19-3793; Rev 0; 8/05 KIT ATION EVALU LE B A IL A AV TFT LCD Step-Up DC-DC Converter ♦ 1.8V to 5.5V Input Supply Range ♦ Built-In 14V, 2.2A, 0.2Ω n-Channel MOSFET ♦ High Efficiency (> 85%) ♦ Fast Transient Response to Pulsed Load ♦ High-Accuracy Output Voltage (1.5%) ♦ Internal Digital Soft-Start ♦ Input Supply Undervoltage Lockout ♦ 1.2MHz Switching Frequency ♦ 0.1µA Shutdown Current ♦ Small 8-Pin TDFN Package Ordering Information Applications PART TEMP RANGE PINPACKAGE PKG CODE MAX8752ETA -40°C to +85°C 8 TDFN 3mm x 3mm T833-2 LCD Monitor Panels IN LX 5 LX TOP VIEW IN VMAIN 8 VIN +1.8V TO +5.5V Pin Configuration SUP Typical Operating Circuit 6 Automotive Displays LDO Notebook Computer Displays 7 The MAX8752 is a high-performance, step-up DC-DC converter that provides a regulated supply voltage for active-matrix thin-film transistor (TFT) liquid-crystal displays (LCDs). The MAX8752 incorporates current-mode, fixed-frequency, pulse-width modulation (PWM) circuitry with a built-in n-channel power MOSFET to achieve high efficiency and fast transient response. The input supply voltage of the MAX8752 is from 1.8V to 5.5V. The MAX8752 operates with a switching frequency of 1.2MHz, allowing the use of ultrasmall inductors and lowESR ceramic capacitors. The current-mode architecture provides fast transient response to the pulsed loads typical of LCD source-driver applications. A compensation pin (COMP) gives users flexibility in adjusting loop dynamics. The 14V internal MOSFET can generate output voltages up to 13V. The internal digital soft-start and current limit effectively control inrush and fault currents. The MAX8752 is available in a 3mm x 3mm 8-pin TDFN package with a maximum height of 8mm. Features FB MAX8752 MAX8752 GND COMP LDO 2 3 4 SHDN GND 1 FB IN SUP COMP SHDN TDFN 3mm x 3mm ________________________________________________________________ Maxim Integrated Products For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. 1 MAX8752 General Description MAX8752 TFT LCD Step-Up DC-DC Converter ABSOLUTE MAXIMUM RATINGS LX, SUP to GND .....................................................-0.3V to +14V IN, SHDN, LDO to GND............................................-0.3V to +6V FB to GND ...................................................-0.3V to (VIN + 0.3V) COMP to GND ..........................................-0.3V to (VLDO + 0.3V) LX Switch Maximum Continuous RMS Current .....................1.6A Continuous Power Dissipation (TA = +70°C) 10-Pin TDFN (derate 18.2mW/°C above +70°C) .......1454mW Operating Temperature Range ...........................-40°C to +85°C Junction Temperature ......................................................+150°C Storage Temperature Range .............................-65°C to +160°C Lead Temperature (soldering, 10s) .................................+300°C 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 (VIN = VSHDN = 2.5V, TA = 0°C to +85°C. Typical values are at TA = +25°C, unless otherwise noted.) PARAMETER CONDITIONS Input Supply Range MIN TYP MAX UNITS 5.5 V 13 V 1.30 1.75 V 0.18 0.35 2 5 1.8 Output Voltage Range IN Undervoltage Lockout Threshold IN Quiescent Current VIN rising, typical hysteresis is 200mV 0.90 VFB = 1.3V, not switching VFB = 1.0V, switching mA IN Shutdown Current SHDN = GND 0.1 10.0 µA LDO Output Voltage 6V ≤ VSUP ≤ 13V, ILDO = 12.5mA 4.6 5.0 5.4 V LDO Undervoltage Lockout VLDO rising, typical hysteresis is 200mV 2.4 2.7 3.0 V LDO Output Current 15 SUP Supply Voltage Range 4.5 SUP Overvoltage-Lockout Threshold VSUP rising, typical hysteresis is 200mV (Note 1) SUP Undervoltage-Lockout Threshold VSUP rising, typical hysteresis is 200mV (Note 2) SUP Supply Current 13.2 LX not switching