MAX8785A 19-0600 Rev 1 5/08 Full-Bridge Controller for Piezoelectric Transformers General Description Features Liquid-crystal display (LCD) enclosures and cold-cathode Resonant, All n-Channel, Full-Bridge Topology fluorescent lamps (CCFL) used in notebook computer Wide Input-Voltage Range (4.5V to 28V) and portable electronic displays are becoming increa- Frequency Sweeping Guarantees CCFL Striking singly narrow, generating the need for a low-profile CCFL power supply. Recent advances in piezoelectric Feed-Forward Control Provides Excellent Line- transformers (PZTs) have made it possible to develop Transient Response smaller, more efficient, and safer backlight inverters for Programmable Maximum Switching Frequency portable displays. Piezoelectric transformers have Analog or Digital DPWM shown better performance than magnetic transformers Lamp-Out Detection with Adjustable Timeout with respect to efficiency, EMI requirements, human safety, and form factor. The MAX8785A is a full-bridge Fault Protection with Adjustable Timeout CCFL controller for piezoelectric transformer-based Primary Current Limit with R Sensing DS(ON) backlight power supplies. The full-bridge topology pro- Strong Gate Drivers Support Large vides a high-spectral purity sinusoidal drive that helps External MOSFETs the PZT convert electrical energy to mechanical and back to electrical energy efficiently. 28-Pin Thin QFN Package The MAX8785A employs a feed-forward control architec- ture that provides excellent line-and-load regulation while Ordering Information maintaining relatively constant switching frequency. The MAX8785A provides protection against open-lamp, PART TEMP RANGE PIN-PACKAGE secondary short-circuit, lamp arcing, and secondary MAX8785AETI+ -40C to +85C 28 Thin QFN* overvoltage with adjustable timeout periods. +Denotes lead-free package. The MAX8785A guarantees lamp striking by sweeping *EP = Exposed pad. the switching frequency from high to low until the lamp is struck. The MAX8785A achieves 10:1 dimming range using a digital pulse-width modulation (DPWM) method. Pin Configuration CCFL brightness can be set with an analog voltage on the CNTL pin or through an external signal at LSYNC. The maximum switching frequency and DPWM frequen- TOP VIEW cy can be adjusted with external resistors. 28 27 26 25 24 23 22 The MAX8785A gate drivers can drive large power MOSFETs typically used in high-power CCFL applica- RATE 1 21 BST1 tions. An internal 5.35V linear regulator powers the + SHDN 2 20 V MOSFET drivers and most of the internal circuitry. The DD1 MAX8785A is available in a low-profile, 28-pin thin QFN CNTL 3 19 GL1 package and operates over the -40C to +85C tempe- LSYNC 4 18 PGND rature range. MAX8785A FLT GL2 5 17 Applications TFLT V 6 16 DD2 Notebook LCDs HF 7 15 BST2 LCD TVs and Monitors 891101 12 13 14 THIN QFN Simplified Operating Circuit appears at end of data sheet. Maxim Integrated Products 1 For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxims website at www.maxim-ic.com. EVALUATION KIT AVAILABLE BATT LF GND PCOMP V COMP CC SEL IFB OLF VFB LX1 LX2 GH1 GH2Full-Bridge Controller for Piezoelectric Transformers ABSOLUTE MAXIMUM RATINGS BATT, RATE, LX1, LX2 to GND .............................-0.3V to +30V COMP, HF, LF, PCOMP, SEL, BST1, BST2 to GND ...............................................-0.3V to +36V TFLT to GND............................................-0.3V to (V + 0.3V) CC BST1 to V , BST2 to V ..................................-0.3V to +30V IFB, VFB, OLF to GND.................................................-6V to +6V DD1 DD2 BST1 to LX1..............................................................-0.3V to +6V PGND to GND .......................................................-0.3V to +0.3V BST2 to LX2..............................................................-0.3V to +6V Continuous Power Dissipation (T = +70C) A V , V , V , CNTL, LSYNC, SHDN, 28-Pin Thin QFN (derate 21.3mW/C above +70C)...1702.1mW CC DD1 DD2 FLT to GND...........................................................-0.3V to +6V Operating Temperature Range ...........................-40C to +85C GH1 to LX...............................................-0.3V to (VBST1 + 0.3V) Junction Temperature......................................................+150C GH2 to LX2.............................................-0.3V to (VBST2 + 0.3V) Storage Temperature Range .............................-65C to +160C GL1 to GND ..............................................-0.3V to (V + 0.3V) Lead Temperature (soldering, 10s) .................................+300C DD1 GL2 to GND ..............................................-0.3V to (V + 0.3V) DD2 Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ELECTRICAL CHARACTERISTICS (V = 12V, V = V = V = 5.35V, R = 190k,T = 0C to +85C, unless otherwise noted. See Figure 1. Typical values BATT DD1 DD2 SHDN RATE A are at T = +25C.) A PARAMETER CONDITIONS MIN TYP MAX UNITS V = V = V = open 5.5 28.0 CC DD1 DD2 BATT Input Voltage Range V V = V = V = BATT 4.5 5.5 CC DD1 DD2 BATT Quiescent Current V = 28V 1.5 6 mA BATT BATT Quiescent Current, Shutdown V = 0, V = 28V 10 25 A SHDN BATT V Output Voltage, Normal Operation 6V < V < 28V, 0 < I < 20mA 5.20 5.35 5.50 V CC BATT LOAD V Output Voltage, Shutdown V = 0, no load 3.5 4.6 5.5 V CC SHDN V rising (leaving lockout) 4.5 CC V Undervoltage-Lockout Threshold V CC V falling (entering lockout) 4.0 CC V Undervoltage-Lockout Hysteresis 150 mV CC GH1, GH2, GL1, and GL2 I = 10mA 1 3 TEST On-Resistance, Low State GH1, GH2, GL1, and GL2 I = 10mA 4 8 TEST On-Resistance, High State V = 24V, V = 19V V = 24V, V BST1 LX1 BST2 LX2 BST1, BST2 Leakage Current 5A = 19V, V = 28V BATT LX1, LX2 Leakage Current V = 28V 5 A BATT Minimum On-Time 240 370 500 ns V = 8V, R = 190k, R = 100k 22 25 28 BATT RATE HF V = 16V, R = 190k, R = 100k 10.8 12.50 14.2 BATT RATE HF Duty Cycle V = 24V, R = 190k, R = 100k 7.5 8.5 9.5 % BATT RATE HF V = 12V, R = 150k, R = 100k 14 BATT RATE HF V = 12V, R = 300k, R = 100k 25 BATT RATE HF Maximum Duty Cycle 45 % Current-Limit Threshold LX1 - PGND, LX2 - PGND 370 400 430 mV Current-Limit Leading-Edge Blanking 240 370 500 ns IFB Regulation Point 760 800 840 mV 0 < V < +3V -2 +2 IFB IFB Input Bias Current A -3V < V < 0 -225 IFB 2 MAX8785A