10
0.01μF
C11
0.1μF
SMBSUS
SMBDATA
SMBCLK
SCL
SDA
SUS
CCI
CCV
BATT
GND
LOT
REF
ILIM
VCC
VDD
BST2
BST1
GH1
LX1
LX2
GL1
PGND
GL2
GH2
VFB
ISEC
IFB
MAX8709B
D1
C7
0.47μF
C5
0.1μF
C6
0.1μF
C1
4.7μF
25V
NH1
NL1
NH2
NL2
C2
1μFT1
1:93 CCFL
C3
15pF
3kV
R3
40.2Ω
1%
C4
22nF
R2
2kΩ
R1
150Ω
1%
Figure 1. Typical Operating Circuit of the MAX8709B
MAX8709B
High-Efficiency CCFL Backlight
Controller with SMBus Interface
10 ______________________________________________________________________________________
SMBus
INTERFACE
BRIGHTNESS
DAC
DPWM
OSC
RAMP
GENERATOR
PEAK
DETECTOR
SUPPLY
CONTROL
LOGIC
MUX
LAMP-OUT
COMP
GMV
GMI
CCV
CLAMP
PK_DET
CLAMP
IMIN
COMP
IMAX
COMP
LX1
LX2
0.5V
0.4V
6mV
400μA
SEC OC
COMP
PWM
COMP
DPWM
COMP
1.25V
MAX8709B
SUS
SDA
SCL
LOT
REF
CCV
CCI
ILIM
REF
IFB
VFB
BATT
GND
BST1
GH1
LX1
BST2
GH2
LX2
GL1
VDD
GL2
PGND
ISEC
REF
VCC
VIN
CCFL
Figure 2. MAX8709B Functional Diagram
MAX8709B
High-Efficiency CCFL Backlight
Controller with SMBus Interface
______________________________________________________________________________________ 11
Typical Operating Circuit
The Typical Operating Circuit of the MAX8709B (Figure 1)
is a complete CCFL backlight inverter for notebook TFT
LCD panels. The circuit works over an input voltage
range of 7V to 24V with an RMS lamp current of 6mA.
The circuit’s maximum RMS open-lamp voltage is limit-
ed to 1600V. Table 1 lists recommended component
options, and Table 2 lists the component suppliers’
contact information.
Detailed Description
The MAX8709B controls a full-bridge resonant inverter to
convert an unregulated DC input into a near-sinusoidal
AC output for powering CCFLs. The lamp brightness is
adjusted by turning the lamp on and off with an internal
DPWM signal. The duty cycle of the DPWM signal is set
through an SMBus-compatible 2-wire serial interface.
Figure 2 shows the functional diagram of the MAX8709B.
Resonant Operation
The MAX8709B 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 cur-
rent flows through MOSFET NH1, DC blocking cap 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 pri-
mary current flows through NL1, C2, the primary side of
T1, and NL2. Once the primary current drops to the mini-
mum 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.
Table 1. Component List
DESIGNATION DESCRIPTION
C1
4.7µF ±20%, 25V X5R
ceramic capacitor (1210)
Murata GRM32RR61E475K
Taiyo Yuden TMK325BJ475MN
TDK C3225X7R1E475M
C2
1µF ±10%, 25V X7R
ceramic capacitor (1206)
Murata GRM31MR71E105K
Taiyo Yuden TMK316BJ105KL
TDK C3216X7R1E105K
C3
15pF ±1pF, 3kV high-voltage
ceramic capacitor (1808)
Murata GRM42D1X3F150J
TDK C4520C0G3F150F
C4
0.022µF ±10%, 16V X7R
ceramic capacitor (0402)
Murata GRP155R71C223K
Taiyo Yuden EMK105BJ223KV
TDK C1005X7R1C223K
C5, C6, C8, C9
0.1µF ±10%, 25V X7R
ceramic capacitors (0603)
Murata GRM188R71E104K
Taiyo Yuden TMK107BJ104KA
TDK C1608X7R1E104K
DESIGNATION DESCRIPTION
C7
0.47µF ±10%, 10V X5R
ceramic capacitor (0603)
Taiyo Yuden LMK107BJ474KA
TDK C1608X5R1A474K
D1
Dual silicon switching diode,
common anode (SOT-323)
Central Semiconductor CMSD2836
Diodes Incorporated BAW56W
NH1/2, NL1/2
30V, 0.095 dual n-channel MOSFETs
(6-pin SOT23)
Fairchild FDC6561AN
R1 150Ω ±1% resistor (0603)
R2 2kΩ ±5% resistor (0603)
R3 39Ω ±1% resistor (0603)
R4, R5 100kΩ ±5% resistors (0603)
T1
CCFL transformer, 1:93 turns ratio
Sumida 5371-400-W1423
TOKO T912MG-1018
Table 2. Component Suppliers
SUPPLIER WEBSITE
Central Semiconductor www.centralsemi.com
Fairchild Semiconductor www.fairchildsemi.com
Murata www.murata.com
Sumida www.sumida.com
Taiyo Yuden www.t-yuden.com
TDK www.components.tdk.com
MAX8709B
High-Efficiency CCFL Backlight
Controller with SMBus Interface
12 ______________________________________________________________________________________
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 cur-
rent 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 volt-
age 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 conditions, which reduce transient power losses
and EMI.
