Micro Linear ML4877ER Lcd desktop backlight lamp driver Datasheet

JULY 2000
NEW!
L
30W CCF
esign
Inverter D
ML4877*
LCD Desktop Backlight Lamp Driver
GENERAL DESCRIPTION
FEATURES
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.
■
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
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.
* THIS PART IS END OF LIFE AS OF JULY 1, 2000
BLOCK DIAGRAM
AZR
2
B ON
19
VDD
13
B SYNC OUT
11
B OFF
20
VDD
HVDD 12
VREF 5
10 L RTD
LINEAR
REGULATOR
DR3
MASTER
BIAS
&
UVLO
DR1
DR1
DR2
14 L GATE 1
DR2
16 L GATE 2
Q
ONE
SHOT
NEG
EDGE
DELAY
S Q
T
Q
R Q
6
L ILIM
8
LEA OUT
–
7
LEA–
+
1
LEA+
+
–
0.5V
ON/OFF 15
RESONANT
THRESHOLD
DETECTOR
SS
Q S
Q R
+
–
VDD
CLK
–
+
OSCILLATOR
SS
17
18
4
9
3
PGND
GND
RT
CT
SS CAP
1
ML4877
PIN CONFIGURATION
ML4877
20-Pin SSOP (R20)
LEA+
1
20
B OFF
AZR
2
19
B ON
SS CAP
3
18
GND
RT
4
17
PGND
VREF
5
16
L GATE 2
L ILIM
6
15
ON/OFF
LEA–
7
14
L GATE 1
LEA OUT
8
13
VDD
CT
9
12
HVDD
L RTD
10
11
B SYNC OUT
TOP VIEW
PIN DESCRIPTION
PIN
NAME
FUNCTION
PIN
NAME
FUNCTION
1
LEA+
Positive input for lamp error amp
11
B SYNC
OUT
Output of MOSFET driver to gate of
synchronous FET catch diode.
2
AZR
Connection to gate of external FET for
high voltage regulator. Internally a
zener diode to ground.
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.
3
SS CAP
Connection of optional external soft
start capacitor
4
RT
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.
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.
9
CT
Oscillator timing capacitor
10
L RTD
Input to resonant threshold detector
2
ML4877
ABSOLUTE MAXIMUM RATINGS
Voltage on HVIDD .................................................... 20V
Current into L RTD ............................................... ±10mA
Junction Temperature .............................................. 150ºC
Storage Temperature Range ....................... -65ºC to 150ºC
Lead Temperature (Soldering 10 sec.) ..................... 260ºC
Thermal Resistance (qJA) .................................... 100ºC/W
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.
Supply Current (ICC) ............................................... 75mA
Output Current, Source or Sink ............................. 250mA
Voltage on Pins LEA+, AZR, SS CAP, RT, VREF,
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
OPERATING CONDITIONS
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
60
70
dB
CURRENT REGULATOR
ERROR AMPLIFIER
Open Loop Gain
Output High
ILOAD = 5µA
Output Low
ILOAD = 25µA
2.8
3.0
0.4
Bandwidth (-3dB)
V
0.7
1
Common Mode Voltage Range
0
MHz
1.0
V
50
100
nA
-5
0
5
mv
750
950
nA
2.5
V
500
550
mV
Input Bias Current
Input Offset Voltage
V
Soft Start Charge Current
VSSCAP = 1V
550
Soft Start Threshold (LEA OUT)
VSSCAP = 1V
2
CURRENT LIMIT COMPARATOR
Current Threshold
450
Input Bias Current