LX switching mA 13.6 13.0 V 14.0 V 1.4 V 1.5 2.0 4 8 mA ERROR AMPLIFIER FB Regulation Voltage ILX = 200mA, T = 0°C to +25°C 1.218 1.240 1.262 ILX = 200mA, T = +25°C to +85°C 1.223 1.240 1.257 0 40 nA 0.05 0.15 %/V 180 280 FB Input Bias Current VFB = 1.24V FB Line Regulation VIN = 1.8V to 5.5V Transconductance 70 Voltage Gain 700 V µS V/V OSCILLATOR Frequency Maximum Duty Cycle 2 1000 1220 1500 kHz 88 92 96 % _______________________________________________________________________________________ TFT LCD Step-Up DC-DC Converter MAX8752 ELECTRICAL CHARACTERISTICS (continued) (VIN = VSHDN = 2.5V, TA = 0°C to +85°C. Typical values are at TA = +25°C, unless otherwise noted.) PARAMETER CONDITIONS MIN TYP MAX UNITS n-CHANNEL MOSFET Current Limit VFB = 1V, 65% duty cycle 1.8 On-Resistance Leakage Current VLX = 12V Current-Sense Transresistance 0.2 2.2 2.6 A 0.2 0.4 Ω 0.1 10 µA 0.3 0.4 V/A SOFT-START Soft-Start Period Soft-Start Step Size 13 ms 0.275 A CONTROL INPUTS SHDN Input Low Voltage VIN = 1.8V to 5.5V SHDN Input High Voltage VIN = 1.8V to 5.5V 0.6 V 0.001 1.000 µA TYP MAX 0.7 × VIN SHDN Input Current V ELECTRICAL CHARACTERISTICS (VIN = VSHDN = 2.5V, TA = -40°C to +85°C. unless otherwise noted.) PARAMETER CONDITIONS Input Supply Range MIN 1.8 Output Voltage Range IN Undervoltage-Lockout Threshold IN Quiescent Current VIN rising, typical hysteresis is 200mV 0.90 VFB = 1.3V, not switching V 13 V 1.75 V 0.35 VFB = 1.0V, switching 5 LDO Output Voltage 6V ≤ VSUP ≤ 13V, ILDO = 12.5mA 4.6 5.4 LDO Undervoltage Lockout VLDO rising, typical hysteresis is 200mV 2.4 3.0 LDO Output Current 15 SUP Supply Voltage Range mA V V mA 4.5 13.0 V 13.2 14.0 V VSUP rising, typical hysteresis is 200mV (Note 2) 1.4 V LX not switching 2.0 SUP Overvoltage-Lockout Threshold VSUP rising, typical hysteresis is 200mV (Note 1) SUP Undervoltage-Lockout Threshold SUP Supply Current UNITS 5.5 LX switching 8 mA ERROR AMPLIFIER FB Regulation Voltage ILX = 200mA 1.210 1.270 V 940 1560 kHz 1.7 2.7 A OSCILLATOR Frequency n-CHANNEL MOSFET Current Limit VFB = 1V, 65% duty cycle On-Resistance Current-Sense Transresistance 0.2 0.4 Ω 0.4 V/A _______________________________________________________________________________________ 3 ELECTRICAL CHARACTERISTICS (continued) (VIN = VSHDN = 2.5V, TA = -40°C to +85°C. unless otherwise noted.) PARAMETER CONDITIONS MIN TYP MAX UNITS 0.6 V CONTROL INPUTS SHDN Input Low Voltage VIN = 1.8V to 5.5V SHDN Input High Voltage 0.7 × VIN VIN = 1.8V to 5.5V V Note 1: Step-up regulator inhibited when VSUP exceeds this threshold. Note 2: Step-up regulator inhibited until VSUP exceeds this threshold. Note 3: Specifications to -40°C are guaranteed by design, not production tested. Typical Operating Characteristics (Circuit of Figure 1, VIN = 2.5V, VMAIN = 10V, TA = +25°C, unless otherwise noted.) EFFICIENCY (%) VIN = 3.3V 75 70 VIN = 1.8V 65 60 80 75 70 VIN = 3.3V 65 60 55 0.5 1000 VIN = 1.8V -1.0 VIN = 3.3V -1.5 -2.0 -3.0 10 100 LOAD CURRENT (mA) 1 1000 10 IN SUPPLY CURRENT vs. SUPPLY VOLTAGE SWITCHING FREQUENCY ERROR vs. INPUT VOLTAGE 50 MAX8752 toc04 0.1 40 IN SUPPLY CURRENT (μA) 0 NORMAL FB -0.1 -0.2 -0.3 -0.4 NO LOAD 30 20 VFB = 1.3V 10 VIN = 1.8V 40 30 VIN = 3.3V 20 10 VIN = 5V -0.5 0 -0.6 1.8 2.8 3.8 4.8 INPUT VOLTAGE (V) 5.8 10,000 50 IN SUPPLY CURRENT (μA) 0.2 100 1000 LOAD CURRENT (mA) IN SUPPLY CURRENT vs. TEMPERATURE MAX8752 toc05 100 LOAD CURRENT (mA) -0.5 VIN = 1.8V 50 10 VIN = 5V 0 -2.5 55 50 4 VIN = 5V 85 80 EFFICIENCY (%) L1 = 3.3μH MAX8752 toc06 90 OUTPUT VOLTAGE ERROR (%) VIN = 5V MAX8752 toc02 85 95 MAX8752 toc01 90 L1 = 2.6μH OUTPUT VOLTAGE ERROR vs. LOAD CURRENT EFFICIENCY vs. LOAD CURRENT MAX8752 toc03 EFFICIENCY vs. LOAD CURRENT SWITCHING FREQUENCY