19-2267; Rev 1; 10/02
High-Efficiency, Wide Brightness
Range, CCFL Backlight Controller
Features
♦ SMBus Slave Address (0x58) for Wide Dimming
Range Inverters
♦ Guaranteed 200Hz to 220Hz DPWM Frequency
♦ Externally Synchronizable DPWM Frequency
♦ Lamp-Out Protection with 1s Timeout
♦ Synchronized to Resonant Frequency
Good Crest Factor for Longer Lamp Life
Ensures Maximum Strike Capability
♦ High Power-to-Light Efficiency
♦ Wide Dimming Range (3 Methods)
Lamp Current Adjust: >3 to 1
DPWM: >10 to 1
Combined: >30 to 1
♦ Feed-Forward for Fast Response to Step Change
of Input Voltage
♦ Wide Input-Voltage Range (4.6V to 28V)
♦ Transformer Secondary Voltage Limiting to
Reduce Transformer Stress
♦ Protected Against Short-Circuit and Other SinglePoint Faults
♦ Dual-Mode Brightness Control Interface
♦ Small Footprint 28-Pin Thin QFN (5mm ✕ 5mm)
Package
Pin Configuration
VCC
BATT
CCV
CCI
IFB
N.C.
VFB
26
25
24
23
22
Multibulb LCD Monitors
27
TOP VIEW
Notebook Computers
28
Applications
Portable Display Electronics
17
LX1
CRF/SDA
6
16
GH1
CTL/SCL
7
15
GL1
13
14
GL2
SMBus is a trademark of Intel Corp.
5
PGND
*Contact factory for availability.
BST1
MODE
12
28 QFN 5 ✕ 5
18
MAX1996A
VDD
-40°C to +85°C
4
11
MAX1996AEGI*
BST2
GND
N.C.
28 Thin QFN 5 ✕ 5
19
10
-40°C to +85°C
3
N.C.
MAX1996AETI
LX2
MINDAC
9
PIN-PACKAGE
GH2
20
8
TEMP RANGE
21
2
N.C.
PART
1
REF
SH/SUS
Ordering Information
ILIM
THIN QFN
5mm × 5mm
________________________________________________________________ Maxim Integrated Products
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.
1
MAX1996A
General Description
The MAX1996A integrated controller is optimized to
drive cold-cathode fluorescent lamps (CCFLs) using
synchronized full-bridge inverter architecture.
Synchronized drive provides near sinusoidal waveforms
over the entire input range to maximize the life of
CCFLs. The controller also operates over a wide inputvoltage range with high efficiency and broad dimming
range.
The MAX1996A includes safety features that limit the
transformer secondary voltage and protect against single-point fault conditions including lamp-out and shortcircuit faults.
The MAX1996A regulates the CCFL brightness in three
ways: linearly controlling the lamp current, digital pulsewidth modulating (DPWM) the lamp current, or using
both methods simultaneously to achieve the widest
dimming range (>30:1). CCFL brightness can be controlled with either an analog voltage or a 2-wire
SMBus™-compatible interface. The MAX1996A 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 synchronizable DPWM
oscillator, and most of the internal circuitry.
The MAX1996A has the same pin configuration as the
MAX1895, but with modified SMBus slave address
(0x58) and command bytes. In addition, the lamp-out
protection timer has been reduced to approximately 1s
and the DPWM frequency is guaranteed from 200Hz to
220Hz over the operating temperature range without
external components or trimming. The MAX1996A is
available in the space-saving 28-pin thin QFN package
and operates over a -40°C to +85°C temperature
range.
MAX1996A
High-Efficiency, Wide Brightness
Range, 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
GH1 to LX1 ...............................................-0.3V to (BST1 + 0.3V)
GH2 to LX2 ...............................................-0.3V to (BST2 + 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)
MINDAC, IFB, CCV, CCI to GND .............................-0.3V to +6V
MODE to GND ...........................................................-6V to +12V
VFB to GND .................................................................-6V to +6V
CRF/SDA, CTL/SCL, SH/SUS to GND ......................-0.3V to +6V
PGND to GND .......................................................-0.3V to +0.3V
Continuous Power Dissipation (TA = +70°C)
28-Pin QFN (derate 20.84mW/°C above +70°C) .......1667mW
Operating Temperature Range ...........................-40°C to +85°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
(VBATT = 12V, MINDAC = GND, VCC = VDD, V SH/SUS = 5.3V, TA = 0°C to +85°C, unless otherwise noted. Typical values are at
TA = +25°C.) (Note 1)
PARAMETER
VBATT Input Voltage Range
CONDITIONS
MIN
5.5
VCC = VDD = open
5.5
28
VBATT = 28V
V SH /SUS = 5.5V
VBATT Quiescent Current, Shutdown
SH /SUS = 0
VCC Output Voltage, Normal Operation
V SH /SUS = 5.5V, 6V < VBATT < 28V
0 < ILOAD < 20mA
VCC Output Voltage, Shutdown
SH /SUS = GND, no load
VCC Undervoltage Lockout (UVLO)
Threshold
VCC rising (leaving lockout)
3.2
VBATT = VCC = 5V
6.0
6
UNITS
V
mA
6
20
µA
5.00
5.35
5.50
V
3.5
4.6
5.5
V
4.5
V
VCC falling (entering lockout)
4.0
Rising edge
0.90
1.75
1.96
2.00
2.04
V
2
6
Ω
VCC UVLO Lockout Hysteresis
200
VCC POR Hysteresis
Falling edge
REF Output Voltage, Normal Operation
4.5V < VCC < 5.5V, ILOAD = 40µA
GH1, GH2, GL1, GL2 On-Resistance
ITEST = 100mA, VCC = VDD = 5.3V
mV
2.70
50
GH1, GH2, GL1, GL2 Maximum Output
Current
BST_ = 12V, LX_ = 7V
Input Resonant Frequency
Guaranteed by design
20
V
mV
1
BST1, BST2 Leakage Current
A
5
µA
300
kHz
Minimum Off-Time
210
315
420
ns
Maximum Off-Time
21.0
31.5
42.0
µs
180
200
220
mV
Maximum Current-Limit Threshold
LX1-GND, LX2-GND (Fixed)
Maximum Current-Limit Threshold
LX1-GND, LX2-GND (Adjustable)
2
MAX
4.6
VBATT Quiescent Current
VCC Power-On Reset (POR) Threshold
TYP
VCC = VDD = VBATT
ILIM = VCC
VILIM = 0.5V
80
100
120
VILIM = 2.0V
370
400
430
_______________________________________________________________________________________
mV
High-Efficiency, Wide Brightness
Range, CCFL Backlight Controller
(VBATT = 12V, MINDAC = GND, VCC = VDD, V SH/SUS = 5.3V, TA = 0°C to +85°C, unless otherwise noted. Typical values are at
TA = +25°C.) (Note 1)
PARAMETER
CONDITIONS
MIN
