JULY 2000 FEATURINGrature Range pe mercial Tem Extended Com 0˚C to 70˚C -2 quipment Handheld E for Portable ML4876* LCD Backlight Lamp Driver with Contrast GENERAL DESCRIPTION FEATURES 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. ■ Backlight lamp driver with differential drive ■ Up to 30% lower power for same light output ■ Low standby current (< 10µA) 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. ■ Improved efficiency (»95%) ■ Allows all N-channel MOSFET drive ■ Low switching losses ■ Resonant threshold detection ■ Buck regulator uses synchronous rectification 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. * THIS PART IS END OF LIFE AS OF JULY 1, 2000 BLOCK DIAGRAM 13 17 GND VDD 11 B SYNC OUT L RTD 10 DR3 LINEAR REGULATOR 6 19 B OFF B ON VDD HVDD 15 18 ONE SHOT MASTER BIAS & UVLO 5 L GATE 1 DR1 DR2 ON/OFF VREF DR1 NEG EDGE DELAY Q 14 L GATE 2 T Q 16 DR2 S Q R Q + LON 0.5V Q S 4 – 16V – Q R + F GATE 12 F ILIM + 20 0.1V FEA– FEA+ 3 Q R – 7 LEA– – + + – 0.2V CLK – 2 RESONANT THRESHOLD DETECTOR Q S – OSCILLATOR + + CT FEA OUT 1 9 LEA OUT 8 1 ML4876 PIN CONFIGURATION ML4876 20-Pin SSOP (R20) FEA OUT 1 20 F ILIM FEA– 2 19 B OFF FEA+ 3 18 B ON L ON 4 17 GND VREF 5 16 L GATE2 ON/OFF 6 15 HVDD LEA– 7 14 L GATE1 LEA OUT 8 13 VDD CT 9 12 F GATE L RTD 10 11 B SYNC OUT TOP VIEW PIN DESCRIPTION PIN NAME DESCRIPTION PIN NAME DESCRIPTION 1 FEA OUT Output of flyback (contrast) error amplifier 11 B SYNC OUT Output of MOSFET driver. Connects to gate of synchronous FET catch diode. 2 FEA– Negative input of flyback (contrast) error amplifier 12 F GATE Connects to gate of MOSFET in primary side of contrast control Positive input of flyback (contrast) error amplifier 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 3 4 FEA+ 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 17 GND Ground 9 CT Oscillator timing capacitor 18 B ON Connection to primary side of gate pulse transformer 10 L RTD Input to resonant threshold detector 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 2 ML4876 ABSOLUTE MAXIMUM RATINGS 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 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 HVDD ..................................................... 20V Current into L RTD ............................................... ±10mA Transient Voltage on B ON .......................................... 9V OPERATING CONDITIONS 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 60 70 MAX UNITS CURRENT REGULATOR Error Amplifier Open Loop Gain Bias Point Closed loop 0.18 0.2 Output High ILOAD = 5µA 2.8 3.0 Output Low ILOAD = 25µA 0.4 Bandwidth (–3dB) dB 0.22 V 0.7 1 Input Voltage Range –0.3 Input Bias Current V V MHz 0.2 VREF V 50 100 nA 500 550 mV 100 nA Current Limit Comparator Current Threshold 450 Input Bias Current VLILIM = 0.1V 50 Propagation Delay (Note 2) 30 ns 4.8 V 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 4.625 4.8 V ONE SHOT Pulse Width 100 150 200 ns DELAY TIMER Delay Time 20 35 55 ns Open Loop Gain 60 70 Offset Voltage –15 FLYBACK REGULATOR Error Amplifier Output High ILOAD = 5µA Output Low ILOAD = 25µA 2.8 dB 15 3.0 0.4 mV V 0.7 V 3 ML4876 ELECTRICAL CHARACTERISTICS SYMBOL (Continued) PARAMETER CONDITIONS MIN TYP MAX UNITS 70 100 130 mV 50 100 nA 125 250 ns FLYBACK REGULATOR (Continued) Current Limit Comparator Threshold Input Bias Current VLILIM = 0.1V Propagation Delay Output Drivers Output High - F Gate VDD = 5V, ILOAD = 12mA Output Low - F Gate ILOAD = 50mA 4.625 4.8 V 0.2 0.375 V 20 50 ns 59 70 81 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 Rise & Fall Time CLOAD = 1000pF 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 0.75 1.1 1.45 V Hysteresis 250 500 750 mV 16 18 20 V Start Up Threshold 3.8 4.1 4.4 V Hysteresis 150 300 450 mV Lamp Out Detect Threshold Under Voltage Detector Logic Interface (On/Off, L ON) VIH 2.6 V VIL Input Bias Current VI = 3V 0.5 V 10 25 µA 5.0 5.35 V Linear Regulator Regulator Voltage (VDD) HVDD = 12V Regulator Source Current External to device 10 Drop Out Voltage IHVDD = 1mA 30 90 mV Drop Out Voltage IHVDD = 5mA 125 275 mA 18 V HVDD Input Voltage Range 4 4.75 5 mA ML4876 ELECTRICAL CHARACTERISTICS SYMBOL (Continued) 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 2.5 2.53 V VREF Load Regulation IVREF = 25µA 10 20 mV 20 30 mV 2.5 2.535 V 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 ML4876 INPUT 5V to 18V T3 CONTRAST VOLTAGE –20V MAX. (ADJUSTABLE) D1 C2 + 22µF 25V C3 1.0µF U2-A L1 NOTE 1 R1 750kΩ Q2 Q1 C1 + 22µF 25V + D3 10V R12 D2 1N4148 5V REF C4 10µF 16V C12, 0.1µF C10 39pF 1kV T2 C9 0.1µF R2 1.6MΩ 1N5817 T1 LAMP U2-B R3 100kΩ Q3 Q4 C8, 0.1µF C11 0.1µF Q5 R16 20 19 18 17 16 15 14 13 12 11 7 8 9 10 R8 4.3kΩ ML4876 1 R4 200kΩ 2 3 4 5 6 C5 C6 R6 47µF 200kΩ 0.47µF CONTRAST ADJUST R7 1.6MΩ C7 47pF R5 200kΩ LON ON/OFF BRIGHTNESS ADJUST 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. Figure 1. ML4876 Typical Application Schematic 6 ML4876 FUNCTIONAL DESCRIPTION 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. CT IC ➞ CLOCK COUT T1 T1 Lm Lm L GATE1 1:N LAMP DRAIN-Q5 CR Q4 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 ML4876 FUNCTIONAL DESCRIPTION (Continued) CONTRAST CONTROL GENERATOR LAMP OUT DETECT 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. 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. 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. 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. 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. 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. 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. 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 FREQUENCY 100KHz 70KHz 50KHz 40KHz 30KHz 20pF 30pF 40pF 50pF 70pF 100pF CT Figure 4. Frequency vs. CT 8 200pF ML4876 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 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) 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 9