ERROR (%) MAX8752 TFT LCD Step-Up DC-DC Converter 0 1.5 2.5 3.5 4.5 SUPPLY VOLTAGE (V) 5.5 -40 -20 0 20 40 TEMPERATURE (°C) _______________________________________________________________________________________ 60 80 TFT LCD Step-Up DC-DC Converter (Circuit of Figure 1, VIN = 2.5V, VMAIN = 10V, TA = +25°C, unless otherwise noted.) SOFT-START (HEAVY LOAD) LOAD TRANSIENT RESPONSE PULSED-LOAD TRANSIENT RESPONSE MAX8752 toc08 MAX8752 toc07 MAX8752 toc09 IMAIN 1A/div IMAIN 200mA/div INDUCTOR CURRENT 1A/div 100mA 0A VMAIN 5V/div INDUCTOR CURRENT 1A/div 0A 0A 0A 10V 10V VMAIN 500mA/div 10V OFFSET 0V INDUCTOR CURRENT 1A/div VMAIN 200mV/div 10V OFFSET 100μs/div 10μs/div SWITCHING WAVEFORMS SUP SUPPLY CURRENT vs. SUP VOLTAGE SUP SUPPLY CURRENT vs. TEMPERATURE INDUCTOR CURRENT 500mA/div ILOAD = 300mA SUP SUPPLY CURRENT (mA) 0V 3.5 3.0 VIN = 1.8V 2.5 2.0 VIN = 5V 1.5 1.0 VIN = 3.3V 4.2 VIN = 3.3V VIN = 1.8V 3.8 VIN = 5V 3.4 0.5 0 0A 4 1μs/div 6 8 10 SUP VOLTAGE (V) 12 3.0 14 -40 -20 0 20 40 TEMPERATURE (°C) 60 80 LDO OUTPUT VOLTAGE vs. LDO CURRENT LDO OUTPUT VOLTAGE vs. TEMPERATURE 5.06 MAX8752 toc14 5.08 MAX8752 toc13 5.08 5.06 5.04 LDO VOLTAGE (V) OUTPUT VOLTAGE (V) ILOAD = 140mA SUP SUPPLY CURRENT (mA) NO LOAD 4.0 LX 5V/div MAX8752 toc11 4.5 MAX8752 toc12 2ms/div MAX8752 toc10 5.04 5.02 5.02 5.00 4.98 5.00 4.96 4.94 4.98 -40 -20 0 20 40 TEMPERATURE (°C) 60 80 0 10 MAX8752 Typical Operating Characteristics (continued) 20 30 LDO CURRENT (mA) 40 50 _______________________________________________________________________________________ 5 TFT LCD Step-Up DC-DC Converter MAX8752 Pin Description PIN NAME FUNCTION COMP Compensation Pin for Error Amplifier. Connect a series resistance and capacitor from COMP to GND. See the Loop Compensation section for component selection guidelines. 2 FB Feedback Pin. The FB regulation voltage is 1.24V nominal. Connect an external resistive voltagedivider between the step-up regulator’s output (VMAIN) and GND, with the center tap connected to FB. Place the divider close to the IC and minimize the trace area to reduce noise coupling. Set VMAIN according to the Output Voltage Selection section. 3 SHDN Shutdown Control Input. Drive SHDN low to turn off the MAX8752. 4 GND Ground 5 LX Switching Node. LX is the drain of the internal MOSFET. Connect the inductor/rectifier diode junction to LX and minimize the trace area for lower EMI. 6 IN Supply Pin. Connect IN to the input supply through a series 100Ω resistor and bypass it to GND with 0.1µF or greater ceramic capacitor. 7 LDO Internal 5V Linear-Regulator Output. This regulator powers all internal circuitry. Bypass LDO to GND with a 0.22µF or greater ceramic capacitor. 8 SUP Linear-Regulator Supply Input. SUP is the supply input of the internal 5V linear regulator. Connect SUP to the step-up regulator output and bypass SUP to GND with a 0.1µF capacitor. BP — 1 Backside Paddle. Connect the backside paddle to analog ground. C11 0.1μF VGOFF -9V/20mA VIN L1 2.6μH +1.8V TO +5.5V VMAIN +10V/240mA D1 R4 100Ω R1 90.9kΩ 1% LX IN R3 40.2kΩ C2 10μF 16V FB C3 0.1μF R2 13kΩ 1% MAX8752 COMP C4 1.2nF C13 0.1μF D3 C12 0.1μF C8 0.1μF C1 10μF 6.3V C10 0.1μF C9 0.1μF D2 VGON 28V/10mA D4 GND C6 20pF SUP C14 0.22μF LDO C7 0.1μF SHDN Figure 1. Typical Applications Circuit 6 _______________________________________________________________________________________ TFT LCD Step-Up DC-DC Converter LOGIC AND DRIVER IN STARTUP OSC GND CURRENT LIMIT SOFTSTART ILIMIT SLOPE COMP ∑ OSCILLATOR SHDN SUP LDO CURRENT SENSE PWM COMPARATOR ERROR AMP LINEAR REGULATOR AND BOOTSTRAP FB MAX8752 1.24V COMP Figure 2. MAX8752 Functional Diagram Detailed Description The MAX8752 is a highly efficient, step-up power supply