Page 1
JULY 2000
NEW!
30W CCFL Inverter Design
ML4877*
LCD Desktop Backlight Lamp Driver
GENERAL DESCRIPTION
The ML4877 is an ideal solution for driving multiple cold
cathode fluorescent tubes (CCFL) used in liquid crystal
display (LCD) backlight applications. It provides dimming
ballast control for the LCD display.
By utilizing differential drive the ML4877 can deliver the
same light output with significantly less input power
compared to existing single ended drive schemes.
Improvements as high as 30% can be realized when using
low power lamps and advanced LCD screen housings. This
increased light output is achieved because the differential
drive configuration is much less sensitive, and therefore
less power is wasted in the capacitive parasitics that exist
in the backlight housing. An additional benefit of this
configuration is an even distribution of light.
The IC includes an adjustable lamp out detect circuit that
latches the IC off when a lamp fault is detected. Also, the
unique architecture of the ML4877 allows the
development of a backlight system that will inherently
meet the UL requirements for safety.
The ML4877 is optimized for large LCD applications
applications where high efficiency is critical to maximize
battery life. The high efficiency is achieved by a resonant
scheme with zero voltage switching.
FEATURES
■ Ideal for 30W inverter designs, 1 to 8 lamp design
■ PWM dimming capability
■ Backlight lamp driver with differential drive
■ Up to 30% lower power for same light output
■ Low standby current (<10µA)
■ Improved efficiency (»95%)
■ Allows all N-channel MOSFET drive
■ Adjustable lamp out detect with latch
■ Resonant threshold detection and synchronous
rectification
■ Positive input for dimming control
* THIS PART IS END OF LIFE AS OF JULY 1, 2000
BLOCK DIAGRAM
AZR
2
REGULATOR
5
PGND18GND
LINEAR
MASTER
BIAS
&
UVLO
17
ON/OFF
HVDD
VREF
12
15
VDD
13
VDD
Q
S
QR
B ON
19
DR3 DR1 DR1
ONE
SHOT
SS
+
–
VDD
CLK
S
Q
RQ
B OFF
RESONANT
THRESHOLD
DETECTOR
OSCILLATOR
4
R
T
B SYNC OUT
20
11
L RTD
10
L GATE 1
DR2
Q
NEG
EDGE
DELAY
9
T
T
Q
DR2
–
+
SS
3
SS CAPC
14
L GATE 2
16
6
+
–
–
+
0.5V
8
7
1
L ILIM
LEA OUT
LEA–
LEA+
1
Page 2
ML4877
PIN CONFIGURATION
PIN DESCRIPTION
LEA+
AZR
SS CAP
R
VREF
L ILIM
LEA–
LEA OUT
C
L RTD
ML4877
20-Pin SSOP (R20)
1
2
3
T
T
4
5
6
7
8
9
10
TOP VIEW
20
19
18
17
16
15
14
13
12
11
B OFF
B ON
GND
PGND
L GATE 2
ON/OFF
L GATE 1
VDD
HVDD
B SYNC OUT
PIN NAME FUNCTION
1 LEA+ Positive input for lamp error amp
2 AZR Connection to gate of external FET for
high voltage regulator. Internally a
zener diode to ground.
3 SS CAP Connection of optional external soft
start capacitor
4R
T
Oscillator timing resistor
5 VREF Voltage reference output
6 L ILIM Input to current limit amplifier
7 LEA- Negative input for lamp error
amplifier
8 LEA OUT Output of lamp error amplifier.
External compensation capacitor
connects between this pin and LEA.
9C
T
Oscillator timing capacitor
10 L RTD Input to resonant threshold detector
PIN NAME FUNCTION
11 B SYNC Output of MOSFET driver to gate of
OUT synchronous FET catch diode.
12 HVDD Battery power input to linear regulator
13 VDD Output of linear regulator. Positive
power for IC.
14 L GATE1 Output of MOSFET driver. Connection
to gate of one side of inverter FET
drive pair.
15 ON/OFF Logic input for chip
16 L GATE2 Output of MOSFET driver. Connection
to gate of one side of inverter FET
drive pair.
17 PGND Power ground
18 GND Signal ground
19 B ON Connection to primary side of gate
pulse transformer
20 B OFF Output of MOSFET driver. Connection
to gate of FET that disables the input
power.
2
Page 3
ABSOLUTE MAXIMUM RATINGS
ML4877
Absolute maximum ratings are those values beyond which
the device could be permanently damaged. Absolute
maximum ratings are stress ratings only and functional
device operation is not implied.
