19-0190; Rev 0; 9/93 3.3V, Step-Down, Current-Mode PWM DC-DC Converters ________________________________Features ♦ Up to 500mA Load Currents The MAX748A/MAX763A also feature cycle-by-cycle current limiting, overcurrent limiting, undervoltage lockout, and programmable soft-start protection. The MAX748A is available in 8-pin DIP and 16-pin wide SO packages; the MAX763A comes in 8-pin DIP and SO packages. ___________________________Applications 5V-to-3.3V Converters Cellular Phones Portable Instruments Hand-Held Computers ♦ 85% to 90% Efficiencies ♦ 1.7mA Quiescent Current (MAX748A) 1.4mA Quiescent Current (MAX763A) ♦ 0.2µA Shutdown Supply Current ♦ 22µH Preselected Inductor Value; No Component Design Required ♦ Overcurrent, Soft-Start, and Undervoltage Lockout Protection ♦ Cycle-by-Cycle Current Limiting ♦ 8-Pin DIP/SO Packages (MAX763A) _________________Ordering Information PART TEMP. RANGE PIN-PACKAGE MAX748ACPA 0°C to +70°C 8 Plastic DIP MAX748ACWE 0°C to +70°C 16 Wide SO MAX748AC/D 0°C to +70°C Dice* MAX748AEPA -40°C to +85°C 8 Plastic DIP MAX748AEWE -40°C to +85°C 16 Wide SO MAX748AMJA -55°C to +125°C 8 CERDIP Ordering Information continued on last page. Computer Peripherals * Contact factory for dice specifications. ____________Typical Operating Circuit INPUT 3.3V TO 16V ____________________Pin Configurations TOP VIEW 22µH V+ LX MAX748A ON/OFF ♦ Guaranteed 159kHz to 219.5kHz Current-Mode PWM SHDN OUTPUT 3.3V SHDN 1 REF OUT 100µF 2 SS 3 MAX748A MAX763A CC 4 REF CC SS GND 8 V+ 7 LX 6 GND 5 OUT DIP Pin Configurations continued on last page. _______________________________________________________________________ Maxim Integrated Products Call toll free 1-800-998-8800 for free samples or literature. 1 MAX748A/MAX763A __________________General Description The MAX748A/MAX763A are 3.3V-output CMOS, stepdown switching regulators. The MAX748A accepts inputs from 3.3V to 16V and delivers up to 500mA. The MAX763A accepts inputs between 3.3V and 11V and delivers up to 500mA. Typical efficiencies are 85% to 90%. Quiescent supply current is 1.4mA (MAX763A), and only 0.2µA in shutdown. Pulse-width-modulation (PWM) current-mode control provides precise output regulation and excellent transient responses. Output voltage accuracy is guaranteed to be ±5% over line, load, and temperature variations. Fixed-frequency switching allows easy filtering of output ripple and noise, as well as the use of small external components. A 22µH inductor works in most applications, so no magnetics design is necessary. MAX748A/MAX763A 3.3V, Step-Down, Current-Mode PWM DC-DC Converters ABSOLUTE MAXIMUM RATINGS Pin Voltages: V+ (MAX748A) ......................................................+17V, -0.3V V+ (MAX763A) ......................................................+12V, -0.3V LX (MAX748A) .................................(V+ - 21V) to (V+ + 0.3V) LX (MAX763A) .................................(V+ - 12V) to (V+ + 0.3V) OUT .................................................................................±25V SS, CC, SHDN ..........................................-0.3V to (V+ + 0.3V) Peak Switch Current (ILX) .....................................................2.0A Reference Current (IREF) ...................................................2.5mA Continuous Power Dissipation (TA = +70°C) 8-Pin Plastic DIP (derate 6.90mW/°C above +70°C)...552mW 8-Pin SO (derate 5.88mW/°C above +70°C) ...............471mW 16-Pin Wide SO (derate 9.52mW/°C above +70°C)....762mW 8-Pin CERDIP (derate 8.00mW/°C above +70°C) .......640mW Operating Temperature Ranges: MAX7_ _ AC_ _ ....................................................0°C to +70°C MAX7_ _ AE_ _ .................................................