19-4672; Rev 3; 3/96 if eal ae ZN yale 3 VAILABLE MM AALM +5V/Adjustable Step-Up Current-Mode DC-DC Converters General Description The MAX731 and MAX752 are fixed and adjustable CMOS, step-up, DC-DC switch-mode regulators. The MAX731 accepts a positive input voltage between +2.5V and +5.25V and converts it to a fixed +5V at 200mA, guaranteed over temperature. Typical full-load efficien- cies are 82% to 87%. |t requires a single inductor value of 22uH to function over the entire range, so no inductor- related design is necessary. The MAX752 is an adjusta- ble version that converts a minimum of +1.8V to any higher voltage up to +15V, at up to 200mA. Typical full-load efficiencies are 85% to 95%. A single 50nH inductor is suitable for the entire range of operating conditions, so no inductor-related design is necessary. The MAX731/MAX752 use current-mode pulse-width modulation (PWM) controllers to provide precise output regulation and low subharmonic noise. Typical no-load supply current is 2mA. A fixed 170kHz oscillator fre- quency allows easy filtering of ripple and noise, and provides for small external components. The MAX731/MAX752 feature cycle-by-cycle current lim- iting, overcurrent limiting, external shutdown, and pro- grammable soft-start protection. For fixed +12V and +15V step-up regulators, refer to the MAX732/MAX733 data sheet. For lower-power step-up applications, refer to the MAX631/632/633 and MAX654- 659 data sheets. Applications +5V-Logic Supply in +3V-Logic System DC-DC Converter Module Replacement Portable Instruments Laptop Computers Distributed Power Systems Cellular Phones Battery-Powered Equipment Pin Configurations Features @ 200mA Load Currents Guaranteed with No External MOSFET @ Step-Up from a 2.5V Input @ 170kHz High-Frequency Current-Mode PWM @ 82% to 87% Typical Efficiencies at Full Load (MAX731) @ 85% to 95% Typical Efficiencies at Full Load (MAX752) @ Small Inductor - No Component Design Required @ 2mA Quiescent Current (MAX752) @ Overcurrent and Soft-Start Protection @ 8-Pin DIP, 16-Pin Wide SO Packages @ Shutdown Pin Ordering Information PART TEMP. RANGE PIN-PACKAGE | MAX731CPA OC to +70C 8 Plastic DIP MAX731CWE OC to +70C 16 Wide SO MAX731C/D OC to +70C Dice* MAX731EPA -40C to +85C 8 Plastic DIP MAX731EWE -40C to +85C 16 Wide SO MAX731MJA -55C to +125C 8 CERDIP Ordering information continued on last page. * Dice are tested at TA = +25C only. Contact factory for availability and processing to MIL-STD-883. Typical Application Circuit 22H SHDN V, 200mA TOP VIEW ux sv im 7 MAM SHON [1] V4 MAX731 V4 MAAXIAA veer [2] spqyz37 |Z] VourNC) Vout | ss[aj Max752 Fejux oc , ce [4] 5] GND SS GND VREF * 150uF x ( )ARE FOR MAXTE2 DIP Ss SO on last page = = MAXIMA Maxim Integrated Products 1 For free samples & the latest literature: http:/www.maxim-ic.com, or phone 1-800-998-8800 2SZXVW/FLEZLXVINMAX731/MAX752 +5V/Adjustable Step-Up Current-Mode DC-DC Converters ABSOLUTE MAXIMUM RATINGS V+, LXtoGND .............0.... VouTtoGND ...... ee, 8S, CC, SHDN toGND............ Peak Switch Current (ILx) .......... Reference Current ({IVREF) ......... Continuous Power Dissipation (TA = +70C) 8-Pin Plastic DIP (derate 9.09mW/C above +70C) . 727mW 16-Pin Wide SO (derate 9.52mW/"C above +70C) .. 762mW 8-Pin CERDIP (derate 8.00mW/C above +70C) ... 640mW Cee eens OC to +70C nbn teens -40C to +85C Cet teens -55C to +125C re -0.3V to +17V Operating Temperature Ranges: Leeks +25V MAX731/752C_ _ .... 70.3V to (V+ + 0.3V) MAX731/752E_ _ Lee tenes 1.5A MAX731/752MJA beeen eee 2.5mA Junction Temperatures: MAX731/752C_ _/E__ MAX731/752MJA eee cece erent +175C Storage Temperature Range Lead Temperature (soldering, 10 sec) Sees -65C to +160C bce e eee eee +300C 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 - MAX731 (Circuit of Figure 1a, VIN = +3V, [LoaD = OmA, Ta = TMIN to TMAX, typical values are at TA = +25C, unless otherwise noted.) PARAMETER CONDITIONS MIN TYP MAX UNITS Minimum Start-Up Input Voltage ILOAD = OMA 18 v ILOAD = 200mA 2.0 2.5 Minimum Operating Voltage, VIN ILOAD = 100mA iA Vv ILOAD = 200mA 2.0 Output Voltage (Note 1) VIN = 2.7V to 4.65V, OMA < ILOAD < 200mA 475 5.00 .25 Vv Output Current 200 mA Line Regulation VIN = 2.7V to 4.65V 0.20 IN Load Regulation ILOAD = OMA to 100mMA 0.005 IMA Efficiency VIN = 3V, ILOAD = 100MA 87 % Supply Current Includes switch current 2.0 4.0 mA SHDN = 0, entire circuit 35 100 Standby Current =!]} - HA SHDN = 0, into V+ 6 Shutdown Input Threshold vil e708 V VIL 0.25 Shutdown Input Leakage Current 1.0 pA Short-Circuit Current 1.5 A LX On Resistance 0.5 Q LX Leakage Current Vps = 5V 1.0 pA Reference Voltage 1.15 1.23 1.30 Vv Reference Drift Ta = TMIN to TMAX 50 pom/C / Oscillator Frequency 125 170 215 kHz Compensation Pin Impedance 20 KQ | Note 1: Circuit will regulate properly with input voltage as high as 5.25V due to voltage drop across the external diode. MAXIM+5V/Adjustable Step-Up Current-Mode DC-DC Converters ELECTRICAL CHARACTERISTICS - MAX752 (Circuit of Figure 1b, R1 and R2 configured for +12V output operation, V+ = 5V, ILOAD = OmA, TA = TMIN to TMAX, typical values are at Ta = +25C, unless otherwise noted.) - PARAMETER CONDITIONS MIN TYP MAX UNITS Minimum Input Voltage ILOAD = OMA 1.8 2.5 Vv MA coe teoma | MAX752C/E 1146 120 12.54 Output Voltage Ora coe Mtn | MAX752M 11.46 120 12.54 V Ona poe vena | MAX752C/E/M 11.46 120 12.54 MAX752C/E 150 V+ = 4.5V to 11.0V Output Current MAX752M 125 mA V+ = 6.0V to 11.0V | MAX752C/E/M 200 Output Voltage Range Vin < VOUT 27 15.75 V Line Regulation V+ = 4.0V to 11.0V 0.20 %N Load Regulation ILOAD = OmA to 100mA 0.0035 IMA Efficiency V+ = 5V, ILOAD = 100MA 88 % Supply Current Includes switch current V7 3.0 mA SHDN = Q, entire circuit 70 100 Standby Current , HA SHDN = 0, into V+ 6 ViH V+-0.5 Shutdown Input Thresnold - Vv VIL 0.25 Shutdown Input Leakage Current 1.0 HA Short-Circuit Current 45 A LX On Resistance 0s Q LX Leakage Current Vps = 12V 1.0 HA Reference Voltage 1.15 1.23 1.30 V Reference Drift Ta = TMIN to TMAX 50 ppm"C Oscillator Frequency 130 170 210 kHz MAXUM ZSLXVW/LELXVNMAX731/MAX752 +5V/Adjustable Step-Up Current-Mode DC-DC Converters t MAX731 SWITCHING WAVEFORMS CONTINUOUS CONDUCTION 2ps/div A: Switch Voltage (LX pin), 2V/div, OV to +5V B: Inductor Current, 500mA/div C: Output Voltage Ripple, 50mV/div Circuit of Fig. 2a, Coyt = 150pF, V+ = 3V, lout = 200mA, Ta = +25C MAX752 SWITCHING WAVEFORMS DISCONTINUOUS CONDUCTION 2usidiv A: Switch Voltage (LX pin), 5V/div, OV to +12.4V B: Inductor Current, 200mA/div C: Output Voltage Ripple, 50mV/div Circuit of Fig. 2b, Cout = 300uF, V+ = 3V, lout = 200mA, Ta = +25C MAX752 LINE TRANSIENT RESPONSE 100us/div A: Vout, 50mV/div, DC-Coupled B: V+, 5V/div, 6.0V to 12,0V Circuit of Fig. 2b, lout = 125mA, Vout = 15V, Ta=+25C 4 MAX752 SWITCHING WAVEFORMS CONTINUOUS CONDUCTION 2us/div A: Switch Voltage (LX pin), 5V/div, OV to H12.4V B: Inductor Current, 500mA/div C: Output Voltage Ripple, 50mV/div Circuit of Fig. 2b, Cour = 300uF, V+ = 3V, lout = 100mA, Ta = +25C MAX731 LINE TRANSIENT RESPONSE B 10ms/div A: Vout, 100mV/div B: VIN, 2V to 3V Circuit of Fig. 2a, ILoap = 200mA Ta=+25C MAX731 LOAD TRANSIENT RESPONSE A g 2ms/div A: Vout,100mV/div B: lout, 180mA/div Circuit of Fig. 2a, Ta = +25C Typical Operating Characteristics MAX731 SWITCHING WAVEFORMS DISCONTINUOUS CONDUCTION betes 2us/div A: Switch Voltage (LX pin), 2V/div, OV to +5V B: Inductor Current, 500mA/div C: Output Voltage Ripple, 50mV/div Circuit of Fig. 2a, Cour = 150uF, V4 = 3V, louT = 100mA, Ta = +25C MAX752 LINE TRANSIENT RESPONSE 100us/div A: Vout, 50mV/div, DC-Coupled B: V+, 5V/div, 6.0V to 9.0V Circuit of Fig. 2b, laut = 200mA, Vout=12V Ta=+25C MAX752 LOAD TRANSIENT RESPONSE 50us/div A: Vout, 50mV/div, DC-Coupled B: lout, 100mA/div, 10MA to 200mA Circuit of Fig. 2b, V+ = +6V, Ta = +25C Vout= 12V MA AXLAAMAX752 LOAD TRANSIENT RESPONSE 5Ous/oiv A: Vout, 50mV/div, D-C Coupled B: jouT, 5OmA/div, 10mA to 125mA Circuit of Fig. 2b, V+ =+6V, TA= 425C VouT = 15V MAX752 EFFICIENCY vs. OUTPUT CURRENT 100 Vee ttV V+=9V _ ot Ca 2 Canis 8 LA a So S80 Ve=4V CIRCUIT OF FIG. 2 Vout = 15V q Ta= 425C 70 Nl 0 100 200 300 400 OUTPUT CURRENT (mA) MAX752 PEAK INDUCTOR CURRENT