The 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 reso-
nant tank circuit can be further simplified to Figure 4(b)
by removing the transformer. CSis the primary series
capacitor, C’Sis the series capacitance reflected to
the secondary, CPis the secondary parallel capacitor,
N is the transformer turns ratio, L is the transformer
secondary leakage inductance, and RLis an idealized
resistance that models the CCFL in normal operation.
T1
C2
VBATT
(a)
NH1
ON
NL1
OFF
NH2
OFF
NL2
ON
LX2LX1
T1
C2
VBATT
(b)
NH1
OFF
NL1
ON
NH2
OFF
NL2
ON
LX2LX1
T1
C2
VBATT
(c)
NH1
OFF
NL1
ON
NH2
ON
NL2
OFF
LX2LX1
T1
C2
VBATT
(d)
NH1
OFF
NL1
ON
NH2
OFF
NL2
ON
LX2LX1
(BODY DIODE TURNS ON FIRST) (BODY DIODE TURNS ON FIRST)
Figure 3. Resonant Operation
MAX8709B
High-Efficiency CCFL Backlight
Controller with SMBus Interface
______________________________________________________________________________________ 13
Figure 5 shows the frequency response of the resonant
tank’s voltage gain under different load conditions. The
primary series capacitor is 1µF, the secondary parallel
capacitor is 15pF, the transformer turns ratio is 1:93,
and the secondary leakage inductance is 260mH.
Notice there are two peaks, fSand fP, in the frequency
response. The first peak, fS, is the series resonant peak
determined by the reflected series capacitor and the
secondary leakage inductance:
The second peak, fP, is the parallel resonant peak deter-
mined by the reflected series capacitor, the parallel
capacitor, and the secondary leakage inductance:
These two frequencies set the lower and upper bound-
aries of resonant operation. When the lamp is off, the
operating point of the resonant tank is close to the paral-
lel resonant peak due to the infinite lamp impedance.
The circuit displays the characteristics of a parallel-
loaded resonant converter, acting like a voltage source
to generate the necessary striking voltage. Theoretically,
the output voltage of the resonant converter keeps
going until the lamp is ionized.
Once the lamp is ionized, the equivalent load resistance
decreases rapidly and the operating point moves toward
the series resonant peak. The series resonant operation
causes the circuit to behave like a current source.
Current and Voltage Control Loops
(CCI, CCV)
The MAX8709B uses a current loop and a voltage loop
to control the power delivered to the CCFL. The current
loop is the dominant loop in regulating the lamp cur-
rent. The voltage loop limits the transformer secondary
voltage and is active during startup, the DPWM off-
time, and open-lamp fault.
Both the current and the voltage loops use transcon-
ductance error amplifiers for regulation. The AC lamp
current is measured with a sense resistor in series with
the CCFL. The voltage across this resistor is applied to
the IFB input and is internally half-wave rectified. The
current-loop transconductance error amplifier com-
pares the rectified IFB voltage with a 400mV internal
threshold to create an error current. The error current
charges and discharges a capacitor connected
between CCI and ground to generate an error voltage
VCCI. Similarly, the AC voltage across the transformer
secondary winding is measured through a capacitive
voltage-divider. The sense voltage is applied to the
VFB input and is internally half-wave rectified. The volt-
age-loop transconductance error amplifier compares
the rectified VFB voltage with a 500mV internal thresh-
old to create an error current. The error current charges
f
LCC
CC
P
SP
SP
'
'
=
+
1
2π
f
LC
S
S
=1
2π'
AC
SOURCE CCFL
CP
L
CS1:N
(a)
AC
SOURCE RL
CP
L
C'S =
(b)
CS
N2
Figure 4. Equivalent Resonant Tank Circuit
FREQUENCY (kHz)
VOLTAGE GAIN (V/V)
80604020
1
2
3
4
0
0 100
RL INCREASING
Figure 5. Frequency Response of the Resonant Tank
MAX8709B
High-Efficiency CCFL Backlight
Controller with SMBus Interface
14 ______________________________________________________________________________________
and discharges a capacitor connected between CCV
and ground to generate an error voltage VCCV. The
lower of VCCI and VCCV takes control and is compared
with an internal ramp signal to set the high-side
MOSFET switch on-time (tON).
Lamp Startup
A CCFL is a gas discharge lamp that is normally driven
in the avalanche mode. To start ionization in a nonion-
ized lamp, the applied voltage (striking voltage) must
be increased to the level required for the start of
avalanche. The striking voltage can be several times
the typical operating voltage.
Because of the resonant topology, the striking voltage
is guaranteed regardless of the temperature. 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 cir-
cuit has a high Q, it is easy to generate high voltages
across the lamp.
Operation during startup differs from the steady-state
condition described in the Current and Voltage Control
Loops section. Upon power-up, VCCI slowly rises,
increasing the duty cycle, which provides soft-start.