designed for TFT-LCD panels. The typical circuit shown in Figure 1 operates from an input voltage as low as 1.8V, and produces a MAIN output of 10V at 220mA from 2.5V input while supporting discrete diode-capacitor charge pumps that produce -9V at 20mA and +28V at 10mA. If the charge-pump outputs are not required, the diodes and capacitors associated with them may be eliminated and the main output increased to 270mA. The MAX8752 employs a current-mode, fixed-frequency, pulse-width modulation (PWM) architecture for fast transient response and low-noise operation. The high switching frequency (1.2MHz) allows the use of lowprofile inductors and ceramic capacitors to minimize the thickness of LCD panel designs. The integrated high-efficiency MOSFET and the IC’s built-in digital soft-start function reduce the number of external components required. The output voltage can be set from VIN to 13V with an external resistive voltage-divider. The MAX8752 regulates the output voltage through a combination of an error amplifier, two comparators, and several signal generators (Figure 2). The error amplifier compares the signal at FB to 1.24V and varies the COMP output. The voltage at COMP determines the At light loads, this architecture allows the MAX8752 to “skip” cycles to prevent overcharging the output capacitor voltage. In this region of operation, the inductor ramps up to a peak value of approximately 250mA, discharges to the output, and waits until another pulse is needed. Output-Current Capability The output-current capability of the MAX8752 is a function of current limit, input voltage, operating frequency, and inductor value. Because of the slope compensation used to stabilize the feedback loop, the inductor current limit depends on the duty cycle. The current limit is determined by the following equation: ILIM = (1.162 - 0.361 x D) x ILIM_EC where ILIM_EC is the current limit specified at 65% duty cycle (see the Electrical Characteristics) and D is the duty cycle. The output current capability depends on the currentlimit value and is governed by the following equation: ⎡ 0.5 x D VIN ⎤ VIN IOUT(MAX) = ⎢ILIM − xη ⎥x f x L V OSC OUT ⎣ ⎦ _______________________________________________________________________________________ 7 MAX8752 LX CLOCK current trip point each time the internal MOSFET turns on. As the load changes, the error amplifier sources or sinks current to the COMP output to set the inductor peak current necessary to service the load. To maintain stability at high duty cycles, a slope-compensation signal is summed with the current-sense signal. On the rising edge of the internal clock, the controller sets a flip-flop, turning on the n-channel MOSFET and applying the input voltage across the inductor. The current through the inductor ramps up linearly, storing energy in its magnetic field. Once the sum of the current-feedback signal and the slope compensation exceed the COMP voltage, the controller resets the flipflop and turns off the MOSFET. Since the inductor current is continuous, a transverse potential develops across the inductor that turns on the diode (D1). The voltage across the inductor then becomes the difference between the output voltage and the input voltage. This discharge condition forces the current through the inductor to ramp back down, transferring the energy stored in the magnetic field to the output capacitor and the load. The MOSFET remains off for the rest of the clock cycle. MAX8752 TFT LCD Step-Up DC-DC Converter where ILIM is the current limit calculated above, η is the regulator efficiency (85% nominal), and D is the duty cycle. The duty cycle when operating at the current limit is: D = VOUT − VIN + VDIODE VOUT − ILIM × RON + VDIODE Table 1. Component List DESIGNATION DESCRIPTION C1 10µF ±10%, 4V X5R ceramic capacitor (0603) TDK C1608X5R0G106K Murata GRM188R60G106M C2 10µF ±10%, 16V X5R ceramic capacitor (1206) TDK C3216X5R1C106K Murata GRM319R61A106K D1 3A, 30V Schottky diode (M-flat) Toshiba CRS02 L1 2.6µH, 2.1A power inductor 3.3µH, 1.7A power inductor Sumida CDRH6D12-3R3 where VDIODE is the rectifier diode forward voltage and RON is the on-resistance of the internal MOSFET. Bootstrapping and Soft-Start The MAX8752 features bootstrapping operation. In normal operation, the internal linear regulator supplies power to the internal circuitry. The input of the linear regulator (SUP) should be directly connected to the output of the step-up regulator. After the input voltage at SUP is above 1.75V, the regulator starts open-loop switching to generate the supply voltage for the linear regulator. The internal reference block turns on when the LDO voltage exceeds 2.7V (typ). When the reference voltage reaches regulation, the PWM controller and the current-limit circuit are enabled and the step-up regulator enters soft-start. During the soft-start, the main step-up regulator directly limits the peak inductor current, allowing from zero up to the full current limit in eight equal current steps. The maximum load current is available after the output voltage reaches regulation (which terminates soft-start), or after the soft-start timer expires (13ms typ). The soft-start routine minimizes the inrush current and voltage overshoot and ensures a well-defined startup behavior. Shutdown The MAX8752 shuts down to reduce the supply current to 0.1µA when SHDN is low. In this mode, the internal reference, error amplifier, comparators, and biasing circuitry turn off and the n-channel MOSFET is turned off. In shutdown, the step-up regulator’s output is connected to IN through the external inductor and rectifier diode. Applications Information Step-up regulators using the MAX8752 can be designed by performing simple calculations for a first iteration. All designs should be prototyped and tested prior to production. Table 1 provides a list of power components for the typical applications circuit. Table 2 lists component suppliers. External component value choice is primarily dictated by the output voltage and the maximum load current, as well as maximum and minimum input voltages. Begin by selecting an inductor value. Once the inductor value and peak current are known, choose the diode and capacitors. Inductor Selection The minimum inductance value, peak current rating, and series resistance are factors to consider when selecting the inductor. These factors influence the converter’s efficiency, maximum output load capability, transient response time, and output voltage ripple. Physical size and cost are also important factors to consider. Table 2. Component Suppliers SUPPLIER PHONE FAX Murata 770-436-1300 770-436-3030 www.murata.com Sumida 847-545-6700 847-545-6720 www.sumida.com TDK 847-803-6100 847-803-6296 www.component.tdk.com Toshiba 949-455-2000 949-859-3963 www.toshiba.com/taec 8 WEBSITE _______________________________________________________________________________________ TFT LCD Step-Up DC-DC Converter In Figure 1, the LCD’s gate-on and gate-off voltages are generated from two unregulated charge pumps driven by the step-up regulator’s LX node. The additional load on LX must therefore be considered in the inductance calculation. The effective maximum output current IMAIN(EFF) becomes the sum of the maximum load current on the step-up regulator’s output plus the contributions from the positive and negative charge pumps: IMAIN(EFF) = IMAIN(MAX) + ηNEG x INEG + (ηPOS + 