19-1042; Rev 0; 10/07 KIT ATION EVALU E L B AVAILA TFT-LCD Step-Up DC-DC Converter The MAX17062 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 MAX17062 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 22V internal MOSFET can generate output voltages up to 20V 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 MAX17062 is available in a 10-pin TDFN package. Features o 90% Efficiency o Adjustable Output from VIN to 20V o 2.6V to 5.5V Input Supply Range o Input Supply Undervoltage Lockout o Pin-Programmable 640kHz/1.2MHz Switching Frequency o Programmable Soft-Start o Improved EMI o FB Regulation Voltage Tolerance < 1% o Small 10-Pin TDFN Package o Thermal-Overload Protection Ordering Information PART TEMP RANGE PINPACKAGE Notebook Computer Displays MAX17062ETB+T -40°C to +85°C 10 TDFN-EP* T1033-2 (3mm x 3mm) LCD Monitor Panels +Denotes a lead-free package. *EP = Exposed pad. T = Tape and reel. Applications LCD TV Panels Pin Configuration Minimal Operating Circuit FREQ IN LX LX VIN 2.6V TO 5.5V SS TOP VIEW 10 9 8 7 6 VOUT 6 LX 8 *EP 2 3 FREQ PGND 5 SHDN PGND 4 5 PGND 4 PGND SHDN FB COMP 3 FB MAX17062 9 2 7 LX IN MAX17062 1 PKG CODE 10 SS AGND COMP 1 EP TDFN (3mm x 3mm) *EP = EXPOSED PAD. ________________________________________________________________ Maxim Integrated Products For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. 1 MAX17062 General Description MAX17062 TFT-LCD Step-Up DC-DC Converter ABSOLUTE MAXIMUM RATINGS LX to AGND ............................................................-0.3V to +22V IN, SHDN, FREQ, FB to AGND..............................-0.3V to +7.5V COMP, SS to AGND ....................................-0.3V to (VIN + 0.3V) PGND to AGND .....................................................-0.3V to +0.3V LX Switch Maximum Continuous RMS Current .....................3.2A Continuous Power Dissipation (TA = +70°C) 10-Pin TDFN (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 = 3V, 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 < V OUT < 20V 4.0 5.5 Output Voltage Range IN Undervoltage-Lockout Threshold IN Quiescent Current IN Shutdown Current Thermal Shutdown MAX UNITS V 20 V 2.45 2.57 V VFB = 1.3V, not switching 0.3 0.6 VFB = 1.0V, switching 1.5 2.5 SHDN = AGND, TA = +25°C 0.01 10.0 SHDN = AGND, TA = +85°C Temperature rising 0.01 VIN rising, typical hysteresis is 50mV 2.30 160 Hysteresis mA μA °C 20 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 1.23 1.24 1.25 V 75 150 225 nA 0.01 0.15 %/V 250 450 110 Voltage Gain Shutdown FB Input Voltage 2400 μS V/V SHDN = AGND 0.05 0.10 0.15 FREQ = AGND 500 640 780 FREQ = IN 1000 1200 1400 88 91 94 % A V OSCILLATOR Frequency Maximum Duty Cycle kHz n-CHANNEL MOSFET Current Limit On-Resistance 4.6 5.3 IN = 5V VFB = 1V, 75% duty cycle, IN = 5V 100 170 IN = 3V 125 210 Leakage Current VLX = 20V Current-Sense Transresistance IN = 5V 3.9 m 11 20 μA 0.09 0.15 0.25 V/A 2 4 SOFT-START Reset Switch Resistance Charge Current 2 VSS = 1.2V _______________________________________________________________________________________ 100 6 μA TFT-LCD Step-Up DC-DC Converter (VIN = V SHDN = 3V, FREQ = 3V, 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 3 SHDN = AGND, TA = +25°C SHDN Input Current V 6 -1 SHDN = AGND, TA = +85°C V 9 +1 0 μA μA ELECTRICAL CHARACTERISTICS (VIN = V SHDN = 3V, FREQ = 3V, TA = -40°C to +85°C, unless otherwise noted.) (Note 1) PARAMETER Input Voltage Range CONDITIONS MIN IN Quiescent Current MAX 2.6 5.5 18V < V OUT < 20V 4.0 5.5 Output Voltage Range IN Undervoltage-Lockout Threshold TYP VOUT < 18V VIN rising, typical hysteresis is 50mV 2.30 UNITS V 20 V 2.57 V VFB = 1.3V, not switching 0.6 VFB = 1.0V, switching 2.5 mA ERROR AMPLIFIER FB Regulation