During this time, VCCV is limited to 150mV above VCCI.
Once the secondary voltage reaches the strike voltage,
the lamp current begins to increase. When the lamp
current reaches the regulation point, VCCI exceeds
VCCV and it reaches steady state.
Feed-Forward Control and
Dropout Operation
The MAX8709B is designed to maintain tight control of
the transformer secondary under all transient condi-
tions including dropout. The feed-forward control
instantaneously adjusts the tON time for changes in
input voltage (VBATT). This feature provides immunity to
input voltage variations and simplifies loop compensa-
tion over wide input voltage ranges. The feed-forward
control also improves the line regulation for short
DPWM on-times and makes startup transients less
dependent on the input voltage.
Feed-forward control is implemented by increasing the
PWM’s internal voltage ramp rate for higher VBATT. This
has the effect of varying tON as a function of the input
voltage while maintaining about the same signal levels
at VCCI and VCCV. Since the required voltage change
across the compensation capacitors is minimal, the
controller’s response to input voltage changes is
essentially instantaneous.
To maximize run time, it may be desirable to allow the
circuit to operate in dropout if the backlight’s perfor-
mance is not critical. When VBATT is very low, the con-
troller loses current regulation and runs at maximum
duty cycle. Under these circumstances, a transient
overvoltage condition could occur when the AC
adapter is suddenly applied to power the circuit. The
feed-forward circuitry minimizes variations in lamp volt-
age due to such input voltage steps. The regulator also
clamps the voltage on VCCI. These two features togeth-
er ensure that overvoltage transients do not appear on
the transformer when leaving dropout.
The VCCI clamp is unique in that it limits VCCI to the
peak voltage of the PWM ramp. As the circuit reaches
dropout, VCCI approaches the PWM ramp’s peak in
order to reach maximum tON. If VBATT decreases fur-
ther, the control loop loses regulation and VCCI tries to
reach its positive supply rail. The clamp on VCCI pre-
vents this from happening and VCCI rides just above
the PWM ramp’s peak. If VBATT continues to decrease,
the feed-forward control reduces the amplitude of the
PWM ramp and the clamp pulls VCCI down. When
VBATT suddenly steps out of dropout, VCCI is still low
and maintains the drive on the transformer at the old
dropout level. The control loop then slowly corrects and
increases VCCI to bring the circuit back into regulation.
DPWM Dimming Control
The MAX8709B controls the brightness of the CCFL by
“chopping” the lamp current on and off using an internal
DPWM signal. The frequency of the DPWM signal is
210Hz. The brightness code set through the SMBus inter-
face determines the duty cycle of the DPWM signal. A
brightness code of 0b00000 corresponds to a 12.5%
duty cycle for the MAX8709B. A brightness code of
0b11111 corresponds to a 100% DPWM duty cycle. The
duty cycle changes by 3.125% per step. Codes 0b00000
to 0b00011 all produce 12.5% for the MAX8709B.
In DPWM operation, the CCI and CCV control loops work
together to regulate the lamp current, limit the secondary
voltage, and control the rising and falling of the lamp cur-
rent. During the DPWM off-cycle, the output of the volt-
age-loop error amplifier (CCV) is set to 1.15V and the
current-loop error-amplifier output (CCI) is high imped-
ance. The high-impedance output acts like a sample-
and-hold circuit to keep VCCI from changing during the
off-cycles. At the beginning of the DPWM on-cycle, VCCV
linearly rises, gradually increasing tON, which provides
soft-start. Once VCCV exceeds VCCI, the current-loop
error amplifier takes control and starts to regulate the
lamp current. In the meantime, VCCV continues to rise
and is limited to 150mV above VCCI. At the end of the
DPWM on-cycle, the CCV capacitor discharges linearly,
gradually decreasing tON and providing soft-stop.
MAX8709B
High-Efficiency CCFL Backlight
Controller with SMBus Interface
______________________________________________________________________________________ 15
POR and UVLO
The MAX8709B includes power-on-reset (POR) and
undervoltage-lockout (UVLO) circuits. The POR resets all
internal registers such as DAC outputs, fault latches,
and all SMBus registers. POR occurs when VCC is below
1.5V. The SMBus input logic thresholds are only guaran-
teed to meet electrical characteristic limits for VCC as
low as 3.5V, but the interface continues to function down
to the POR threshold.
The UVLO is activated and disables both high-side and
low-side switch drivers when VCC is below 4.2V (typ).
Low-Power Shutdown (SUS)
When the MAX8709B is placed in shutdown, all functions
of the IC are turned off except for the 5.3V linear regulator
that powers all internal registers and the SMBus inter-
face. The SMBus interface is accessible in shutdown. In
shutdown, the linear-regulator output voltage drops to
about 4.5V and the supply current is 6µA (typ), which is
the required power to maintain all internal register states.
While in shutdown, lamp-out detection and short-circuit
detection latches are reset. The device can be placed
into shutdown either by writing to the shutdown-mode
register or pulling SUS low.
