Page 1
JULY 2000
FEATURING
Extended Commercial Temperature Range
-20˚C to 70˚C
for Portable Handheld Equipment
ML4876*
LCD Backlight Lamp Driver with Contrast
GENERAL DESCRIPTION
The ML4876 is an ideal solution for driving small cold
cathode fluorescent tubes (CCFL) used in liquid crystal
display (LCD) backlight applications. It provides the
dimming ballast control and the contrast control for the
LCD display.
By utilizing differential drive the ML4876 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 ML4876 is optimized for portable applications where
high efficiency is critical to maximize battery life. The
high efficiency is achieved by a resonant scheme with
zero voltage switching.
BLOCK DIAGRAM
13 17 18 19 11
HVDD
15
6
ON/OFF
VREF
5
LON
4
F GATE
12
F ILIM
20
0.1V
FEA–
2
FEA+
3
VDD GND
LINEAR
REGULATOR
MASTER
BIAS
&
UVLO
+
–
–
+
VDD
Q
S
QR
Q
QR
–
+
S
+
–
ONE
SHOT
16V
B ON B OFF B SYNC OUT
DR3 DR1 DR1
S
RQ
FEATURES
■ 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
■ Low switching losses
■ Resonant threshold detection
■ Buck regulator uses synchronous rectification
* THIS PART IS END OF LIFE AS OF JULY 1, 2000
DR2
Q
CLK
NEG
EDGE
DELAY
RESONANT
THRESHOLD
DETECTOR
OSCILLATOR
Q
T
Q
–
+
DR2
L RTD
L GATE 1
L GATE 2
0.5V
–
+
–
LEA–
+
0.2V
10
14
16
7
C
FEA OUT
1 9 8
T
LEA OUT
1
Page 2
ML4876
PIN CONFIGURATION
PIN DESCRIPTION
FEA OUT
FEA–
FEA+
L ON
VREF
ON/OFF
LEA–
LEA OUT
C
L RTD
ML4876
20-Pin SSOP (R20)
1
2
3
4
5
6
7
8
T
9
10
TOP VIEW
20
19
18
17
16
15
14
13
12
11
F ILIM
B OFF
B ON
GND
L GATE2
HVDD
L GATE1
VDD
F GATE
B SYNC OUT
PIN NAME DESCRIPTION
1 FEA OUT Output of flyback (contrast) error
amplifier
2 FEA– Negative input of flyback
(contrast) error amplifier
3 FEA+ Positive input of flyback (contrast)
error amplifier
4 L ON Logic input. A ”0“ on this pin
disables the lamp driver section
only
5 VREF Voltage reference output
6 ON/OFF Logic input. A ”0“ on this pin
disables the linear regulator
7 LEA– Negative input for lamp error
amplifier
8 LEAOUT Output of lamp error amplifier
9C
T
Oscillator timing capacitor
10 L RTD Input to resonant threshold detector
PIN NAME DESCRIPTION
11 B SYNC OUT Output of MOSFET driver.
Connects to gate of synchronous
FET catch diode.
12 F GATE Connects to gate of MOSFET in
primary side of contrast control
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 HVDD Battery power input to linear
regulator
16 L GATE2 Output of MOSFET driver.
Connection to gate of one side of
inverter FET drive pair
17 GND Ground
18 B ON Connection to primary side of gate
pulse transformer
19 B OFF Output of MOSFET driver.
Connection to gate of FET that
disables the input power.
