9-3910; Rev 0; 1/06 Low-Cost, Multiple-Output Power Supply for LCD Monitors/TVs The MAX8728 generates all the supply rails for thin-film transistor (TFT) liquid-crystal display (LCD) panels in TVs and monitors. It includes step-down and step-up regulators, positive and negative charge pumps, and a dualmode, logic-controlled high-voltage switch control block. The MAX8728 can operate from input voltages from 7V to 13.
MAX8728 Low-Cost, Multiple-Output Power Supply for LCD Monitors/TVs ABSOLUTE MAXIMUM RATINGS IN, INL, SUPP to GND ............................................-0.3V to +14V SUPP to IN ..........................................................................±0.3V DRVP to GNDP.........................................-0.3V to VSUPP + 0.3V CTL, EN, SHDN, OUT1, VL, VCC to GND ................-0.3V to +6V COMP, FB1, FB2, FBN, FBP, FSEL, DEL, THR, MODE, REF to GND ........................-0.3V to VCC + 0.
Low-Cost, Multiple-Output Power Supply for LCD Monitors/TVs (Circuit of Figure 1, VIN = VINL = VSUPP = 12V, VOUT1 = +3.3V, VSRC = 28V, GND1 = GND2 = GNDP = GND = 0, IREF = 0, TA = 0°C to +85°C. Typical values are at TA = +25°C, unless otherwise noted.) PARAMETER CONDITIONS MIN TYP MAX UNITS STEP-DOWN REGULATOR OUT1 Voltage in Fixed Mode VIN = 7.0V to 13.2V, EN = VCC, ILOAD = 0.5A (Note 1) 3.25 3.30 3.35 V FB1 Regulation Voltage in Adjustable Modes 20% to 35% duty cycle, EN = VCC, ILOAD = 0.
MAX8728 Low-Cost, Multiple-Output Power Supply for LCD Monitors/TVs ELECTRICAL CHARACTERISTICS (continued) (Circuit of Figure 1, VIN = VINL = VSUPP = 12V, VOUT1 = +3.3V, VSRC = 28V, GND1 = GND2 = GNDP = GND = 0, IREF = 0, TA = 0°C to +85°C. Typical values are at TA = +25°C, unless otherwise noted.) PARAMETER CONDITIONS Current-Sense Transresistance MIN TYP MAX UNITS 0.6 1.2 1.8 V/A 0.5 1.0 LX2 On-Resistance Soft-Start Period 3 Ω ms POSITIVE CHARGE-PUMP REGULATOR FBP Regulation Voltage 1.
Low-Cost, Multiple-Output Power Supply for LCD Monitors/TVs (Circuit of Figure 1, VIN = VINL = VSUPP = 12V, VOUT1 = +3.3V, VSRC = 28V, GND1 = GND2 = GNDP = GND = 0, IREF = 0, TA = 0°C to +85°C. Typical values are at TA = +25°C, unless otherwise noted.
MAX8728 Low-Cost, Multiple-Output Power Supply for LCD Monitors/TVs ELECTRICAL CHARACTERISTICS (Circuit of Figure 1, VIN = VINL = VSUPP = 12V, VOUT1 = +3.3V, VSRC = 28V, GND1 = GND2 = GNDP = GND = 0, IREF = 0, TA = -40°C to +85°C.) (Note 2) PARAMETER CONDITIONS MIN TYP MAX UNITS GENERAL IN, INL Input Voltage Range For VL regulator operation IN Standby Supply Current VIN = 7V to 13.2V, EN = SHDN = GND FSEL = GND 7.0 13.2 V 0.
Low-Cost, Multiple-Output Power Supply for LCD Monitors/TVs (Circuit of Figure 1, VIN = VINL = VSUPP = 12V, VOUT1 = +3.3V, VSRC = 28V, GND1 = GND2 = GNDP = GND = 0, IREF = 0, TA = -40°C to +85°C.) (Note 2) PARAMETER CONDITIONS MIN TYP MAX UNITS 1.97 2.02 V 1.71 1.78 V 0.4 V CHARGE-PUMP REGULATORS FBP Regulation Voltage FBN Regulation Voltage VREF - VFBN SEQUENCE CONTROL SHDN Input Low Voltage SHDN Input High Voltage 2 V EN Turn-On Threshold 0.95 1.10 V DEL Turn-On Threshold 0.95 1.
