MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
General Description
The MAX8759 integrated cold-cathode fluorescent lamp
(CCFL) inverter controller is designed to drive CCFLs
using a full-bridge resonant inverter. The resonant operation ensures reliable striking and provides near-sinusoidal
waveforms over the entire input range. The controller
operates over a wide input-voltage range of 4.5V to
28V with high power to light efficiency. The device also
includes safety features that effectively protect against
single-point fault conditions such as lamp-out, secondary
overvoltage, and secondary short-circuit faults.
The MAX8759 provides accurate lamp-current regulation
(±2.5%) for superior CCFL inverter performance. The lamp
current is adjustable with an external resistor; 10:1 dimming range can be achieved by turning the CCFL on and
off using a digital pulse-width modulation (DPWM) method,
while maintaining the lamp-current constant. The MAX8759
provides three mechanisms for controlling brightness:
2-wire SMBus™-compatible interface, external ambientlight sensor (ALS), or system PWM control. The MAX8759
supports Intel display power-saving technology (DPST) to
maximize battery life. The device includes two lamp-current
feedback input pins that support dual-lamp applications with
a minimum number of external components.
The MAX8759 controls a full-bridge inverter for maximum
efficiency and directly drives four external n-channel
power MOSFETs. An internal 5.35V linear regulator powers the MOSFET drivers and most of the internal circuitry.
The MAX8759 is available in a space-saving, 28-pin, thin
QFN package and operates over a -40°C to +85°C temperature range.
Applications
●● Notebooks
●● LCD Monitors
Features
●● Accurate Dimming Control Using SMBus, PWM
Interface, or Ambient Light Sensor
●● 10:1 Dimming Range with 256-Step Resolution
●● Resonant-Mode Operation
• Longer Lamp Life with Near Sinusoidal Lamp
Current Waveform
• Guaranteed Striking Capability
• High-Power-to-Light Efficiency
●● Wide Input-Voltage Range (4.5V to 28V)
●● Input Feed-Forward for Excellent Line Rejection
●● ±2.5% Lamp-Current Regulation
●● Adjustable 1.5% Accurate DPWM Frequency
●● Dual Lamp-Current Feedback Inputs
●● Comprehensive Fault Protection
• Secondary Voltage Limiting
• Primary Current Limit with Lossless Sensing
• Lamp-Out Protection with Adjustable Timeout
• Secondary Short-Circuit Protection
●● Small 28-Pin, 5mm x 5mm, Thin QFN Package
Minimal Operating Circuit
INPUT VOLTAGE
7.5V TO 24V
27
11
VCC
PINPKG
PACKAGE
CODE
28 Thin QFN-EP*
MAX8759ETI+ -40°C to +85°C
T2855-6
5mm x 5mm
+Denotes a lead-free package.
*EP = Exposed paddle.
Pin Configuration appears at end of data sheet.
VDD
SMB_DATA
SMB_CLOCK
PWM INPUT
2
3
7
BST2
DEL
BST1
VCC
GH1
SDA
LX1
LX2
SCL
GL1
PGND1
PGND2
PWMO
FREQ
GL2
GH2
IFB1
ALS SUPPLY
ALS OUTPUT
5
IFB2
VALS
VFB
ISEC
6
VCC
24
18
17
16
26
20
PWMI
8
9
21
GND
TEMP RANGE
SMBus is a trademark of Intel Corp.
19-3874; Rev 3; 4/14
28
BATT
MAX8759
Ordering Information
PART
1
ALS
COMP
TFLT
19
23
CCFL
22
25
12
13
14
15
10
4
VCC
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
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
FREQ, VCC, VDD to GND........................................-0.3V to +6V
SDA, SCL to GND....................................................-0.3V to +6V
ALS, COMP, PWMI, PWMO,
TFLT, DEL, VALS 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)
IFB1, IFB2, ISEC, VFB to GND..................................-3V to +6V
PGND1, PGND2 to GND......................................-0.3V to +0.3V
Continuous Power Dissipation (TA = +70°C)
28-Pin Thin QFN 5mm x 5mm
(derate 21.3mW/°C above +70°C).............................1702mW
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
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, TA = 0°C to +85°C. Typical values are at TA = +25°C, unless otherwise noted.)
PARAMETER
BATT Input Voltage Range
CONDITIONS
MAX
4.5
5.5
VCC = VDD = open
5.5
28.0
VBATT = 28V
VBATT = VCC = 5V
MAX8759 is enabled
BATT Quiescent Current, Shutdown
MAX8759 is disabled
VCC Output Voltage, Normal Operation
MAX8759 is enabled, 6V < VBATT < 28V,
0 < ILOAD < 10mA
VCC Output Voltage, Shutdown
MAX8759 is disabled, no load
2.5
UNITS
V
mA
100
200
µA
5.2
5.35
5.5
V
3.5
4.3
5.5
V
VCC rising (leaving lockout)
VCC falling (entering lockout)
5
5
4.3
3.7
VCC Undervoltage Lockout Hysteresis
VCC POR Threshold
TYP
VCC = VDD = VBATT
BATT Quiescent Current
VCC Undervoltage Lockout Threshold
MIN
230
Rising edge
VCC POR Hysteresis
V
mV
1.75
V
50
mV
GH1, GH2, GL1, GL2 On-Resistance,
Low State
ITEST = 100mA, VCC = VDD = 5V
3
6
Ω
GH1, GH2, GL1, GL2 On-Resistance,
High State
ITEST = 100mA, VCC = VDD = 5V
10
18
Ω
BST1, BST2 Leakage Current
VBST_ = 12V, VLX_ = 7V
4
10
µA
Resonant Frequency Range
Guaranteed by design
80
kHz
500
700
ns
Minimum On-Time
30
350
Maximum Off-Time
40
60
80
µs
Current-Limit Threshold
LX1 - PGND1, LX2 - PGND2
415
430
445
mV
Zero-Current-Crossing Threshold
LX1 - PGND1, LX2 - PGND2
3
8
13
mV
Current-Limit Leading-Edge Blanking
IFB1, IFB2 Input-Voltage Range
350
-3
ns
+3
V
IFB1 Regulation Point
765
785
805
mV
IFB2 Regulation Point
780
800
820
mV
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Maxim Integrated │ 2
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
Electrical Characteristics (continued)
(Circuit of Figure 1, VBATT = 12V, VCC = VDD, TA = 0°C to +85°C. Typical values are at TA = +25°C, unless otherwise noted.)
PARAMETER
IFB1, IFB2 Input Bias Current
CONDITIONS
0 < VIFB1,2 < 3V
-3
-3V < VIFB1,2 < 0
-230
IFB1, IFB2 Lamp-Out Threshold
IFB1, IFB2 to COMP Transconductance
MIN
0.5V < VCOMP < 4V
COMP Output Impedance
TYP
MAX
+3
UNITS
µA
575
600
625
mV
60
100
160
µS
6
12
24
MΩ
COMP Discharge Current During
Overvoltage or Overcurrent Fault
VVFB = 2.6V or VISEC = 1.5V
500
1000
2000
µA
COMP Discharge Current During
DPWM Off-Time
VCOMP = 1.5V
90
110
130
µA
DPWM Rising-to-Falling Ratio
VIFB1,2 = 0
+3
V
ISEC Input Voltage Range
-3
ISEC Overcurrent Threshold
ISEC Input Bias Current
2.5
1.18
VISEC = 1.25V
VFB Input Voltage Range
1.21
-0.3
-4
1.26
V
+0.3
µA
+4
V
VFB Input Impedance
150
300
450
MΩ
VFB Overvoltage Threshold
2.2
2.3
2.4
V
210
240
280
mV
VFB Undervoltage Threshold
VFB Undervoltage Delay
DPWM Oscillator Frequency
RFREQ = 169kΩ
250
RFREQ = 169kΩ, TA = +25°C to +85°C
207
210
213
RFREQ = 169kΩ
205
210
215
RFREQ = 340kΩ
106
RFREQ = 100kΩ
PWMO Output Impedance
PWMI Input High Voltage
40
PWMI Input Hysteresis
kΩ
0.7
V
V
300
PWMI Input Frequency Range
mV
-0.3
+0.3
µA
5
50
kHz
5
LSB
%
PWMI Full-Range Accuracy
PWMI duty cycle = 100%
98
100
PWMI duty cycle = 50%
48
50
52
PWMI duty cycle = 0%
9.7
10.0
10.3
ALS Full-Adjustment Range
0
1.8
V
5
LSB
-0.1
+0.1
µA
5.50
V
+3
µA
60
Ω
ALS Full-Range Accuracy
ALS Input Bias Current
VALS Output Voltage
MAX8759 is enabled, 6V < VBATT < 28V,
ILOAD = 1mA
VALS Leakage Current
MAX8759 is disabled, VALS = GND
VALS On-Resistance
MAX8759 is enabled
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60
2.1
PWMI Input Bias Current
Hz
343
20
PWMI Input Low Voltage
PWMI Brightness Setting
µs
5.10
5.30
-3
30
Maxim Integrated │ 3
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
Electrical Characteristics (continued)
(Circuit of Figure 1, VBATT = 12V, VCC = VDD, TA = 0°C to +85°C. Typical values are at TA = +25°C, unless otherwise noted.)