-40°C to +85°C MAX7_ _ AMJA_ _ ..........................................-55°C to +125°C Junction Temperatures: MAX7_ _ AC/E ..............................................................+150°C MAX7_ _ AM .................................................................+175°C Storage Temperature Range ............................-65°C to +160°C Lead Temperature (soldering, 10sec) ............................+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 (Circuit of Figure 3, V+ = 5V, ILOAD = 0mA, TA = TMIN to TMAX, unless otherwise noted.) PARAMETER CONDITIONS Input Voltage Range Output Voltage 2 MAX748A TYP MAX MIN MAX763A TYP MAX 3.3 16.0 3.3 11.0 C/E temp. ranges, V+ = 4.0V to 16V, 0mA < ILOAD < 300mA 3.135 3.3 3.465 M temp. range, V+ = 4.0V to 16V, 0mA < ILOAD < 250mA 3.135 3.3 3.465 C/E temp. ranges, V+ = 4.75V to 16V, 0mA < ILOAD < 500mA 3.135 3.3 3.465 M temp. range, V+ = 4.75V to 16V, 0mA < ILOAD < 400mA 3.135 3.3 3.465 3.135 3.3 3.465 M temp. range, V+ = 4.0V to 11V, 0mA < I LOAD < 250mA 3.135 3.3 3.465 C/E temp. ranges, V+ = 4.75V to 11V, 0mA < ILOAD < 500mA 3.135 3.3 3.465 M temp. range, V+ = 4.75V to 11V, 0mA < ILOAD < 400mA 3.135 3.3 3.465 ILOAD = 0mA to 500mA UNITS V V C/E temp. ranges, V+ = 4.0V to 11V, 0mA < ILOAD < 300mA Line Regulation Load Regulation MIN 0.13 0.13 %/V 0.001 0.001 %/mA ________________________________________________________________________________________________ 3.3V, Step-Down, Current-Mode PWM DC-DC Converters (Circuit of Figure 3, V+ = 5V, ILOAD = 0mA, TA = TMIN to TMAX, unless otherwise noted.) PARAMETER CONDITIONS Efficiency V+ = 5V MIN MAX748A TYP MAX MIN MAX763A TYP MAX ILOAD = 300mA 88 88 ILOAD = 100mA 90 90 UNITS % Supply Current Includes switch current 1.7 3.0 1.4 2.5 mA Shutdown Current SHDN = 0V (Note 1) 0.2 100.0 0.2 100.0 µA Shutdown Input Threshold VIH 2.0 2.0 VIL 0.25 1.0 1.0 Shutdown Input Leakage Current Short-Circuit Current V 0.25 1.2 Undervoltage Lockout V+ falling 2.7 LX On Resistance ILX = 500mA 1.0 LX Leakage Current V+ = 12V, LX = 0 Reference Voltage TA = +25°C Reference Drift TA = TMIN to TMAX 1.2 3.0 1.22 159 180 Compensation Pin Impedance 3.0 V Ω 1.0 10 1.30 1.15 nA 1.22 50 Oscillator Frequency A 2.7 10 1.15 µA 1.30 50 212.5 159 200 7500 V ppm/°C 212.5 kHz Ω 7500 Note 1: The standby current typically settles to 10µA (over temperature) within 2 seconds; however, to decrease test time, the part is guaranteed at a 100µA maximum value. ________________________________________________Typical Operating Characteristics (Circuit of Figure 3, TA = +25°C, VOUT = 3.3V, unless otherwise noted.) MAXIMUM OUTPUT CURRENT vs. SUPPLY VOLTAGE MAX763A EFFICIENCY vs. OUTPUT CURRENT 100 1000 MAX748A 100 VIN = 7.0V 95 VIN = 5.0V VIN = 4.5V 95 800 400 EFFICIENCY (%) 600 85 80 VIN = 11.0V 75 60 4 6 8 10 12 SUPPLY VOLTAGE (V) VIN = 16V 75 VIN = 9V 14 16 VIN = 12V 65 65 0 2 80 NOTE 2 NOTE 2 0 85 70 70 200 VIN = 6V 90 90 MAX763A EFFICIENCY (%) MAXIMUM OUTPUT CURRENT (mA) MAX748A EFFICIENCY vs. OUTPUT CURRENT 0 100 200 300 400 500 600 700 800 OUTPUT CURRENT (mA) NOTE 2 60 0 100 200 300 400 500 600 700 800 900 OUTPUT CURRENT (mA) _________________________________________________________________________________________________ 3 MAX748A/MAX763A ELECTRICAL CHARACTERISTICS (continued) ____________________________Typical Operating Characteristics (continued) (Circuit of Figure 3, TA = +25°C, VOUT = 3.3V, unless otherwise noted.) MAX763A QUIESCENT