Voltage on HVIDD .................................................... 20V
Current into L RTD ............................................... ±10mA
Junction Temperature.............................................. 150ºC
Storage Temperature Range .......................-65ºC to 150ºC
Lead Temperature (Soldering 10 sec.) ..................... 260ºC
Supply Current (ICC) ............................................... 75mA
Thermal Resistance (qJA).................................... 100ºC/W
Output Current, Source or Sink ............................. 250mA
Voltage on Pins LEA+, AZR, SS CAP, RT, VREF,
OPERATING CONDITIONS
L ILIM, LEA-, LEA OUT, CT, B SYNC OUT, VDD,
L GATE 1, ON/OFF, L GATE 2, PGND, GND,
B ON, B OFF ............................... –0.3V to VDD +0.3V
Temperature Range
ML4877C ................................................... 0ºC to 70ºC
ML4877E...............................................–20ºC to 70ºC
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, VDD = 5V ±5%, TA= Operating Temperature Range, CT = 47pF, RT = 82kW (Note 1)
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
CURRENT REGULATOR
ERROR AMPLIFIER
Open Loop Gain 60 70 dB
Output High I
Output Low I
Bandwidth (-3dB) 1 MHz
= 5µA 2.8 3.0 V
LOAD
= 25µA 0.4 0.7 V
LOAD
Common Mode Voltage Range 0 1.0 V
Input Bias Current 50 100 nA
Input Offset Voltage -5 0 5 mv
Soft Start Charge Current V
Soft Start Threshold (LEA OUT) V
CURRENT LIMIT COMPARATOR
Current Threshold 450 500 550 mV
Input Bias Current V
Propagation Delay (Note 2) 150 250 ns
OUTPUT DRIVERS
Output High - B SYNC OUT, B OFF VDD 5V, I
Output Low - B SYNC OUT, B OFF I
Rise & Fall time - B SYNC OUT, B OFF C
Output High - B ON VDD 5V, I
Output Low - B ON I
Fall Time - B ON C
ONE SHOT
= 1V 550 750 950 nA
SSCAP
= 1V 2 2.5 V
SSCAP
= 0.1V 50 100 nA
ILIM
= 12mA 4.625 4.8 V
LOAD
12mA 0.2 0.375 V
LOAD
=100pF 20 50 ns
LOAD
= 12mA 4.625 4.8 V
LOAD
50mA 0.2 0.375 V
LOAD
= 2400pF (Note 2) 45 80 ns
LOAD
Pulse Width 100 150 200 ns
3
Page 4
ML4877
ELECTRICAL CHARACTERISTICS (Continued)
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
DELAY TIMER
Delay Time 20 35 55 ns
HIGH VOLTAGE INVERTER
Oscillator
Nominal Frequency 68 80 92 kHz
Discharge Current VCT = 2V 500 700 900 µA
Peak Voltage 2.3 2.5 2.7 V
Valley Voltage 0.8 1 1.2 V
Output Drivers
Output High - L GATE 1, 2 VDD = 5V, I
Output Low - L GATE 1, 2 I
Rise & Fall Time - L GATE 1, 2 C
Resonant Threshold Detector
Threshold 0.45 0.8 1.15 V
Hysteresis 0.15 03 0.45 mV
Lamp Out Detect
Threshold -2 VDD 2 %
Latch Inhibit Threshold (SSCAP) LRTD > VDD + 0.1V 2.5 V
Under Voltage Detector
Start Up Threshold 3.8 4.1 4.4 V
Hysteresis 150 300 450 mV
Logic Interface (ON/OFF)
V
IH
V
IL
Input Bias Current ON/OFF = 3V 10 25 µA
Linear Regulator
12mA 4.625 4.8 V
LOAD
= 50mA 0.2 0.375 V
LOAD
=1000pF 20 50 ns
LOAD
2.5 V
0.5 V
Aux Zener Reference Voltage (AZR) I
Regulator Voltage (VDD) HVDD = 12V 4.75 5.0 5.35 V
Regulator Source Current External to device 10 mA
Drop Out Voltage I
Drop Out Voltage l
HVDD Input Voltage Range 5 18 V
= 10µA 12.3 13.5 14.7 V
AZR
= 1mA 30 90 mA
HVDD
= 5mA 125 275 mA
HVDD
4
Page 5
ML4877
ELECTRICAL CHARACTERISTICS (Continued)
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
BIAS
VDD Supply Current ON/OFF = “I”, no load 375 450 µA
VDD Supply Current ON/OFF = “0”, HVDD = 12V 1 10 µA
VREF Load Regulation I
VREF Output Voltage TA = 25ºC 2.47 2.5 2.53 V
VREF Line Regulation 20 30 mV
VREF Line, Load, Temp 2.465 2.5 2.535 V
Note 1: Limits are guaranteed by 100% testing, sampling, or correlation with worst case test conditions.