PARAMETER
Zero-Crossing Delay
Maximum Zero-Crossing Delay
DEL Disable Threshold
TFLT Charge Current
TFLT Trip Threshold
CONDITIONS
MIN
TYP
MAX
VBATT = 9V, RTHR = 120kW
0
0.15
0.30
VBATT = 12V, RTHR = 120kW
1.50
1.80
2.10
VBATT = 18V, RTHR = 120kW
3.2
3.8
4.4
DEL rising
4.5
µs
V
3.8
VISEC < 1.25V and VIFB < 540mV; VFLT = 2V
0.9
1.0
1.1
VISEC < 1.25V and VIFB > 660mV; VFLT = 2V
-1.5
-1.2
-0.8
VISEC > 1.25V and VIFB > 660mV; VFLT = 2V
115
135
155
Rising edge
3.7
4
4.3
V
0.7
V
SDA, SCL, Input High Voltage
2.1
SDA, SCL, Input Hysteresis
-1
VSDA = 0.4V
SMBus Frequency
µA
V
100
SDA, SCL, Input Bias Current
SMBus Free Time
µs
DEL falling
SDA, SCL, Input Low Voltage
SDA Output Low Sink Current
UNITS
mV
+1
µA
100
kHz
4
mA
10
tBUF
tHIGH
4.7
4
µs
tLOW
tSU:STA
4.7
µs
4.7
µs
4
µs
STOP Condition Setup Time from SCL
tHD:STA
tSU:STO
4
µs
SDA Valid to SCL Rising-Edge Setup
Time, Slave Clocking in Data
tSU:DAT
250
ns
SCL Falling Edge to SDA Transition
tHD:DAT
0
ns
SCL Falling Edge to SDA Valid, Reading
Out Data
tDV
200
ns
SCL Serial Clock High Period
SCL Serial Clock Low Period
START Condition Setup Time
START Condition Hold Time
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1
µs
Maxim Integrated │ 4
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
Electrical Characteristics
(Circuit of Figure 1, VBATT = 12V, VCC = VDD, TA = -40°C to +85°C).(Note 1)
PARAMETER
BATT Input Voltage Range
CONDITIONS
MIN
TYP
MAX
VCC = VDD = VBATT
4.5
5.5
VCC = VDD = open
5.5
28.0
VBATT = 28V
5
VBATT = VCC = 5V
5
UNITS
V
BATT Quiescent Current
MAX8759 is enabled
VCC Output Voltage, Normal Operation
MAX8759 is enabled, 6V < VBATT < 28V,
0 < ILOAD < 10mA
5.2
5.5
V
VCC Output Voltage, Shutdown
MAX8759 is disabled, no load
3.5
5.5
V
VCC Undervoltage Lockout Threshold
VCC rising (leaving lockout)
VCC falling (entering lockout)
4.3
3.7
mA
V
GH1, GH2, GL1, GL2 On-Resistance,
Low State
ITEST = 100mA, VCC = VDD = 5V
6
Ω
GH1, GH2, GL1, GL2 On-Resistance,
High State
ITEST = 100mA, VCC = VDD = 5V
18
Ω
Resonant Frequency Range
Guaranteed by design
30
80
kHz
350
700
ns
Minimum On-Time
Maximum Off-Time
Current-Limit Threshold
LX1 - PGND1, LX2 - PGND2
Zero-Current Crossing Threshold
LX1 - PGND1, LX2 - PGND2
IFB1, IFB2 Input Voltage Range
IFB1 Regulation Point
IFB2 Regulation Point
IFB1, IFB2 Input Bias Current
-3V < VIFB1,2 < 0
IFB1, IFB2 Lamp-Out Threshold
IFB1, IFB2 to COMP Transconductance
0.5V < VCOMP < 4V
COMP Output Impedance
40
80
µs
410
450
mV
3
13
mV
-3
+3
V
760
810
mV
775
825
mV
-230
µA
565
635
mV
60
160
µS
6
25
MΩ
COMP Discharge Current During
Overvoltage or Overcurrent Fault
VVFB = 2.6V or VISEC = 1.5V
500
2000
µA
COMP Discharge Current During
DPWM Off-Time
VCOMP = 1.5V
90
130
µA
-3
+3
V
1.18
1.26
V
ISEC Input Voltage Range
ISEC Overcurrent Threshold
VFB Input Voltage Range
VFB Input Impedance
-4
+4
V
150
450
MΩ
VFB Overvoltage Threshold
2.2
2.4
V
VFB Undervoltage Threshold
210
280
mV
203
217
Hz
20
60
kΩ
DPWM Oscillator Frequency
PWMO Output Impedance
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RFREQ = 169kΩ
Maxim Integrated │ 5
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
Electrical Characteristics (continued)
(Circuit of Figure 1, VBATT = 12V, VCC = VDD, TA = -40°C to +85°C).(Note 1)
PARAMETER
CONDITIONS
MIN
PWMI Input Low Voltage
PWMI Input High Voltage
5
MAX8759 is enabled
TFLT Trip Threshold
%
9.7
10.3
0
1.8
V
5.10
5.50
V
60
Ω
VBATT = 9V, RTHR = 100kΩ
0
0.3
VBATT = 12V, RTHR = 100kΩ
1.50
2.10
VBATT = 16V, RTHR = 100kΩ
3.2
4.4
DEL rising
DEL falling
4.5
3.9
µs
µs
V
VISEC < 1.25V and VIFB < 540mV; VFLT = 2V
0.8
1.2
VISEC < 1.25V and VIFB > 660mV; VFLT = 2V
-1.5
-0.8
VISEC > 1.25V and VIFB > 660mV; VFLT = 2V
115
155
Rising edge
3.7
4.3
V
0.7
V
SDA, SCL, Input Low Voltage
SDA, SCL, Input High Voltage
SDA Output Low-Sink Current
V
52
PWMI duty cycle = 0%
VALS On-Resistance
TFLT Charge Current
V
48
PWMI duty cycle = 50%
MAX8759 is enabled, 6V < VBATT < 28V,
ILOAD = 1mA
DEL Disable Threshold
0.7
kHz
98
VALS Output Voltage
Maximum Zero-Crossing Delay
UNITS
50
PWMI duty cycle = 100%
ALS Full-Adjustment Range
Zero-Crossing Delay
MAX
2.1
PWMI Input Frequency Range
PWMI Brightness Setting
TYP
VSDA = 0.4V
SMBus Frequency
µA
2.1
V
4
mA
10
100
kHz
SMBus Free Time
tBUF
4.7
µs
SCL Serial Clock High Period
tHIGH
4
µs
SCL Serial Clock Low Period
tLOW
4.7
µs
START Condition Setup Time
tSU:STA
4.7
µs
START Condition Hold Time
tHD:STA
4
µs
STOP Condition Setup Time from SCL
tSU:STO
4
µs
SDA Valid to SCL Rising-Edge Setup
Time, Slave Clocking in Data
tSU:DAT
250
ns
SCL Falling Edge to SDA Transition
tHD:DAT
0
ns
SCL Falling Edge to SDA Valid,
Reading Out Data
tDV
200
ns
Note 1: Specifications to -40°C are guaranteed by design, not production tested.
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Maxim Integrated │ 6
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
Typical Operating Characteristics
(Circuit of Figure 1, VIN = 12V, VCC = VDD, TA = +25°C, unless otherwise noted.)
LOW-INPUT VOLTAGE OPERATION
(VIN = 8.0V)
HIGH-INPUT VOLTAGE OPERATION
(VIN = 20.0V)
MAX8759 toc01
A: VFB, 2V/div
B: LX1, 10V/div
10µs/div
LINE TRANSIENT RESPONSE
(8V TO 20V)
MAX8759 toc02
MAX8759 toc03
A
A
B
B
C
C
C
D
D
D
C: LX2, 10V/div
D:IFB, 2V/div
A: VFB, 2V/div
B: LX1, 20V/div
10µs/div
C: LX2, 20V/div
D: IFB, 2V/div
MAX8759 toc05
MAX8759 toc04
B
A: VIN, 10V/div
B: COMP, 2V/div
MINIMUM BRIGHTNESS STARTUP WAVEFORM
(SMBus MODE, BRIGHTNESS REGISTER = 0x00)
LINE TRANSIENT RESPONSE
(20V TO 8V)
A
100µs/div
C: IFB, 2V/div
D: LX1, 20V/div
MINIMUM BRIGHTNESS DPWM OPERATION
(SMBus MODE, BRIGHTNESS REGISTER = 0x00)
MAX8759 toc06
A
A
A
B
B
B
C
C
C
D
A: VIN, 10V/div
B: COMP, 2V/div
100µs/div
C: IFB, 2V/div
D: LX1, 20V/div
A: VFB, 2V/div
B: COMP, 1V/div
50% BRIGHTNESS DPWM OPERATION
(SMBus MODE, BRIGHTNESS REGISTER = 0x80)
MAX8759 toc07
2ms/div
C: IFB, 2V/div
DPWM SOFT-START
A: VFB, 2V/div
B: COMP, 1V/div
2ms/div
C: IFB, 2V/div
DPWM SOFT-STOP
MAX8759 toc08
MAX8759 toc09
A
A
A
B
B
B
C
C
A: VFB, 2V/div
B: COMP, 1V/div
2ms/div
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C: IFB, 2V/div
A: VFB, 2V/div
B: COMP, 1V/div
40µs/div
C: IFB, 2V/div
C
A: VFB, 2V/div
B: COMP, 1V/div
40µs/div
C: IFB, 2V/div
Maxim Integrated │ 7
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN = 12V, VCC = VDD, TA = +25°C, unless otherwise noted.)