Typical Operating Characteristics (Circuit of Figure 1. VIN = VINL = VSUPP = 12V, AVDD = 13.5V, VGON = 28V, VGOFF = -6V, VOUT1 = 3.3V, FSEL = GND, TA = +25°C, unless otherwise noted.) NORMALIZED STEP-DOWN REGULATOR OUTPUT VOLTAGE vs. LOAD CURRENT STEP-DOWN REGULATOR EFFICIENCY vs. LOAD CURRENT 85 EN = GND 3.35 OUTPUT VOLTAGE (V) EFFICIENCY (%) 80 EN = GND 75 70 65 MAX8728 toc02 3.40 MAX8728 toc01 90 EN = VL 3.30 3.25 EN = VL 3.20 60 3.15 55 3.10 50 0.01 0.1 1 LOAD CURRENT (A) 0.01 10 0.
Low-Cost, Multiple-Output Power Supply for LCD Monitors/TVs MAX8728toc08 STEP-UP REGULATOR LOAD TRANSIENT RESPONSE (100mA TO 600mA) MAX8728toc07 STEP-UP REGULATOR SOFT-START (HEAVY LOAD) EN, 2V/div 0V GATE, 5V/div LOAD CONTROL, 5V/div 0V AVDD, AC, 200mV/div AVDD, 5V/div 0V INDUCTOR CURRENT, 500mA/div INDUCTOR CURRENT, 500mA/div 0V 0V STEP-UP REGULATOR PULSED-LOAD TRANSIENT RESPONSE (100mA TO 1A) TIMER-DELAY OVERCURRENT PROTECTION MAX8728toc10 10μs/div MAX8728toc09 1ms/div LOAD CONTROL, 5V/div
Typical Operating Characteristics (continued) (Circuit of Figure 1. VIN = VINL = VSUPP = 12V, AVDD = 13.5V, VGON = 28V, VGOFF = -6V, VOUT1 = 3.3V, FSEL = GND, TA = +25°C, unless otherwise noted.) POSITIVE CHARGE-PUMP OUTPUT VOLTAGE ERROR vs. INPUT VOLTAGE (VOUT = 28V) REF VOLTAGE vs. REF CURRENT REF VOLTAGE (V) 1.998 1.997 1.996 0.5 1.995 IOUT = 10mA 0 -0.5 -1.0 IOUT = 50mA -1.5 -2.0 -2.5 -3.0 1.994 0 20 40 60 REF CURRENT (μA) 80 10.0 100 POSITIVE CHARGE-PUMP OUTPUT VOLTAGE ERROR vs.
Low-Cost, Multiple-Output Power Supply for LCD Monitors/TVs NEGATIVE CHARGE-PUMP LOAD TRANSIENT RESPONSE MAX8728toc20 MAX8728toc19 POWER-UP SEQUENCE VGON, 20V/div, 10kΩ LOAD AVDD, 5V/div, 150Ω LOAD VGOFF, AC, 50mV/div OUT1, 5V/div, 100Ω LOAD EN, CEN = 0.01μF, 2V/div VGOFF, 5V/div, 10kΩ LOAD IVGOFF, 50mA/div DEL, CDEL = 0.01μF 2V/div 40μs/div 2ms/div INL SUPPLY CURRENT vs. TEMPERATURE INL SUPPLY CURRENT vs.
Low-Cost, Multiple-Output Power Supply for LCD Monitors/TVs MAX8728 Pin Description 12 PIN NAME FUNCTION 1 GND1 Step-Down Regulator and Negative Charge-Pump Power Ground 2 OUT1 Step-Down Regulator Output Sense Input. OUT1 is the inverting input to the internal current-sense amplifier. Connect OUT1 directly to the step-down regulator output. 3 DRVN 4 CTL 5 IN 6 LX1 Step-Down Regulator Switching Node. LX1 is the source of the internal high-side MOSFET.