Note 2: Actual load is 1200pF. The 2:1 transformer reflects an effective 2400pF.
= 25µA 10 20 mV
LOAD
5
Page 6
ML4877
5 TO 18V
U2-A
IN
C7
C8
Q1
HVDD
12
VREF
5
C3
1.0µF
ON/OFF
15
R1
AZR
2
LINEAR
REGULATOR
MASTER
BIAS
& UVLO
Q S
QR
*OPTIONAL
SEE NOTE
VDD
13
SS
C1
VDD
D1
T2
C9
B ON B ON B SYNC
DR3 DR1
ONE
SHOT
+
–
VDD
CLK
Q2
2019
DR1
S Q
SQ
RESONANT
THRESHOLD
DETECTOR
OSCILLATOR
11
NEG
EDGE
DELAY
R7
10k
100µh
C6
0.1µF
R6
100k
1.6MΩ
L1
C11
T1
39pF
LAMP
Q4
R6
O.5Ω
R4
OPTIONALORU2-B
L RTD
10
GATE1
DR2
Q
T
–
+
DR2
Q
SS
14
GATE2
16
L ILIM
6
+
–
0.5V
LEA OUT
8
C4
LEA–
7
1
0.047µF
R5
100kΩ
–
+
82kΩ
4 9 317 18
R
T
R2
C
C5
47pF
T
SS CAP
C2
0.1µF
PGND
GND
Figure 1. Typical Application Schematic for the ML4877
6
Page 7
FUNCTIONAL DESCRIPTION
ML4877
The ML4877 consists of a PWM regulator, a lamp driver/
inverter, a linear regulator and control circuits. This IC, in
conjunction with external components, converts a DC
battery voltage into the high voltage and high frequency
AC signal required to start and drive miniature cold
cathode fluorescent lamps. Typical application circuits
are shown in Figure 1 and Figure 5. Note: Please read the
Power Sequencing section below prior to using the
ML4877.
LAMP DRIVER
The lamp driver, sometimes referred to as a lamp inverter,
is comprised of a PWM regulator and a Royer type
inverter circuit to drive the lamp. The PWM regulator, in a
buck configuration, controls the magnitude of the lamp
current to provide the dimming capability. Figure 2 shows
a simplified circuit to more easily illustrate the operation
of the circuit.
Due to the presence of the buck inductor, L1, the circuit
shown in Figure 2 is essentially a current fed parallel
loaded resonant circuit. Lm is the primary inductance of
the output transformer, T1, which tunes with the resonant
capacitor CR to set the resonant frequency of the inverter.
The oscillator frequency is always set lower than the natural
resonant frequency to ensure synchronization. The current
source IC models the current through the buck inductor L1.
The MOSFETs, (Q3 and Q4) are alternately turned on with
a constant 50% duty cycle signal (L GATE1, L GATE2) at
one-half the frequency of the oscillator. In this way each
transistor pulses, or excites, the resonant tank on each
half cycle. The combination of these two signals appear
across the primary winding of the output transformer as a
sinusoidal waveform. This voltage is multiplied by the
step-up turns ratio of the output transformer and impressed
across the lamp.
The output transitions are controlled by feedback through
the L RTD pin by sensing the voltage at the center tap of
the output transformer. Each time this signal reaches the
minimum resonant threshold detection point an internal
clock pulse is generated to keep the system synchronized.
Figure 3 shows some of these representative waveforms at
the important nodes of the circuit.
The PWM regulator is comprised of a MOSFET (U2-A),
inductor L1, and the gate control and drive circuitry as
shown in Figure 1. A signal with a constant pulse width of
I 50ns is applied to the primary of the 2:1 pulse
transformer T2, rectified by diode D1, and used to charge
the gate capacitance of U2-A, thereby turning it on. The
turn off is controlled by discharging this capacitance
through MOSFET Q2. The pulse width of the signal on the
gate of Q2 (B OFF) varies according to the difference of
the amplitude of the feedback signal on LEA+, and LEA–.
The signal on LEA– is proportional to the AC current
flowing in the lamp, while the signal on LEA+ is a
function of the brightness control setting. The AC lamp
current feedback signal is developed by monitoring the
current through resistor R6 in the common source
connection of the inverter MOSFETs, Q3 and Q4. The
lamp current, and therefore brightness, is adjusted by
varying the voltage applied to R4, at the brightness adjust
control point. Increasing this voltage increases the brightness.