OPEN-LAMP VOLTAGE
LIMITING AND TIMEOUT
SECONDARY SHORT-CIRCUIT
PROTECTION AND TIMEOUT
MAX8759 toc011
A
A
B
B
C
C
C: TFLT, 5V/div
DPWM FREQUENCY
vs. RFREQ
250
200
150
100
100
150
200
250
300
ILAMP = 4mA
4
6.2
6.1
5.9
5.8
5.7
5
10
15
20
5.6
25
5
10
15
20
NORMALIZED BRIGHTNESS
vs. SMBus BRIGHTNESS SETTING
NORMALIZED BRIGHTNESS
vs. PWMI DUTY CYCLE
NORMALIZED BRIGHTNESS
vs. ALS VOLTAGE
40
20
20
40
60
80
BRIGHTNESS SETTING (%)
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100
80
60
40
20
0
0
0.2
0.4
0.6
PWMI DUTY RATIO
0.8
1.0
100
25
MAX8759 toc18
MAX8759 toc17
100
NORMALIZED BRIGHTNESS (%)
60
25
6.0
INPUT VOLTAGE (V)
MAX8759 toc16
NORMALIZED BRIGHTNESS (%)
20
RMS LAMP CURRENT (ILAMP = 6mA)
vs. INPUT VOLTAGE
ILAMP = 5mA
5
15
VIN (V)
ILAMP = 6mA
6
10
INPUT VOLTAGE (V)
80
0
5
RFREQ (kΩ)
100
0
40
C: TFLT, 1V/div
ILAMP = 7mA
7
3
350
NORMALIZED BRIGHTNESS (%)
50
50
MAX8759 toc15
8
RMS LAMP CURRENT (mA)
300
50
60
RMS LAMP CURRENT
vs. INPUT VOLTAGE
MAX8759 toc13
350
DPWM FREQUENCY (Hz)
A: ISEC, 2V/div
B: COMP, 1V/div
RMS LAMP CURRENT (mA)
A: VFB, 2V/div
B: COMP, 500mV/div
70
30
2ms/div
MAX8759 toc14
200ms/div
SWITCHING FREQUENCY (kHz)
80
MAX8759 toc12
MAX8759 toc010
SWITCHING FREQUENCY
vs. INPUT VOLTAGE
80
60
40
20
0
0
0.4
0.8
1.2
1.6
2.0
VALS (V)
Maxim Integrated │ 8
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN = 12V, VCC = VDD, TA = +25°C, unless otherwise noted.)
NORMALIZED BRIGHTNESS
vs. SMBus BRIGHTNESS AND PWMI DUTY CYCLE
60
40
20
0
0.8
VALS = 1.8V
SMB = 0xFF
0.2
0.4
0.6
0.8
VALS = 1.8V
0.6
0.4
0.2
SMB = 0x80
0
MAX8759 toc20
80
1.0
MAX8759 toc19
NORMALIZED BRIGHTNESS (%)
100
NORMALIZED BRIGHTNESS
vs. ALS VOLTAGE AND PWMI DUTY CYCLE
0
1.0
VALS = 0.8V
0
0.2
0.4
PWMI DUTY RATIO
0.6
0.8
ALS TRANSIENT RESPONSE
(ALSDEL1 = ALSDEL0 = 0)
VCC LINE REGULATION
MAX8759 toc21
MAX8759 toc22
5.35
A
VCC VOLTAGE (V)
5.34
A: ALS, 1V/div
5.33
5.32
5.31
B
1s/div
5.30
8
12
B: COMP, 1V/div
VIN = 12V
2
4
6
5.30
5.29
5.28
8
LOAD CURRENT (mA)
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MAX8759 toc24
5.31
VCC VOLTAGE (V)
VCC VOLTAGE (V)
5.32
0
24
5.32
MAX8759 toc23
VIN = 24V
5.33
5.30
20
VCC VOLTAGE
vs. TEMPERATURE
5.35
5.31
16
INPUT VOLTAGE (V)
VCC LOAD REGULATION
5.34
1.0
PWMI DUTY RATIO
10
12
5.27
-40
-20
0
20
40
60
80
TEMPERATURE (°C)
Maxim Integrated │ 9
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
Pin Description
PIN
NAME
1
BATT
Supply Input. BATT is the input to the internal 5.35V linear regulator that powers the device. Bypass BATT
to GND with a 0.1µF ceramic capacitor.
2
SDA
SMBus Serial Data Input
3
SCL
SMBus Serial Clock Input
4
TFLT
Fault-Timer Adjustment Pin. Connect a capacitor from TFLT to GND to set the time-out periods for openlamp and secondary overcurrent faults.
5
VALS
Ambient-Light-Sensor Supply Pin. Bypass VALS to GND with a 0.1µF capacitor.
6
ALS
Ambient-Light-Sensor Input
7
PWMI
DPST Control Input
8
PWMO
DPST Buffer Output. Connect a capacitor between PWMO and GND. The capacitor forms a lowpass filter
with an internal 40kΩ (typ) resistor for filtering the DPST signal.
9
FREQ
Chopping-Frequency Adjustment Pin. Connect a resistor from FREQ to GND to set the DPWM frequency:
fDPWM = 210Hz x 169kΩ / RFREQ.
10
COMP
Transconductance Error Amplifier Output. A compensation capacitor connected between COMP and GND
sets the rise and fall time of the lamp-current envelope in DPWM operation.
11
DEL
Adaptive Zero-Crossing-Delay Adjustment Pin. Connect a resistor between DEL and GND to adjust the
range of the zero-crossing delay. Connecting DEL to VCC disables the zero-crossing delay function.
IFB1
Lamp-Current-Feedback Input. The IFB1 sense signal is internally full-wave rectified. IFB1 is compared
with IFB2 and the larger is used for lamp-current regulation. The average value of the rectified signal is
regulated to 785mV (typ) by controlling the on-time of high-side switch. An open-lamp fault is generated if
the peak voltage of IFB1 is below 600mV for a fault delay period set by TFLT.
IFB2
Lamp-Current-Feedback Input. The IFB2 sense signal is internally full-wave rectified. IFB1 is compared
with IFB2 and the larger is used for lamp-current regulation. The average value of the rectified signal is
regulated to 800mV (typ) by controlling the on-time of high-side switch. An open-lamp fault is generated if
the peak voltage of IFB2 is below 600mV for a fault-delay period set by TFLT. IFB2 input can be disabled
by connecting IFB2 to VCC.
14
VFB
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 striking and lamp-out
fault. When the peak voltage on VFB exceeds the internal overvoltage threshold, the controller turns on an
internal current sink, discharging the COMP capacitor to limit the switch on-time. The VFB pin is also used
to detect a secondary undervoltage condition. If the peak voltage on VFB is below 230mV continuously for
250µs during the DPWM ON period, the MAX8759 shuts down.
15
ISEC
Transformer Secondary Current-Feedback Input. A current-sense resistor connected between the lowvoltage end of the transformer secondary and the ground sets the maximum secondary current during
short-circuit fault. When the peak voltage on ISEC exceeds the internal overcurrent threshold, the controller
turns on an internal current sink discharging the COMP capacitor.
16
LX1
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.
17
GH1
High-Side MOSFET NH1 Gate Driver Output
12
13
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FUNCTION
Maxim Integrated │ 10
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
Pin Description (continued)
PIN
NAME
18
BST1
19
PGND1
20
GL1
Low-Side MOSFET NL1 Gate Driver Output
21
VDD
Low-Side Gate-Driver Supply Input. Connect VDD to the output of the internal linear regulator (VCC).
Bypass VDD with a 0.1µF capacitor to PGND.
22
GL2
Low-Side MOSFET NL2 Gate-Driver Output
23
PGND2
24
BST2
GH2 Gate-Driver Supply Input. Connect a 0.1µF capacitor from LX2 to BST2.
25
GH2
High-Side MOSFET NH2 Gate-Driver Output
26
LX2
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.
27
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.
28
VCC
5.35V/10mA Internal Linear-Regulator Output. VCC is the supply voltage for the device. Bypass VCC with a
0.47µF ceramic capacitor to GND.
—
EP
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FUNCTION
GH1 Gate-Driver Supply Input. Connect a 0.1µF capacitor from LX1 to BST1.
Power Ground. PGND1 is the return for the GL1 gate driver.
Power Ground. PGND2 is the return for the GL2 gate driver.
Exposed Backside Pad. Connect PAD to GND.