Low-Cost, Multiple-Output Power Supply for LCD Monitors/TVs PIN NAME FUNCTION 21 DRN High-Voltage Switch Control Input. DRN is the drain of the internal high-voltage p-channel MOSFET connected to GON. See the High-Voltage Switch Control section for details. 22 THR GON Falling Regulation Adjustment Input. Connect THR to the center of a resistive voltage-divider between a reference supply and GND to adjust the GON falling regulation set point.
MAX8728 Low-Cost, Multiple-Output Power Supply for LCD Monitors/TVs VIN 10.8V TO 13.2V L1 6.4μH P1 C2 0.22μF C7 1μF C24 1000pF C3 10μF 16V 9 10 C1 10μF 16V C8 0.1μF OUT1 3.3V/2A 6 D2 1 2 32 R12 10.0kΩ, 1% C25 47pF ON/OFF 11 30 C11 0.1μF C12 0.22μF 12 24 R10 158kΩ 1% LX2 GND2 COMP LX1 GND1 3 C16 0.1μF OUT1 CTL FB1 MODE SHDN 26 R2 20kΩ 1% 25 23 R3 160kΩ 31 DRN DEL SRC FROM TCON 4 29 C14 220pF GNDP D6 R6 2kΩ VGON 28V/50mA 20 19 C15 1μF FBP R5 22.
Low-Cost, Multiple-Output Power Supply for LCD Monitors/TVs The typical operating circuit (Figure 1) of the MAX8728 is a complete power-supply system for TFT LCD panels in monitors and TVs. The circuit generates a +3.3V logic supply, a +13.5V source driver supply, a +28V positive gate driver supply, and a -6V negative gate driver supply from a 12V ±10% input supply and operates at 1.5MHz. Table 1 lists some selected components and Table 2 lists the contact information for component suppliers.
MAX8728 Low-Cost, Multiple-Output Power Supply for LCD Monitors/TVs AVDD VIN INL GATE LX2 VL VL STEP-UP REGULATOR VL REGULATOR BST GND2 IN FB2 COMP OUT1 LX1 STEP-DOWN REGULATOR SHDN EN SEQUENCE CONTROL GND1 VL DEL OUT1 CTL FB1 THR FSEL THERMAL SHUTDOWN OSCILLATOR ON/OFF LCD ENABLE FROM TCON MODE VCC REF REFERENCE DRN FAULT LOGIC AND TIMER SWITCH CONTROL BLOCK GND GON SRC MAX8728 FBN NEGATIVE REGULATOR POSITIVE REGULATOR IN FBP DRVN SUPP DRVP VGOFF VIN Figure 2
Low-Cost, Multiple-Output Power Supply for LCD Monitors/TVs Low-Frequency Operation The step-down regulator of the MAX8728 enters into low-frequency operating mode if the voltage on OUT1 is below 1.3V. In the low-frequency mode, the switching frequency of the step-down regulator is 1/6 the oscillator frequency. This feature prevents potentially uncontrolled inductor current if OUT1 is overloaded or shorted to ground.
MAX8728 Low-Cost, Multiple-Output Power Supply for LCD Monitors/TVs The soft-start feature effectively limits the inrush current during startup (see the Step-Up Regulator Soft-Start Waveforms in the Typical Operating Characteristics). The MAX8728 monitors FB2 for undervoltage conditions. If the voltage is continuously below 90% of the nominal regulation point for approximately 50ms, the MAX8728 sets a fault latch, shutting down all outputs except VL, REF, and the step-down regulator.
Low-Cost, Multiple-Output Power Supply for LCD Monitors/TVs The negative charge-pump regulator is enabled when SHDN and EN are logic high and the step-down regulator reaches regulation. Each time it is enabled, the negative charge-pump regulator goes through a soft-start routine by ramping down its internal reference voltage from 2V to 250mV in 128 steps. The soft-start period is 3ms (typ) and FBN fault detection is disabled during this period.
MAX8728 Low-Cost, Multiple-Output Power Supply for LCD Monitors/TVs turns off and Q2 turns on, connecting GON to DRN. GON can then be discharged through a resistor connected between DRN and GND or AVDD. Q2 turns off and stops discharging GON when VGON reaches 10 times the voltage on THR. When VMODE is less than 0.9 x VREF, the switch control block works in the second mode. The rising edge of VCTL turns on Q1 and turns off Q2, connecting GON to SRC.