OSCILLATOR
The frequency of the oscillator in the ML4877 is set by
selecting the values Of CT and RT. Figure 4 shows the
I
C
➞
T1
LmLm
C
R
Q3 Q4
T1
1:N
Figure 2. Kelvin Sense Connections
C
OUT
LAMP
C
T
CLOCK
L GATE1
DRAIN-Q4
L GATE2
DRAIN-Q3
T1-CNTR-PRI
SOURCE OF
U2-A
Figure 3. Operating Waveforms
of the Lamp Driver Section
7
Page 8
ML4877
FUNCTIONAL DESCRIPTION (Continued)
oscillator frequency versus the value of RT for different
values Of CT. This nomograph may be used to select the
appropriate value of RT and CT to achieve the desired
oscillator frequency for the ML4877.
LINEAR REGULATOR
A linear voltage regulator is provided to power the low
voltage and low current control circuitry on the ML4877.
This is typically used when there is no separate 5V supply
available at the inverter board. For operation up to 18V,
the linear regulator is used by connecting the HVDD pin
to the input battery voltage. For operation over 18V, a
MOSFET, and a resistor (Q and R1, Figure 1) are
connected as shown. The MOSFET is required to stand off
the high voltage. The AZR pin is just a zener diode to
ground used to bias the gate of Q1.
LAMP OUT DETECT
In those cases when there is no lamp connected, or the
connection is faulty, the output voltage of the lamp driver
circuit will tend to rise to a high level in an attempt to
start the nonexistent lamp. The lamp out detect circuit on
the ML4877 will detect this condition by sensing a
voltage proportional to the center tap voltage on the
primary of the output transformer, T1 on the L RTD pin.
The ration of resistors R7 and R8 sets the lamp out detect
threshold. When the voltage on the L RTD pin exceeds
VDD, an internal latch is set and the lamp driver goes
into a shutdown mode. The logic control pin ON/OFF
must be cycled low, then high to reset the latch and
return the lamp driver to the normal state. The input to the
lamp out latch is inhibited by the signal on the soft start
pin. The latch will not be set until the voltage on SS CAP
(pin 3) rises to more than 4.2V nominally.
By selecting the appropriate value the AC lamp current
can be set to slowly increase with a controlled time
constant. The capacitor value can be calculated
according to the following formula.
C = (3 X 10-7)TS (1)
Where TS = Duration of the soft start sequence in seconds
LOGIC CONTROL
The ML4877 is controlled by a single logic input, ON/
OFF. A logic level high on this pin enables the lamp
driver. A logic zero puts the circuit into a very low power
state.
POWER SEQUENCING
It is important to observe correct power and logic input
sequencing when powering up the ML4877. The following
procedure must be observed to avoid damaging the
device.
1. Apply the battery power to HVDD, or
2. If HVDD is not used. Apply the VDD voltage. With
HVDD connected the VDD voltage is supplied by the
internal regulator on the ML4877.
3. Apply a logic high to the ON/OFF input.
Please refer to Application Note 32 for detailed
application information beyond what is presented here.
APPLICATIONS SECTION
SOFT START
The capability to control the start up behavior is achieved
by setting the value of a single capacitor, C2 in Figure 1.
1000
C = 30pF
C = 46pF
100
FRQUENCY (kHz)
10
10 100 1000
C = 81pF
C = 120pF
RESISTANCE (kΩ)
Figure 4. Oscillator Frequency Nomograph
8
HIGH POWER INVERTER
The ML4877 is easily adapted to high power CCFL
inverter designs. Figure 5 displays a schematic of a 30W
ML4877 application. This particular design employs PWM
dimming in order to extend dimming range.
The 30W inverter design is ideal for applications between
the 20W and 30W range. Deep dimming capability is
achieved via PWM technique with no flicker and no popon effects. Uniform intensity can be maintained across 1
to 8 lamps to well below 5%.
Figure 6 provides a top view of an example of a ML4877
30W design. This design can be modified for 1 to 8 lamps
and contains a PWM dimming interface using standard
low cost components.
For the latest application notes and other information,
visit the Micro Linear website at www.microlinear.com.