Maxim Integrated │ 11
MAX8759
INPUT VOLTAGE
7.5V TO 24V
Low-Cost, SMBus, CCFL Backlight Controller
F1
2A
21
C7
0.1µF
27
11
VCC
28
C9
0.47µF
SMB_DATA
3
PWM INPUT
VDD
BST2
DEL
BST1
VCC
GH1
7
8
SDA
LX1
LX2
SCL
GL1
PGND1
PWMO
PGND2
GL2
FREQ
R3
169kΩ
1%
ALS SUPPLY
GH2
IFB1
5
IFB2
VALS
VFB
C15
0.1µF
ISEC
ALS OUTPUT
6
C8
0.47µF
24
18
C2
2.2µF
FDC6561AN
17
C10
0.1µF
16
26
N1A
N1B
N2A
N2B
20
ALS
COMP
TFLT
19
C3
2.2µF
C11
0.1µF
T1
1:110
PWMI
C12
1µF
9
C1
10µF
25V
21
GND
MAX8759
2
SMB_CLOCK
BATT
FDC6561AN
C4
10pF
3kV
CCFL
23
22
25
12
13
VCC
14
15
C6
68nF
10
4
C14
0.22µF
R2
3.9kΩ
C5
10nF
R1
150Ω
1%
C13
6.8nF
Figure 1. Typical MAX8759 Single-Lamp Operating Circuit
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Maxim Integrated │ 12
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
Typical Operating Circuit
The MAX8759 typical operating circuit (Figure 1) is a single-lamp CCFL backlight inverter for notebook computer
TFT LCD panels. The input voltage range of the circuit
is from 7.5V to 24V. The maximum RMS lamp current is
set to 6mA and the maximum RMS striking voltage is set
to 1800V. Table 1 lists some important components and
Table 2 lists the component suppliers’ contact information.
Detailed Description
The MAX8759 controls a full-bridge resonant inverter to
convert an unregulated DC input into a high-frequency AC
output for powering CCFLs. The resonant operation maximizes striking capability and provides near-sinusoidal
waveforms over the entire input range to improve CCFL
lifetime. The lamp brightness is adjusted by turning the
Table 1. List of Important Components
DESIGNATION
C1
C2, C3
C4
NH1/2, NL1/2
T1
DESCRIPTION
10µF ±20%, 25V X5R ceramic capacitor
(1210)
Murata GRM32DR61E106M
TDK C3225X5R1E106M
2.2µF ±10%, 25V X5R ceramic capacitors
(0805)
Murata GRM21BR61E225K
TDK C2012X5R1E225K
10pF ±10%, 3kV HV ceramic capacitor
(1808)
Kemet C1808C100KHGAC
TDK C4520C0G3F100F
Dual n-channel MOSFETs, 30V, 0.095W,
6-pin SOT23
Fairchild FDC6561AN
CCFL transformer, 1:110 turns ratio
TMP UI9.8L type
Table 2. Component Suppliers
SUPPLIER
WEBSITE
Fairchild
Semiconductor
www.fairchildsemi.com
Kemet
www.kemet.com
Murata
www.murata.com
TDK
www.components.tdk.com
TMP
www.tmp.com
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lamp on and off with a DPWM signal. The DPWM frequency can be accurately adjusted with a resistor. The
brightness of the lamp is proportional to the duty cycle of
the DPWM signal, which is controlled either with a 2-wire
SMBus-compatible interface, with an external ALS, or
with an external PWM signal. The device also includes
safety features that effectively protect against single-point
fault conditions such as lamp-out and secondary shortcircuit faults. An internal 5.35V linear regulator powers the
MOSFET drivers and most of the internal circuitry. Figure
2 is the functional diagram of the MAX8759 and Figure 3
is the detailed diagram of the SMBus and ALS input block.
Resonant Operation
The MAX8759 drives four n-channel power MOSFETs
that make up the zero-voltage-switching (ZVS) full-bridge
inverter as shown in Figure 4. Assume that NH1 and NL2
are on at the beginning of a switching cycle as shown in
Figure 4(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 is turned off, the primary current
forward biases the body diode of NL1, which clamps the
LX1 voltage just below ground as shown in Figure 4(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 4(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 is turned off, the primary current
forward biases the body diode of NL2, which clamps
the LX2 voltage just below ground as shown in Figure
4(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 4(a). Note that switching transitions on
all four power MOSFETs occur under ZVS conditions,
which reduces transient power losses and EMI.
Maxim Integrated │ 13
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
LINEAR
REGULATOR
BATT
BIAS EN
GND
RDY
UVLO
COMPARATOR
4.3V
VCC
RDY
VALS
MAX8759
VCC
OC
COMP
ISEC
OC
1.21V
S
ZX
Q
R
OPEN-LAMP
COMP
MIN
TFLT
135µA
FAULT
LATCH
1µA
FW
MAX
FW
VREF
COMP
SHUTDOWN
UV
COMP
ERROR
AMP
1000µA
BST1
100µA
DH
2.3V
OV
COMP
DWPM
OSC
FREQ
SCL
ALS
PWMI
ALS
ADC
40kΩ
MIN
TON
PWM
COMP
8-BIT
COUNTER
Q
R
TON FF
S
Q
DPWM
LATCH
SMBus
SDA
GH1
LX1
OC
VFB
PWMO
VFB
BATT
Q
R
230mV
600mV
IFB1
IFB2
S
4V
S
BRIGHTNESS
CONTROL
R
BST2
GATE-DRIVER
CONTROL
STATE
MACHINE
DH
GH2
LX2
VDD
DL
GL1
PGND2
Q
ZERO-CROSS
ZX DETECTIONS LX_
AND
DELAY BLOCK
DPWM
COMP
PWM
ADC
DL
GL2
PGND1
MUX
ILIM
COMP
400mV
DEL
Figure 2. MAX8759 Functional Diagram
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Maxim Integrated │ 14
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
PWMO
BUFFER
A
PWMI
D
MUX
"1"
PWM_MD
DIGITAL
MULTIPLIER
BUFFER
MUX
DPWM
SETTING
ALS
A
DIGITAL
POT
D
PWM_SEL
ALS
CLAMP
OFFSET
MUX
ALS_CTL
FAULT/
STATUS
REGISTER
0X02
SDA
SCL
ALS
STATUS
REGISTER
0X04
ALS
LOW-LIMIT
REGISTER
0X05
ALS
HIGH-LIMIT
REGISTER
0X06
SMBus
INTERFACE
BRIGHT
CONTROL
REGISTER
0X00
DEVICE
CONTROL
REGISTER
INVERTER
ON/OFF
0X01
SMBus AND AMBIENT-LIGHT-SENSOR INPUT BLOCK
Figure 3. MAX8759 SMBus and Ambient-Light-Sensor Input Block
A simplified CCFL inverter circuit is shown in Figure 5 (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 5(b) by removing the
transformer. CS is the primary series capacitor, CS’ 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.
Figure 6 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
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 =
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1
2π LC′S
Maxim Integrated │ 15
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
VBATT
NH1
ON
VBATT
NH2
OFF
T1
NH1
OFF
NH2
ON
T1
C2
C2
LX1
LX2
NL1
OFF
LX1
NL2
ON
LX2
NL1
ON
NL2
OFF
(a)
(c)
VBATT
VBATT
NH1
OFF
NH2
OFF
T1
NH1
OFF
NH2
OFF
T1
C2
C2
LX1
LX2
NL1
ON
LX1
NL2
ON
LX2
NL1
ON
NL2
ON
(BODY DIODE TURNS ON FIRST)
(BODY DIODE TURNS ON FIRST)
(b)
(d)
Figure 4. Resonant Operation
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 =
1
2π L
C′SC P
C′S + C P
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
displays the characteristics of a parallel-loaded resonant
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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 increases until the lamp is
ionized or until it reaches the IC’s secondary voltage limit.
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 MAX8759 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
Maxim Integrated │ 16
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
CS
L
1:N
4
AC
SOURCE
CCFL
VOLTAGE GAIN (V/V)
CP
(a)
C
C'S = S2
N
L
AC
SOURCE
CP
3
2
1
0
RL
RL INCREASING
0
20
40
60
80
100
FREQUENCY (kHz)
(b)
Figure 5. Equivalent Resonant Tank Circuit
Figure 6. Frequency Response of the Resonant Tank
lamp current is sensed with a resistor connected in series
with the low-voltage terminal of the lamp. The MAX8759
has two lamp-current feedback inputs (IFB1 and IFB2) to
support dual-lamp application. The voltages across the
sense resistors are fed to the IFB1 and IFB2 inputs and
are internally full-wave rectified. The transconductance
error amplifier selects the higher one of the two feedback
signals and compares the rectified voltage with an 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).
Feed-forward control is implemented by increasing the
internal voltage ramp rate for higher VBATT. This has the
effect of varying tON as a function of the input voltage
while maintaining approximately the same signal levels at
VCOMP. Since the required voltage change across the compensation capacitor is minimal, the controller’s response to
input voltage changes is essentially instantaneous.
