ML4877* Desktop Backlight Lamp Driver GENERAL DESCRIPTION

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ML4877* Desktop Backlight Lamp Driver GENERAL DESCRIPTION

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ML4877*
Desktop Backlight Lamp Driver
GENERAL DESCRIPTION
ML4877 ideal solution driving multiple cold cathode fluorescent tubes (CCFL) used liquid crystal display (LCD) backlight applications. provides dimming ballast control display. utilizing differential drive ML4877 deliver same light output with significantly less input power compared existing single ended drive schemes. Improvements high realized when using power lamps advanced screen housings. This increased light output achieved because differential drive configuration much less sensitive, therefore less power wasted capacitive parasitics that exist backlight housing. additional benefit this configuration even distribution light. includes adjustable lamp detect circuit that latches when lamp fault detected. Also, unique architecture ML4877 allows development backlight system that will inherently meet requirements safety. ML4877 optimized large applications applications where high efficiency critical maximize battery life. high efficiency achieved resonant scheme with zero voltage switching.
FEATURES
Ideal inverter designs, lamp design dimming capability Backlight lamp driver with differential drive lower power same light output standby current (<10µA) Improved efficiency (95%) Allows N-channel MOSFET drive Adjustable lamp detect with latch Resonant threshold detection synchronous rectification Positive input dimming control
THIS PART LIFE JLLY 2000
BLOCK DIAGRAM
HVDD LINEAR REGULATOR SYNC
GATE
VREF
MASTER BIAS UVLO
SHOT
EDGE DELAY
GATE
0.5V
ILIM
ON/OFF OSCILLATOR PGND RESONANT THRESHOLD DETECTOR
LEA- LEA+
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ML4877
CONFIGURATION
ML4877 20-Pin SSOP (R20)
LEA+ VREF ILIM LEA- PGND GATE ON/OFF GATE HVDD SYNC
VIEW
DESCRIPTION
NAME FUNCTION NAME FUNCTION
LEA+
Positive input lamp error Connection gate external high voltage regulator. Internally zener diode ground. Connection optional external soft start capacitor
SYNC HVDD GATE1
Output MOSFET driver gate synchronous catch diode. Battery power input linear regulator Output linear regulator. Positive power Output MOSFET driver. Connection gate side inverter drive pair. Logic input chip Output MOSFET driver. Connection gate side inverter drive pair. Power ground Signal ground Connection primary side gate pulse transformer Output MOSFET driver. Connection gate that disables input power.
VREF ILIM LEALEA
Oscillator timing resistor Voltage reference output Input current limit amplifier Negative input lamp error amplifier Output lamp error amplifier. External compensation capacitor connects between this LEA. Oscillator timing capacitor Input resonant threshold detector
ON/OFF GATE2
PGND
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ML4877
ABSOLUTE MAXIMUM RATINGS
Absolute maximum ratings those values beyond which device could permanently damaged. Absolute maximum ratings stress ratings only functional device operation implied. Supply Current (ICC) 75mA Output Current, Source Sink 250mA Voltage Pins LEA+, AZR, CAP, VREF, ILIM, LEA-, OUT, SYNC OUT, VDD, GATE ON/OFF, GATE PGND, GND, -0.3V +0.3V Voltage HVIDD Current into ±10mA Junction Temperature Storage Temperature Range Lead Temperature (Soldering sec.) Thermal Resistance (JA)
OPERATING CONDITIONS
Temperature Range ML4877C
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, ±5%, Operating Temperature Range, 47pF, (Note
SYMBOL CURRENT REGULATOR ERROR AMPLIFIER Open Loop Gain Output High Output Bandwidth (-3dB) Common Mode Voltage Range Input Bias Current Input Offset Voltage Soft Start Charge Current Soft Start Threshold (LEA OUT) CURRENT LIMIT COMPARATOR Current Threshold Input Bias Current Propagation Delay OUTPUT DRIVERS Output High SYNC OUT, Output SYNC OUT, Rise Fall time SYNC OUT, Output High Output Fall Time SHOT Pulse Width ILOAD 12mA ILOAD 12mA CLOAD =100pF ILOAD 12mA ILOAD 50mA CLOAD 2400pF (Note 4.625 4.625 0.375 0.375 VILIM 0.1V (Note VSSCAP VSSCAP ILOAD ILOAD 25µA PARAMETER CONDITIONS UNITS
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ML4877
ELECTRICAL CHARACTERISTICS