SUPPLY CURRENT vs. SUPPLY VOLTAGE MAX748A QUIESCENT SUPPLY CURRENT vs. SUPPLY VOLTAGE 1000 IOUT = OmA IOUT = 0mA 3.5 TA = +125°C 3.0 2.5 2.0 TA = -55°C 1.5 TA = +25°C 1.0 PEAK INDUCTOR CURRENT (mA) 4.0 QUIESCENT SUPPLY CURRENT (mA) QUIESCENT SUPPLY CURRENT (mA) PEAK INDUCTOR CURRENT vs. OUTPUT CURRENT 5.0 4.5 4.0 TA = -55°C 3.0 TA = +125°C 2.0 TA = +25°C 1.0 800 VIN = 11V 600 400 VIN = 6V 200 VIN = 8V 0.5 0 0 0 1 2 3 4 5 6 7 8 SUPPLY VOLTAGE (V) 0 0 9 10 11 12 2 3.5 250 3.0 225 MAX748A 200 MAX763A 150 125 4 6 8 10 12 SUPPLY VOLTAGE (V) 14 OUTPUT VOLTAGE vs. SUPPLY VOLTAGE 4.0 2.0 1.5 1.0 16 3.0 ILOAD = 200mA 2.5 2.0 ILOAD = 500mA 1.5 ILOAD = 300mA 1.0 IOUT = 0mA V+ = 5V 0.5 0 0 -60 -40 -20 0 2.0 20 40 60 80 100 120 140 2.5 3.0 3.5 OSCILLATOR FREQUENCY vs. TEMPERATURE 300 3.0 2.5 VIN = 3.5V 2.0 VIN = 3.0V 1.5 1.0 OSCILATOR FREQUENCY (kHz) VIN = 4.0V 3.5 250 IOUT = 100mA V+ = 5V MAX748A 200 150 MAX763A 100 50 0.5 0 0 0 50 100 150 200 250 300 350 400 450 500 OUTPUT CURRENT (mA) 4.0 SUPPLY VOLTAGE (V) TEMPERATURE (°C) 4.0 OUTPUT VOLTAGE (V) ILOAD = 50mA 3.5 OUTPUT VOLTAGE vs. OUTPUT CURRENT 4 0 50 100 150 200 250 300 350 400 450 500 550 OUTPUT CURRENT (mA) 16 2.5 100 2 14 NOTES 3, 4 0.5 0 6 8 10 12 SUPPLY VOLTAGE (V) OUTPUT VOLTAGE (V) SHUTDOWN CURRENT (µA) 275 175 4 SHUTDOWN CURRENT vs. TEMPERATURE OSCILLATOR FREQUENCY vs. SUPPLY VOLTAGE OSCILLATOR FREQUENCY (kHz) MAX748A/MAX763A 3.3V, Step-Down, Current-Mode PWM DC-DC Converters -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) _______________________________________________________________________________________ 4.5 5.0 3.3V, Step-Down, Current-Mode PWM DC-DC Converters SWITCHING WAVEFORMS, CONTINUOUS CONDUCTION SWITCHING WAVEFORMS, DISCONTINUOUS CONDUCTION 6V 6V A A 0V 0V 400mA 200mA B B 0mA 0mA C C 2µs/div 2µs/div A: SWITCH VOLTAGE (LX PIN), 5V/div, 0V TO +6V B: INDUCTOR CURRENT, 200mA/div C: OUTPUT VOLTAGE RIPPLE, 50mV/div A: SWITCH VOLTAGE (LX PIN), 5V/div, 0V TO +6V B: INDUCTOR CURRENT, 100mA/div C: OUTPUT VOLTAGE RIPPLE, 50mV/div V+ = 6V, IOUT = 250mA V+ = 6V, IOUT = 75mA LINE-TRANSIENT RESPONSE LOAD-TRANSIENT RESPONSE A A 500mA 10V 7V B B 0V 0mA 5ms/div 5ms/div A: VOUT, 50mV/div B: V+, 5V/div, 7.0V TO 10.0V A: VOUT, 50mV/div B: IOUT, 200mA/div, 0mA TO 500mA IOUT = 350mA V+ = 6V Note 2: Operation beyond the specifications listed in the Electrical Characteristics may exceed the power dissipation ratings of the device. Note 3: Wide temperature range circuit of Figure 5 using Sprague surface-mount capacitors. Note 4: Standby current includes all external component leakage currents. Capacitor leakage currents dominate at TA = +85°C. _______________________________________________________________________________________ 5 MAX748A/MAX763A _________________________________Typical Operating Characteristics (continued) (Circuit of Figure 3, TA = +25°C, VOUT = 3.3V, unless otherwise noted.) MAX748A/MAX763A 3.3V, Step-Down, Current-Mode PWM DC-DC Converters _______________________________________________________________________Pin Description PIN # 16-PIN WIDE SO 8-PIN DIP/SO (MAX748A) NAME FUNCTION Shutdown—active low. Connect to ground to power down chip; tie to V+ for normal operation. Output voltage falls to 0V when SHDN is low. 1 2 SHDN 2 3 REF 3 7 SS Soft-Start. Capacitor between SS and GND provides soft-start and short-circuit protection. 4 8 CC Compensation Capacitor Input externally compensates the outer (voltage) feedback loop. Connect to OUT with a 330pF capacitor. 5 9 OUT Output-Voltage Sense Input provides regulation feedback sensing. Connect to +3.3V output. 