Feed-Forward Control
The MAX8759 is designed to maintain tight control of
the lamp current under all transient conditions. The feedforward control instantaneously adjusts the on-time for
changes in input voltage (VBATT). This feature provides
immunity to input-voltage variations and simplifies loop
compensation over wide input-voltage ranges. The feedforward control also improves the line regulation for
short DPWM on-times and makes startup transients less
dependent on the input voltage.
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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 MAX8759’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 voltage across the lamp.
Maxim Integrated │ 17
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
Dimming Control
to 1.8V. The MAX8759 compares the ALS input voltage
against user-programmable low and high limits. When the
ALS input voltage is below the low limit, the brightness is
clamped to the ALS low limit. When the ALS input voltage
is above the high limit, the brightness is clamped to the
ALS high limit. If the minimum ALS setting is below 10%,
the brightness is clamped to 10%. Figure 7 shows the
brightness change as a function of the ALS voltage.
In DPWM operation, the COMP voltage controls the
dynamics of the lamp-current envelope. At the beginning
of the DPWM ON cycle, the average value of the lampcurrent feedback signal is below the regulation point, so
the transconductance error amplifier sources current into
the COMP capacitor. The switch on-time (tON) gradually
increases as VCOMP rises, which provides soft-start. At
the end of the DPWM ON cycle, the MAX8759 turns on a
110µA internal current source. The current source linearly
discharges the COMP capacitor, gradually decreasing
tON, and providing soft-stop.
The ALS input voltage is sampled every DPWM period
and is loaded in ALS status register 0x04. The analog
voltage on the ALS pin is converted into an 8-bit digital
code. The total number of brightness levels is 256. One
step change results in a 0.391% change in the DPWM
duty cycle.
The MAX8759 controls the brightness of the CCFL by
“chopping” the lamp current on and off using a low-frequency (between 100Hz and 350Hz) DPWM signal. The
frequency of the internal DPWM oscillator is adjustable
through a resistor connected between the FREQ pin and
GND. The CCFL brightness is proportional to the DPWM
duty cycle, which can be adjusted from 10.15% to 100%.
The DPWM frequency can be set with an external resistor. Connect a resistor between FREQ and GND. The
DPWM frequency is given by the following equation:
f DPWM= 210Hz × 169kW / R FREQ
The adjustable range of the DPWM frequency is between
100Hz and 350Hz (RFREQ is between 100kΩ and 350kΩ).
The MAX8759 has three ways for brightness control. The
brightness can be controlled by a 2-wire serial interface
(SMBus), by an external PWM signal, or by an external
ambient-light sensor signal. There are five operating
modes, which can be selected by setting bits 1 to 3 in
device control register 0x01 (see the SMBus Register
Definitions section for details).
ALS Mode
The MAX8759 can work with several types of ambientlight sensors. The ideal ambient-light sensors should
have a linear response to ambient light and should have
a spectral response equivalent to that of the human
eye. Ambient-light sensors must provide filtering of lowfrequency harmonics found in the electrical spectrum of
the many light sources. The ALS’s output should be a DC
analog voltage that is linearly proportional to the ambient
luminance.
In ALS mode, the MAX8759 sets the brightness based
on the analog voltage on the ALS pin. The ALS pin is
connected to the output of an external ambient-light sensor. The usable input-voltage range of the ALS pin is 0
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PWM Mode
In PWM mode, the MAX8759 sets the brightness based
on the duty cycle of the PWMI signal. The absolute minimum brightness is 10%. If the PWMI duty cycle is less
than 10%, the brightness stays at 10%. The frequency
range of the PWMI signal is between 5kHz and 50kHz
when the PWMO capacitor is 1µF.
SMBus Mode
In SMBus mode, the MAX8759 sets the brightness based
on the brightness control register (0x00). The brightness
control register contains 8 bits and supports 256 brightness levels. A setting of 0xFF for register 0x00 sets the
inverter to the maximum brightness. A setting of 0x00 for
register 0x00 sets the inverter to the minimum brightness
(10%).
ALS with DPST Mode
In ALS with DPST mode, the MAX8759 sets the brightness based on the analog voltage on the ALS pin and
duty cycle at the PWMI pin. The MAX8759 lowers the ALS
brightness setting by an additional amount that is proportional to the duty cycle of the PWMI signal. For example,
if the ALS brightness setting is 80% and the duty cycle of
PWMI signal is 60%, the resulting brightness setting is
80% x 60% = 48%.
SMBus with DPST Mode
In SMBus with DPST mode, the MAX8759 sets the brightness based on the brightness control register (0x00). The
MAX8759 lowers the SMBus brightness setting by an
additional amount that is proportional to the duty cycle of
the PWMI signal. For example, if the brightness control
register is set to 0x80 (corresponding to 50% brightness
setting) and the duty cycle of the PWMI signal is 60%, the
resulting brightness setting is 50% x 60% = 30%.
Maxim Integrated │ 18
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
Fault Protections
Secondary Voltage Limiting (VFB)
Lamp-Out Protection
For safety, the MAX8759 monitors the lamp-current feedback inputs (IFB1 and IFB2) to detect faulty or open CCFL
tubes. As described in the Lamp-Current Regulation section, the voltage on IFB1 and IFB2 is internally full-wave
rectified. If the rectified IFB1 or IFB2 voltage is below
600mV, the MAX8759 charges the TFLT capacitor with 1µA.
The MAX8759 sets the fault latch and the device is shut
down when the voltage on TFLT exceeds 4V. Unlike the
normal shutdown mode, the linear regulator output (VCC)
remains at 5.35V. Clearing bit 0 of the device control register (0x01) or cycling the input power clears the fault latch.
During the fault-delay period, the current control loop tries
to maintain the lamp-current regulation by increasing the
high-side MOSFET on-time. Because the lamp impedance
is very high when it is open, 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 MAX8759 turns on a 1000µA 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 during the lamp-out delay period.
Primary Overcurrent Protection
The MAX8759 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
PGND_. If the voltage exceeds the current-limit threshold
(430mV, typ), the regulator immediately turns off the highside switch to prevent the transformer primary current
from increasing further.
NORMALIZED BRIGHTNESS
1.0
0.8
Secondary Undervoltage Protection (VFB)
The MAX8759 senses the VFB voltage for undervoltage
condition. During the DPWM ON period, if the VFB voltage is below the undervoltage threshold (230mV, typ)
continuously for an internal delay period (250µs typ, for
RFREQ = 169kΩ), the MAX8759 shuts down.
Secondary Current Limit (ISEC)
The secondary current limit provides fail-safe current
limiting in case of a short circuit or leakage from the lamp
high-voltage terminal to ground that prevents the current
control loop from functioning properly. ISEC monitors
the voltage across a sense network placed between
the transformer’s low-voltage secondary terminal and
ground. The ISEC voltage is continuously compared to
the ISEC regulation threshold (1.21V, typ). Any time the
ISEC voltage exceeds the threshold, the MAX8759 turns
on a 1000µA current source that discharges the COMP
capacitor, reducing the on-time of the high-side switches.
At the same time, the MAX8759 charges the TFLT capacitor with a 135µA current. The MAX8759 sets the fault
latch and shuts down when the voltage on TFLT exceeds
4V. Clearing bit 0 of the device control register (0x01) or
cycling the input power clears the fault latch.
Linear Regulator Output (VCC)
0.6
The internal linear regulator steps down the DC input voltage at BATT pin to 5.35V (typ). The linear regulator supplies
power to the internal control circuitry of the MAX8759 and
is also used to power the MOSFET drivers by connecting
VCC to VDD. The VCC voltage drops to 4.5V in shutdown.
0.4
0.2
0
The MAX8759 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 formed by C4 and
C5 in Figure 1. The voltage across C5 is fed to the VFB
input. An overvoltage comparator compares the VFB peak
voltage with a 2.3V (typ) internal threshold. Once the
VFB peak voltage exceeds the overvoltage threshold, the
MAX8759 turns on an internal 1000µA current source that
discharges the COMP capacitor. The high-side MOSFET’s
on-time shortens as the COMP voltage decreases, limiting
the transformer secondary’s peak voltage at the threshold
determined by the capacitive voltage-divider.