Lamp-Out Protection
For safety, the MAX8709B monitors the lamp-current
feedback (IFB) to detect faulty or open CCFL tubes and
secondary short circuits in the lamp and IFB sense
resistor. If the voltage on IFB is continuously below 30%
of the LOT voltage for greater than 1.22s (typ), the
MAX8709B latches off the full bridge. Unlike the normal
shutdown mode, the linear-regulator output (VCC)
remains at 5.3V. Toggling SUS or cycling the input
power reactivates the device.
During the 1.22s delay, VCCI slowly rises, increasing
tON in an attempt to maintain lamp current regulation.
As VCCI rises, VCCV rises with it until the secondary
voltage reaches its preset limit. At this point, VCCV
stops and limits the secondary voltage by limiting tON.
Because VCCV is limited to 150mV above VCCI, the volt-
age control loop is able to quickly limit the secondary
voltage. Without this clamping feature, the transformer
voltage overshoots to dangerous levels because VCCV
takes time to slew down from its supply rail.
Primary Overcurrent Protection (ILIM)
The MAX8709B senses 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 reg-
ulator turns off the high-side switch at the opposite side of
the primary to prevent the transformer primary current
from increasing further.
The current-limit threshold can be adjusted using the
ILIM input. Connect a resistive voltage-divider between
REF or VCC and GND with the midpoint connected to
ILIM. The current-limit threshold measured between
LX_ and GND is 1/5 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 leak-
age from the lamp high-voltage terminal to ground, pre-
vents the CCI 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 com-
pared to the ISEC regulation threshold (1.25V typ). Any
time the ISEC voltage exceeds the threshold, a con-
trolled current is drawn from CCI to reduce the on-time
of the bridge’s high-side switches.
Reference Output (REF)
The reference output is nominally 2V, and can source at
least 40µA (see the Typical Operating Characteristics).
Bypass REF with a 0.22µF ceramic capacitor connected
between REF and GND.
Linear-Regulator Output (VCC)
The internal linear regulator steps down the DC input volt-
age to 5.3V (typ). The linear regulator supplies power to
the internal control circuitry of the MAX8709B and can
also be used to power the MOSFET drivers by connect-
ing VCC directly to VDD. The VCC voltage drops to 4.5V in
shutdown.
MAX8709B
High-Efficiency CCFL Backlight
Controller with SMBus Interface
16 ______________________________________________________________________________________
SMBus Interface (SDA, SCL)
The MAX8709B supports an Intel SMBus-compatible 2-
wire digital interface. SDA is the bidirectional data line
and SCL is the clock line of the 2-wire interface corre-
sponding respectively to SMBDATA and SMBCLK lines of
the SMBus. SDA and SCL are Schmidt-triggered inputs
that can accommodate slow edges; however, the rising
and falling edges should still be faster than 1µs and
300ns, respectively. The MAX8709B use the write-byte,
read-byte, and receive-byte protocols (Figure 6). The
SMBus protocols are documented in System Manage-
ment Bus Specification V1.1 and are available at
http://www.SMBus.org/.
The MAX8709B is a slave-only device and responds to
the 7-bit address 0b01011000 (i.e., with the R/Wbit clear
indicating a write, this corresponds to 0x58). The
MAX8709B has three functional registers: a 5-bit bright-
ness register (BRIGHT4–BRIGHT0), a 3-bit shutdown-
mode register (SHMD2–SHMDE0), and a 2-bit status
register (STATUS1–STATUS0). In addition, the device has
three identification (ID) registers: an 8-bit chip ID register,
an 8-bit chip revision register, and an 8-bit manufacturer
ID register.
1B
ACK
1B7 BITS
ADDRESS ACK
1B
WR
8 BITS
DATA
1B
ACK P
8 BITS
SCOMMAND
Write-Byte Format
Receive-Byte Format
SLAVE ADDRESS COMMAND BYTE: SELECTS
WHICH REGISTER YOU ARE
WRITING TO
DATA BYTE: DATA GOES INTO THE
REGISTER SET BY THE COMMAND BYTE
1B
ACK
1B7 BITS
ADDRESS ACK
1B
WR S
1B
ACK
8 BITS
DATA
7 BITS
ADDRESS
1B
RD
1B8 BITS
/// PS COMMAND
SLAVE ADDRESS
SLAVE ADDRESS
COMMAND BYTE: SENDS COMMAND
WITH NO DATA; USUALLY USED FOR
ONE-SHOT COMMAND
COMMAND BYTE: SELECTS
WHICH REGISTER YOU ARE
READING FROM
SLAVE ADDRESS: REPEATED
DUE TO CHANGE IN DATA-
FLOW DIRECTION
DATA BYTE: READS FROM
THE REGISTER SET BY THE
COMMAND BYTE
1B
ACK
7 BITS
ADDRESS
1B
RD
8 BITS
DATA
1B
/// PS
DATA BYTE: READS DATA FROM
THE REGISTER COMMANDED BY
THE LAST READ-BYTE OR WRITE-
BYTE TRANSMISSION; ALSO USED
FOR SMBUS ALERT RESPONSE
RETURN ADDRESS
S = START CONDITION SHADED = SLAVE TRANSMISSION WR = WRITE = 0
P = STOP CONDITION ACK= ACKNOWLEDGED = 0 RD = READ =1
/// = NOT ACKNOWLEDGED = 1
1B
ACK
7 BITS
ADDRESS
1B
WR
8 BITS
COMMAND
1B
ACK PS
Send-Byte Format
Read-Byte Format
Figure 6. SMBus Protocols
MAX8709B
High-Efficiency CCFL Backlight
Controller with SMBus Interface
______________________________________________________________________________________ 17
Communication starts with the master signaling the
beginning of a transmission with a START condition,
which is a high-to-low transition on SDA while SCL is
high. When the master has finished communicating with
the slave, the master issues a STOP condition, which is
a low-to-high transition on SDA while SCL is high. The
bus is then free for another transmission. Figures 7 and
8 show the timing diagrams for signals on the 2-wire
interface. The address byte, command byte, and data
byte are transmitted between the START and STOP con-
ditions. The SDA state is allowed to change only while
SCL is low, except for the START and STOP conditions.
