19-3480; Rev 0; 10/04 KIT ATION EVALU LE B A IL A AV TFT-LCD Step-Up DC-DC Converter The MAX8727 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 MAX8727 incorporates currentmode, fixed-frequency, pulse-width modulation (PWM) circuitry with a built-in n-channel power MOSFET to achieve high efficiency and fast transient response. Users can select 640kHz or 1.2MHz operation using a logic input pin (FREQ). The high switching frequencies allow the use of ultra-small inductors and low-ESR ceramic capacitors. The current-mode architecture provides fast transient response to pulsed loads. A compensation pin (COMP) gives users flexibility in adjusting loop dynamics. The 30V internal MOSFET can generate output voltages up to 24V from an input voltage between 2.6V and 5.5V. Soft-start slowly ramps the input current and is programmed with an external capacitor. The MAX8727 is available in a 10-pin thin DFN package. Features ♦ 90% Efficiency ♦ Adjustable Output from VIN to 24V ♦ 2.6V to 5.5V Input Supply Range ♦ Input Supply Undervoltage Lockout ♦ Pin-Programmable 640kHz/1.2MHz Switching Frequency ♦ Programmable Soft-Start ♦ 0.1µA Shutdown Current ♦ Small 10-Pin Thin DFN Package Applications Notebook Computer Displays Ordering Information PART LCD Monitor Panels MAX8727ETB Pin Configuration TEMP RANGE PIN-PACKAGE -40°C to +85°C 10 Thin DFN 3mm x 3mm Minimal Operating Circuit VIN 2.6V TO 5.5V VOUT TOP VIEW 6 LX COMP 1 10 SS FB 2 9 FREQ MAX8727 8 IN 4 7 LX 5 6 LX SHDN 3 GND GND 8 7 LX FB IN 2 MAX8727 9 3 10 FREQ GND 5 SHDN GND SS 4 COMP 1 THIN DFN 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 MAX8727 General Description MAX8727 TFT-LCD Step-Up DC-DC Converter ABSOLUTE MAXIMUM RATINGS LX to GND ..............................................................-0.3V to +26V IN, SHDN, FREQ, FB to GND ...................................-0.3V to +6V COMP, SS to GND .........................................-0.3V to VIN + 0.3V LX Switch Maximum Continuous RMS Current .....................2.4A Continuous Power Dissipation (TA = +70°C) 10-Pin Thin DFN (derate 24.4mW/°C above +70°C) ....1951mW 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 = V SHDN = 3V, FREQ = GND, TA = 0°C to +85°C. Typical values are at TA = +25°C, unless otherwise noted.) (Note 1) PARAMETER Input Voltage Range CONDITIONS MIN TYP VOUT < 18V 2.6 5.5 18V < VOUT < 24V 4.0 5.5 Output Voltage Range IN Undervoltage-Lockout Threshold IN Quiescent Current IN Shutdown Current MAX VIN rising, typical hysteresis is 50mV UNITS V 24 V 2.38 2.57 V 0.225 0.440 2 5 0.1 10.0 1.22 1.24 1.26 V 50 125 250 nA 0.05 0.15 %/V 200 300 2.20 VFB = 1.3V, not switching VFB = 1.0V, switching SHDN = GND mA µA ERROR AMPLIFIER FB Regulation Voltage Level to produce VCOMP = 1.24V FB Input Bias Current VFB = 1.24V FB Line Regulation Level to produce VCOMP = 1.24V, VIN = 2.6V to 5.5V Transconductance 100 Voltage Gain Shutdown FB Input Voltage 700 µS V/V SHDN = GND 0.05 0.10 0.15 FREQ = GND 540 640 740 FREQ = IN 1000 1220 1500 87 90 93 V OSCILLATOR Frequency Maximum Duty Cycle kHz % n-CHANNEL MOSFET Current Limit VFB = 1V, 75% duty cycle 3.0 On-Resistance Leakage Current VLX = 24V Current-Sense Transresistance 0.11 3.8 4.6 A 125 250 Ω 30 45 µA 0.21 0.31 V/A 100 Ω 7.5 µA SOFT-START Reset Switch Resistance Charge Current 2 VSS = 1.2V 2.5 4.5 _______________________________________________________________________________________ TFT-LCD Step-Up DC-DC Converter (VIN = V SHDN = 3V, FREQ = GND, TA = 0°C to +85°C. Typical values are at TA = +25°C, unless otherwise noted.) (Note 1) PARAMETER CONDITIONS MIN TYP MAX UNITS 0.3 × VIN V CONTROL INPUTS SHDN, FREQ Input Low Voltage VIN = 2.6V to 5.5V SHDN, FREQ Input High Voltage VIN = 2.6V to 5.5V SHDN, FREQ Input Hysteresis VIN = 2.6V to 5.5V 0.7 × VIN 0.1 × VIN FREQ Pulldown Current SHDN Input Current V 2.3 SHDN = GND V 6.0 9.5 µA 0.001 1 µA TYP MAX UNITS ELECTRICAL CHARACTERISTICS (VIN = V SHDN = 3V, FREQ = GND, TA = -40°C to +85°C, unless otherwise noted.) (Note 