6 10, 11 GND Ground* 7 12, 13, 14 LX Drain of internal P-channel power MOSFET* 8 1,15,16 V+ Supply Voltage Input. Bypass to GND with 1µF ceramic and large-value electrolytic capacitor in parallel. The 1µF capacitor must be as close to the GND and V+ pins as possible.* 4, 5, 6 N.C. Reference Voltage Output (+1.23V) supplies up to 100µA for external loads. Bypass to GND with a 0.047µF capacitor. No Connect—no internal connections to these pins. *16-pin wide SO package: All pins sharing the same name must be connected together externally. _________________Detailed Description Programmable Soft-Start The MAX748A/MAX763A switch-mode regulators use a current-mode pulse-width-modulation (PWM) control system in a step-down (buck) regulator topography. They convert an unregulated DC input voltage from 4V to 11V (MAX763A) or from 4V to 16V (MAX748A) to a regulated 3.3V output at 300mA. For loads less than 300mA, V+ may be less than 4.0V (see the Output Voltage vs. Supply Voltage graph in the Typical Operating Characteristics ). The current-mode PWM architecture provides cycle-by-cycle current limiting, improved load-transient response, and simpler outerloop design. Figure 2 shows a capacitor connected to the soft-start (SS) pin to ensure orderly power-up. A typical value is 0.047µF. SS controls both the SS timing and the maximum output current that can be delivered while maintaining regulation. The charging capacitor slowly raises the clamp on the error-amplifier output voltage, limiting surge currents at power-up by slowly increasing the cycle-by-cycle current-limit threshold. Table 1 lists timing characteristics for selected capacitor values and circuit conditions. The overcurrent comparator trips when the load exceeds approximately 1.2A. When either an undervoltage or overcurrent fault condition is detected, an SS cycle is actively initiated, which triggers an internal transistor to discharge the SS capacitor to ground. An SS cycle is also enabled at power-up and when coming out of shutdown mode. The controller consists of two feedback loops: an inner (current) loop that monitors the switch current via the current-sense resistor and amplifier, and an outer (voltage) loop that monitors the output voltage through the error amplifier (Figure 1). The inner loop performs cycle-by-cycle current limiting, truncating the power transistor on-time when the switch current reaches a predetermined threshold. This threshold is determined by the outer loop. For example, a sagging output voltage produces an error signal that raises the threshold, allowing the circuit to store and transfer more energy during each cycle. 6 Overcurrent Limiting The overcurrent comparator triggers when the load current exceeds approximately 1.2A. On each clock cycle, the output FET turns on and attempts to deliver current until cycle-by-cycle or overcurrent limits are exceeded. Note that the SS capacitor must be greater than 0.01µF for overcurrent protection to function properly. A typical value is 0.047µF. _______________________________________________________________________________________ 3.3V, Step-Down, Current-Mode PWM DC-DC Converters SHDN MAX748A/MAX763A MAX748A 3.3V to 16.0V MAX763A 3.3V to 11.0V VIN C3 150µF C2 1.0µF V+ OVERCURRENT COMPARATOR SLOPE COMPENSATION BIAS GEN RAMP GEN Σ OUT C5 330pF RSENSE ERROR AMP CURRENT SENSE AMP F/F CC R Q S 1.23V BANDGAP LX PWM COMPARATOR REF C6 1000pF 200kHz OSC 1M ±35% L1 22µH VOUT 3.3V D1 1N5817 C4 150µF MAX748A MAX763A SS CLAMP UNDERVOLTAGE LOCKOUT SS C1 0.047µF VUVLO GND Figure 1. Detailed Block Diagram with