1) x IPOS I POS is the positive charge-pump output current, assuming the pump source for IPOS is VMAIN. Calculate the approximate inductor value using the typical input voltage (VIN), the maximum output current (IMAIN(MAX)), the expected efficiency (ηTYP) taken from an appropriate curve in the Typical Operating Characteristics, and an estimate of LIR based on the above discussion: 2 ⎞ ⎛η ⎛ V ⎞ ⎛ VMAIN − VIN TYP ⎞ L = ⎜ IN ⎟ ⎜ ⎟ ⎟ ⎜ ⎝ VMAIN ⎠ ⎝ IMAIN(MAX) × fOSC ⎠ ⎝ LIR ⎠ Choose an available inductor value from an appropriate inductor family. Calculate the maximum DC input current at the minimum input voltage VIN(MIN) using conservation of energy and the expected efficiency at that operating point (ηMIN) taken from an appropriate curve in the Typical Operating Characteristics: IIN(DC, MAX) = IMAIN(MAX) × VMAIN VIN(MIN) × ηMIN Calculate the ripple current at that operating point and the peak current required for the inductor: IRIPPLE = VIN(MIN) × (VMAIN − VIN(MIN) ) L × VMAIN × fOSC I IPEAK = IIN(DC, MAX) + RIPPLE 2 The inductor’s saturation current rating and the MAX8752’s LX current limit (ILIM) should exceed IPEAK and the inductor’s DC current rating should exceed IIN(DC,MAX). For good efficiency, choose an inductor with less than 0.1Ω series resistance. Considering the Typical Applications Circuit (Figure 1), the maximum load current (IMAIN(MAX)) is 180mA with a 10V output and a typical input voltage of 2.5V: IMAIN(EFF) = 180mA + 1 x 20mA + 3 x 10mA = 230mA where IMAIN(MAX) is the maximum main output current, nNEG is the number of negative charge-pump stages, nPOS is the number of positive charge-pump stages, INEG is the negative charge-pump output current, and _______________________________________________________________________________________ 9 MAX8752 The maximum output current, input voltage, output voltage, and switching frequency determine the inductor value. Very high inductance values minimize the current ripple and therefore reduce the peak current, which decreases core losses in the inductor and I 2R 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. Finding the best inductor involves choosing the best compromise between circuit efficiency, inductor size, and cost. The equations used here include a constant, LIR, which is the ratio of the inductor peak-to-peak ripple current to the average DC inductor current at the full-load current. The best trade-off between inductor size and circuit efficiency for step-up regulators generally has an LIR between 0.3 and 0.5. However, depending on the AC characteristics of the inductor core material and ratio of inductor resistance to other power path resistances, the best LIR can shift up or down. If the inductor resistance is relatively high, more ripple can be accepted to reduce the number of turns required and increase the wire diameter. If the inductor resistance is relatively low, increasing inductance to lower the peak current can decrease losses throughout the power path. If extremely thin high-resistance inductors are used, as is common for LCD panel applications, the best LIR can increase to between 0.5 and 1.0. Once a physical inductor is chosen, higher and lower values of the inductor should be evaluated for efficiency improvements in typical operating regions. MAX8752 TFT LCD Step-Up DC-DC Converter Choosing an LIR of 0.5 and estimating efficiency of 80% at this operating point: 2 ⎛ 2.5V ⎞ ⎛ 10V − 2.5V ⎞ ⎛ 0.80 ⎞ L = ⎜ ⎟ ⎜ ⎟ ≈ 2.6μH ⎟ ⎜ ⎝ 10V ⎠ ⎝ 0.23A × 1.2MHz ⎠ ⎝ 0.50 ⎠ Using the circuit’s minimum input voltage (2.2V) and estimating efficiency of 75% at that operating point: IIN(DC, MAX) = 0.23A × 10V 2.2V × 0.75 ≈ 1.4 A The ripple current and the peak current are: IRIPPLE = 