1) PARAMETER Input Voltage Range CONDITIONS MIN VOUT < 18V 2.6 5.5 18V < VOUT < 24V 4.0 5.5 Output Voltage Range IN Undervoltage-Lockout Threshold IN Quiescent Current IN Shutdown Current VIN rising, typical hysteresis is 50mV 2.20 VFB = 1.3V, not switching V 24 V 2.57 V 0.44 mA VFB = 1.0V, switching 5 SHDN = GND 10 µA 1.260 V ERROR AMPLIFIER FB Regulation Voltage Level to produce VCOMP = 1.24V FB Input Bias Current VFB = 1.24V 250 nA FB Line Regulation Level to produce VCOMP = 1.24V, VIN = 2.6V to 5.5V 0.15 %/V 100 300 µS 0.05 0.15 V Transconductance Shutdown FB Input Voltage SHDN = GND 1.215 OSCILLATOR Frequency FREQ = GND 490 770 FREQ = IN 900 1600 86 94 Maximum Duty Cycle kHz % n-CHANNEL MOSFET Current Limit VFB = 1V, 75% duty cycle 3.0 On-Resistance Current-Sense Transresistance 0.11 5.1 A 250 mΩ 0.31 V/A 100 Ω 7.5 µA SOFT-START Reset Switch Resistance Charge Current VSS = 1.2V 2.5 _______________________________________________________________________________________ 3 MAX8727 ELECTRICAL CHARACTERISTICS (continued) ELECTRICAL CHARACTERISTICS (continued) (VIN = V SHDN = 3V, FREQ = GND, TA = -40°C to +85°C, unless otherwise noted.) (Note 1) PARAMETER CONDITIONS MIN TYP MAX UNITS 0.3 × VIN V CONTROL INPUTS SHDN, FREQ Input Low Voltage VIN = 2.6V to 5.5V SHDN, FREQ Input High Voltage VIN = 2.6V to 5.5V 0.7 × VIN V Note 1: Specifications to -40°C are guaranteed by design, not production tested. Typical Operating Characteristics (Circuit of Figure 1. VIN = 5V, VMAIN = 15V, TA = +25°C unless otherwise noted.) VIN = 3.3V 60 VIN = 5.0V VIN = 3.3V 60 50 10 1000 100 VIN = 5.0V -1.0 VIN = 3.3V -2.0 1 10 1000 100 1 10 1000 100 LOAD CURRENT (mA) LOAD CURRENT (mA) LOAD CURRENT (mA) SWITCHING FREQUENCY vs. INPUT VOLTAGE SUPPLY CURRENT vs. SUPPLY VOLTAGE SUPPLY CURRENT vs. TEMPERATURE (SWITCHING) SUPPLY CURRENT (mA) FREQ = IN 1100 1000 900 800 SWITCHING 3.0 2.5 2.0 1.5 1.0 FREQ = GND 700 3.5 3.0 3.5 4.0 4.5 INPUT VOLTAGE (V) 5.0 5.5 3 VIN = 3.3V 2 1 0 500 VIN = 5.0V NONSWITCHING 0.5 600 4 MAX8727 toc06 1200 SUPPLY CURRENT (mA) 1300 5 MAX8727 toc05 4.0 MAX8727 toc04 1400 2.5 -0.5 -1.5 50 1 4 80 70 0 LOAD REGULATION (%) VIN = 5.0V 70 L = 6.8µH 90 EFFICIENCY (%) 80 LOAD REGULATION 0.5 MAX8727 toc02 L = 3.6µH 90 EFFICIENCY (%) 100 MAX8727 toc01 100 EFFICIENCY vs. LOAD CURRENT (640kHz OPERATION) MAX8727 toc03 EFFICIENCY vs. LOAD CURRENT (1.2MHz OPERATION) SWITCHING FREQUENCY (kHz) MAX8727 TFT-LCD Step-Up DC-DC Converter 0 2.5 3.0 3.5 4.0 4.5 SUPPLY VOLTAGE (V) 5.0 5.5 -40 -20 0 20 40 60 TEMPERATURE (°C) _______________________________________________________________________________________ 80 100 TFT-LCD Step-Up DC-DC Converter SOFT-START (RLOAD = 30Ω) LOAD-TRANSIENT RESPONSE (ILOAD = 50mA TO 550mA) MAX8727 toc07 MAX8727 toc08 15V VOUT 5mV/div AC-COUPLED IOUT 500mA/div 50mA VOUT 5V/div OV INDUCTOR CURRENT 1A/div INDUCTOR CURRENT 1A/div OA OA 1ms/div 1µs/div PULSED LOAD-TRANSIENT RESPONSE (ILOAD = 100mA TO 1.1A) SWITCHING WAVEFORMS (ILOAD = 600mA) MAX8727 toc09 MAX8727 toc10 15V VOUT 5mV/div AC-COUPLED IOUT 1A/div 0.1A 1µs/div LX 10V/div OV INDUCTOR CURRENT 1A/div INDUCTOR CURRENT 1A/div OA OA 1µs/div _______________________________________________________________________________________ 5 MAX8727 Typical Operating Characteristics (continued) (Circuit of Figure 1. VIN = 5V, VMAIN = 15V, TA = +25°C unless otherwise noted.) TFT-LCD Step-Up DC-DC Converter MAX8727 Pin Description PIN NAME 1 COMP FUNCTION Compensation Pin for Error Amplifier. Connect a series RC from COMP to ground. See the Loop Compensation section for component selection guidelines. Feedback Pin. The FB regulation voltage is 1.24V nominal. Connect an external resistive voltage-divider between the step-up regulator’s output (VOUT) 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 VOUT according to the Output Voltage Selection section. 2 FB 3 SHDN Shutdown Control Input. Drive SHDN low to turn off the MAX8727. 4 GND Ground. Connect pins 4 and 5 directly together. 