External Components Table 1. Typical Soft-Start Times (Circuit of Figure 3, C4 = 150µF) FROM SHDN SS CLAMP SS 1M ±35% C1 Circuit Cond. Soft-Start Time (ms) vs. C1 (µF) V+ (V) IOUT (mA) C1 = 0.01 C1 = 0.047 C1 = 0.1 C1 = 0.47 8 0 1 4 7 12* 0 1 2 3 12 6 8 200 10 33 50 200 12* 200 7 17 20 80 8 300 13 44 65 325 12* 300 8 25 35 140 1.23V * MAX748A only MAX748A MAX763A Figure 2. Soft-Start Circuitry Block Diagram _______________________________________________________________________________________ 7 MAX748A/MAX763A 3.3V, Step-Down, Current-Mode PWM DC-DC Converters Table 3. External Component Suppliers Production Method Surface Mount Inductors Capacitors Sumida CD105 series Matsuo 267 series Coiltronics CTX series Sprague 595D/293D series Coilcraft DT series High Performance/ Miniature Through-Hole Sumida RCH895 series Sanyo OS-CON series (very low ESR) Through-Hole Renco RL1284 series Nichicon PL series (low ESR) Phone and FAX Numbers: Coilcraft Coiltronics Matsuo Nichicon USA: USA: USA: Japan: USA: Japan: (708) 639-6400, (305) 781-8900, (714) 969-2491, (06) 332-0871 (708) 843-7500, (03) 3607-5111, FAX: (708) 639-1469 FAX: (305) 782-4163 FAX: (714) 960-6492 USA: USA: USA: USA: (516) 586-5566, (0720) 70-1005, (603) 224-1961, (708) 956-0666, FAX: FAX: FAX: FAX: (516) 586-5562 (0720) 70-1174 (603) 224-1430 (708) 956-0702 FAX: (708) 843-2798 FAX: (03) 3607-5428 Undervoltage Lockout The undervoltage lockout feature monitors the supply voltage at V+ and allows operation to start when V+ rises above 2.95V. When V+ falls, operation continues until the supply voltage falls below 2.7V (typ). When an undervoltage condition is detected, control logic turns off the output power FET and discharges the SS capacitor to ground. This prevents partial turn-on of the power MOSFET and avoids excessive power dissipation. The control logic holds the output power FET off until the supply voltage rises above approximately 2.95V, at which time an SS cycle begins. When the input voltage exceeds the undervoltage lockout threshold, switching action will occur, but the output will not be regulated until the input voltage exceeds 3.3V (no load). The exact input voltage required for regulation depends on load conditions (see the Output Voltage vs. Supply Voltage graph in the Typical Operating Characteristics). Shutdown Mode The MAX748A/MAX763A are held in shutdown mode by keeping SHDN at ground. In shutdown mode, the output drops to 0V and the output power FET is held in an off state. The internal reference also turns off, which causes the SS capacitor to discharge. Typical supply current in shutdown mode is 0.2µA. The actual design limit for shutdown current is much less than the 100µA specified in the Electrical Characteristics. However, testing to tighter limits is prohibitive because the current takes several seconds to settle to a final value. For 8 Renco Sanyo Sprague Elec. Co. Sumida normal operation, connect SHDN to V+. Coming out of shutdown mode initiates an SS cycle. Continuous-/DiscontinuousConduction Modes The input voltage, output voltage, load current, and inductor value determine whether the IC operates in continuous or discontinuous mode. As the inductor value or load current decreases, or the input voltage increases, the MAX748A/MAX763A tend to operate in discontinuous-conduction mode (DCM). In DCM, the inductor current slope is steep enough so it decays to zero before the end of the transistor off-time. In continuous-conduction mode (CCM), the inductor current never decays to zero, which is typically more efficient than DCM. CCM