2.2V × (10V − 2.2V) 2.6μH × 10V × 1.2MHz IPEAK = 1.4 A + 0.55A 2 ≈ 0.55A ≈ 1.7A Output Capacitor Selection The total output voltage ripple has two components: the capacitive ripple caused by the charging and discharging of the output capacitance, and the ohmic ripple due to the capacitor’s equivalent series resistance (ESR): VRIPPLE = VRIPPLE(C) + VRIPPLE(ESR) VRIPPLE(C) ≈ IMAIN COUT ⎛ VMAIN − VIN ⎞ ⎜ V ⎟ , and ⎝ MAIN fOSC ⎠ VRIPPLE(ESR) ≈ IPEAK RESR(COUT) where I PEAK is the peak inductor current (see the Inductor Selection section). For ceramic capacitors, the output voltage ripple is typically dominated by VRIPPLE(C). The voltage rating and temperature characteristics of the output capacitor must also be considered. 10 Input Capacitor Selection The input capacitor (CIN) reduces the current peaks drawn from the input supply and reduces noise injection into the IC. A 10µF ceramic capacitor is used in the Typical Applications Circuit (Figure 1) because of the high source impedance seen in typical lab setups. Actual applications usually have much lower source impedance since the step-up regulator often runs directly from the output of another regulated supply. Typically, CIN can be reduced below the values used in the Typical Applications Circuit. Ensure a low noise supply at IN by using adequate C IN . Alternatively, greater voltage variation can be tolerated on CIN if IN is decoupled from CIN using an RC lowpass filter (see R3 and C3 in Figure 1). Rectifier Diode Selection The MAX8752’s high switching frequency demands a high-speed rectifier. Schottky diodes are recommended for most applications because of their fast recovery time and low forward voltage. The diode should be rated to handle the output voltage and the peak switch current. Make sure that the diode’s peak current rating is at least IPEAK calculated in the Inductor Selection section and that its breakdown voltage exceeds the output voltage. Output Voltage Selection The MAX8752 operates with an adjustable output from VIN to 13V. Connect a resistive voltage-divider from the output (VMAIN) to GND with the center tap connected to FB (see Figure 1). Select R2 in the 10kΩ to 50kΩ range. Calculate R1 with the following equation: ⎛V ⎞ R1 = R2 × ⎜ MAIN − 1⎟ ⎝ VFB ⎠ where VFB, the step-up regulator’s feedback set point, is 1.24V (typ). Place R1 and R2 close to the IC. ______________________________________________________________________________________ TFT LCD Step-Up DC-DC Converter RCOMP ≈ CCOMP ≈ CCOMP2 ≈ 264 × VIN × VOUT × COUT L × IMAIN(EFF) VOUT × COUT 10 × IMAIN(MAX) × RCOMP 0.02 × RESR × L × IMAIN(EFF) VIN × VOUT For the ceramic output capacitor, where ESR is small, CCOMP2 is optional. The best gauge of correct loop compensation is by inspecting the transient response of the MAX8752. Adjust RCOMP and CCOMP as necessary to obtain optimal transient performance. PC Board Layout and Grounding Careful PC board layout is important for proper operation. Use the following guidelines for good PC board layout: 1) Minimize the area of high-current loops by placing the inductor, rectifier diode, and output capacitors near the input capacitors and near the LX and GND pins. The high-current input loop goes from the positive terminal of the input capacitor to the inductor, to the IC’s LX pin, out the IC’s GND pin, and to the input capacitor’s negative terminal. The highcurrent output loop is from the positive terminal of the input capacitor to the inductor, to the rectifier diode (D1), to the positive terminal of the output capacitors, reconnecting between the output- capacitor and input-capacitor ground terminals. Connect these loop components with short, wide connections. Avoid using vias in the high-current paths, especially the ground paths. If vias are unavoidable, use many vias in parallel to reduce resistance and inductance. 