5 GND Ground. Connect pins 4 and 5 directly together. 6 LX Switch Pin. LX is the drain of the internal MOSFET. Connect the inductor/rectifier diode junction to LX and minimize the trace area for lower EMI. Connect pins 6 and 7 directly together. 7 LX Switch Pin. LX is the drain of the internal MOSFET. Connect the inductor/rectifier diode junction to LX and minimize the trace area for lower EMI. Connect pins 6 and 7 directly together. 8 IN Supply Pin. Bypass IN with a minimum 1µF ceramic capacitor directly to GND. 9 FREQ Frequency-Select Input. When FREQ is low, the oscillator frequency is set to 640kHz. When FREQ is high, the frequency is 1.2MHz. This input has a 5µA pulldown current. SS Soft-Start Control Pin. Connect a soft-start capacitor (CSS) to this pin. Leave open for no soft-start. The softstart capacitor is charged with a constant current of 4.5µA. Full current limit is reached after t = 2.5 × 105 CSS. The soft-start capacitor is discharged to ground when SHDN is low. When SHDN goes high, the soft-start capacitor is charged to 0.4V, after which soft-start begins. 10 L1 3.6µH VIN 4.5V TO 5.5V C1 10µF 6.3V R3 10Ω D1 6 LX 8 C3 1µF R1 309kΩ 1% 7 LX FB IN 3 10 C6 33nF C8 4.7µF 25V R2 28.0kΩ 1% FREQ GND 5 SHDN GND SS C7 4.7µF 25V 2 MAX8727 9 C2 4.7µF 25V 4 COMP 1 R4 100kΩ C5 39pF C4 330pF Figure 1 Typical Operating Circuit 6 _______________________________________________________________________________________ VOUT 15V/600mA TFT-LCD Step-Up DC-DC Converter 4µA MAX8727 SKIP COMPARATOR SHDN IN BIAS SOFTSTART SKIP COMP ERROR AMPLIFIER SS ERROR COMPARATOR FB ∞ LX CONTROL AND DRIVER LOGIC 1.24V N CLOCK GND OSCILLATOR FREQ SLOPE COMPENSATION Σ CURRENT SENSE 5µA MAX8727 Figure 2. MAX8727 Functional Diagram Detailed Description The MAX8727 is a highly efficient power supply that employs a current-mode, fixed-frequency, pulse-width modulation (PWM) architecture for fast transient response and low-noise operation. The device 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 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 command 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. At light loads, this architecture allows the MAX8727 to “skip” cycles to prevent overcharging the output voltage. In this region of operation, the inductor ramps up to a peak value of approximately 50mA, discharges to the output, and waits until another pulse is needed again. Output Current Capability The output current capability of the MAX8727 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 = (1.26 - 0.35 x D) x ILIM_EC where ILIM_EC is the current limit specified at 75% 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 × D × VIN ⎤ VIN IOUT(MAX) = ⎢ILIM − × η ⎥ × fOSC × L ⎥⎦ VOUT ⎢⎣ 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 where VDIODE is the rectifier diode forward voltage and RON is the on-resistance of the internal MOSFET. _______________________________________________________________________________________ 7 MAX8727 TFT-LCD Step-Up DC-DC Converter Soft-Start Shutdown The MAX8727 can be programmed for soft-start upon power-up with an external capacitor. When the shutdown pin is taken high, the soft-start capacitor (CSS) is immediately charged to 0.4V. Then the capacitor is charged at a constant current of 4.5µA (typ). During this time, the SS voltage directly controls the peak inductor current, allowing 0A at VSS = 0.4V to the full current limit at VSS = 1.5V. The maximum load current is available after the soft-start is completed. When the SHDN pin is taken low, the softstart capacitor is discharged to ground. The MAX8727 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. The step-up regulator’s output is connected to IN by the external inductor and rectifier diode. Frequency Selection The MAX8727’s frequency can be user selected to operate at either 640kHz or 1.2MHz. Connect FREQ to GND for 640kHz operation. For a 1.2MHz switching frequency, connect FREQ to IN. This allows the use of small, minimum-height external