0
0.4
0.8
1.2
1.6
VALS (V)
Figure 7. Normalized Brightness vs. ALS Voltage
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2.0
POR and UVLO
The MAX8759 includes power-on reset (POR) and undervoltage lockout (UVLO) features. POR resets the fault
latch and sets all the SMBus registers to their POR
Maxim Integrated │ 19
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
WRITE-BYTE FORMAT
S
ADDRESS
WR
ACK
COMMAND
ACK
DATA
ACK
P
—
7 BITS
1b
1b
8 BITS
1b
8 BITS
1b
—
SLAVE ADDRESS
COMMAND BYTE: SELECTS
WHICH REGISTER YOU ARE
WRITING TO
READ-BYTE FORMAT
DATA BYTE: DATA GOES INTO THE
REGISTER SET BY THE COMMAND BYTE
S
ADDRESS
WR
ACK
COMMAND
ACK
S
ADDRESS
RD
ACK
DATA
///
P
—
7 BITS
1b
1b
8 BITS
1b
—
7 BITS
1b
1b
8 BITS
1b
—
SLAVE ADDRESS
COMMAND BYTE: SELECTS
WHICH REGISTER YOU ARE
READING FROM
SEND-BYTE FORMAT
SLAVE ADDRESS: REPEATED
DUE TO CHANGE IN DATAFLOW DIRECTION
RECEIVE-BYTE FORMAT
S
ADDRESS
WR
ACK
—
7 BITS
1b
1b
COMMAND
ACK
P
S
8 BITS
1b
—
—
COMMAND BYTE: SENDS COMMAND WITH NO DATA; USUALLY
USED FOR ONE-SHOT COMMAND
S = START CONDITION
P = STOP CONDITION
DATA BYTE: READS FROM THE
REGISTER SET BY THE COMMAND
BYTE
SHADED = SLAVE TRANSMISSION
ACK = ACKNOWLEDGED = 0
ADDRESS
RD
ACK
7 BITS
1b
1b
SLAVE ADDRESS
DATA
///
P
8 BITS
1b
—
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
WR = WRITE = 0
RD = READ = 1
/// = NOT ACKNOWLEDGED = 1
Figure 8. SMBus Protocols
values. POR occurs when VCC rises above 1.75V (typ).
The UVLO occurs when VCC is below 4.2V (typ). The
MAX8759 disables both high-side and low-side switch
drivers below the UVLO threshold.
Low-Power Shutdown
The MAX8759 is placed into shutdown by clearing bit 0
of the device control register (0x01).When the MAX8759
is shut down, all functions of the IC are turned off except
the 5.35V linear regulator. In shutdown, the linear regulator output voltage drops to 4.5V and the supply current is
6µA (typ). While in shutdown, the fault latch is reset. The
device can be reenabled by setting bit 0 of the device
control register to 1.
Ambient-Light-Sensor Supply Pin (VALS)
The MAX8759 provides the supply voltage of the ALS
through the VALS pin. VALS is internally connected to
the 5.35V linear regulator output through a p-channel
MOSFET. The p-channel MOSFET is turned on when
the MAX8759 is enabled and turned off when the part is
disabled. Bypass VALS to ground with a minimum 0.1µF
ceramic capacitor. Place the capacitor as close to the ALS
supply input as possible.
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SMBus Interface (SDA, SCL)
The MAX8759 supports an SMBus-compatible 2-wire
digital interface. SDA is the bidirectional data line and
SCL is the clock line of the 2-wire interface corresponding respectively to SMBDATA and SMBCLK lines of the
SMBus. SDA and SCL have Schmidt-triggered inputs that
can accommodate slow edges; however, the rising and
falling edges should still be faster than 1µs and 300ns,
respectively. The MAX8759 uses the write-byte and readbyte protocols (Figure 8). The SMBus protocols are documented in System Management Bus Specification V1.08
and are available at http://www.sbs-forum.org/.
The MAX8759 is a slave-only device and responds to the
7-bit address 0b0101100. The read and write commands
can be distinguished by adding ONE more bit (R/W bit)
to the end of the 7-bit slave address, with one indicating
read and zero indicating write. The MAX8759 has seven
registers: a brightness control register (0x00), a device
control register (0x01), a fault/status register (0x02),
an identification register (0x03), an ALS status register
(0x04), an ALS low-limit register (0x05), and an ALS highlimit register (0x06). The MAX8759 only acknowledges
these seven registers.
Maxim Integrated │ 20
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
A
tLOW
B
tHIGH
C
D
E
F
G
H
I
J
K
L
M
SMBCLK
SMBDATA
tSU:STA tHD:STA
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 .
tSU:DAT
tHD:DAT
tHD:DAT
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.
tSU:STO tBUF
J = ACKNOWLEDGE CLOCKED INTO MASTER.
K = ACKNOWLEDGE CLOCK PULSE.
L = STOP CONDITION, DATA EXECUTED BY SLAVE.
M = NEW START CONDITION .
Figure 9. SMBus Write Timing
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-tohigh transition on SDA while SCL is high. The bus is then
free for another transmission. Figures 9 and 10 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 conditions. 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 MAX8759 since either the master or the slave
acknowledges the receipt of the correct byte during the
ninth clock. If the MAX8759 receives its correct slave
address followed by R/W = 0, it expects to receive 1 or
2 bytes of information (depending on the protocol). If the
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device detects a START or STOP condition prior to clocking in the bytes of data, it considers this an error condition
and disregards all the data. If the transmission is completed correctly, the registers are updated immediately
after a STOP (or RESTART) condition. If the MAX8759
receives its correct slave address followed by R/W = 1,
it expects to clock out the register data selected by the
previous command byte.
SMBus Register Definitions
All MAX8759 registers are byte wide and accessible
through the read/write byte protocols mentioned in the
previous section. Their bit assignments are provided in
the following sections with reserved bits containing a
default value of zero.
Table 3 summarizes the register assignments, as well as
each register’s POR state. During shutdown, the serial
interface remains fully functional.
Maxim Integrated │ 21
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
A
tLOW
B
C
tHIGH
D
E
F
G
H
J
I
K
SMBCLK
SMBDATA
tSU:STA tHD:STA
tSU:DAT
A = START CONDITION.
B = MSB OF ADDRESS CLOCKED INTO SLAVE.
C = LSB OF ADDRESS CLOCKED INTO SLAVE.
D = R/W BIT CLOCKED INTO SLAVE.
tHD:DAT
tSU:DAT
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.
tSU:STO
tBUF
I = ACKNOWLEDGE CLOCK PULSE.
J = STOP CONDITION.
K = NEW START CONDITION.
Figure 10. SMBus Read Timing
Table 3. Commands Description
SMBus
COMMAND POR
PROTOCOL
BYTE
STATE
DATA-REGISTER BIT ASSIGNMENT
BIT 7
(MSB)
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
(LSB)
BR7
BRT6
BRT5
BR4
BRT3
BRT2
BRT1
BRT0
ALS_CTL
PWM_MD
PWM_
SEL
LAMP_CTL
Read and
Write
0x00
0xFF
Read and
Write
0x01
0x00
Reserved Reserved ALSDEL1 ALSDEL0
Read Only
0x02
N/A
Reserved Reserved Reserved Reserved LAMP_STAT OV_CURR Reserved
Read Only
0x03
0x01
MFG4
MFG3
MFG2
MFG1
MFG0
REV2
REV1
REV0
Read Only
0x04
0x00
ALS7
ALS6
ALS5
ALS4
ALS3
ALS2
ALS1
ALS0
Read and
Write
0x05
0x00
ALSLL7
ALSLL6
ALSLL5
ALSLL4
ALSLL3
ALSLL2
ALSLL1
ALSLL0
Read and
Write
0x06
0xFF
ALSHL7
ALSHL6
ALSHL5
ALSHL4
ALSHL3
ALSHL2
ALSHL1
ALSHL0
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FAULT
Maxim Integrated │ 22
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
Brightness Control Register [0x00]
(POR = 0xFF)
The brightness control register of the MAX8759 contains
8 bits and supports 256 brightness levels. A write-byte
cycle to register 0x00 sets the brightness level if the
inverter is in SMBus mode. A write-byte cycle to register
0x00 has no effect if the inverter is not in SMBus mode. A
read-byte cycle to register 0x00 returns the current brightness level regardless of the operation mode. A setting of
0xFF for register 0x00 sets the inverter to the maximum
brightness. A setting of 0x00 for register 0x00 sets the
inverter to the minimum brightness.
BIT 7 (R/W)
BIT 6 (R/W)
BIT 5 (R/W)
BIT 4 (R/W)
BIT 3 (R/W)
BIT 2 (R/W)
BIT 1 (R/W)
BIT 0 (R/W)
BRT7
BRT6
BRT5
BRT4
BRT3
BRT2
BRT1
BRT0
BRT[7..0]: 256 brightness levels.
Device Control Register [0x01] (POR = 0x00)
This register has a single bit that controls the inverter ON/
OFF state, 3 bits that control the operating mode of the
inverter, and 2 bits for setting ALS delay time. The remaining bits are reserved for future use.
BIT 7
BIT 6
BIT 5 (R/W)
BIT 4 (R/W)
BIT 3 (R/W)
BIT 2 (R/W)
BIT 1 (R/W)
BIT 0 (R/W)
Reserved
Reserved
ALSDEL1
ALSDEL0
ALS_CTL
PWM_MD
PWM_SEL
LAMP_CTL
ALSDEL1: ALS delay select bit.
ALSDEL0: ALS delay select bit.
ALS_CTL: Ambient-light-sensor select bit (1 = use ALS, 0 = not use ALS).
PWM_MD: PWM mode select bit (1 = absolute brightness, 0 = percentage change).
PWM_SEL: Brightness control select bit (1 = control by PWM, 0 = control by SMBus).
LAMP_CTL: Inverter on/off bit (1 = on, 0 = off).
A value of 1 written to LAMP_CTL turns on the lamp as
quickly as possible. A value of zero written to LAMP_CTL
immediately turns off the lamp.
where BRTCURRENT is the current brightness setting from
either ALS or SMBus without influence from the PWM
input and DPWM is the duty cycle of the PWM signal.