allows the MAX748A/MAX763A to deliver maximum load current, and is also slightly less noisy than DCM, because it doesn’t exhibit the ringing that occurs when the inductor current reaches zero. Internal Reference The +1.23V bandgap reference supplies up to 100µA at REF. A 1000pF bypass capacitor from REF to GND is required. Oscillator The MAX748A/MAX763A’s internal oscillator is guaranteed to operate in the 159kHz to 212.5 kHz range over temperature for V+ = 5V. Temperature stability over the military temperature range is about 0.04%/°C. _______________________________________________________________________________________ 3.3V, Step-Down, Current-Mode PWM DC-DC Converters C2 1.0µF C3 150µF 8 1 V+ SHDN LX 7 D1 1N5817 MAX748A MAX763A OUT 3 SS C1 0.047µF CC GND 6 MAX748A/MAX763A INPUT MAX748A 3.3V TO 16.0V V MAX763A 3.3V TO 11.0V IN 5 4 L1 22µH OUTPUT 3.3V C5 330pF OPTIONAL 21kHz LOWPASS OUTPUT FILTER L2 25µH OUTPUT FILTER OUTPUT C7 2.2µF C4 150µF REF 2 C6 1000pF Figure 3. Standard 3.3V Step-Down Application Circuit Using Through-Hole Components (commercial temperture range) Table 2. Component Table for Wide Temperature Applications C1(µF) C2(µF) C3(µF) C4(µF) C5(pF) C6(pF) L1(µH) ThroughHole 0.047 1.0 150* 220* 330 1000 22 SO 1.0 68** 100*** 330 1000 22 0.047 * Sanyo OS-CON Series (very low ESR) ** 16V or greater maximum voltage rating. *** 6.3V or greater maximum voltage rating. ____________Applications Information Fixed +3.3V Step-Down Converter Application Figure 3 shows the standard 3.3V step-down circuit with components shown for commercial temperature range applications. Figures 4, 5, and Table 2 suggest external component values for both SO and through-hole wide temperature range applications. These circuits are useful in systems that require high current and high efficiency and are powered by an unregulated supply, such as a battery or wall-plug AC-DC adapter. The MAX748A delivers a guaranteed 300mA for input voltages of 4V to 16V, and a guaranteed 500mA for input voltages of 4.75V to 16V with 800mA typical output currents. The MAX763A delivers a guaranteed 300mA for input voltages of 4V to 11V, a guaranteed 500mA for input voltages of 4.75V to 11V, and has 700mA typical output currents. The MAX748A/ MAX763A operate from an input down to 3V (the upper limit of undervoltage lockout), but with some reduction in output voltage and maximum output current. Inductor Selection The MAX748A/MAX763A require no inductor design because they are tested in-circuit, and are guaranteed to deliver the power specified in the Electrical Characteristics with high efficiency using a single 22µH inductor. The 22µH inductor’s incremental saturation current rating should be greater than 1A for 500mA load operation. Table 3 lists inductor types and suppliers for various applications. The surface-mount inductors have nearly equivalent efficiencies to the larger through-hole inductors. Output Filter Capacitor Selection The primary criterion for selecting the output filter capacitor is low effective series resistance (ESR). The product of the inductor-current variation and the output capacitor’s ESR determines the amplitude of the sawtooth ripple seen on the output voltage. Minimize the output filter capacitor’s ESR to maintain AC stability. _______________________________________________________________________________________ 9 MAX748A/MAX763A 3.3V, Step-Down, Current-Mode PWM DC-DC Converters INPUT MAX748A 3.3V TO 16.0V MAX763A 3.3V TO 11.0V 1 VIN C2 1.0µF 8 C3* 150µF (16V) V+ SHDN LX OUT SS C1 0.047µF CC GND 6 C3* 68µF (16V) 5 4 V+ SHDN