Minimum Current-Crossing Threshold
LX1-GND, LX2-GND
MAX
6
Current-Limit Leading-Edge Blanking
D/A Converter Resolution
TYP
210
Guaranteed monotonic
315
UNITS
mV
420
5
ns
Bits
MINDAC Input Voltage Range
0
2
V
MINDAC Input Bias Current
-2
+2
µA
4.0
V
1.7
V
MINDAC Digital PWM Disable Threshold
MINDAC = VCC
IFB Input Voltage Range
368
30
50
70
180
200
220
-2
125
1V < VCCI < 2.5V
CCI Output Impedance
VFB Input Voltage Range
VFB = 0V
VFB Regulation Point
VFB to CCV Transconductance
150
+2
µA
175
mV
100
µS
20
MΩ
-2
+2
V
+0.5
µA
530
mV
490
510
40
-10
CCV Output Impedance
µS
+10
20
No AC signal on MODE
200
210
32kHz AC signal on MODE
250
100kHz AC signal on MODE
781
MODE-to-DPWM Sync Ratio
fMODE/fDPWM
128
Lamp-Out Detection Timeout Timer
(Note 2)
VIFB < 0.1V
No AC signal on MODE
1.14
1.22
32kHz AC signal on MODE
1.02
100kHz AC signal on MODE
0.33
MODE Operating Voltage Range
mV
-0.5
1V < VCCV < 2.7V
VFB Zero-Voltage Crossing Threshold
Digital PWM Chop-Mode Frequency
408
VMINDAC = 1V, DAC code = 00000 binary
IFB Lamp-Out Threshold
VFB Input Bias Current
388
VMINDAC = 0V, DAC code = 00100 binary
IFB Input Bias Current
IFB to CCI Transconductance
3.5
0
VMINDAC = 0V, DAC code = 11111 binary
IFB Regulation Point
2.4
mV
MΩ
220
Hz
1.30
s
-5.5
11.0
V
-1
+1
µA
0.6
V
2.6
V
MODE Input Current
MODE = GND or VCC
Positive Analog Interface Mode,
MODE = GND Threshold (VCTL/SCL = 0V
Sets Minimum Brightness)
Sync clock average value on MODE to sync
DPWM oscillator, not in shutdown (Note 3)
Negative Analog Interface Mode,
MODE = REF Threshold (VCTL/SCL = 0V
Sets Maximum Brightness = 0V)
Sync clock average value on MODE to sync
DPWM oscillator, not in shutdown (Note 3)
1.4
SMBus Interface Mode, MODE = VCC
Threshold
Sync clock average value on MODE to sync
DPWM oscillator, not in shutdown (Note 3)
VCC 0.6
V
_______________________________________________________________________________________
3
MAX1996A
ELECTRICAL CHARACTERISTICS (continued)
MAX1996A
High-Efficiency, Wide Brightness
Range, CCFL Backlight Controller
ELECTRICAL CHARACTERISTICS (continued)
(VBATT = 12V, MINDAC = GND, VCC = VDD, V SH/SUS = 5.3V, TA = 0°C to +85°C, unless otherwise noted. Typical values are at
TA = +25°C.) (Note 1)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
MODE AC Signal Amplitude
Peak-to-peak (Note 4)
2
5
V
MODE AC Signal Synchronization Range
Chopping oscillator synchronized to MODE
32
100
kHz
2.7
5.5
V
-1
+1
0
VCRF/SDA
CRF/SDA Input Range
CRF/SDA Input Current
VCRF/SDA = 5.5V, SH /SUS = VCC
VCRF/SDA = 5.5V, SH /SUS = 0V
CTL/SCL Input Range
CTL/SCL Input Current
MODE = REF or GND
A/D Converter Resolution
Guaranteed monotonic
20
-1
+1
5
A/D Converter Hysteresis
2.1
SH /SUS Input Bias Current
-1
+1
0.8
SDA, SCL Input High Voltage
2.1
SDA, SCL Input Hysteresis
V
V
mV
300
SDA, SCL Input Low Voltage
µA
LSB
0.8
SH /SUS Input High Voltage
SH /SUS Input Hysteresis
V
Bits
1
SH /SUS Input Low Voltage
µA
µA
V
V
300
mV
SDA Output Low Sink Current
VCRF/SDA = 0.4V
4
mA
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
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
700
ns
ELECTRICAL CHARACTERISTICS
(VBATT = 12V, MINDAC = GND, VCC = VDD, V SH/SUS = 5.3V, TA = -40°C to +85°C, unless otherwise noted.) (Note 1)
PARAMETER
VBATT Input Voltage Range
4
CONDITIONS
MIN
TYP
MAX
VCC = VDD = VBATT
4.6
5.5
VCC = VDD = open
5.5
28.0
VBATT Quiescent Current
V SH/SUS = 5.5V
VBATT Quiescent Current, Shutdown
V SH/SUS = 0V
VBATT = 28V
6
VBATT = VCC = 5V
6
_______________________________________________________________________________________
20
UNITS
V
mA
µA
High-Efficiency, Wide Brightness
Range, CCFL Backlight Controller
(VBATT = 12V, MINDAC = GND, VCC = VDD, V SH/SUS = 5.3V, TA = -40°C to +85°C, unless otherwise noted.) (Note 1)
PARAMETER
CONDITIONS
VCC Output Voltage, Normal Operation
VCC Output Voltage, Shutdown
MIN
MAX
UNITS
V SH/SUS = 5.5V, 6V < VBATT < 28V,
0 < ILOAD < 20mA
5.0
5.5
V
SH/SUS = GND, no load
3.5
5.5
V
VCC rising (leaving lockout)
VCC UVLO Threshold
TYP
VCC rising (entering lockout)
4.5
4
V
VCC POR Threshold
Rising edge
0.9
2.7
V
REF Output Voltage, Normal Operation
4.5V < VCC < 5.5V, ILOAD = 40µA
1.96
2.04
V
GH1, GH2, GL1, GL2 On-Resistance
ITEST = 100mA
10
Ω
Maximum Current-Limit Threshold
LX1-GND, LX2-GND (Fixed)
ILIM = VCC
180
220
mV
VILIM = 0.5V
80
120
VILIM = 2.0V
360
440
0
1.7
V
VMINDAC = 0V, DAC code = 11111 binary
335
440
mV
-2
+2
µA
IFB Lamp-Out Threshold
120
180
mV
VFB Input Voltage Range
-2
+2
V
-0.5
0.5
µA
VFB Regulation Point
480
540
mV
VFB Zero-Voltage Crossing Threshold
-20
+20
mV
0.8
V
Maximum Current-Limit Threshold
LX1-GND, LX2-GND (Adjustable)
IFB Input Voltage Range
IFB Regulation Point
IFB Input Bias Current
VFB Input Bias Current
VFB = 0V
SHVSUS Input Low Voltage
SHVSUS Input High Voltage
2.1
SDA, SCL Input Low Voltage
V
0.8
SDA, SCL Input High Voltage
SDA Output Low Sink Current
VCRF/SDA = 0.4V
mV
V
2.1
V
4
mA
Note 1: Specifications to -40°C are guaranteed by design based on final test characterization results.
Note 2: Corresponds to 256 DPWM cycles or 32768 MODE cycles.
Note 3: The MODE pin thresholds are only valid while the part is operating. When in shutdown, VREF = 0 and the part only differentiates between SMB mode and ADC mode. When in shutdown and with ADC mode selected, the CRF/SDA and CTL/SCL
pins are at high impedance and do not cause extra supply current when their voltages are not at GND or VCC.
Note 4: The amplitude is measured with the following circuit:
VAMPLITUDE > 2V
500pF
MODE
10kΩ
_______________________________________________________________________________________
5
MAX1996A
ELECTRICAL CHARACTERISTICS (continued)
Typical Operating Characteristics
(VBATT = 12V, VCTL = VCRF, VMINDAC = 1V, MODE = GND, circuit of Figure 1, Table 4.)