Voltage Level to produce VCOMP = 1.24V 1.253 V FB Input Bias Current VFB = 1.24V 225 nA FB Line Regulation Level to produce VCOMP = 1.24V, VIN = 2.6V to 5.5V 0.15 %/V 110 450 μS 0.05 0.15 V Transconductance Shutdown FB Input Voltage SHDN = AGND 1.227 OSCILLATOR Frequency FREQ = AGND 450 830 FREQ = IN 950 1500 87 95 % 5.3 A Maximum Duty Cycle kHz n-CHANNEL MOSFET Current Limit VFB = 1V, 75% duty cycle, IN = 5V On-Resistance Current-Sense Transresistance 3.9 IN = 5V 170 IN = 3V 210 IN = 5V 0.09 0.25 m V/A SOFT-START Reset Switch Resistance Charge Current VSS = 1.2V 2 100 6 μA _______________________________________________________________________________________ 3 MAX17062 ELECTRICAL CHARACTERISTICS (continued) ELECTRICAL CHARACTERISTICS (continued) (VIN = V SHDN = 3V, FREQ = 3V, 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: Limits are 100% tested at TA = +25°C. Maximum and minimum limits over temperature are guaranteed by design. Typical Operating Characteristics (Circuit of Figure 1. VIN = 5V, VMAIN = 15V, TA = +25°C, unless otherwise noted.) EFFICIENCY vs. LOAD CURRENT (VIN = 5V, VOUT = 15V) 90 fOSC = 1.2MHz L = 2.7μH 80 LOAD REGULATION (%) 80 1.0 fOSC = 1.2MHz L = 2.7μH 70 MAX17062 toc03 fOSC = 640kHz L = 4.7μH EFFICIENCY (%) fOSC = 640kHz L = 4.7μH 70 100 MAX17062 toc02 90 LOAD REGULATION (VOUT = 15V) EFFICIENCY vs. LOAD CURRENT (VIN = 3.3V, VOUT = 9V) MAX17062 toc01 100 EFFICIENCY (%) 0.5 VIN = 5.0V 0 VIN = 3.3V -0.5 60 60 -1.0 50 50 1 10 1 1000 100 10 LOAD CURRENT (mA) SUPPLY CURRENT vs. SUPPLY VOLTAGE FREQ = IN 1100 1000 900 800 700 3.5 SUPPLY CURRENT (mA) 1200 MAX17062 toc05 4.0 MAX17062 toc04 1300 100 LOAD CURRENT (mA) LOAD CURRENT (mA) 1400 3.0 SWITCHING 2.5 2.0 1.5 1.0 FREQ = GND 0.5 600 500 NONSWITCHING 0 2.5 3.0 3.5 4.0 4.5 INPUT VOLTAGE (V) 4 10 1 1000 100 SWITCHING FREQUENCY vs. INPUT VOLTAGE SWITCHING FREQUENCY (kHz) MAX17062 TFT-LCD Step-Up DC-DC Converter 5.0 5.5 2.5 3.0 3.5 4.0 4.5 5.0 SUPPLY VOLTAGE (V) _______________________________________________________________________________________ 5.5 1000 TFT-LCD Step-Up DC-DC Converter SOFT-START (RLOAD = 30Ω) LOAD-TRANSIENT RESPONSE (ILOAD = 50mA TO 550mA) MAX17062 toc06 MAX17062 toc07 15V VOUT 500mV/div AC-COUPLED 0V VOUT 5V/div IOUT 500mA/div 50mA OV INDUCTOR CURRENT 1A/div INDUCTOR CURRENT 2A/div OA 0A 2ms/div 100μs/div L = 2.7μH RCOMP = 47kΩ CCOMP1 = 560pF PULSED LOAD-TRANSIENT RESPONSE (ILOAD = 100mA TO 1.1A) SWITCHING WAVEFORMS (ILOAD = 600mA) MAX17062 toc08 MAX17062 toc09 15V VOUT 200mV/div AC-COUPLED LX 10V/div 0V IOUT 1A/div 0.1A INDUCTOR CURRENT 1A/div INDUCTOR CURRENT 1A/div 0A 10μs/div 0A 1μs/div L = 2.7μH RCOMP = 47kΩ CCOMP1 = 560pF _______________________________________________________________________________________ 5 MAX17062 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 MAX17062 Pin Description PIN NAME 1 COMP 2 FB 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 AGND, 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. 3 SHDN Shutdown Control Input. Drive SHDN low to turn off the MAX17062. 4, 5 PGND Power Ground. Connect pins 4 and 5 directly together. 6, 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 together. 8 IN Supply Pin. Bypass IN with a minimum 1μF ceramic capacitor directly to AGND. 9 FREQ 10 SS EP AGND 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 6μA pulldown current. 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 4μA. Full current limit is reached when the voltage of SS pin is charged to 1.5V, which is the current-limit time, t = 2.4 10 5 C SS. 