LX MAX748A MAX763A OUT OUTPUT 3.3V SS C1 0.047µF CC GND 6 C6 1000pF *OS-CON Series (very low ESR) 5 C5 330pF 3 C4* 220µF (10V) 7 D1 1N5817 L1 22µH REF 2 8 1 C5 330pF 3 C2 1.0µF 7 D1 1N5817 MAX748A MAX763A INPUT MAX748A 3.3V TO 16.0V V MAX763A 3.3V TO 11.0V IN 4 L1 22µH OUTPUT 3.3V C4* 100µF (6.3V) REF 2 C6 1000pF *Sprague 293D or 595D Series-16V. See Table 3 for alternative suppliers. Figure 4. Standard 3.3V Step-Down Application Circuit Using Through-Hole Components (all temperature ranges) Figure 5. Standard 3.3V Step-Down Application Circuit Using Surface-Mount Components (Commercial and Extended Industrial Temperature Ranges) The capacitor’s ESR should be less than 0.25Ω to keep the output ripple less than 50mVp-p over the entire current range (using a 22µH inductor). Capacitor ESR usually rises at low temperatures, but OS-CON capacitors provide very low ESR below 0°C. Table 3 lists capacitor suppliers. in Figures 6-9 have been tested successfully over a wide range of operating conditions. The 1µF input bypass capacitor must be positioned as close to the V+ and GND pins as possible. Also, place the output capacitor as close to the OUT and GND pins as possible. The traces connecting ground to the input and output filter capacitors and to the catch diode must be short to reduce inductance. Use an uninterrupted ground plane if possible. Other Components The catch diode should be a Schottky or high-speed silicon rectifier with a peak current rating of at least 1.0A for full-load (500mA) operation. The 1N5817 is a good choice. The 330pF outer-loop compensation capacitor provides the widest input voltage range and best transient characteristics. Printed Circuit Layouts A good layout is essential for stable, low-noise operation. The layouts and component placement diagrams 10 Output-Ripple Filtering A simple lowpass pi-filter (Figure 3) can be added to the output to reduce output ripple to about 5mVp-p. The cutoff frequency shown is 21kHz. Since the filter inductor is in series with the circuit output, minimize the filter inductor’s resistance so the voltage drop across it is not excessive. ______________________________________________________________________________________ 3.3V, Step-Down, Current-Mode PWM DC-DC Converters Figure 7. DIP PC Layout, Component Side (1X Scale) Figure 8. DIP PC Layout, Solder Side (1X Scale) Figure 9. DIP PC Layout, Drill Guide (1X Scale) ______________________________________________________________________________________ MAX748A/MAX763A Figure 6. DIP PC Layout, Through-Hole Component Placement Diagram (1X Scale) 11 MAX748A/MAX763A 3.3V, Step-Down, Current-Mode PWM DC-DC Converters _____Pin Configurations (continued) ____________________Chip Topographies 0.131" TOP VIEW 3.327mm SHDN 1 REF 2 SS 3 MAX748A MAX763A CC 4 8 V+ 7 LX 6 GND 5 OUT SS REF CC SHDN 0.116" SO V+ 2.946mm V+ V+ 1 16 V+ SHDN 2 15 V+ REF 3 14 LX OUT 13 LX GND N.C. 4 MAX748A N.C. 5 12 LX N.C. 6 11 GND SS 7 10 GND CC 8 9 OUT Wide SO V+ V+ GND LX LX LX MAX748A TRANSISTOR COUNT: 298 SUBSTRATE CONNECTION: V+ 0.072" 1.829mm SHDN V+ V+ V+ _Ordering Information (continued) PART TEMP. RANGE MAX763ACPA 0°C to +70°C 8 Plastic DIP PIN-PACKAGE MAX763ACSA 0°C to +70°C 8 SO MAX763AC/D 0°C to +70°C Dice* MAX763AEPA -40°C to +85°C 8 Plastic DIP MAX763AESA -40°C to +85°C 8 SO MAX763AMJA -55°C to +125°C 8 CERDIP REF LX LX LX * Contact factory for dice specifications. 0.116" 2.946mm SS GND CC OUT MAX763A TRANSISTOR COUNT: 281 SUBSTRATE CONNECTION: V+ 12 ______________________________________________________________________________________