HIGH INPUT-VOLTAGE OPERATION
(VBATT = 20V)
MAX1996 toc01
FEED-FORWARD COMPENSATION
MAX1996 toc02
VFB
2V/div
MAX1996 toc03
20V
VBATT
10V
VFB
2V/div
IFB
2V/div
IFB
2V/div
LX1
10V/div
LX1
10V/div
LX2
10V/div
LX2
10V/div
VFB
2V/div
IFB
2V/div
LX1
10V/div
10µs/div
10µs/div
20µs/div
STARTUP
SYNCHRONIZED DPWM
(fMODE = 100kHz, DPWM = 50%)
SYNCHRONIZED DPWM
(fMODE = 32kHz, DPWM = 50%)
MAX1996 toc06
MAX1996 toc05
MAX1996 toc04
12V
VBATT
0V
VFB
2V/div
IFB
2V/div
IBATT
500mA/div
IFB
1V/div
IFB
1V/div
VFB
1V/div
VFB
1V/div
LX1
10V/div
LX1
10V/div
LX2
10V/div
LX2
10V/div
1ms/div
1ms/div
1ms/div
LAMP-OUT PROTECTION
LAMP-OUT VOLTAGE LIMITING
VCC vs. VBATT
MAX1996 toc08
MAX1996 toc07
6
NORMAL OPERATION
1s
5
VSECONDARY
2kV/div
VSECONDARY
2kV/div
SHUTDOWN
4
VFB
2V/div
LAMP REMOVED
2ms/div
IFB
1V/div
VFB
2V/div
3
2
1
LAMP REMOVED
200ms/div
IFB
1V/div
0
0
5
10
15
VBATT (V)
6
MAX1996 toc09
LOW INPUT-VOLTAGE OPERATION
(VBATT = 8V)
VCC (V)
MAX1996A
High-Efficiency, Wide Brightness
Range, CCFL Backlight Controller
_______________________________________________________________________________________
20
25
High-Efficiency, Wide Brightness
Range, CCFL Backlight Controller
VCC vs. TEMPERATURE
VCC LOAD REGULATION
MAX1996 toc10
5.40
4.6
5.36
4.5
5.35
NORMAL OPERATION
5.25
4.3
5.20
4.2
5.15
4.1
4.0
5.10
1
10
100
4.55
4.50
5.34
NORMAL OPERATION
5.33
4.45
5.32
4.40
SHUTDOWN VCC (V)
4.4
VCC (V)
VCC (V)
5.30
SHUTDOWN VCC (V)
SHUTDOWN
0.1
4.60
SHUTDOWN
5.35
0.01
MAX1996 toc11
4.35
5.31
-40
-15
10
35
60
85
TEMPERATURE (°C)
ILOAD (mA)
Pin Description
PIN
NAME
FUNCTION
1
ILIM
Current-Limit Threshold Adjustment. Bias ILIM with a resistive voltage-divider between REF or VCC
and GND. The current-limit threshold measured between LX_ and GND is 1/5th the voltage at ILIM;
ILIM adjustment range is 0V to 3V. Connect ILIM to VCC to set the default current-limit threshold to
0.2V.
2
REF
2V Reference Output. Bypass REF to GND with a 0.1µF capacitor. REF is discharged to GND when
shut down.
3
MINDAC
4
GND
5
MODE
6
CRF/SDA
DAC Zero-Scale Input. VMINDAC sets the D/A converter’s minimum-scale output voltage. Disable
DPWM by connecting MINDAC to VCC.
System Ground. The GND input to the maximum and minimum current-limit comparators. The
comparators sense the low-side FET NL1 and NL2 for zero-current crossing and current limit.
Interface Selection Input and Sync Input for DPWM Chopping. The average voltage on the MODE
pin selects one of three CCFL brightness control interfaces:
MODE = VCC enables SMBus serial interface.
MODE = GND enables the analog interface (positive analog interface mode), VCTL/SCL = 0V sets
minimum brightness.
MODE = REF enables the analog interface (reverse analog interface mode), VCTL/SCL = 0V sets
maximum brightness.
An AC clocking signal superimposed on the DC average MODE pin voltage can be used to
synchronize the DPWM chopping frequency. See Synchronizing the DPWM Frequency.
Reference and Serial Data Input. In analog interface mode, pin 6 is the reference input to the 5-bit
brightness control ADC. Bypass CRF to GND with a 0.1µF capacitor. In SMBus interface mode,
SDA is an SMBus serial data input/open-drain output.
_______________________________________________________________________________________
7
MAX1996A
Typical Operating Characteristics (continued)
(VBATT = 12V, VCTL = VCRF, VMINDAC = 1V, MODE = GND, circuit of Figure 1, Table 4.)
High-Efficiency, Wide Brightness
Range, CCFL Backlight Controller
MAX1996A
Pin Description (continued)
PIN
NAME
7
CTL/SCL
Brightness Control and Serial Clock Input. In analog interface mode, pin 7 is a CCFL brightness
control input. CTL varies from 0V to REF to linearly control lamp brightness. In SMBus interface
mode, SCL is an SMBus serial clock input.
8
SH/SUS
Shutdown and Suspend Mode Control. In analog interface mode, pin 8 is an active-low shutdown
input. In SMBus interface mode, pin 8 is an SMBus suspend control input.
9, 10, 11, 23
N.C.
No Connection. Not internally connected.
12
VDD
Power Supply for Gate Drivers. Connect VDD to the output of the linear regulator (VCC). Bypass VDD
with a 0.1µF capacitor to PGND.
13
PGND
14
GL2
Low-Side FET NL2 Gate-Driver Output
15
GL1
Low-Side FET NL1 Gate-Driver Output
16
GH1
High-Side FET NH1 Gate-Driver Output
17
LX1
Switching Node Connection. LX1 is the internal lower supply rail for the GH1 high-side gate driver.
LX1 is also the sense input to the current comparators.
18
BST1
High-Side FET NH1 Driver Bootstrap Input. Connect BST1 through a diode to VDD and through a
0.1µF capacitor to LX1 (Figure 1).
19
BST2
High-Side FET NH2 Driver Bootstrap Input. 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 lower supply rail for the GH2 high-side gate driver.
LX2 is also the sense input to the current comparators.
21
GH2
High-Side FET 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 voltagedivider 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 150mV for 1s, then the MAX1996A signals an open-lamp fault.
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 in which fullbridge applies the input voltage (BATT) to transformer network. Connect CCI to GND through a
0.1µF capacitor. CCI is internally discharged to GND in shutdown.
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. Connect CCV to GND
with a 10nF capacitor. The CCV voltage controls the time interval that the full bridge applies the
input voltage (BATT) to transformer network. CCV is internally discharged to GND in shutdown.
27
BATT
Supply Input. Input to the internal 5.3V linear regulator that provides power (VCC) to the chip.
Bypass BATT to GND with a 0.1µF capacitor.
28
VCC
5.3V Linear-Regulator Output. VCC is the supply voltage for the MAX1996A. Bypass VCC to GND
with a 0.47µF ceramic capacitor. VCC can also be connected to BATT if VBATT < 5.5V.
25
8
FUNCTION
Power Ground. Gate-driver current flows through this pin.