2) Create a power ground island (PGND) consisting of the input and output capacitor grounds and GND. Connect all of these together with short, wide traces or a small ground plane. Maximizing the width of the power ground traces improves efficiency and reduces output voltage ripple and noise spikes. Create an analog ground plane (AGND) consisting of the feedback divider’s ground, the COMP capacitor’s ground, and the IC’s exposed backside pad near pin 1. Connect the AGND and PGND islands by connecting the GND pin directly to the exposed backside pad. Make no other connections between these separate ground planes. 3) Place the feedback voltage-divider resistors as close to FB as possible. The divider’s center trace should be kept short. Placing the resistors far away causes the FB trace to become an antenna that can pick up switching noise. Avoid running the feedback trace near LX. 4) Place the SUP and LDO bypass capacitors and the IN bypass capacitors (C3 in Figure 1) if within 5mm of their respective pins. Connect their ground terminals to GND through the IC’s exposed back paddle near GND (pin4). 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 LX node while keeping it wide and short. Keep the LX node away from the feedback node and other sensitive nodes. Use DC traces as shield if necessary. Refer to the MAX8752 evaluation kit for an example of proper board layout. Chip Information TRANSISTOR COUNT: 3091 PROCESS: BiCMOS ______________________________________________________________________________________ 11 MAX8752 Loop Compensation The voltage-feedback loop needs proper compensation to prevent excessive output ripple and poor efficiency caused by instability. This is done by connecting a resistor (RCOMP) and capacitor (CCOMP) in series from COMP to GND, and another capacitor (CCOMP2) from COMP to GND. RCOMP is chosen to set the high-frequency integrator gain for fast transient response, while CCOMP is chosen to set the integrator zero to maintain loop stability. The second capacitor, CCOMP2, is chosen to cancel the zero introduced by output-capacitance ESR. For optimal performance, choose the components using the following equations: 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.) 6, 8, &10L, DFN THIN.EPS MAX8752 TFT LCD Step-Up DC-DC Converter D2 D A2 PIN 1 ID N 0.35x0.35 b PIN 1 INDEX AREA E [(N/2)-1] x e REF. E2 DETAIL A e k A1 CL CL A L L e e PACKAGE OUTLINE, 6,8,10 & 14L, TDFN, EXPOSED PAD, 3x3x0.80 mm -DRAWING NOT TO SCALE- 21-0137 G 1 2 COMMON DIMENSIONS MIN. MAX. D 0.70 2.90 0.80 3.10 E A1 2.90 0.00 3.10 0.05 L k 0.20 0.40 0.25 MIN. A2 0.20 REF. SYMBOL A PACKAGE VARIATIONS PKG. CODE N D2 E2 e JEDEC SPEC b [(N/2)-1] x e DOWNBONDS ALLOWED T633-1 6 1.50±0.10 2.30±0.10 0.95 BSC MO229 / WEEA 0.40±0.05 1.90 REF NO T633-2 6 1.50±0.10 2.30±0.10 0.95 BSC MO229 / WEEA 0.40±0.05 1.90 REF NO T833-1 8 1.50±0.10 2.30±0.10 0.65 BSC MO229 / WEEC 0.30±0.05 1.95 REF NO T833-2 8 1.50±0.10 2.30±0.10 0.65 BSC MO229 / WEEC 0.30±0.05 1.95 REF NO T833-3 8 1.50±0.10 2.30±0.10 0.65 BSC MO229 / WEEC 0.30±0.05 1.95 REF YES T1033-1 10 1.50±0.10 2.30±0.10 0.50 BSC MO229 / WEED-3 0.25±0.05 2.00 REF NO T1433-1 14 1.70±0.10 2.30±0.10 0.40 BSC ---- 0.20±0.05 2.40 REF YES T1433-2 14 1.70±0.10 2.30±0.10 0.40 BSC ---- 0.20±0.05 2.40 REF NO PACKAGE OUTLINE, 6,8,10 & 14L, TDFN, EXPOSED PAD, 3x3x0.80 mm -DRAWING NOT TO SCALE- 21-0137 G 2 2 Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time. 12 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 © 2005 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products, Inc.