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. Exposed Pad. Connect to AGND. VIN 4.5V TO 5.5V C1 4.7μF 10V L1 2.7μH C2 4.7μF 10V C7 10μF 25V R1 10Ω 8 LX IN LX C3 1μF MAX17062 3 9 10 C4 33nF VOUT +15V/600mA D1 SHDN PGND FREQ SS PGND FB AGND COMP C8 10μF 25V 6 7 5 4 R4 221kΩ 2 EP R3 20kΩ 1 R2 47kΩ C5 560pF C6 OPEN Figure 1. Typical Operating Circuit 6 _______________________________________________________________________________________ TFT-LCD Step-Up DC-DC Converter 4μA MAX17062 SKIP COMPARATOR SHDN IN BIAS SOFTSTART SKIP COMP ERROR AMPLIFIER SS ERROR COMPARATOR FB ∞ LX CONTROL AND DRIVER LOGIC 1.24V N CLOCK PGND OSCILLATOR FREQ SLOPE COMPENSATION Σ CURRENT SENSE 6μA MAX17062 Figure 2. MAX17062 Functional Diagram Detailed Description The MAX17062 is a highly efficient power supply that employs a current-mode, fixed-frequency, 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 MAX17062 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 MAX17062 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 table) 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 MAX17062 TFT-LCD Step-Up DC-DC Converter Soft-Start The MAX17062 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 4μA (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. Frequency Selection The MAX17062’s frequency can be user selected to operate at either 640kHz or 1.2MHz. Connect FREQ to AGND 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. Shutdown The MAX17062 shuts down to reduce the supply current to 0.01μA 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. Thermal-Overload Protection Thermal-overload protection prevents excessive power dissipation from overheating the MAX17062. When the junction temperature exceeds TJ = +160°C, a thermal sensor immediately activates the fault protection, which shuts down the MAX17062, allowing the device to cool down. Once the device cools down by approximately 20°C, the MAX17062 starts up automatically. Applications Information Step-up regulators using the MAX17062 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 (Figure 1). 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 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. 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 Table 1. Component List DESIGNATION DESCRIPTION C1, C2 4.7μF ±10%, 10V X5R ceramic capacitors (0603) TDK C1608X5RIA475K C7, C8 10μF±10%, 25V X5R ceramic capacitors (1210) TDK C3225X5RIE106K D1 3A, 30V Schottky diode (M-Flat) Toshiba CMS03 L1 2.7μH ±20% power inductor TOKO FDV0630-2R7M Table 2. Component Suppliers SUPPLIER PHONE FAX TDK 847-803-6100 847-390-4405 TOKO 847-297-0070 847-699-7864 www.tokoam.com Toshiba 949-455-2000 949-859-3963 www.toshiba.com/taec 8 WEBSITE www.component.tdk.com _______________________________________________________________________________________ TFT-LCD Step-Up DC-DC Converter ⎛ V ⎞ L = ⎜ IN ⎟ ⎝ VMAIN ⎠ 2⎛ ⎞ ⎛η ⎞ VMAIN − VIN ⎜ ⎟ ⎜ TYP ⎟ ⎜I ⎟ ⎝ MAIN(MAX) × fOSC ⎠ ⎝ LIR ⎠ The inductor’s saturation current rating and the MAX17062’s LX current limit (I LIM ) should exceed I PEAK , 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 operating circuit (Figure 1), the maximum load current (IMAIN(MAX)) is 600mA with a 15V output and a typical input voltage of 5V. Choosing an LIR of 0.5 and estimating efficiency of 85% at this operating point: 2 ⎛ 5V ⎞ ⎛ 15V − 5V ⎞ ⎛ 0.85 ⎞ L=⎜ ≈ 2.7μH ⎝ 15V ⎟⎠ ⎜⎝ 0.6 A × 1.2MHz ⎟⎠ ⎜⎝ 0.50 ⎟⎠ Using the circuit’s minimum input voltage (4.5V) and estimating efficiency of 85% at that operating point: IIN(DC, MAX) = IIN(DC, MAX) = VIN(MIN) × ηMIN Calculate the ripple current at that operating point and the peak current required for the inductor: IRIPPLE = IRIPPLE = ≈ 2.35A 4.5V × (15V − 4.5V) ≈ 0.97A 2.7μH × 15V × 1.2MHz IPEAK = 2.35A + 0.97A ≈ 2.84A 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) VIN(MIN) × (VMAIN − VIN(MIN) ) ⎛V I − VIN ⎞ VRIPPLE(C) ≈ MAIN ⎜ MAIN COUT ⎝ VMAIN fOSC ⎟⎠ L × VMAIN × fOSC I IPEAK = IIN(DC, MAX) + RIPPLE 2 4.5V × 0.85 The ripple current and the peak current are: 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: IMAIN(MAX) × VMAIN 0.6A × 15V and: 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 MAX17062 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. 