Data is transmitted in 8-bit words and is sampled on the
rising edge of SCL. Nine clock cycles are required to
transfer each byte in or out of the MAX8709B since
either the master or the slave acknowledges the receipt
of the correct byte during the ninth clock. If the
MAX8709B receives the correct slave address followed
by R/W= 0, it expects to receive 1 or 2 bytes of informa-
tion (depending on the protocol). If the device detects a
START or STOP condition prior to clocking in the bytes
of data, it considers this an error condition and disre-
gards all the data.
If the transmission is completed correctly, the registers
are updated immediately after a STOP (or RESTART)
condition. If the MAX8709B receives its correct slave
address followed by R/W= 1, it expects to clock out the
register data selected by the previous command byte.
SMBCLK
AB CD
EFG H
IJK
SMBDATA
tSU:STA tHD:STA
tLOW tHIGH
tSU:DAT tHD:DAT
tHD:DAT tSU:STO tBUF
A = START CONDITION
B = MSB OF ADDRESS CLOCKED INTO SLAVE
C = LSB OF ADDRESS CLOCKED INTO SLAVE
D = R/W BIT CLOCKED INTO SLAVE
E = SLAVE PULLS SMBDATA LINE LOW
LM
F = ACKNOWLEDGE BIT CLOCKED INTO MASTER
G = MSB OF DATA CLOCKED INTO SLAVE
H = LSB OF DATA CLOCKED INTO SLAVE
I = SLAVE PULLS SMBDATA LINE LOW
J = ACKNOWLEDGE CLOCKED INTO MASTER
K = ACKNOWLEDGE CLOCK PULSE
L = STOP CONDITION, DATA EXECUTED BY SLAVE
M = NEW START CONDITION
Figure 7. SMBus Write Timing
SMBCLK
A = START CONDITION
B = MSB OF ADDRESS CLOCKED INTO SLAVE
C = LSB OF ADDRESS CLOCKED INTO SLAVE
D = R/W BIT CLOCKED INTO SLAVE
AB CD
EFG H
IJ
SMBDATA
tSU:STA tHD:STA
tLOW tHIGH
tSU:DAT tHD:DAT tSU:DAT tSU:STO tBUF
K
E = SLAVE PULLS SMBDATA LINE LOW
F = ACKNOWLEDGE BIT CLOCKED INTO MASTER
G = MSB OF DATA CLOCKED INTO MASTER
H = LSB OF DATA CLOCKED INTO MASTER
I = ACKNOWLEDGE CLOCK PULSE
J = STOP CONDITION
K = NEW START CONDITION
Figure 8. SMBus Read Timing
MAX8709B
High-Efficiency CCFL Backlight
Controller with SMBus Interface
18 ______________________________________________________________________________________
SMBus Commands
The MAX8709B registers are accessible through several
different redundant commands (i.e., the command byte
in the read-byte and write-byte protocols), which can
be used to read or write the brightness, SHMD, status,
or ID registers.
Table 3 summarizes the command byte’s register
assignments, as well as each register’s power-on state.
The MAX8709B also supports the receive-byte protocol
for quicker data transfers. This protocol accesses the
register configuration pointed to by the last command
byte. Immediately after power-up, the data byte
returned by the receive-byte protocol is the inverted
contents of the brightness register, left justified
(i.e., BRIGHT4 is in the most-significant-bit position of
the data byte) with the 3 remaining bits containing a
one, STATUS1, and STATUS0. This gives the same
result as using the read-word protocol with
0b10XXXXXX (0xAA and 0xA9) command. Use caution
with the shorter protocols in multimaster systems, since
a second master could overwrite the command byte
without informing the first master. During shutdown, the
serial interface remains fully functional.