VILIM = 0.1V
50
100
nA
Propagation Delay
(Note 2)
150
250
ns
OUTPUT DRIVERS
Output High - B SYNC OUT, B OFF
VDD 5V, ILOAD = 12mA
4.625
Output Low - B SYNC OUT, B OFF
ILOAD 12mA
0.2
0.375
V
Rise & Fall time - B SYNC OUT, B OFF
CLOAD =100pF
20
50
ns
Output High - B ON
VDD 5V, ILOAD = 12mA
Output Low - B ON
ILOAD 50mA
0.2
0.375
V
Fall Time - B ON
CLOAD = 2400pF (Note 2)
45
80
ns
150
200
ns
4.625
4.8
V
4.8
V
ONE SHOT
Pulse Width
100
3
ML4877
ELECTRICAL CHARACTERISTICS
SYMBOL
(Continued)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
20
35
55
ns
68
80
92
kHz
500
700
900
µA
Peak Voltage
2.3
2.5
2.7
V
Valley Voltage
0.8
1
1.2
V
4.625
4.8
DELAY TIMER
Delay Time
HIGH VOLTAGE INVERTER
Oscillator
Nominal Frequency
Discharge Current
VCT = 2V
Output Drivers
Output High - L GATE 1, 2
VDD = 5V, ILOAD 12mA
V
Output Low - L GATE 1, 2
ILOAD = 50mA
0.2
0.375
V
Rise & Fall Time - L GATE 1, 2
CLOAD =1000pF
20
50
ns
Resonant Threshold Detector
Threshold
0.45
0.8
1.15
V
Hysteresis
0.15
03
0.45
mV
-2
VDD
2
%
Lamp Out Detect
Threshold
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
2.5
V
VIL
Input Bias Current
0.5
ON/OFF = 3V
V
10
25
µA
Linear Regulator
Aux Zener Reference Voltage (AZR)
IAZR = 10µA
12.3
13.5
14.7
V
Regulator Voltage (VDD)
HVDD = 12V
4.75
5.0
5.35
V
Regulator Source Current
External to device
10
Drop Out Voltage
IHVDD = 1mA
30
90
mA
Drop Out Voltage
lHVDD = 5mA
125
275
mA
18
V
HVDD Input Voltage Range
4
5
mA
ML4877
ELECTRICAL CHARACTERISTICS
SYMBOL
(Continued)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
375
450
µA
1
10
µA
10
20
mV
2.5
2.53
V
20
30
mV
2.5
2.535
V
BIAS
VDD Supply Current
ON/OFF = “I”, no load
VDD Supply Current
ON/OFF = “0”, HVDD = 12V
VREF Load Regulation
ILOAD = 25µA
VREF Output Voltage
TA = 25ºC
2.47
VREF Line Regulation
VREF Line, Load, Temp
2.465
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.
5
ML4877
U2-A
5 TO 18V
IN
*OPTIONAL
SEE NOTE
C7
C8
R1
T2
D1
U2-B
Q2
C1
OR
OPTIONAL
C9
L1
100µh
Q1
R7
10k
2
VDD
13
B ON
19
B ON
20
B SYNC
11
T1
L RTD
VDD
HVDD
12
DR3
DR1
DR1
ONE
SHOT
VREF
C3
1.0µF
S
Q
S
Q
C6
DR2
GATE1
14
DR2
GATE2
16
0.1µF
Q
NEG
EDGE
DELAY
MASTER
BIAS
& UVLO
5
39pF
10
LINEAR
REGULATOR
T
Q
Q4
L ILIM
6
+
R6
O.5Ω
–
0.5V
ON/OFF
LEA OUT
8
15
1
–
+
+
–
R
–
Q
VDD
+
S
RESONANT
THRESHOLD
DETECTOR
7
R5
100kΩ
SS
CLK
17
PGND
OSCILLATOR
18
4
GND
RT
R2
82kΩ
9
3
CT
C5
47pF
SS CAP
C2
0.1µF
Figure 1. Typical Application Schematic for the ML4877
6
R6
100k
C4
LEA– 0.047µF
SS
Q
C11
LAMP
AZR
R4
1.6MΩ
ML4877
FUNCTIONAL DESCRIPTION
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
CT
IC
➞
COUT
T1
CLOCK
T1
Lm Lm
1:N
LAMP
L GATE1
CR
DRAIN-Q4
L GATE2
Q3
Q4
DRAIN-Q3
T1-CNTR-PRI
SOURCE OF
U2-A
Figure 2. Kelvin Sense Connections
Figure 3. Operating Waveforms
of the Lamp Driver Section
7
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.