components while maintaining low output noise. FREQ has an internal pulldown, allowing the user the option of leaving FREQ unconnected for 640kHz operation. Applications Information Step-up regulators using the MAX8727 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 L is known, choose the diode and capacitors. Inductor Selection Table 1. Component List DESIGNATION DESCRIPTION C1 10µF ±10%, 6.3V X5R ceramic capacitor (0805) Murata GRM21BR60J106K Taiyo Yuden JMK212BJ106KD C2, C7, C8 4.7µF±20%, 25V X7R ceramic capacitors (1206) Murata GRM31CR71E475M D1 3A, 30V Schottky diode (M-Flat) Toshiba CMS02 L1 3.6µH ±30% power inductor Sumida CDRH6D26-3R6NC 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, transientresponse time, and output voltage ripple. Physical size and cost are also important factors to be considered. 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 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. Finding the best inductor involves choosing the best compromise between circuit efficiency, inductor size and cost. Table 2. Component Suppliers SUPPLIER Murata PHONE FAX 770-436-1300 770-436-3030 WEBSITE www.murata.com Sanyo 619-661-4143 619-661-1055 www.sanyovideo.com Sumida 847-545-6700 847-545-6720 www.sumida.com Taiyo Yuden 800-348-2496 847-925-0899 www.t-yuden.com Toshiba 949-455-2000 949-859-3963 www.toshiba.com/taec 8 _______________________________________________________________________________________ TFT-LCD Step-Up DC-DC Converter Once a physical inductor is chosen, higher and lower values of the inductor should be evaluated for efficiency improvements in typical operating regions. 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 L = ⎜ IN ⎟ ⎜ ⎟ ⎜ TYP ⎟ ⎜ ⎟ ⎝ 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) = Considering the typical operating circuit, the maximum load current (IMAIN(MAX)) is 600mA with a 15V output and a typical input voltage of 5V. Choosing an LIR of 0.35 and estimating efficiency of 85% at this operating point: 2 ⎛ 5V ⎞ ⎛ 15V − 5V ⎞ ⎛ 0.85 ⎞ L = ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ≈ 3.6µH ⎝ 15V ⎠ ⎝ 0.6A × 1.2MHz ⎠ ⎝ 0.35 ⎠ Using the circuit’s minimum input voltage (4.5V) and estimating efficiency of 85% at that operating point: IIN(DC, MAX) = VIN(MIN) × ηMIN VIN(MIN) × (VMAIN − VIN(MIN) ) L × VMAIN × fOSC I IPEAK = IIN(DC, MAX) + RIPPLE 2 0.6A × 15V 4.5V × 0.85 ≈ 2.35A The ripple current and the peak current are: IRIPPLE = 4.5V × (15V − 4.5V) ≈ 0.73A 3.6µH × 15V × 1.2MHz IPEAK = 2.35A + 0.73A ≈ 2.70A 2 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) IMAIN(MAX) × VMAIN Calculate the ripple current at that operating point and the peak current required for the inductor: IRIPPLE = The inductor’s saturation current rating and the MAX8727’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. VRIPPLE(C) ≈ IMAIN ⎛ VMAIN − VIN ⎞ ⎜ ⎟, and COUT ⎝ VMAIN 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. _______________________________________________________________________________________ 9 MAX8727 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 the 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. MAX8727 TFT-LCD Step-Up DC-DC Converter 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 operating 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 operating circuit. Ensure a low noise supply at IN by using adequate CIN. 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 MAX8727’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. sen to cancel the zero introduced by output-capacitance ESR. For optimal performance, choose the components using the following equations: RCOMP ≈ CCOMP ≈ CCOMP2 ≈ 315 × VIN × VOUT × COUT L × IMAIN(MAX) VOUT × COUT 10 × IMAIN(MAX) × RCOMP 0.0036 × RESR × L × IMAIN(MAX) 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 MAX8727. Adjust RCOMP and CCOMP as necessary to obtain optimal transient performance. Soft-Start Capacitor The soft-start capacitor should be large enough that it does not reach final value before the output has reached regulation. Calculate CSS to be: Output Voltage Selection The MAX8727 operates with an adjustable output from VIN to 24V. 