Page 9
ML4877
GND
V
DD
DIMMING
F1 4A
C24
220µF
R7
10kΩ
C22
220µF
J1
1
2
3
C21
1nF
R13
10kΩ
C19
4.7nF
R8
91kΩ
C18
1nF
Q5
2N3904
C20
0.1µF
C13
1µF
R15
1kΩ
R14
1kΩ
Q4
IRF7416
C23
1nF
2N3904
C17
4.7nF
R4
6.2kΩ
R18
200Ω
Q6
1
2
3
4
5
6
7
8
9
10
C16
47pF
Q7
2N3904
R16
20Ω
2N3906
LEA+
AZR
SS CAP
R
T
VREF
L ILIM
LEA–
LEA OUT
C
T
L RTD
R17
20Ω
Q8
ML4877
U2
B SYNC OUT
B OFF
B ON
GND
PGND
L GATE 2
ON/OFF
L GATE 1
VDD
HVDD
L1
22µH
CR6
SK34MSCT
20
19
18
17
16
15
14
13
12
11
R5
30kΩ
CR5
R6
7.5kΩ
IRLR2905
R9
143kΩ
C14
1µF
Q1
R11
0.2Ω
1W
R3
10kΩ
C12
1µF
C1
0.22µF
MKS-10
63V
4, 10
9
Q2
IRLR2905
R12
0.2Ω
1W
T1
CR1
5.1V
J2
C2
13
5
8
12
68pF
C3
68pF
C4
68pF
C5
68pF
C6
68pF
C7
68pF
C8
68pF
C9
68pF
1kV
1kV
1kV
1kV
1kV
1kV
1kV
1kV
1
2
3
4
5
6
7
8
9
10
11
J3
1
2
3
4
5
6
7
8
9
10
11
R1
10kΩ
2
3
1N4148
1
–
+
CR3
U1B
J1
2
CR4
1N4148
J2
1
2
R2
10kΩ
CR2
1N4148
1
Q3
2N3906
R6
390kΩ
0.033µF
PWM Control
C10
C15
1µF
R10
10kΩ
8
5
+
6
–
U1A
4
7
C11
1µF
Figure 5. 30W Backlight CCFL Inverter with PWM Dimming
9
Page 10
ML4877
Figure 6. 30W CCFL Inverter Board, 1 to 8 Lamps
10
Page 11
PHYSICAL DIMENSIONS inches (millimeters)
Package: R20
20-Pin SSOP
0.279 - 0.289
(7.08 - 7.34)
20
ML4877
PIN 1 ID
1
0.026 BSC
(0.65 BSC)
0.066 - 0.070
(1.68 - 1.78)
0.009 - 0.015
(0.23 - 0.38)
SEATING PLANE
ORDERING INFORMATION
0.205 - 0.213
(5.20 - 5.40)
0.068 - 0.078
(1.73 - 1.98)
0.301 - 0.313
(7.65 - 7.95)
0.002 - 0.008
(0.05 - 0.20)
0º - 8º
0.022 - 0.038
(0.55 - 0.95)
0.004 - 0.008
(0.10 - 0.20)
PART NUMBER TEMPERATURE RANGE PACKAGE
ML4877CR (END OF LIFE) 0°C to 70°C Molded SSOP (R20)
ML4877ER (OBSOLETE) –20ºC to 70ºC Molded SSOP (R20)
© Micro Linear 1998. is a registered trademark of Micro Linear Corporation. All other trademarks are the property of their respective owners.
Products described herein may be covered by one or more of the following U.S. patents: 4,897,611; 4,964,026; 5,027,116; 5,281,862; 5,283,483; 5,418,502;
5,508,570; 5,510,727; 5,523,940; 5,546,017; 5,559,470; 5,565,761; 5,592,128; 5,594,376; 5,652,479; 5,661,427; 5,663,874; 5,672,959; 5,689,167; 5,714,897;
5,717,798; 5,742,151; 5,747,977; 5,754,012; 5,757,174; 5,767,653; 5,777,514; 5,793,168; 5,798,635; 5,804,950; 5,808,455; 5,811,999; 5,818,207; 5,818,669;
5,825,165; 5,825,223. Japan: 2,598,946; 2,619,299; 2,704,176; 2,821,714. Other patents are pending.
Micro Linear reserves the right to make changes to any product herein to improve reliability, function or design. Micro Linear does not assume any liability
arising out of the application or use of any product described herein, neither does it convey any license under its patent right nor the rights of others. The circuits
contained in this data sheet are offered as possible applications only. Micro Linear makes no warranties or representations as to whether the illustrated circuits
infringe any intellectual property rights of others, and will accept no responsibility or liability for use of any application herein. The customer is urged to consult
with appropriate legal counsel before deciding on a particular application.
10/29/98Printed in U.S.A.
DS4877-01
2092 Concourse Drive
San Jose, CA 95131
Tel: (408) 433-5200
Fax: (408) 432-0295
www.microlinear.com
11
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