Table 3. Commands Description
DATA REGISTER BIT ASSIGNMENT
SMBus
PROTOCOL
COMMAND
BYTE*
POR
STATE BIT 7
(MSB) BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0
(LSB)
Read and
Write
0x01
0b0XXX XX01 0x17 0 0 0 BRIGHT4
(MSB) BRIGHT3 BRIGHT2 BRIGHT1 BRIGHT0
(LSB)
Read and
Write
0x02
0b0XXX XX10 0xF9 STATUS1 STATUS0 1 1 1 SHMD2 SHMD1 SHMD0
Read Only 0x03
0b0XXX XX11 0x0C ChipID7
0
ChipID6
0
ChipID5
0
ChipID4
0
ChipID3
1
ChipID2
1
ChipID1
0
ChipID0
1
Read Only 0x04
0b0XXX XX00 0x00 ChipRev7
0
ChipRev6
0
ChipRev5
0
ChipRev4
0
ChipRev3
0
ChipRev2
0
ChipRev1
0
ChipRev0
0
Read and
Write
0xAA
0b10XX XXX0 0x40 BRIGHT4
(MSB) BRIGHT3 BRIGHT2 BRIGHT1 BRIGHT0
(LSB) 0STATUS1 STATUS0
Read and
Write
0XA9
0b10XX XXX1 0x40 BRIGHT4
(MSB) BRIGHT3 BRIGHT2 BRIGHT1 BRIGHT0
(LSB) 0STATUS1 STATUS0
Read Only 0xFE
0b11XX XXX0 0x4D MfgID7
0
MfgID6
1
MfgID5
0
MfgID4
0
MfgID3
1
MfgID2
1
MfgID1
0
MfgID0
1
Read Only 0xFF
0b11XX XXX1 0x0C ChipID7
0
ChipID6
0
ChipID5
0
ChipID4
0
ChipID3
1
ChipID2
1
ChipID1
0
ChipID0
1
*The hexadecimal command byte shown is recommended for maximum forward compatibility with future products.
X = Don’t care.
MAX8709B
High-Efficiency CCFL Backlight
Controller with SMBus Interface
______________________________________________________________________________________ 19
Brightness Register [BRIGHT4 – BRIGHT0]
(POR = 0b10111)
The 5-bit brightness register corresponds to the 5-bit
brightness code used in dimming control (See the
Dimming Control section). BRIGHT4 - BRIGHT0 =
0b11111 sets minimum brightness and BRIGHT4 -
BRIGHT0 = 0b00000 sets maximum brightness. Note that
the brightness-register polarity of command bytes 0xA9
and 0xAA are inverted from that of command byte 0x01.
Shutdown-Mode Register [SHMD2–SHMD0]
(POR = 0b001)
The 3-bit shutdown-mode register configures the oper-
ation of the device when the SUS pin is toggled as
described in Table 4. The shutdown-mode register can
also be used to directly shut off the CCFL regardless of
the state of SUS (Table 5).
Status Register [STATUS1–STATUS0]
(POR = 0b11)
The status register returns information on fault condi-
tions. If the MAX8709B detects that VIFB does not
exceed 30% of VLOT continuously for 1.22s, the IC
latches STATUS1 to zero. STATUS1 is reset to 1 by tog-
gling SUS or by toggling the input power.
STATUS0 reports 1 as long as no overcurrent condi-
tions are detected. If an overcurrent condition is detect-
ed in any given DPWM period, STATUS0 is cleared for
the duration of the following DPWM period. If an over-
current condition is not detected in any given DPWM
period, STATUS0 is set for the duration of the following
DPWM period. Note that the status-register polarity of
command bytes 0xA9 and 0xAA are inverted from that
of command byte 0x02.
ID Registers
The ID registers return information on the manufacturer
chip ID and the chip revision number. The MAX8709B is
the first-generation advanced CCFL controller and its
ChipRev is 0x00. Reading from MfgID register returns
0x4D, which is the ASCII code for M (for Maxim). The
ChipID register returns 0x0D. Writing to these registers
has no effect.
Table 5. SUS and SHMD Register Truth
Table
SUS SHMD2 SHMD1 SHMD0 OPERATING MODE
0 0 X 0 Operate
0 0 X 1 Shutdown, STATUS1 set
1 0 0 X Operate
1 0 1 X Shutdown, STATUS1 set
X 1 X X Shutdown, STATUS1 set
Table 6. Status-Register Bit Descriptions (Read Only, Writes Have No Effect)
BIT NAME POR
STATE DESCRIPTION
1 STATUS1 1
STATUS1 = 0 (or STATUS1 = 1) means that a lamp-out condition has been detected. The
STATUS1 bit stays clear even after the lamp-out condition has gone away. The only way to set
STATUS1 is to shut off the lamp by programming the shutdown-mode register or by toggling SUS.
0 STATUS0 1
STATUS0 = 0 (or STATUS0 = 1) means that an overcurrent condition was detected during the
previous DPWM period. STATUS0 = 1 means that an overcurrent condition was not detected
during the previous DPWM period.
Table 4. SHMD Register Bit Descriptions
BIT NAME POR
STATE DESCRIPTION
2 SHMD2 0 SHMD2 = 1 forces the lamp off and sets STATUS1. SHMD2 = 0 allows the lamp to operate,
although it may still be shut down by SUS (depending on the state of SHMD1 and SHMD0).