_______________________________________________________________________________________
High-Efficiency, Wide Brightness
Range, CCFL Backlight Controller
MAX1996A
VIN
5V TO 28V
C1
NH1
NH2
D1
C2
T1
CCFL
C3
NL1
NL2
GL2
GH2
PGND
BATT
GH1
GL1
C4
LX1
R1
R2
LX2
C5
C6
BST1
BST2
VCC
VDD
D2-2
D2-1
C7
MAX1996A
GND
VFB
CCV
IFB
MODE
SH/SUS
CRF/SDA
CTL/SCL
CCI
REF
C9
MINDAC
C8
ILIM
R3
ON/OFF
R4
C10
REFERENCE INPUT
CONTROL INPUT
Figure 1. Standard Application Circuit
Detailed Description
The MAX1996A is optimized to drive CCFLs using a
synchronized full-bridge inverter architecture. The drive
to the full-bridge MOSFETs is synchronized to the resonant frequency of the tank circuit so that the CCFL’s
full-strike voltage develops for all operating conditions.
The synchronized architecture provides near sinusoidal
drive waveforms over the entire input range to maximize the life of CCFLs. The MAX1996A operates over a
wide input voltage range (4.6V to 28V), achieves high
efficiency, and maximizes dimming range.
The MAX1996A regulates the brightness of a CCFL in
three ways:
1) Linearly controlling the lamp current.
2) Digitally pulse-width modulating (or chopping) the
lamp current (DPWM).
3) Using both methods simultaneously for widest dimming range.
DPWM is implemented by pulse-width modulating the
lamp current at a rate faster than the eye can detect.
The MAX1996A includes a 5.3V linear regulator to
power the drivers for full-bridge switches, the synchronizable DPWM oscillator, and most of the internal circuitry. The MAX1996A is very flexible and can be
controlled with an analog interface or with an SMBus
interface.
_______________________________________________________________________________________
9
MAX1996A
High-Efficiency, Wide Brightness
Range, CCFL Backlight Controller
VBATT
VBATT
NH1
ON
NH2
OFF
NH1
OFF
NH2
ON
T1
T1
C2
C2
LX1
LX1
LX2
LX2
NL1
NL2
NL1
NL2
OFF
ON
ON
OFF
(a)
(c)
VBATT
VBATT
NH1
OFF
NH2
OFF
NH1
OFF
NH2
OFF
T1
T1
C2
C2
LX1
LX2
LX1
NL2
ON
NL1
ON
LX2
NL1
ON
NL2
ON
(BODY DIODE TURNS ON FIRST)
(BODY DIODE TURNS ON FIRST)
(b)
(d)
Figure 2. Resonant Operation
Resonant Operation
The MAX1996A drives the four N-channel power
MOSFETs that make up the zero-voltage switching
(ZVS) full-bridge inverter as shown in Figure 1. The LX1
and LX2 switching nodes are AC coupled to the primary side of the transformer.
Assume that NH1 and NL2 are turned on at the beginning of the cycle as shown in Figure 2(a). The primary
current flows through MOSFET NH1, DC blocking cap
C2, the primary side of transformer T1, and finally MOSFET NL2. During this interval, the primary current ramps
up until the controller turns off NH1. When NH1 is off,
the primary current forward biases the body diode of
NL1 and brings the LX1 node down as shown in Figure
2(b). When the controller turns on NL1, its drain-tosource 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 finally NL2. Once the primary current drops
10
to the minimum current threshold (6mV/RDSON), 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 2(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 off,
the primary current forward biases the body diode of
NL2, and brings the LX2 node down as shown in Figure
2(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 2(a).
______________________________________________________________________________________
High-Efficiency, Wide Brightness
Range, CCFL Backlight Controller
AC
SOURCE
CP
CS/(NxN)
AC
SOURCE
CCFL
1:N
LI
CP
RB
Figure 3. Equivalent Circuit
Note that switching transitions on all four power
MOSFETs occur under ZVS conditions, which reduces
transient power losses and EMI.
The equivalent circuit of the resonant tank is shown in
Figure 3. The resonant frequency is determined by the
RLC resonant tank elements: CS, CP, LL, and RB. CS is
the series capacitance on the primary side of the transformer. CP is the parallel cap on the transformer’s secondary. L L is the transformer secondary leakage
inductance. RB is an idealized resistance that models
the CCFL load in normal operation.
Current and Voltage-Control Loops
The MAX1996A uses a current loop and a voltage loop
to control the energy applied to the CCFL. The current
loop is the dominant control in setting the lamp brightness. The rectified lamp current is measured with a
sense resistor in series with the CCFL. The voltage
across this resistor is applied to the IFB input to regulate
the average lamp current. The voltage loop controls the
voltage across the lamp and is active during the beginning of DPWM on-cycles and the open-lamp fault condition. It limits the energy applied to the resonant network
once the transformer secondary voltage is above the
threshold of 500mV average measured at VFB.
Both voltage and current circuits use transconductance-error amplifiers to compensate the loops. The
voltage-error amplifier creates an error current based
upon the voltage difference between VFB and the internal reference level (typically 500mV) (Figure 4). The
error current is then used to charge and discharge a
capacitor at the CCV output to create an error voltage
VCCV. The current loop produces a similar signal at CCI
based on the voltage difference between IFB and the
dimming control signal. This signal is set by either the
SMBus interface or the analog interface (see the
Dimming Range section). This error voltage is called
VCCI. In normal operation, the current loop is in control
of the regulator so long as VCCI is less than VCCV. The
control signal is compared with an internal ramp signal
to set the high-side switch on time (tON).
When DPWM is employed, the two control loops work
together to limit the transformer voltage and to allow a
wide dimming range with good line rejection. During the
DPWM off-cycle, VCCV is set to 1.2V and the currentloop error amplifier output is high impedance. VVFB is
set to 0.6V to create a soft-start at the beginning of each
DPWM on-cycle in order to avoid overshoot on the transformer’s secondary. When the transconductance amplifier in the current loop is high impedance, it acts like a
sample-and-hold circuit to keep VCCI from changing
during the off-cycles. This action allows the current-control loop to regulate the average lamp current.
See the Current-Sense Resistor and the Voltage-Sense
Capacitors sections for information regarding setting
the current- and voltage-loop thresholds.
Startup
Operation during startup differs from the steady-state
condition described in the Current and Voltage-Control
Loops section. Upon power-up, V CCI slowly rises,
increasing the duty cycle, which provides soft-start.
During this time, VCCV, which is the faster control loop,
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. With MINDAC = VCC, DPWM is disabled
and the current loop remains in control regulating the
lamp current.
Feed-Forward Control
The MAX1996A has a feed-forward control circuit,
which influences both control loops. Feed-forward control instantly adjusts the tON time to changes in input
voltage. This feature provides immunity to changes in
input voltage at all brightness levels and makes compensation over wide input ranges easier. The feed-forward circuit improves line regulation for short DPWM
on-times and makes startup transients less dependent
on input voltage.
Feed-forward control is implemented by varying the
internal voltage ramp rate. This has the effect of varying
tON as a function of 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
change in VBATT is essentially instantaneous.