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: CSS > 21 × 10 −6 × COUT × ⎛ ⎞ 2 VOUT − VIN × VOUT ⎜ ⎟ ⎜⎜ V × I ⎟ − I × V INRUSH OUT OUT ⎟⎠ ⎝ IN ⎛V ⎞ R1 = R2 × ⎜ MAIN − 1⎟ ⎝ VFB ⎠ where COUT is the total output capacitance including any bypass capacitor on the output bus, VOUT is the maximum output voltage, IINRUSH is the peak inrush current allowed, IOUT is the maximum output current during power-up, and VIN is the minimum input voltage. The load must wait for the soft-start cycle to finish before drawing a significant amount of load current. The duration after which the load can begin to draw maximum load current is: where VFB, the step-up regulator’s feedback set point, is 1.24V (typ). Place R1 and R2 close to the IC. 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 cho- 10 tMAX = 6.77 x 105 x CSS ______________________________________________________________________________________ TFT-LCD Step-Up DC-DC Converter C10 1µF V2 +28V C9 1µF L1 3.6µH VIN 4.5V TO 5.5V C1 10µF 6.3V R4 10Ω D1 6 LX 8 C5 1µF R1 309kΩ 1% 7 LX FB IN 3 10 C7 4.7µF 25V C8 4.7µF 25V VOUT 15V/600mA R2 28.0kΩ 1% FREQ GND 5 SHDN GND SS C2 4.7µF 25V 2 MAX8727 9 MAX8727 D2 V3 -14V D3 C7 C8 0.1µF 0.1µF 4 COMP 1 C4 33nF R3 100kΩ C6 39pF C3 330pF Figure 3. Multiple-Output TFT-LCD Power Supply Multiple-Output Power Supply for TFT LCD Figure 3 shows a power supply for active-matrix TFTLCD flat-panel displays. Output-voltage transient performance is a function of the load characteristic. Add or remove output capacitance (and recalculate compensation-network component values) as necessary to meet the required transient performance. Regulation performance for secondary outputs (V2 and V3) depends on the load characteristics of all three outputs. 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 of GND, and to the input capacitor’s negative terminal. The high-current output loop is from the positive terminal of the input capacitor to the inductor, to the rectifier diode (D1), and 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. 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 pins. 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 ground connection, the COMP and SS capacitor ground connections, and the device’s exposed backside pad. Connect the AGND and PGND islands by connecting the GND pins directly to the exposed backside pad. Make no other connections between these separate ground planes. ______________________________________________________________________________________ 11 MAX8727 TFT-LCD Step-Up DC-DC Converter 3) Place the feedback voltage-divider-resistors as close to the FB pin 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 IN pin bypass capacitor as close to the device as possible. The ground connection of the IN bypass capacitor should be connected directly to GND pins 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 LX node while keeping it wide and short. Keep the LX node away from the feedback node and analog ground. Use DC traces as a shield if necessary. Refer to the MAX8727 evaluation kit for an example of proper board layout. Chip Information TRANSISTOR COUNT: 2746 PROCESS: BiCMOS 12 ______________________________________________________________________________________ TFT-LCD Step-Up DC-DC Converter 6, 8, &10L, DFN THIN.EPS D N PIN 1 INDEX AREA E E2 DETAIL A C L A C L 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 SYMBOL MIN. MAX. A 0.70 0.80 D 2.90 3.10 E 2.90 3.10 A1 0.00 0.05 L 0.20 0.40 k 0.25 MIN. A2 0.20 REF. 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 21-0137 -DRAWING NOT TO SCALE- 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. Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 13 © 2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products, Inc. MAX8727 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.)