20 F ILIM Input to current limit comparator
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Page 3
ABSOLUTE MAXIMUM RATINGS
ML4876
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 Any Other Pin ............... –0.3V to VDD +0.3V
Junction Temperature ..............................................150°C
Storage Temperature Range ..................... –65°C to 150°C
Lead Temperature (Soldering 10 sec.) .....................260°C
Thermal Resistance (qJA) Plastic SSOP ............... 100°C/W
Supply Current (ICC) ............................................... 75mA
Output Current, Source or Sink ............................. 250mA
OPERATING CONDITIONS
Voltage on HVDD ..................................................... 20V
Current into L RTD ............................................... ±10mA
Transient Voltage on B ON .......................................... 9V
Temperature Range
ML4876C ................................................... 0°C to 70°C
ML4876E ............................................... –20°C to 70°C
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, VDD = 5V ±5%, CT = 47pF, TA = Operating Temperature Range (Note 1)
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
CURRENT REGULATOR
Error Amplifier
Open Loop Gain 60 70 dB
Bias Point Closed loop 0.18 0.2 0.22 V
Output High I
Output Low I
Bandwidth (–3dB) 1 MHz
= 5µA 2.8 3.0 V
LOAD
= 25µA 0.4 0.7 V
LOAD
Input Voltage Range –0.3 0.2 VREF V
Input Bias Current 50 100 nA
Current Limit Comparator
Current Threshold 450 500 550 mV
Input Bias Current V
Propagation Delay (Note 2) 30 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 Pulse Width 100 150 200 ns
DELAY TIMER Delay Time 20 35 55 ns
FLYBACK REGULATOR
Error Amplifier
= 0.1V 50 100 nA
LILIM
= 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
Open Loop Gain 60 70 dB
Offset Voltage –15 15 mV
Output High I
Output Low I
= 5µA 2.8 3.0 V
LOAD
= 25µA 0.4 0.7 V
LOAD
3
Page 4
ML4876
ELECTRICAL CHARACTERISTICS
(Continued)
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
FLYBACK REGULATOR
(Continued)
Current Limit Comparator
Threshold 70 100 130 mV
Input Bias Current V
Propagation Delay 125 250 ns
= 0.1V 50 100 nA
LILIM
Output Drivers
Output High - F Gate VDD = 5V, I
Output Low - F Gate I
Rise & Fall Time C
= 50mA 0.2 0.375 V
LOAD
= 1000pF 20 50 ns
LOAD
= 12mA 4.625 4.8 V
LOAD
HIGH VOLTAGE INVERTER
Oscillator
Nominal Frequency 59 70 81 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 0.75 1.1 1.45 V
Hysteresis 250 500 750 mV
Lamp Out Detect
Threshold 16 18 20 V
Under Voltage Detector
Start Up Threshold 3.8 4.1 4.4 V
Hysteresis 150 300 450 mV
Logic Interface (On/Off, L ON)
VIH 2.6 V
VIL 0.5 V
Input Bias Current VI = 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
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 I
HVDD Input Voltage Range 5 18 V
= 1mA 30 90 mV
HVDD
= 5mA 125 275 mA
HVDD
4
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ML4876
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
SUPPLY
VDD Supply Current (No Load) On/Off = 3V, LON = 3V 0.45 0.6 mA
VDD Supply Current On/Off = 3V, LON = 0V 200 350 µA
VDD Supply Current On/Off = “0”, HVDD = 12V 10 µA
VREF Output Voltage TA = 25°C 2.47 2.5 2.53 V
VREF Load Regulation I
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.
(Continued)
= 25µA 10 20 mV
VREF
5
Page 6
ML4876
INPUT 5V to 18V
CONTRAST
VOLTAGE
–20V MAX.
(ADJUSTABLE)
C1
22µF
25V
R2
1.6MΩ
R3
100kΩ
C11
0.1µF
T3
D1
+
Q1
R12
C2
22µF
25V
200kΩ
R4
+
C3
1.0µF
R1
750kΩ
Q2
D3
10V
Q3
20 19 18 17 16 15 14 13 12 11
NOTE 1
5V REF
C4
+
10µF
C9
16V
0.1µF
C8, 0.1µF
ML4876
C5
0.47µF
U2-A
1N4148
1N5817
D2
10987654321
T2
C6
47µF
U2-B
4.3kΩ
R8
C12, 0.1µF
R6
200kΩ
Q4
L1
C10
39pF
1kV
T1
LAMP
Q5
R16
R5
200kΩ
CONTRAST
ADJUST
C7
47pF
LON ON/OFF
NOTE 1
R1, D3, Q2 ARE OPTIONAL AND ALLOWS A BATTERY
VOLTAGE RANGE FROM 7V TO 28V. REMOVING THESE
COMPONENTS AND CONNECTING DIRECTLY TO THE
INPUT VOLTAGE ALLOWS 5.0 TO 18V.
R7
1.6MΩ
BRIGHTNESS
ADJUST
Figure 1. ML4876 Typical Application Schematic
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Page 7
FUNCTIONAL DESCRIPTION
ML4876
The ML4876 consists of a PWM regulator, a lamp driver/
inverter, a linear regulator, a flyback 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. In
addition it generates the DC voltage for the contrast
requirements of LCD screens. A typical application circuit
is shown in Figure 1. Please refer to Application Note 32
for detailed application information beyond what is
presented here.
Note:
Please read the Power Sequencing section below
prior to using the ML4876.