The PWM_SEL bit determines whether the SMBus or
PWM input should control brightness when the inverter is
not in ALS mode. This bit has no effect when ALS_CTL
is set to 1.
When PWM_MD bit is 1, the PWM input has no effect on
the brightness setting unless the inverter is in PWM mode.
The PWM_MD bit selects the manner in which the PWM
input is to be interpreted. When this bit is zero, the PWM
input reflects a percentage change in the current brightness (i.e., DPST mode) and follows the following equation:
DPST brightness = BRTCURRENT x DPWM
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When ALS_CTL is 1, the inverter controls brightness based
primarily on the light reading from the ALS. However, the
ALS brightness setting can be modified if the PWM_MD bit
is set to zero. When the ALS_CTL bit is zero, the inverter
controls the brightness according to the PWM input (PWM
mode), the SMBus setting (SMBus mode), or a combination of the two (SMBus mode with DPST).
Maxim Integrated │ 23
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
The relationships among these 3 control bits can be
thought of as specifying an operating mode for the inverter. The defined modes are shown in Table 4. Note that
depending on the settings of some bits, other bits have
no effect and are don’t-care bits—they are shown with a
value of X in Table 4. For example, when the ALS_CTL bit
is 1, the value of PWM_SEL has no effect on the operation of the inverter, so its value is shown as X.
ALSDEL0 and ALSDEL1 set the delay time required
to change the brightness in ALS mode. This delay time
is necessary for smooth transitions during brightness
change. Table 5 shows the available delays.
Note that the behavior of register 0x00 (brightness control
register) is affected by certain combinations of the control
bits as shown in Table 4.
When SMBus mode is selected, register 0x00 reflects the
last value written to it. However, when any non-SMBus
mode is selected, register 0x00 reflects the current brightness value based on the current mode of operation.
Table 4. Operating Modes Selected by
Device Control Register Bits 3, 2, and 1
ALS_CTL
PWM_MD
PWM_SEL
1
1
X
ALS mode
1
0
X
ALS mode with DPST
0
X
1
PWM mode
0
1
0
SMBus mode
0
0
0
SMBus mode with
DPST
Table 5. Delay Time Selected by Device
Control Register Bits 5, 4
ALSDEL1
ALSDEL0
DELAY TIME
(ms)
N
PERIODS
1
1
25
5
1
0
15
3
0
1
10
2
0
0
20 (default)
4
Fault/Status Register [0x02] (POR = 0x00)
This register has 3 status bits that allow monitoring
the inverter’s operating state. Bit 0 is a logical OR of
open-lamp fault and overcurrent fault. Bit 2 indicates
secondary/UL overcurrent fault. Bit 3 always indicates
the current lamp on/off status. The value of this bit is
one whenever both lamp 1 and lamp 2 are on. The value
of this bit is zero whenever lamp 1 or lamp 2 is off. The
MODE
remaining bits are reserved for future use. All reserved
bits return a zero when read. All the bits in this register
are read only. A write-byte cycle to register 0x02 has
no effect. Write zero to bit 0 of register 0x01 to clear the
fault bit.
BIT 7 (R)
BIT 6 (R)
BIT 5 (R)
BIT 4 (R)
BIT 3 (R)
BIT 2 (R)
BIT 1 (R)
BIT 0 (R)
Reserved
Reserved
Reserved
Reserved
LAMP_STAT
OV_CURR
Reserved
FAULT
LAMP_STAT: Lamp status bit (1 = lamp 1 and lamp 2 are on, 0 = lamp 1 or lamp 2 is off).
OV_CURR: Secondary/UL overcurrent fault (1 = secondary/UL overcurrent fault, 0 = no secondary/UL overcurrent).
FAULT: Fault bit (1 = open-lamp or primary overcurrent fault, 0 = no fault).
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Maxim Integrated │ 24
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
Identification Register [0x03] (POR = 0x01)
The identification register contains two bit fields to denote
the manufacturer and the silicon revision. The bit field
widths allow up to 32 vendors with up to eight silicon revisions each. This register is read only. A write-byte cycle to
register 0x03 has no effect.
BIT 7 (R)
BIT 6 (R)
BIT 5 (R)
BIT 4 (R)
BIT 3 (R)
BIT 2 (R)
BIT 1 (R)
BIT 0 (R)
MFG4
MFG3
MFG2
MFG1
MFG0
REV2
REV1
REV0
MFG[4..0]: Manufacturer ID (the vendor ID for Maxim is 0).
REV[2..0]: Silicon rev (revs 0–7 allowed for silicon revisions).
ALS Status Register [0x04] (POR = 0x00)
The ALS should return a value reflecting the brightness
setting based on the ALS input. The register has 8 bits
that define a full range of 256 brightness levels. The
register is read only and a write-byte cycle has no effect.
A read-byte cycle to register 0x04 returns the current
reading of ALS, regardless of the operating mode set in
register 0x01.
BIT 7 (R)
BIT 6 (R)
BIT 5 (R)
BIT 4 (R)
BIT 3 (R)
BIT 2 (R)
BIT 1 (R)
BIT 0 (R)
ALS7
ALS6
ALS5
ALS4
ALS3
ALS2
ALS1
ALS0
ALS[7..0]: 256 steps of ambient-light sensor reading.
ALS Low-Limit Register [0x05] (POR = 0x00)
The value in this read-write register reflects the lowest
possible brightness value the inverter can set based on
inputs from the ALS. Users can change this value so that
they can control the effect of ALS. A write-byte cycle to
register 0x05 sets the lowest possible brightness value
that can be set based on ALS inputs. If the brightness
setting due to ALS is lower than the value written to this
register, the inverter immediately increases the brightness
setting to the newly written value. A read-byte cycle to register 0x05 returns the current minimum brightness value
that can be set based on ALS inputs.
BIT 7 (R/W)
BIT 6 (R/W)
BIT 5 (R/W)
BIT 4 (R/W)
BIT 3 (R/W)
BIT 2 (R/W)
BIT 1 (R/W)
BIT 0 (R/W)
ALSLL7
ALSLL6
ALSLL5
ALSLL4
ALSLL3
ALSLL2
ALSLL1
ALSLL0
ALSLL[7..0]: The lowest brightness setting that can be set based on ALS inputs.
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Maxim Integrated │ 25
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
ALS High-Limit Register [0x06] (POR = 0xFF)
The value in this read-write register reflects the highest
possible brightness value the inverter can set based on
inputs from the ALS. Users can change this value so that
they can control the effect of ALS. A write-byte cycle to
register 0x06 sets the highest possible brightness value
that can be set based on ALS inputs. If the brightness
setting due to ALS is higher than the value written to this
register, the inverter immediately decreases the brightness setting to the newly written value. A read-byte cycle
to register 0x06 returns the current maximum brightness
value that can be set based on ALS inputs. The default
value of register 0x06 is 0xFF, which corresponds to the
maximum brightness.
BIT 7 (R/W)
BIT 6 (R/W)
BIT 5 (R/W)
BIT 4 (R/W)
BIT 3 (R/W)
BIT 2 (R/W)
BIT 1 (R/W)
BIT 0 (R/W)
ALSHL7
ALSHL6
ALSHL5
ALSHL4
ALSHL3
ALSHL2
ALSHL1
ALSHL0
ALSHL[7..0]: The highest brightness setting that can be set based on ALS inputs.
Applications Information
MOSFETs
The MAX8759 requires four external n-channel power
MOSFETs: NL1, NL2, NH1, and NH2 to form a fullbridge inverter circuit. The controller 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 MAX8759 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 ZVS 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 (see the Resonant
Operation section). 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 MAX8759 senses the lamp current flowing through
sense resistors connected between the low-voltage ter-
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minals of the lamps and ground. The voltages across
the sense resistors are fed to IFB1 and IFB2 and are
internally full-wave rectified. The MAX8759 controls the
desired lamp current by regulating the average of the
rectified IFB_ voltages. To set the RMS lamp current in a
single-lamp application, determine the value of the sense
resistor as follows:
R1 =
π × 785mV
2 2 × ILAMP(RMS)
where ILAMP(RMS) is the desired RMS lamp current and
785mV is the typical value of the IFB1 regulation point
specified in the Electrical Characteristics table. To set the
RMS lamp current to 6mA, the value of the sense resistor
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, influences the actual RMS lamp current. Use a true RMS current meter to make final adjustments.
Setting the Secondary Voltage Limit
The MAX8759 limits the transformer secondary voltage
during startup and lamp-out faults. The secondary voltage
is sensed through the capacitive voltage-divider formed
by C4 and C5 (Figure 1). The VFB voltage is proportional to the CCFL voltage. The selection of the parallel
resonant capacitor C1 is described in the Transformer
Design and Resonant Component Selection section. C4
is usually between 10pF and 22pF. After the value of C4
is determined, select C5 using the following equation
Maxim Integrated │ 26
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
to set the desired maximum RMS secondary voltage
VLAMP(RMS)_MAX:
=
C5
2 × VLAMP(RMS)_MAX
2.3V
× C4
where the 2.3V is the typical value of the VFB peak voltage when the lamp is open. To set the maximum RMS
secondary voltage to 1800V when C4 is 10pF, use 10nF
for C5.