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
LINEAR REGULATOR
(1)
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.
Where TS = Duration of the soft start sequence in seconds
LAMP OUT DETECT
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.
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.
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
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
HIGH POWER INVERTER
The capability to control the start up behavior is achieved
by setting the value of a single capacitor, C2 in Figure 1.
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.
C
=
pF
=
100
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%.
30
C
=
pF
46
C
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.
C
pF
81
FRQUENCY (kHz)
1000
=
F
0p
12
10
10
100
1000
RESISTANCE (kΩ)
Figure 4. Oscillator Frequency Nomograph
8
For the latest application notes and other information,
visit the Micro Linear website at www.microlinear.com.
ML4877
J2
J1
1
2
3
GND
VDD
DIMMING
3
T1 1
5
8
4, 10
L1
22µH
Q4
IRF7416
F1 4A
R17
20Ω
C20
0.1µF
C24
220µF
C22
220µF
C23
1nF
R13
10kΩ
R16
20Ω
C18
1nF
C13
1µF
C17
4.7nF
R4
6.2kΩ
1
2
3
4
5
6
7
8
9
10
LEA+
B OFF
AZR
B ON
SS CAP
GND
RT
ML4877 PGND
VREF
L GATE 2
L ILIM
ON/OFF
LEA–
U2 L GATE 1
LEA OUT
VDD
CT
HVDD
L RTD
B SYNC OUT
1
R2
10kΩ
3
+ U1B
1
CR4
1N4148
R1
10kΩ
R11
0.2Ω
1W
R12
0.2Ω
1W
20
19
18
17
16
15
14
13
12
11
J3
C6
68pF 1kV
C7
68pF 1kV
C8
68pF 1kV
C9
68pF 1kV
1
2
3
4
5
6
7
8
9
10
11
R3
10kΩ
R9
143kΩ
C14 C12
1µF 1µF
CR1
5.1V
J2
2
–
Q2
IRLR2905
1
2
3
4
5
6
7
8
9
10
11
C16
47pF
J1
2
Q1
IRLR2905
12
Q5
2N3904
R8
91kΩ
1
R6
7.5kΩ
Q8
2N3906
Q6
2N3904
C19
4.7nF
R7
10kΩ
R5
30kΩ
9
CR5
C21
1nF
R14
1kΩ
CR6
SK34MSCT
Q7
2N3904
R18
200Ω
R15
1kΩ
C1
0.22µF
MKS-10
63V
C2
68pF 1kV
C3
68pF 1kV
C4
68pF 1kV
C5
68pF 1kV
R6
390kΩ
5
6
CR2
1N4148
2
8
+
–
U1A
4
Q3
2N3906
C10
0.033µF
CR3
1N4148
C15
1µF
7
C11
1µF
R10
10kΩ
PWM Control
Figure 5. 30W Backlight CCFL Inverter with PWM Dimming
9
ML4877
Figure 6. 30W CCFL Inverter Board, 1 to 8 Lamps
10
ML4877
PHYSICAL DIMENSIONS
inches (millimeters)
Package: R20
20-Pin SSOP
0.279 - 0.289
(7.08 - 7.34)
20
0.205 - 0.213
(5.20 - 5.40)
0.301 - 0.313
(7.65 - 7.95)
PIN 1 ID
1
0.026 BSC
(0.65 BSC)
0.068 - 0.078
(1.73 - 1.98)
0º - 8º
0.066 - 0.070
(1.68 - 1.78)
0.009 - 0.015
(0.23 - 0.38)
SEATING PLANE
0.002 - 0.008
(0.05 - 0.20)
0.022 - 0.038
(0.55 - 0.95)
0.004 - 0.008
(0.10 - 0.20)
ORDERING INFORMATION
© Micro Linear 1998.
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)
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.
10/29/98 Printed 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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