______________________________________________________________________________________
11
MAX1996A
CS
MAX1996A
High-Efficiency, Wide Brightness
Range, CCFL Backlight Controller
REFERENCE
INPUT
CRF/SDA
LAMP CURRENT AND
DPWM CONTROL
MINDAC
CTL/SCL
SMBus
CONTROL
INPUT
BATT
MODE
INPUT
VOLTAGE
DPWM
OSC
VCC
DPWM
COMP
SUPPLY
REF
SH/SUS
MINDAC = VCC
Y = 1, N =0
GND
0.15V
CCV
BST1
0.5V
PWM COMP
GH1
GMV
VFB
LX1
CCV
CLAMP
CONTROL
LOGIC
RAMP
GENERATOR
PEAK
DETECTOR
GMI
IFB
PK_DET
CLAMP
GH2
IMIN COMP
LX2
4mV
LX2
LX1
BST2
CCFL
CCI
GL1
MUX
VDD
REF
GL2
ILMIT
IMAX COMP
PGND
GND
MAX1996A
Figure 4. Functional Diagram
Transient Overvoltage Protection
from Dropout
The MAX1996A is designed to maintain tight control of
the transformer secondary under all transient conditions
including dropout. To maximize run time, it is desirable
to allow the circuit to operate in dropout at extremely
low battery voltages where the backlight’s performance
is not critical. When VBATT is very low, the controller
can lose regulation and run at maximum duty cycle.
Under these circumstances, a transient overvoltage
condition can occur when the AC adapter is suddenly
applied to power the circuit. The feed-forward circuitry
minimizes variations in lamp voltage due to such input
voltage steps. The regulator also clamps the voltage on
VCCI. Both features ensure that overvoltage transients
12
do not appear on the transformer when leaving
dropout.
The VCCI clamp is unique in that it limits at the peaks of
the voltage-ramp generator. As the circuit reaches
dropout, VCCI approaches the peaks of the ramp generator in order to reach maximum tON. If VBATT decreases
further, the control loop loses regulation and VCCI tries to
reach its positive supply rail. The clamp on VCCI prevents this from happening and VCCI rides just above the
peaks of the PWM ramp. If VBATT continues to decrease,
the feed-forward PWM ramp generator loses amplitude
and the clamp drags VCCI down with it to a voltage
below where VCCI would have been if the circuit were not
in dropout. When VBATT suddenly steps out of dropout,
VCCI is still low and the MAX1996A maintains the drive
on the transformer at the old dropout level. The control
______________________________________________________________________________________
High-Efficiency, Wide Brightness
Range, CCFL Backlight Controller
DIGITAL INTERFACE
PIN
ANALOG INTERFACE
MODE = REF
VCTL/SCL = 0 = maximum brightness
MODE = VCC
SH/SUS
SMBus suspend
CRF/SDA
SMBus data I/O
CTL/SCL
SMBus clock input
MODE = GND
VCTL/SCL = 0 = minimum brightness
Logic level shutdown control input
Reference input for minimum brightness
Reference input for maximum
brightness
Analog control input to set brightness (range from 0 to CRF/SDA)
loop then slowly corrects the lamp current by increasing
VCCI, which brings the circuit back into regulation.
Interface Selection
Table 1 describes the functionality of SH/SUS, CRF/
SDA, and CTL/SCL in each of the MAX1996A’s three
interface modes. The MAX1996A features both an
SMBus digital interface and an analog interface. Note
that the MODE signal can also synchronize the DPWM
frequency. (See Synchronizing the DPWM Frequency.)
Dimming Range
The brightness is controlled by either the Analog Interface
(see the Analog Interface section) or the SMBus Interface
(see the SMBus Interface section). The brightness of the
CCFL is adjusted in the following three ways:
1) Lamp-current control, where the magnitude of the
average lamp current is adjusted.
2) DPWM control, where the average lamp current is
pulsed to the set level with a variable duty cycle.
3) The combination of the first two methods.
In each of the three methods, a 5-bit brightness code is
generated from the selected interface and is used to
set the lamp current and/or DPWM duty cycle.
The 5-bit brightness code defines the lamp current
level with 00000\b representing minimum lamp current
and 11111\b representing maximum lamp current. The
average lamp current is measured across an external
sense resistor (see the Current-Sense Resistor section).
The voltage on the sense resistor is measured at IFB.
The brightness code adjusts the regulation voltage at
IFB (VIFB). The minimum average VIFB is VMINDAC/5,
where VMINDAC varies between 0 to 2V, and the maximum average is set by the following formula:
VIFB = VREF ✕ 31 / 160 + VMINDAC / 160,
which is between 387.5mV and 400mV.
If VIFB does not exceed 150mV peak (which is about
47.7mV/R1 RMS lamp current) for greater than 1s, the
MAX1996A assumes a lamp-out condition and shuts
down (see the Lamp-Out Detection section).
The equation relating brightness code to IFB regulation
voltage is:
VIFB = VREF ✕ n / 160 + VMINDAC ✕ (32 - n) / 160
where n is the brightness code.
To always use maximum average lamp current when
using DPWM control, set VMINDAC to VREF.
DPWM control is similar to lamp-current control in that it
also responds to the 5-bit brightness code. A brightness code of 00000\b corresponds to a 9% DPWM duty
cycle and a brightness code of 11111\b corresponds to
a 100% DPWM duty cycle. The duty cycle changes by
3.125% per step, but codes 00000\b to 00011\b all produce 9% (Figure 5).
To disable DPWM and always use 100% duty cycle, set
VMINDAC to VCC. Note that with DPWM disabled, the
equations shown above should assume VMINDAC = 0
instead of VMINDAC = VCC. Table 2 describes MINDAC’s functionality and Table 3 shows some typical
settings for the brightness adjustment.
In normal operation, VMINDAC is set between zero and
VREF and the MAX1996A uses both lamp-current control and DPWM control to vary the lamp brightness
(Figure 6). In this mode, lamp-current control regulates
the average lamp current during a DPWM on-cycle.
Analog Interface and Brightness Code
The MAX1996A’s analog interface uses an internal ADC
with 1-bit hysteresis to generate the brightness code
used to dim the lamp (see the Dimming Range section).
CTL/SCL is the ADC’s input and CRF/SDA is its reference voltage. The ADC can operate in either positivescale ADC mode or negative-scale ADC mode. In
positive-scale ADC mode, the brightness code increases from 0 to 31 as VCTL increases from zero to VCRF.
In negative-scale mode, the brightness scale decreases
from 31 to zero as VCTL increases from zero to VCRF.
______________________________________________________________________________________
13
MAX1996A
Table 1. Interface Modes
MAX1996A
High-Efficiency, Wide Brightness
Range, CCFL Backlight Controller
COMBINED POWER LEVEL (BOTH
DPWM AND LAMP-CONTROL CURRENT)
DPWM SETTINGS
100
100
90
COMBINED POWER LEVEL (%)
90
DPWM DUTY CYCLE (%)
80
70
60
50
40
30
20
80
70
60
50
40
30
20
10
10
0
0
0
4
8
12
16
20
24
28
0
32
4
8
12
16
20
24
28
32
BRIGHTNESS CODE
BRIGHTNESS CODE
Figure 6. Combined Power Level
Figure 5. DPWM Settings
Table 2. MINDAC Functionality
CONDITION
FUNCTION
MINDAC = VCC
DPWM disabled (always on 100% duty cycle). Operates in lamp-current control only.
(Use VMINDAC = 0 in the equations.)
MINDAC = REF
DPWM control enabled, duty cycle ranges from 9% to 100%.
Lamp-current control is disabled (always maximum current).
0 ≤ VMINDAC < VREF
The device uses both lamp-current control and DPWM.