Setting the Secondary Current Limit
The MAX8759 limits the secondary current even if the
IFB_ sense resistors are shorted or transformer secondary current finds its way to ground without passing
through the sense resistors. ISEC monitors the peak
voltage across the sense network (R2 and C6 in Figure
1) connected between the low-voltage terminal of the
transformer secondary winding and ground. Using an
RC- sense network instead of a single-sense resistor
makes the secondary current-limit frequency dependent.
The UL safety standard requires the AC peak current in
a limited-current circuit should not exceed 0.7mA for frequencies below 1kHz. For frequencies above 1kHz, the
limit of 0.7mA is multiplied by the value of the frequency
in kilohertz but should not exceed 70mA peak when the
frequency is equal to or above 100kHz. To meet the UL
current-limit specifications, determine the value of R2
using the current limit at 1kHz and determine the value of
C6 using the current limit at 100kHz:
R2 >
1.23V
=
1.75kW
0.7mA
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 700V and the minimum DC input voltage is
7.5V, the turns ratio should be greater than 104. The turns
ratio of the transformer used in the circuit of Figure 1 is 110.
The next step in the design procedure is to determine
the desired operating frequency range. The MAX8759
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
transformer secondary leakage inductance L, the primary
series DC blocking capacitors (CS), and the secondary
parallel resonant capacitor CP. 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. The
leakage inductance values can have large tolerance and
significant variations among different batches. It is best to
work directly with transformer vendors on leakage inductance requirements. The MAX8759 works best when the
secondary leakage inductance is between 250mH and
350mH. Series capacitor CS sets the minimum operating
frequency, which is approximately two times the series
resonant peak frequency. Choose:
≤
π 2 × f 2 MIN × L
70mA
C6 <
90nF
=
2π × 100kHz × 1.23V
where fMIN is the minimum operating frequency range. In
the circuit of Figure 1, the transformer’s turns ratio is 110
where 1.23V is the typical value of the ISEC peak voltage
and its secondary leakage inductance is approximately
when the transformer secondary is shorted. The circuit of
300mH. To set the minimum operating frequency to
Figure 1 uses 3.9kΩ for R2 and 68nF for C6.
30kHz, the total series capacitance needs to be less than
4.5µF. Therefore, two 2.2µF capacitors (C2 and C3) are
Transformer Design and Resonant
used in Figure 1.
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 follows:
N≥
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VLAMP(RMS)
0.9 × VIN(MIN)
Parallel capacitor CP sets the maximum operating frequency, which is also the parallel resonant peak frequency. Choose:
CS
CP ≥
2
2
4π × f MAX × L × C S − N 2
In the circuit of Figure 1, to set the maximum operating
frequency to 100kHz, CP needs to be larger than 8.6pF.
A 10pF high-voltage capacitor (C4) is used in Figure 1.
Maxim Integrated │ 27
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
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 >
D MAX × VIN(MAX)
B S × S × f MIN
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, maintaining lamp-current regulation, and during transients caused by changing the input
voltage. To maintain stable operation, the COMP capacitor (CCOMP) needs to be at least 3.3nF.
The COMP capacitor also limits the dynamics of the lampcurrent envelope in DPWM operation. At the end of the
DPWM on cycle, the MAX8759 turns on a 110µA internal
current source to linearly discharge the COMP capacitor.
Use the following equation to set the fall time:
110µA × t FALL
C COMP =
VCOMP
where tFALL is the fall time of the lamp-current envelope
and VCOMP is the COMP voltage when the lamp current
is in regulation. At the beginning of the DPWM on cycle,
the COMP capacitor is charged by a transconductance
error amplifier. The rise time is about three times longer
than the fall time.
Setting the Fault-Delay Time
The TFLT capacitor determines the delay time for both
the open-lamp fault and secondary short-circuit fault. The
MAX8759 charges the TFLT capacitor with a 1µA current
source during an open-lamp fault and charges the TFLT
capacitor with a 135µA current source during a secondary
short-circuit fault. Therefore, the secondary short-circuit
fault delay time is approximately 135 times shorter than that
of open-lamp fault. The MAX8759 sets the fault latch when
the TFLT voltage reaches 4V. Use the following equations
to calculate the open-lamp fault delay (TOPEN_LAMP) and
secondary short-circuit fault delay TSEC_SHORT):
TOPEN_LAMP =
www.maximintegrated.com
C TFLT × 4V
1µA
TSEC_SHORT =
C TFLT × 4V
135µA
Bootstrap Capacitors
The high-side gate drivers are powered using two bootstrap circuits. The MAX8759 integrates the bootstrap
diodes so only two 0.1µF bootstrap capacitors are
needed. Connect the capacitors (C10 and C11 in Figure
1) between LX1 and BST1, and between LX2 and BST2
to complete the bootstrap circuits.
Dual-Lamp Operating Circuit
The MAX8759 includes two lamp current feedback input
pins that support dual-lamp applications with a minimum
number of external components. Figure 11 shows the typical dual-lamp operating circuit.
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 highvoltage sections of the layout need to be well separated
from the control circuit. Most layouts for single-lamp 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 can simplify this task. Quiet analog ground is
used for VCC, COMP, FREQ, and TFLT.
3) Route high-speed switching nodes away from sensitive analog areas (VCC, COMP, FREQ, and TFLT).
Make all pin-strap control input connections 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 (underneath) the
8-pin SO package.
Maxim Integrated │ 28
MAX8759
INPUT VOLTAGE
7.5V TO 21V
Low-Cost, SMBus, CCFL Backlight Controller
F1
2A
VCC
1
C7
0.1µF
27
11
28
VCC
C9
0.47µF
SMB_DATA
3
PWM INPUT
VDD
BST2
DEL
BST1
VCC
GH1
7
SDA
LX1
LX2
SCL
GL1
PWMO
C12
1µF
PGND2
GL2
GH2
9
FREQ
R3
169kΩ
1%
IFB1
IFB2
ISEC
ALS SUPPLY
5
C8
1µF
24
18
17
16
26
C10
0.1µF
N1A
N1B
C2
2.2µF
1
2
N2A
C4
10pF
3kV
D3
N2B
19
HV
LV
FDS6990A
22
C3
2.2µF
25
COMP
TFLT
R14
100kΩ
T2
1:110
12
R1
150Ω
1%
1
HV
LV
C5
10pF
3kV
13
R9
180kΩ
15
R10
20kΩ
VFB
C6
1nF
2
VALS
ALS
D4
R7
390kΩ
1%
23
R12
180kΩ
6
T1
1:110
C11
0.1µF
20
C15
0.1µF
ALS OUTPUT
FDS6990A
PWMI
PGND1
8
C1
22µF
25V
21
GND
MAX8759
2
SMB_CLOCK
BATT
14
R8
390kΩ
1%
C17
1nF
R15
100kΩ
R2
150kΩ
1%
R13
20kΩ
10
4
C14
0.22µF
C13
6.8nF
Figure 11. Typical MAX8758 Dual-Lamp Operating Circuit
www.maximintegrated.com
Maxim Integrated │ 29
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
6) Ensure the feedback connections are short and direct.
To the extent possible, IFB1, IFB2, VFB, and ISEC
connections should be far away from the high-voltage
traces and the transformer.
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.
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.
Pin Configuration
TRANSISTOR COUNT: 16,138
GL1
PGND1
BST1
GH1
LX1
ISEC
PROCESS: BiCMOS
VDD
TOP VIEW
Chip Information
21
20
19
18
17
16
15
GL2
22
14
VFB
PGND2
23
13
IFB2
BST2
24
12
IFB1
11
DEL
10
COMP
9
FREQ
8
PWMO
GH2
25
LX2
26
GND
27
*EXPOSED PADDLE
1
2
3
4
5
6
7
SDA
SCL
TFLT
VALS
ALS
PWMI
28
BATT
VCC
MAX8759
Package Information
For the latest package outline information and land patterns
(footprints), go to www.maximintegrated.com/packages. Note
that a “+”, “#”, or “-” in the package code indicates RoHS status
only. Package drawings may show a different suffix character, but
the drawing pertains to the package regardless of RoHS status.
PACKAGE
TYPE
PACKAGE
CODE
OUTLINE
NO.
LAND
PATTERN NO.
28 TQFN
T2855+6
21-0140
90-0026
TQFN 5mm x 5mm
www.maximintegrated.com
Maxim Integrated │ 30
MAX8759
Low-Cost, SMBus, CCFL Backlight Controller
Revision History
REVISION
NUMBER
REVISION
DATE
3
4/14
DESCRIPTION
Removed automotive reference in Applications section
PAGES
CHANGED
1
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim Integrated’s website at www.maximintegrated.com.
Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent licenses
are implied. Maxim Integrated reserves the right to change the circuitry and specifications without notice at any time. The parametric values (min and max limits)
shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.
Maxim Integrated and the Maxim Integrated logo are trademarks of Maxim Integrated Products, Inc.
© 2014 Maxim Integrated Products, Inc. │ 31
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