1 SHMD1 0 When SUS = 0, this bit has no effect. SUS = 1 and SHMD1 = 1 forces the lamp off and sets STATUS1.
SUS = 1 and SHMD1 = 0 allows the lamp to operate, although it may still be shut down by the SHMD2 bit.
0 SHMD0 1 When SUS = 1, this bit has no effect. SUS = 0 and SHMD0 = 1 forces the lamp off and sets STATUS1.
SUS = 0 and SHMD0 = 0 allows the lamp to operate, although it may still be shut down by the SHMD2 bit.
MAX8709B
High-Efficiency CCFL Backlight
Controller with SMBus Interface
20 ______________________________________________________________________________________
Table 7. CCFL Specifications
SPECIFICATION SYMBOL UNITS DESCRIPTION
CCFL Minimum
Striking Voltage
(Kick-Off Voltage)
VSTRIKE VRMS
Although CCFLs typically operate at less than 550VRMS, a higher voltage (1000VRMS
and up) is required initially to start the tube. The strike voltage is typically higher at
cold temperatures and at the end of life of the tube. Resonant operation and the
high Q of the resonant tank generate the required strike voltage of the lamp.
CCFL Typical
Operating Voltage
(Lamp Voltage)
VLAMP VRMS
Once a CCFL has been struck, the lamp voltage required to maintain light output
falls to approximately 550VRMS. Short tubes may operate on as little as 250VRMS.
The operating voltage of the CCFL stays relatively constant, even as the tube’s
brightness is varied.
CCFL Operating
Current
(Lamp Current)
ILAMP mARMS The desired RMS AC current through a CCFL is typically 6mARMS. DC current is not
allowed through CCFLs. The sense resistor, R1, sets the lamp current.
CCFL Maximum
Frequency
(Lamp Frequency)
f kHz
The maximum AC-lamp-current frequency. The circuit should be designed to
operate the lamp below this frequency. The MAX8709B is designed to operate
between 20kHz and 100kHz.
Applications Information
To select the correct component values for the
MAX8709B, several CCFL parameters must be specified.
(Table 7).
MOSFETs
The MAX8709B requires four external n-channel power
MOSFETs (NL1, NL2, NH1, and NH2) to form a full-bridge
inverter circuit to drive the transformer primary. The regu-
lator senses the on-state drain-to-source voltage of the
two low-side MOSFETs NL1 and NL2 to detect the trans-
former 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. Select dual logic-level n-channel MOSFETs
with low RDS(ON) to minimize conduction loss for
NL1/NL2 and NH1/NH2. The regulator utilizes the energy
stored in the transformer’s primary leakage inductance to
softly turn on each of 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. ZVS effectively eliminates the instantaneous turn-on
loss of MOSFETs caused by COSS (drain-to-source
capacitance) and parasitic capacitance discharge, and
improves efficiency and reduces switching-related EMI.
Setting the Lamp Current
The MAX8709B senses the lamp current flowing through
a resistor R1 (Figure 1) connected between the low-volt-
age terminal of the lamp and ground. The voltage across
R1 is fed to IFB and is internally rectified. The MAX8709B
controls the desired lamp current by regulating the
average of the half-wave rectified IFB voltage. To set
the RMS lamp current, determine R1 as follows:
where ILAMP(RMS) is the desired RMS lamp current and
400mV is the typical value of the IFB regulation point
specified in the Electrical Characteristics table. To set
the RMS lamp current to 6mA, the value of R1 should
be 148Ω. The closest standard 1% resistors are 147Ω
and 150Ω. The precise shape of the lamp-current
waveform, which is dependent on lamp parasitics, influ-
ences the actual RMS lamp current. Use a true RMS
current meter to make final adjustments to R1.
Setting the Secondary Voltage Limit
The MAX8709B limits the transformer secondary voltage
during lamp striking 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 to 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:
where 510mV is the typical value of the VFB regulation
threshold specified in the Electrical Characteristics
CV
mV C
LAMP RMS MAX
42
510 13=×
×
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟×
()_
π-
RmV
ILAMP RMS
1400
2
=×
×
π
()
MAX8709B
High-Efficiency CCFL Backlight
Controller with SMBus Interface
______________________________________________________________________________________ 21
table. If C3 is 15pF, C4 needs to be 21.2nF to set the
desired maximum RMS secondary voltage to 1600V.
The closest standard value of C4 is 22nF.
The resistor R2 is used to set the VFB DC bias point to
0V. Choose the value of R2 as follows:
where fSW is the nominal resonant operating frequency.
Setting the Secondary Current Limit
The MAX8709B limits the secondary current even if the
IFB sense resistor 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:
where ISEC(RMS)_MAX is the desired maximum RMS
transformer secondary current during fault conditions,
and 1.25V is the typical value of the ISEC regulation
point specified in the Electrical Characteristics table.
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 transformer turns ratio.
The ratio must be high enough to support the CCFL
operating voltage at the minimum supply voltage. The
transformer turns-ratio N can be calculated as follows:
where VLAMP(RMS) is the maximum RMS lamp voltage
in normal operation, and VIN(MIN) is the minimum DC
input voltage.