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 (Q4 and Q5) 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
150ns is applied to the primary of the 2:1 pulse
transformer T2, rectified by diode D2, 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 U2-B. The pulse width of the signal on
the gate of U2-B (B OFF) varies according to the
amplitude of the feedback signal on LEA–, which is
proportional to the AC current flowing in the lamp. This
feedback signal is developed by monitoring the current
through resistor R6 in the common source connection of
the inverter MOSFETs (Q4 and Q5). The lamp current, and
therefore brightness, is adjusted by varying the voltage
applied to R7 at the brightness adjust control point.
Increasing this voltage decreases the brightness.
C
T
I
C
➞
T1
LmLm
C
R
Q4 Q5
1:N
C
T1
OUT
LAMP
CLOCK
L GATE1
DRAIN-Q5
L GATE2
DRAIN-Q4
T1-CNTR-PRI
SOURCE OF
U2-A
Figure 2. Simplified Lamp Driver Circuit Figure 3. Operating Waveforms of the
Lamp Driver Section
7
Page 8
ML4876
FUNCTIONAL DESCRIPTION
(Continued)
CONTRAST CONTROL GENERATOR
The contrast voltage generator is a separate regulator in a
flyback configuration. In conjunction with the external
transformer (T3), MOSFET (Q1), diode (D1), and assorted
capacitors and resistors, it provides an adjustable DC
output contrast voltage necessary to drive LCD screens.
The voltage is adjusted by controlling the voltage applied
to R5 at the contrast adjustment point.
The contrast voltage can be made either positive or
negative simply by changing the connection of the
external components. The schematic shown in Figure 1 is
connected for a negative voltage. Please refer to
Application Note 32 for the circuit connection for a
positive output voltage.
OSCILLATOR
The frequency of the oscillator in the ML4876 is set by
selecting the value of CT.
Figure 4 shows the oscillator frequency versus the value
of CT. This nomograph may be used to select the
appropriate value of CT to achieve the desired oscillator
frequency.
LINEAR REGULATOR
A linear voltage regulator is provided to power the low
voltage and low current control circuitry on the ML4876.
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 HVDD to the
input battery voltage. For operation over 18V, a MOSFET
and a resistor (Q2 and R1, Figure 1) are connected as
shown. The MOSFET is required to stand off the high
voltage.
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 ML4876 will detect this condition by sensing the
center tap voltage on the primary of the output
transformer (T1) on the L RTD pin. When this voltage
exceeds 16V, an internal latch is set and the lamp driver
goes into a shutdown mode. The logic control pin L ON
must be cycled low, then high to reset the latch and
return the lamp driver to the normal state.
LOGIC CONTROL
The ML4876 is controlled by a two logic inputs, L ON and
ON/OFF. A logic level high on the L ON pin enables just
the lamp driver. A logic zero on the L ON pin disable the
lamp driver only. A logic level high on the ON/OFF pin
enable the complete circuit. A logic level low on the
ON/OFF pin 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 ML4876. The following
procedure must be observed to avoid damaging the
device.
1. Apply the battery power to HVDD
2. Apply the VDD voltage (if HVDD is not used). With
HVDD connected this voltage is supplied by the
internal regulator on the ML4876.
3. Apply a logic high to the ON/OFF input. This will
enable the internal linear regulator to ensure the VDD
supply is on (when HVDD is used).
4. Apply a logic high to the L ON input.
200KHz
100KHz
70KHz
50KHz
40KHz
FREQUENCY
30KHz
20pF 30pF 40pF 50pF 70pF 100pF 200pF
C
T
Figure 4. Frequency vs. C
T
8
Page 9
ML4876
PHYSICAL DIMENSIONS
0.279 - 0.289
(7.08 - 7.34)
20
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)
inches (millimeters)
Package: R20
20-Pin SSOP
0.205 - 0.213
(5.20 - 5.40)
0.068 - 0.078
(1.73 - 1.98)
SEATING PLANE
0.002 - 0.008
(0.05 - 0.20)
0.301 - 0.313
(7.65 - 7.95)
0º - 8º
0.022 - 0.038
(0.55 - 0.95)
0.004 - 0.008
(0.10 - 0.20)
ORDERING INFORMATION
PART NUMBER TEMPERATURE RANGE PACKAGE
ML4876CR (END OF LIFE) 0°C to 70°C 20-Pin Molded SSOP (R20)
ML4876ER (OBSOLETE) –20°C to 70°C 20-Pin 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;. Japan: 2,598,946; 2,619,299; 2,704,176. 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.
DS4876-01
2092 Concourse Drive
San Jose, CA 95131
Tel: (408) 433-5200
Fax: (408) 432-0295
www.microlinear.com
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