SYMBOL DELAY TIMER Delay Time HIGH VOLTAGE INVERTER Oscillator Nominal Frequency Discharge Current Peak Voltage Valley Voltage Output Drivers Output High GATE Output GATE Rise Fall Time GATE Resonant Threshold Detector Threshold Hysteresis Lamp Detect Threshold Latch Inhibit Threshold (SSCAP) Under Voltage Detector Start Threshold Hysteresis Logic Interface (ON/OFF) Input Bias Current Linear Regulator Zener Reference Voltage (AZR) Regulator Voltage (VDD) Regulator Source Current Drop Voltage Drop Voltage HVDD Input Voltage Range IAZR 10µA HVDD External device IHVDD lHVDD 12.3 4.75 13.5 14.7 5.35 ON/OFF LRTD 0.1V 0.45 0.15 1.15 0.45 ILOAD 12mA ILOAD 50mA CLOAD =1000pF 4.625 0.375 PARAMETER
(Continued)
CONDITIONS UNITS
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ML4877
ELECTRICAL CHARACTERISTICS
SYMBOL BIAS Supply Current Supply Current VREF Load Regulation VREF Output Voltage VREF Line Regulation VREF Line, Load, Temp
Note Note
(Continued)
CONDITIONS UNITS
PARAMETER
ON/OFF "I", load ON/OFF "0", HVDD ILOAD 25µA 2.47
2.465
2.53 2.535
Limits guaranteed 100% testing, sampling, correlation with worst case test conditions. Actual load 1200pF. transformer reflects effective 2400pF.
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ML4877
U2-A *OPTIONAL NOTE HVDD LINEAR REGULATOR SHOT 1.0µF MASTER BIAS UVLO ILIM
U2-B OPTIONAL 100µh
SYNC
0.1µF
39pF LAMP
GATE1 GATE2
VREF
EDGE DELAY
100k
0.5V ON/OFF
LEA- 0.047µF 100k
OSCILLATOR
PGND
47pF 0.1µF
Figure Typical Application Schematic ML4877
RESONANT THRESHOLD DETECTOR
1.6M
REV. 10/12/2000
ML4877
FUNCTIONAL DESCRIPTION
ML4877 consists regulator, lamp driver/ inverter, linear regulator control circuits. This conjunction with external components, converts battery voltage into high voltage high frequency signal required start drive miniature cold cathode fluorescent lamps. Typical application circuits shown Figure Figure Note: Please read Power Sequencing section below prior using ML4877. LAMP DRIVER lamp driver, sometimes referred lamp inverter, comprised regulator Royer type inverter circuit drive lamp. regulator, buck configuration, controls magnitude lamp current provide dimming capability. Figure shows simplified circuit more easily illustrate operation circuit. presence buck inductor, circuit shown Figure essentially current parallel loaded resonant circuit. primary inductance output transformer, which tunes with resonant capacitor resonant frequency inverter. oscillator frequency always lower than natural resonant frequency ensure synchronization. current source models current through buck inductor MOSFETs, alternately turned with constant duty cycle signal GATE1, GATE2) one-half frequency oscillator. this each transistor pulses, excites, resonant tank each half cycle. combination these signals appear across primary winding output transformer sinusoidal waveform. This voltage multiplied step-up turns ratio output transformer impressed across lamp. output transitions controlled feedback through sensing voltage center output transformer. Each time this signal reaches minimum resonant threshold detection point internal clock pulse generated keep system synchronized. Figure shows some these representative waveforms important nodes circuit. regulator comprised MOSFET (U2-A), inductor gate control drive circuitry shown Figure signal with constant pulse width 50ns applied primary pulse transformer rectified diode used charge gate capacitance U2-A, thereby turning turn controlled discharging this capacitance through MOSFET pulse width signal gate OFF) varies according difference amplitude feedback signal LEA+, LEA-. signal LEA- proportional current flowing lamp, while signal LEA+ function brightness control setting. lamp current feedback signal developed monitoring current through resistor common source connection inverter MOSFETs, lamp current, therefore brightness, adjusted varying voltage applied brightness adjust control point. Increasing this voltage increases brightness. OSCILLATOR frequency oscillator ML4877 selecting values Figure shows
COUT
CLOCK
GATE1
LAMP
DRAIN-Q4
GATE2
DRAIN-Q3
T1-CNTR-PRI SOURCE U2-A
Figure Kelvin Sense Connections
Figure Operating Waveforms Lamp Driver Section
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ML4877
FUNCTIONAL DESCRIPTION