The next step in the design procedure is to determine
the desired operating frequency range. The MAX8709B
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 opera-
tion, the resonant frequency range must be within the
operating frequency range specified by the CCFL lamp
transformer manufacturers. As discussed in the
Resonant Operation section, the resonant frequency
range is determined by the transformer secondary leak-
age inductance L, the primary series DC-blocking
capacitor C2, and the secondary parallel resonant
capacitor C3. Since it is difficult to control the trans-
former leakage inductance, the resonant tank design
should be based on the existing secondary leakage
inductance of the selected CCFL transformer. The leak-
age inductance values usually have large tolerance
and significant variations among different batches. It is
best to work directly with transformer vendors on leak-
age inductance requirements. The MAX8709B 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:
where fMIN is the minimum operating frequency range.
Parallel capacitor C3 sets the maximum operating fre-
quency, which is also the parallel resonant peak fre-
quency. Choose:
The transformer core saturation also needs to be con-
sidered 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:
where DMAX is the maximum duty cycle (approximately
0.8) of the high-side switches, VIN(MAX) is the maximum
DC input voltage, BSis the saturation flux density of the
core, and S is the minimal cross-section area of the core.
Compensation Design
The CCI capacitor sets the speed of the current loop
that is used during startup, maintaining lamp current
regulation, and during transients caused by changing
the input voltage. The typical CCI value is 0.1µF. Larger
values increase the transient-response delays. Smaller
values speed up transient response, but extremely
small values can cause loop instability.
The CCV capacitor sets the speed of the voltage loop
that affects soft-start and soft-stop during DPWM opera-
tion, and voltage loop stability during startup and open-
lamp conditions. The typical CCV capacitor value is
10nF. Use the smallest value of CCV that gives an
NDV
BSf
MAX IN MAX
SMIN
1>×
××
()
CC
fLCN
MAX
32
42
22 2
≥×××( ) π-
CN
fL
MIN
2
2
22
≤××π
NV
V
LAMP RMS
IN MIN
=×
()
()
. 09
RV
ISEC RMS MAX
3125
2
=×
.
()_
RfC
SW
210
24
=××π
MAX8709B
High-Efficiency CCFL Backlight
Controller with SMBus Interface
22 ______________________________________________________________________________________
acceptable fault transient response and does not cause
excessive ringing at the beginning of a DPWM pulse.
Larger CCV values reduce transient overshoot but can
reduce light output at low-DPWM duty cycles by increasing
the time required to reach the tube strike voltage.
Other Components
The external bootstrap circuits formed by D1 and
C5/C6 in Figure 1 power the high-side MOSFET drivers.
Connect BST1/BST2 through a signal-level silicon
diode to VDD, and bypass it to LX1/LX2 with a 0.1µF
ceramic capacitor.
Layout Guidelines
Careful PC board layout is critical to achieve stable
operation. The high-voltage section and the switching
section of the circuit require particular attention. The
high-voltage sections of the layout need to be well sep-
arated from the control circuit. Most layouts for single-
lamp notebook displays are constrained to the long
and narrow form factor, so this separation occurs natu-
rally. Follow these guidelines for good PC board layout:
1) Keep the high-current paths short and wide, espe-
cially at the ground terminals. This is essential for
stable, jitter-free operation, and high efficiency.
2) Utilize 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 exposed backside pad to the QFN package.
Using separate copper islands for these grounds
may simplify this task. Quiet analog ground is used
for REF, CCV, CCI, and ILIM (if a resistive voltage-
divider is used).
3) Route high-speed switching nodes away from sen-
sitive analog areas (CCI, CCV, REF, VFB, IFB, ISEC,
ILIM). Make all pin-strap control input connections
(ILIM, etc.) to analog ground or VCC 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. With 8-pin
SO MOSFETs, this is best done by routing power
to the MOSFETs from outside using the top copper
layer, while connecting GND and LX inside (under-
neath) the 8-pin SO package.
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-volt-
age traces and the transformer.
7) To the extent possible, high-voltage trace clear-
ance 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 sepa-
rated to prevent arcing. Moving these traces to
opposite sides of the board can be beneficial in
some cases (see Figure 9).
NOTE: DUAL MOSFET N2 IS MOUNTED ON THE BOTTOM SIDE OF THE PC BOARD DIRECTLY UNDER N1.
HIGH-CURRENT PRIMARY CONNECTION HIGH-VOLTAGE SECONDARY CONNECTION
LAMP
N1 N2
T1
C4
C2
D1
R2
C3
Figure 9. High-Voltage Components Layout Example
MAX8709B
High-Efficiency CCFL Backlight
Controller with SMBus Interface
______________________________________________________________________________________ 23
Chip Information
TRANSISTOR COUNT: 7116
PROCESS: BiCMOS
Pin Configuration
MAX8709B
High-Efficiency CCFL Backlight
Controller with SMBus Interface
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.
24 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2007 Maxim Integrated Products is a registered trademark of Maxim Integrated Products, Inc.
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.)
QFN THIN.EPS