Table 3. Brightness Adjustment Ranges
SMBus
DAC
OUTPUT
DPWM
DUTY
CYCLE (%)
COMBINED
POWER
LEVEL (%)
MODE = REF,
VCRF/SDA = 0
Bright [4:0] = 11111
Full-scale
DAC output =
387.5mV
100
100
MODE = REF,
VCRF/SDA = VCTL/SCL,
VMINDAC = 1/3VREF
Bright [4:0] = 00000
VMINDAC = 1/3VREF
Zero-scale
DAC output =
VMINDAC/5
9
3
POSITIVE-SCALE
ADC MODE
NEGATIVE-SCALE
ADC MODE
Maximum
Brightness
MODE = GND,
VCRF/SDA =
VCTL/SCL
Minimum
Brightness
MODE = GND,
VCRF/SDA = 0,
VMINDAC = 1/3VREF
RANGE
Note: The current level range is solely determined by the MINDAC to REF ratio and is externally set.
14
______________________________________________________________________________________
High-Efficiency, Wide Brightness
Range, CCFL Backlight Controller
VCCA
CONVENTIONAL
INTERFACE
VCTL(TH) = (n + 2) / 33 VCRF (Positive-Scale ADC
mode, MODE = GND)
VCTL(TH) = (33 - n) / 33 VCRF (Negative-Scale ADC
mode, MODE = REF)
VCTL’s negative threshold is the voltage required to
transition the brightness code as VCTL decreases and
can be calculated as follows:
VCTL(TH) = n / 33 VCRF (Positive-Scale ADC mode,
MODE = GND)
VCTL(TH) = (31 - n) / 33 VCRF (Negative-Scale ADC
mode, MODE = REF)
where n is the brightness code. See Figure 7 for a
graphical representation of the thresholds.
31
BRIGHTNESS CODE
30
29
3
2
1
0
1
33
2
33
3
33
4
33
VCTL
VCRF
1
32
33
31
33
30
33
29
33
VCTL
VCRF
Figure 7. Brightness Code
30
33
31
33
32
33
1
(MODE = GND)
3
33
(MODE = REF)
2
33
MAX1996A
The analog interface’s internal ADC uses 1-bit hysteresis to keep the lamp from flickering between two codes.
V CTL ’s positive threshold (V CTL(TH) ) is the voltage
required to transition the brightness code as V CTL
increases and can be calculated as follows:
VCCB
DIMMING
CONTROL
CIRCUIT
MIN
DIM
CIRCUIT
VCTL
INVERTER
CONTROLLER
0 TO VMAX
VCCA
VCCB
VCRF
MAX1996A
INTERFACE
DIMMING
CONTROL
CIRCUIT
VCTL
MAX1996A
MINDAC
REF
Figure 8. Analog Interface for Dimming
See the Digital Interface section for instructions on
using the SMBus interface.
Unlike conventional dimming control circuits that have
separate supplies and require additional minimum
brightness circuitry, the MAX1996A provides dedicated
pins for dimming control. The advantages of the
MAX1996A’s analog interface are illustrated in Figure 8.
The analog interface is very simple in that the output
voltage range of the dimming control circuit matches
the input voltage range of the inverter control IC. With
this method, it is possible to guarantee the maximum
dimming range (Figure 9). For the conventional interface, the control voltage and the input voltage have different ranges. To avoid nonuniform lighting across the
CCFL tube, or the thermometer effect, the lower limits of
maximum and minimum control voltages have to be
above the upper limits of the maximum and minimum
input voltages, respectively. Therefore, the useful dimming range is reduced. For the MAX1996A’s analog
interface, the control voltage has the same range as the
input voltage, so the useful dimming range is maximized.
Synchronizing the DPWM Frequency
1
33
0
MODE has two functions: one is to select the interface
mode as described in the Interface Selection section
and the other is to synchronize the DPWM chopping
frequency to an external signal to prevent unwanted
artifacts in the display screen.
To synchronize the DPWM frequency, connect MODE to
VCC, REF, or GND through a 10kΩ resistor. Then connect
______________________________________________________________________________________
15
MAX1996A
High-Efficiency, Wide Brightness
Range, CCFL Backlight Controller
MAX BRIGHTNESS
CONTROL VOLTAGE
TOLERANCE
MAX BRIGHTNESS
INPUT VOLTAGE
CONVENTIONAL INTERFACE
TYPICAL DIMMING
RANGE
MIN BRIGHTNESS
CONTROL VOLTAGE
TYPICAL DIMMING
RANGE LOST
MIN BRIGHTNESS
INPUT VOLTAGE
GND
MAX BRIGHTNESS
CONTROL VOLTAGE
MAX BRIGHTNESS
INPUT VOLTAGE
TOLERANCE
MAX1996A
INTERFACE
MIN BRIGHTNESS
CONTROL VOLTAGE
TYPICAL DIMMING RANGE
MIN BRIGHTNESS
INPUT VOLTAGE
GND
Figure 9. Useful Dimming Range
a 500pF capacitor from an AC signal source to MODE as
shown in Figure 10. The amplitude of the AC signal must
be at least 2VP-P but no greater than 5VP-P for accurate
operation. The transition time of the AC signal should be
less than 200µs. The synchronization range is 32kHz to
100kHz, which corresponds to a DPWM frequency range
of 250Hz to 781Hz (128 MODE pulses per DPWM cycle).
High DPWM frequencies limit the dimming range. See the
Loop Compensation section for more information concerning high DPWM frequencies.
A simple oscillator circuit as shown in Figure 11 can be
used to generate the synchronization signal. The core of
the oscillator is the MAX9031, which is a low-cost, single16
supply comparator in a 5-pin SC70 package. The VCC
and REF of the MAX1996A provide the supply voltage
and the reference voltage for the oscillator. The positive
threshold of the oscillator is: VTH+ = (VCC + VREF)/2. The
negative threshold is given by: VTH- = VREF/2. The frequency of the oscillator is:
f=
1
VTH+ (VCC − VTH− )
RCln
VTH− (VCC − VTH+ )
For C = 330pF and R = 13kΩ, the resulting oscillator frequency is 100kHz. For C = 330pF and R = 39kΩ, the
oscillator frequency is 32kHz.
______________________________________________________________________________________
High-Efficiency, Wide Brightness
Range, CCFL Backlight Controller
REF
VCC
100kΩ
1%
ADC10kΩ
100kΩ
1%
REF
MAX1996A
VL
MAX1996A
MODE
MAX9031
SMBus
TO MODE
ADC+
R
500pF
GND
DPWM
SYNCHRONIZATION
SIGNAL
C
Figure 10. DPWM Synchronization
Figure 11. Simple RC Oscillator
POR and UVLO
The MAX1996A includes POR and UVLO circuits. The
POR resets all internal registers such as DAC output,
fault conditions, and all SMBus registers. POR occurs
when VCC is below 1.5V. The SMBus input-logic thresholds are only guaranteed to meet electrical characteristic limits for V CC 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
When the MAX1996A 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 interface. 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 by either writing to
the shutdown mode register (SMBus mode only) or with
SH/SUS.
Lamp-Out Detection
For safety, the MAX1996A monitors the lamp current to
detect the open-lamp fault. When the peak voltage on
IFB drops below 150mV (IFB regulation point must be
set above 48mV) the lamp-out timer starts. Before the
timer times out, VCCI increases the secondary voltage 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 voltage control loop is able to quickly limit the secondary voltage.