Low-Cost, Multiple-Output Power Supply for LCD Monitors/TVs MAX8728 REF SWITCH CONTROL 5μA DEL FAULT SHDN Q4 MAX8728 EN GATE DONE SRC 0.5 x VREF Q1 GON 9R 1kΩ R 50μA REF R 4mA Q2 DRN THR 4R MODE 1kΩ 5R Q3 CTL Figure 5.
MAX8728 Low-Cost, Multiple-Output Power Supply for LCD Monitors/TVs Fault Protection During steady-state operation, if any output of the four regulators (step-down regulator, step-up regulator, positive charge-pump regulator, and negative chargepump regulator) does not exceed its respective faultdetection threshold, the MAX8728 activates an internal fault timer.
Low-Cost, Multiple-Output Power Supply for LCD Monitors/TVs VOUT1_ RIPPLE = VOUT1_ RIPPLE(ESR) + VOUT1_ RIPPLE(C) VOUT1_ RIPPLE(ESR) = IOUT1_ RIPPLE × RESR _ OUT1 IOUT1_ RIPPLE VOUT1_ RIPPLE(C) = 8 × COUT1 × fSW where I OUT1 _ RIPPLE is defined in the Step-Down Regulator, Inductor Selection section, COUT1 is output capacitance, and R ESR _ OUT1 is the ESR of output capacitor COUT1. In Figure 1’s circuit, the inductor ripple current is 0.6A.
MAX8728 Low-Cost, Multiple-Output Power Supply for LCD Monitors/TVs rent ripple and therefore reduce the peak current, which decreases core losses in the inductor and I2R losses in the entire power path. However, large inductor values also require more energy storage and more turns of wire, which increase physical size and can increase I2R losses in the inductor. Low inductance values decrease the physical size but increase the current ripple and peak current.
Low-Cost, Multiple-Output Power Supply for LCD Monitors/TVs To further optimize transient response, vary RCOMP in 20% steps and CCOMP in 50% steps while observing transient response waveforms. Input-Capacitor Selection The input capacitor reduces the current peaks drawn from the input supply and reduces noise injection into the IC. Two 10µF ceramic capacitors are used in the Typical Applications Circuit (Figure 1) because of the high-source impedance seen in typical lab setups.
MAX8728 Low-Cost, Multiple-Output Power Supply for LCD Monitors/TVs Charge-Pump Output Capacitor Decreasing the flying capacitance reduces the output ripple. Increasing the output capacitance reduces the output ripple and improves the transient response.
Low-Cost, Multiple-Output Power Supply for LCD Monitors/TVs 4) Place the INL pin and REF pin bypass capacitors as close to the device as possible. The ground connection of the INL bypass capacitor should be connected directly to the GND pin with a wide trace. 5) Minimize the length and maximize the width of the traces between the output capacitors and the load for best transient responses. 6) Minimize the size of the LX1 and LX2 nodes while keeping them wide and short.
Package Information (The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.maxim-ic.com/packages.) QFN THIN.EPS MAX8728 Low-Cost, Multiple-Output Power Supply for LCD Monitors/TVs D2 D b CL 0.10 M C A B D2/2 D/2 k L MARKING AAAAA E/2 E2/2 CL (NE-1) X e E DETAIL A PIN # 1 I.D. e/2 E2 PIN # 1 I.D. 0.35x45° e (ND-1) X e DETAIL B e L1 L CL CL L L e e 0.10 C A C 0.
Low-Cost, Multiple-Output Power Supply for LCD Monitors/TVs Package Information (continued) (The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.maxim-ic.com/packages.) COMMON DIMENSIONS EXPOSED PAD VARIATIONS PKG. 16L 5x5 20L 5x5 28L 5x5 32L 5x5 40L 5x5 SYMBOL MIN. NOM. MAX. MIN. NOM. MAX. MIN. NOM. MAX. MIN. NOM. MAX. MIN. NOM. MAX. A A1 A3 b D E e PKG. CODES 0.70 0.75 0.80 0.70 0.75 0.80 0.70 0.75 0.80 0.