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: MAX17062 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. Two 4.7μF ceramic capacitors are 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 R1 and C3 in Figure 1). Rectifier Diode Selection The MAX17062’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 MAX17062 operates with an adjustable output from VIN to 20V. Connect a resistive voltage-divider from the output (VMAIN) to AGND 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. 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 AGND, and another capacitor (CCOMP2) from COMP to AGND. R COMP is chosen to set the highfrequency integrator gain for fast transient response, 10 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 outputcapacitance 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 MAX17062. 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: CSS > 21 × 10 −6 × COUT × ⎛ ⎞ 2 VOUT − VIN × VOUT ⎜ ⎟ ⎜⎜ V × I ⎟ INRUSH − IOUT × VOUT ⎟⎠ ⎝ IN 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: tMAX = 2.4 x 105 x CSS ______________________________________________________________________________________ TFT-LCD Step-Up DC-DC Converter C11 0.22μF C12 0.1μF 3 C1 4.7μF 10V C2 4.7μF 10V 2 C15 0.22μF 3 2 R5 10Ω 8 C3 1μF VGON +29V 1 L1 2.7μH VIN 4.5V TO 5.5V D3 C14 0.1μF MAX17062 D2 1 VGOFF -15V U1 IN D1 VOUT +15V/600mA C7 10μF 25V 6 LX 7 LX C8 10μF 25V MAX17062 PGND 5 3 R1 100kΩ 9 10 SHDN PGND FREQ FB SS COMP C4 33nF AGND 4 R4 221kΩ 2 EP R3 20kΩ 1 R2 47kΩ C6 OPEN C5 560pF 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 (VGON and VGOFF) depends on the load characteristics of all three outputs. PCB Layout and Grounding Careful PCB layout is important for proper operation. Use the following guidelines for good PCB 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 PGND pins. The high-current output 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 LX switch node to the rectifier diode (D1) to the output capacitors, and reconnecting negative terminals of output capacitors to PGND of the IC. This loop has very high di/dt, and it is critical to minimize the area of this loop. 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 PGND pins. Connect all 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 PGND pins directly to the exposed backside pad. Make no other connections between these separate ground planes. ______________________________________________________________________________________ 11 MAX17062 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 AGND 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 MAX17062 Evaluation Kit for an example of proper board layout. Chip Information TRANSISTOR COUNT: 3612 PROCESS: BiCMOS 12 ______________________________________________________________________________________ TFT-LCD Step-Up DC-DC Converter 6, 8, &10L, DFN THIN.EPS ______________________________________________________________________________________ 13 MAX17062 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.) 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.) 6, 8, &10L, DFN THIN.EPS MAX17062 TFT-LCD Step-Up DC-DC Converter 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. 14 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 © 2007 Maxim Integrated Products is a registered trademark of Maxim Integrated Products, Inc.