Without this clamping feature, the transformer voltage
would overshoot to dangerous levels because VCCV
would take more time to slew down from its supply rail. If
the peak voltage on IFB does not rise above 150mV
before timeout, the MAX1996A shuts down the full
bridge.
Overcurrent Fault Detection
and Protection
The MAX1996A senses overcurrent faults on each
switching cycle. The current comparator monitors the
voltage drop from LX_ to GND. If the voltage exceeds
the current-limit threshold, the regulator turns off the
high-side switch to prevent the transformer primary current from increasing further.
Applications Information
The MAX1996A’s standard application circuit, shown in
Figure 1, regulates the current of a 4.5W CCFL. The
IC’s analog voltage interface sets the lamp brightness
with a greater than 30 to 1 power adjustment range.
This circuit operates from a wide supply voltage range
of 4.6V to 28V. Typical applications for this circuit
include notebook, desktop monitor, and car navigation
displays. Table 4 shows the recommended components for the power stage of the 4.5W application. To
select the correct component values, several C CFL
parameters (Table 6) and the DC input characteristics
must be specified.
MOSFETs
The MAX1996A requires four external switches—NL1,
NL2, NH1, and NH2—to form a full bridge to drive CCFL.
The regulator senses drain-to-source voltage of NL1 and
NL2 to detect the transformer primary minimum current
crossing and overcurrent fault condition. RDSON of NL1
and NL2 should be matched. Select a dual logic-level N-
______________________________________________________________________________________
17
MAX1996A
High-Efficiency, Wide Brightness
Range, CCFL Backlight Controller
Table 4. Components for the Standard Application Circuit
DESIGNATION
C1
DESCRIPTION
1µF, 25V X7R ceramic capacitor
C3
15pF, 3.1kV high-voltage ceramic
capacitor
C5–C8, C10
C9
D1
D2
TMK325BJ475MN
Taiyo Yuden
www.t-yuden.com
C3225X7R1E475M
TDK
www.tdk.com
TMK316BJ105KL
C3216X7R1E105K
Taiyo Yuden
TDK
GHM1038-SL-150J-3K
Murata
www.murata.com
C4520C0G3F150K
TDK
0.015µF, 16V X7R ceramic capacitor
0.1µF,10V X5R ceramic capacitors
0.01µF, 16V X7R ceramic capacitor
100mA dual-series diode
100mA dual Schottky
diode common anode
EMK105BJ153KV
Taiyo Yuden
GRM36X7R153K016
Murata
LMK105BJ104MV
Taiyo Yuden
GRM36X5R104K010
Murata
C10055R1A104K
TDK
ECJ-0EB1C103K
Panasonic
www.panasonic.com
MMBD4148SE
Fairchild Semiconductor
www.fairchildsemi.com
MMBD7000
General Semiconductor
www.gensemi.com
CMPD7000
Central Semiconductor
www.centralsemi.com
BAT54AW
Diodes Incorporated
www.diodes.com
CMSSH-3A
Central Semiconductor
FDC6561AN
Fairchild Semiconductor
Dual N-channel MOSFETs
(30V, 0.095Ω, SOT23-6)
TPC6201
Toshiba
www.toshiba.com
150Ω ±1% resistor
—
—
R2
2kΩ ±5% resistor
—
—
R3
100kΩ ±1% resistor
—
—
R4
49.9kΩ ±1% resistor
—
—
T1
1:100 transformer
T912MG-1018
Toko
www.tokoam.com
NH1/NL1,
NH2/NL2
R1
channel MOSFET with low RDSON to minimize conduction
loss for NL1/NL2 and NH1/NH2 (Fairchild FDC6561). The
regulator softly turns on each of four switches in the full
bridge. ZVS occurs when the external power MOSFETs
are turned on while their respective drain-to-source voltages are near zero volts. ZVS effectively eliminates the
MOSFET transition losses caused by CRSS (drain-to18
MANUFACTURER
4.7µF, 25V X5R ceramic capacitor
C2
C4
RECOMMENDED DEVICE
source capacitance) and parasitic capacitance discharge. ZVS improves efficiency and reduces switching-related EMI.
Current-Sense Resistor
The MAX1996A regulates the CCFL average current
through sense resistor R1 in Figure 1. The voltage at
______________________________________________________________________________________
High-Efficiency, Wide Brightness
Range, CCFL Backlight Controller
Voltage-Sense Capacitors
The MAX1996A limits the transformer secondary voltage
during open-lamp fault through the capacitive divider
C3/C4. The voltage of VFB is proportional to CCFL voltage. To set the maximum RMS secondary transformer
voltage, choose C3 around 10pF to 22pF, and select C4
such that C4 = VT(MAX)/1.11V ✕ C3, where VT(MAX) comprises the maximum RMS secondary transformer voltage
(above the strike voltage). R2 sets the VFB DC bias point
to zero volts. Choose R2 =10/(C4 ✕ 6.28 ✕ FSW), where
FSW is the nominal resonant operating frequency.
Loop Compensation
CCI sets the speed of the current loop that is used during startup, maintaining lamp-current regulation, and
during transients, caused by changing the lamp-current settling. The typical CCI capacitor value is 0.1µF.
Larger values limit lamp-current overshoot, but
increase setting time. Smaller values speed up its
response time, but extremely small values can lead to
instability.
CCV sets the speed of the voltage loop that affects startup, DPWM transients, and operation in an open-tube
fault condition. If DPWM is not used, the voltage control
loop should only be active during startup or an openlamp fault. The CCV capacitors typical value is 0.01µF.
Use the smallest value of CCV capacitor necessary to
set an acceptable fault-transient response and not cause
excessive ringing at the beginning of a DPWM pulse.
Larger CCV capacitor values reduce transient overshoot,
but can degrade regulation at low DPWM duty cycles by
increasing the delay to strike voltage.
Resonant Components
The MAX1996A works well with air-gap transformers
with turns ratio N in the order of NP:NS = 1:90 to 1:100
for most applications. The transformer secondary resonant frequency must be controlled. A low-profile CCFL
transformer typically operates between 50kHz (Fmin)
and 200kHz (F max ). Transformer T1, DC blocking
capacitor C2, parallel capacitor C3, and the CCFL
lamp form a resonant tank. The resonant frequency is
determined by the transformer secondary leakage
inductance L, C2, and C3. The tank is a bandpass filter
whose lower frequency is bounded by L, N, and C2. N
is the transformer’s turns ratio. Choose C2 ≤ N2 (10 ✕
F2MIN ✕ L). The upper frequency is bounded by L and
C3. Choose C3 ≥ 1/(40 ✕ F2MIN ✕ L).
Other Components
The high-side MOSFET drivers (GH1 and GH2) are
powered by the external bootstrap circuit formed by
D2, C5, and C6. Connect BST1/BST2 through a dual
signal-level Schottky diode D2 to VDD, and connect it to
LX1/LX2 with 0.1µF ceramic capacitors. Use a dualseries signal-level diode (D1) to generate the half-wave
rectified current-sense voltage across R1. The current
through these diodes is the lamp current.
Dual-Lamp Regulator
The MAX1996A can be used to drive two CCFL tubes
as shown in Figure 12. See Table 5 for component
selection. The circuit consists of two identical transformers with primary windings connected in parallel
and secondary windings in series. The two transformers can also be replaced with a single transformer,
which has one primary winding and two secondary
windings. The advantage of the series secondary windings is that the same current flows through both lamps,
resulting in approximately the same brightness.