(Continued) selecting appropriate value lamp current slowly increase with controlled time constant. capacitor value calculated according following formula. 10-7)TS oscillator frequency versus value different values This nomograph used select appropriate value achieve desired oscillator frequency ML4877. LINEAR REGULATOR linear voltage regulator provided power voltage current control circuitry ML4877. This typically used when there separate supply available inverter board. operation 18V, linear regulator used connecting HVDD input battery voltage. operation over 18V, MOSFET, resistor Figure connected shown. MOSFET required stand high voltage. just zener diode ground used bias gate LAMP DETECT those cases when there lamp connected, connection faulty, output voltage lamp driver circuit will tend rise high level attempt start nonexistent lamp. lamp detect circuit ML4877 will detect this condition sensing voltage proportional center voltage primary output transformer, pin. ration resistors sets lamp detect threshold. When voltage exceeds VDD, internal latch lamp driver goes into shutdown mode. logic control ON/OFF must cycled low, then high reset latch return lamp driver normal state. input lamp latch inhibited signal soft start pin. latch will until voltage (pin rises more than 4.2V nominally. SOFT START capability control start behavior achieved setting value single capacitor, Figure
1000
Where Duration soft start sequence seconds LOGIC CONTROL ML4877 controlled single logic input, OFF. logic level high this enables lamp driver. logic zero puts circuit into very power state. POWER SEQUENCING important observe correct power logic input sequencing when powering ML4877. following procedure must observed avoid damaging device. Apply battery power HVDD, HVDD used. Apply voltage. With HVDD connected voltage supplied internal regulator ML4877. Apply logic high ON/OFF input. Please refer Application Note detailed application information beyond what presented here.
APPLICATIONS SECTION
HIGH POWER INVERTER ML4877 easily adapted high power CCFL inverter designs. Figure displays schematic ML4877 application. This particular design employs dimming order extend dimming range. inverter design ideal applications between range. Deep dimming capability achieved technique with flicker popon effects. Uniform intensity maintained across lamps well below Figure provides view example ML4877 design. This design modified lamps contains dimming interface using standard cost components. latest application notes other information, visit Micro Linear website www.microlinear.com.
FRQUENCY (kHz)
RESISTANCE
1000
Figure Oscillator Frequency Nomograph
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ML4877
DIMMING 68pF 68pF 68pF 68pF 68pF 68pF 68pF 68pF
22µH 220µF 220µF 0.1µF IRF7416 0.22µF MKS-10 IRLR2905
SK34MSCT
2N3904
IRLR2905
2N3904
2N3906
7.5k
2N3904
4.7nF
4.7nF 6.2k
LEA+ ML4877 PGND VREF GATE ILIM ON/OFF LEA- GATE HVDD SYNC
143k
5.1V
47pF
390k
1N4148
REV. 10/12/2000
1N4148 2N3906 0.033µF
1N4148
Control
Figure Backlight CCFL Inverter with Dimming
ML4877
Figure CCFL Inverter Board, Lamps
REV. 10/12/2000
ML4877
PHYSICAL DIMENSIONS
inches (millimeters)
Package: 20-Pin SSOP
0.279 0.289 (7.08 7.34)
0.205 0.213 (5.20 5.40)
0.301 0.313 (7.65 7.95)
0.026 (0.65 BSC) 0.068 0.078 (1.73 1.98)
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
PART NUMBER ML4877CR (END LIFE) ML4877ER (OBSOLETE) TEMPERATURE RANGE 70°C PACKAGE Molded SSOP (R20) Molded SSOP (R20)
DISCLAIMER FAIRCHILD SEMICONDUCTOR RESERVES RIGHT MAKE CHANGES WITHOUT FURTHER NOTICE PRODUCTS HEREIN IMPROVE RELIABILITY, FUNCTION DESIGN. FAIRCHILD DOES ASSUME LIABILITY ARISING APPLICATION PRODUCT CIRCUIT DESCRIBED HEREIN; NEITHER DOES CONVEY LICENSE UNDER PATENT RIGHTS, RIGHTS OTHERS. LIFE SUPPORT POLICY FAIRCHILD'S PRODUCTS AUTHORIZED CRITICAL COMPONENTS LIFE SUPPORT DEVICES SYSTEMS WITHOUT EXPRESS WRITTEN APPROVAL PRESIDENT FAIRCHILD SEMICONDUCTOR CORPORATION. used herein: Life support devices systems devices systems which, intended surgical implant into body, support sustain life, whose failure perform when properly used accordance with instructions provided labeling, reasonably expected result significant injury user. www.fairchildsemi.com
REV. 10/12/2000
critical component component life support device system whose failure perform reasonably expected cause failure life support device system, affect safety effectiveness.
2000 Fairchild Semiconductor Corporation

ML4877* Desktop Backlight Lamp Driver GENERAL DESCRIPTION

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