In normal operation, C12 is charged to approximately
6V biasing N1 on, which permits current to flow in the
loop as follows: in the first half cycle, current flows
through the secondary winding of T1, CCFL1, diode
D1, MOSFET N1, sense resistor R1, zener diode D4
(forward bias), CCFL2, and finally returning to T2. In the
second half cycle, the lamp current flows through T2,
CCFL2, D4 (breakdown), D3 (forward bias), CCFL1,
and back to T1.
The roundabout path of current flow is necessary in
order to detect an open-lamp condition when either
CCFL is removed. If CCFL1 is open, the lamp current
cannot flow through sense resistor R1. When IFB drops
below 150mV, the controller detects the condition and
shuts down after a 1s delay. During the delay, current
can flow from T2 through CCFL2, D4 (breakdown), and
R6 back to T2. If CCFL2 is removed, the voltage across
D4 drops to zero and C11 is discharged through R5.
N1 is biased off, which forces the voltage at IFB to drop
to zero once again. During the 1s turn-off delay, current
flows from T1 to CCFL1 through D3 (breakdown) and
R6 back to T1. D3 clamps the drain of N1 enabling the
use of a MOSFET with modest breakdown characteristics.
______________________________________________________________________________________
19
MAX1996A
IFB is the half-wave rectified representation of the current through the lamp. The inverter regulates the average voltage at IFB, which is controlled by either the
analog interface or the SMBus interface. To set the
maximum lamp RMS current, determine R1 as follows:
R1 = 0.444V/ICCFL, RMS, MAX, where ICCFL, RMS, MAX
is the maximum RMS lamp current. MINDAC and the
wave shape influence the actual maximum RMS lamp
current. If necessary, use an RMS current meter to
make final adjustments to R1.
MAX1996A
High-Efficiency, Wide Brightness
Range, CCFL Backlight Controller
VIN
5V TO 28V
C1
NH1
NH2
C2
D1
T1
C3
NL1
N1
CCFL
R1
D3
D6
NL2
C4
R2
C11
R6
R7
GL2
GH2
PGND
BATT
LX1
GH1
GL1
R6
T2
LX2
C5
C6
BST2
BST1
C13
VDD
VCC
C7
C12
D4
D2-2
D2-1
R5
D5
CCFL2
MAX1996A
GND
VFB
CCV
IFB
MODE
SH/SUS
CRF/SDA
CTL/SCL
CCI
REF
C9
MINDAC
C8
ILIM
R3
ON/OFF
R4
C10
REFERENCE INPUT
CONTROL INPUT
Figure 12. Dual-Lamp Application Circuit
The secondary voltages of both transformers are monitored through the two identical capacitive voltagedividers (C3/C4 and C13/C11). Dual-diode D6 rectifies
the two sensed voltages and passes the signal to the
VFB pin. A full-wave rectified sinusoidal waveform
appears at the VFB pin. The RMS value of this new VFB
signal is greater than the half-wave rectified signal in
the single-lamp application. To compensate for the
waveform change and the forward-voltage drop in the
diodes, the capacitive voltage-divider ratio must be
decreased. Choose C3 around 10pF to 22pF, and
select C4 according to C4 = VT, MAX/1.33V ✕ C3, where
VT, MAX is the maximum transformer secondary RMS
voltage.
20
Layout Guidelines
Careful PC board layout is critical to achieve low
switching losses and clean, stable operation. The highvoltage and switching-power stages require particular
attention (Figure 13). The high-voltage sections of the
layout need to be well separated from the control circuit. Most layouts are constrained to long narrow PC
boards, 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.
______________________________________________________________________________________
High-Efficiency, Wide Brightness
Range, CCFL Backlight Controller
DESIGNATION
C1
C2
C3, C13
C4, C11
C5–C8, C10, C12
C9
D1, D5
D2
D3, D4
D6
N1
NH1/NL1,
NH2/NL2
DESCRIPTION
RECOMMENDED DEVICE
MAX1996A
Table 5. Components for the Dual-Lamp Application Circuit
MANUFACTURER
TMK325BJ475MN
Taiyo Yuden
www.t-yuden.com
C3225X7R1E475M
TDK
www.tdk.com
4.7µF, 25V X5R ceramic capacitor
1µF, 25V X7R ceramic capacitor
15pF, 3.1kV high-voltage ceramic
capacitors
0.015µF, 16V X7R ceramic capacitors
TMK316BJ105KL
Taiyo Yuden
C3216X7R1E105K
TDK
GHM1038-SL-150J-3K
Murata
www.murata.com
C4520C0G3F150K
TDK
EMK105BJ153KV
Taiyo Yuden
GRM36X7R153K016
Murata
LMK105BJ104MV
Taiyo Yuden
GRM36X5R104K010
Murata
C1005X5R1A104K
TDK
ECJ-0EB1C103K
Panasonic
www.panasonic.com
MMBD4148
Fairchild Semiconductor
www.fairchildsemi.com
IMBD4148
General Semiconductor
www.gensemi.com
MMBD4148
Diodes Incorporated
www.diodes.com
BAT54AW
Diodes Incorporated
CMSSH-3A
Central Semiconductor
www.centralsemi.com
6.2V zener diodes
CMPZ5234B
Central Semiconductor
BZX84C6V2
Diodes Incorporated
Dual diode, common cathode
CMPD2838
Central Semiconductor
BAV70
Diodes Incorporated
0.1µF, 10V X5R ceramic capacitors
0.01µF, 16V X7R ceramic capacitor
100mA diodes
100mA dual Schottky diode, common
anode
N-channel MOSFET (SOT23)
Dual N-channel MOSFETs
(30V, 0.095Ω, SOT23-6)
2N7002
Fairchild Semiconductor
2N7002
General Semiconductor
2N7002
Central Semiconductor
FDC6561AN
Fairchild Semiconductor
TPC6201
Toshiba
www.toshiba.com
R1
150Ω ±1% resistor
—
—
R2, R6
2kΩ ±5% resistors
—
—
R3
100kΩ ±1% resistor
—
—
R4
49.9kΩ ±1% resistor
—
—
______________________________________________________________________________________
21
MAX1996A
High-Efficiency, Wide Brightness
Range, CCFL Backlight Controller
Table 5. Components for the Dual-Lamp Application Circuit
DESIGNATION
DESCRIPTION
RECOMMENDED DEVICE
MANUFACTURER
R5
1kΩ ±5% resistor
—
—
R7
20kΩ ±5% resistor
—
—
T1, T2
1:100 transformers
T912MG-1018
Toko
www.tokoam.com
C4
D1
C2
N1
N2
T1
HIGH-CURRENT PRIMARY CONNECTION
C3
R2
LAMP
HIGH-VOLTAGE SECONDARY CONNECTION
NOTE: DUAL MOSFET N2 IS MOUNTED ON THE BOTTOM SIDE OF THE PC BOARD DIRECTLY UNDER N1.
Figure 13. Layout Example
Table 6. CCFL Specifications
SPECIFICATION
CCFL Minimum Strike Voltage
(Kick-Off Voltage)
CCFL Typical Operating
Voltage (Lamp Voltage)
SYMBOL
VS
VL
UNITS
DESCRIPTION
VRMS
Although CCFLs typically operate at
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