vs. OUTPUT CURRENT 1000 = 800 = V+ = 6V & 4 = 600 s Ve =9V y 2 * YL V+=4V 5 400 4 {|__| 2 f 8 L LT | cIRCUIT OF FIG. 2, x Vout = 12V, = 200 11 =50uH, 4 C4 = 300nf, Ta= 425C 0 ij 1 1 0 400 200 300 400 OUTPUT CURRENT (mA) PAAXIMM 100 90 EFFICIENCY (%), 80 70 6.0 NO-LOAD SUPPLY CURRENT (mA) 5.0 40 3.0 20 1.0 0 100 +5V/Adjustable Step-Up Current-Mode DC-DC Converters JAX73 EFFICIENCY vs. OUTPUT CURRENT 200 1 A= +25C 300 400 OUTPUT CURRENT (mA) MAX731 NO-LOAD SUPPLY CURRENT vs. SUPPLY VOLTAGE Ta = +28C 500 00 20 25 30 35 40 45 50 SUPPLY VOLTAGE (V) MAX752 PEAK INDUCTOR CURRENT vs. OUTPUT CURRENT 4200 _ 1000 z =& 5 V+=6V Z 800 I cf V+=9V 3 600 | V+=4V 1 = = YA ha S //) | B 400 Aton 2 Ke ea CIRCUIT OF FIG. 2b, a I four = 15) s Li =50pH & 200 C4 = 300pF Ta=+25'C 0 i i I l 0 100 200 300 400 500 OUTPUT CURRENT (mA) CURRENT (mA) EFFICIENCY (%) MAXIMUM OUTPUT CURRENT (mA) Typical Operating Characteristics (continued) MAX752 EFFICIENCY vs. OUTPUT CURRENT 100 90 80 CIRCUIT OF FIG. 2b =12V Ta= 425C 70 9 50 100 150 200 250 300 OUTPUT CURRENT (mA) MAX731 PEAK INDUCTOR CURRENT AND MAXIMUM OUTPUT CURRENT vs. SUPPLY VOLTAGE 1400 PEAK 1200 1000 800 MAXIMUM DC OUTPUT CURRENT 600 200 Ta=+25C 0 2.0 25 3.0 35 SUPPLY VOLTAGE (V) 40 45 MAX752 MAXIMUM OUTPUT CURRENT vs. SUPPLY VOLTAGE 500 Vout = 15V 400 Vout = 12V 300 200 100 CIRCUIT OF FIG. 2b Ta = +25C 2 4 6 8 10 SUPPLY VOLTAGE (V) 12 14 CSZLXVW/FEZLXVWNMAX731/MAX752 +5V/Adjustable Step-Up Current-Mode DC-DC Converters Table 1a. MAX731 Typical Soft-Start Times Vin = 3V, CouT = 150uF Cgs (uF) Delay (ms) 0.1 10 0.2 20 0.5 50 1.0 100 2.0 160 5.0 170 NOTE: SOFT-START TIMES ARE +35%. C1 1S THE SOFT-START CAPACITOR; C4 IS THE OUTPUT CAPACITOR Table 1b. MAX752 Typical Soft-Start Times VOU av Oa e SOF SOFT-START TIME (ms) vs. C1 (uF) V+ (V) lout (mA) O.4uF 0.47,F 1.0nF 45 0 55 115 125 6.0 0 40 80 70 9.0 0 30 60 45 45 100 90 350 780 6.0 100 60 210 445 2.0 100 30 60 60 45 200 175 715 1690 6.0 200 85 340 760 9.0 200 30 75 125 von SV OA SOc SOFT-START TIME (ms) vs. C1 (uF) V+ (V) lout (mA) Our 0.47uF 1.0uF 45 0 90 210 250 6.0 0 65 135 150 9.0 - 0 35 65 50 12.0 0 30 50 35 45 75 155 680 4380 6.0 75 105 425 880 9.0 75 45 160 305 12.0 75 30 50 35 45 125 235 1125 2260 6.0 425 135 595 1255 9.0 125 55 230 475 12.0 125 30 50 40 NOTE: SOFT-START TIMES ARE +35%. C1 1S THE SOFT-START CAPACITOR; C4 IS THE OUTPUT CAPACITOR 6 MAXIM+5V/Adjustable Step-Up Current-Mode DC-DC Converters Pin Description 8-PIN DIP | 16-PINSO| NAME FUNCTION 1,4, 10, 15 N.C. No Connect - no internal connection j 2 SHDN | Shutdown - active low. Ground to power-down the IC; tie to V+ for normal operation. Output power FET is held off when SHDN is iow. 2 3 VREF Reference Voltage Output (+ 1.23V) supplies up to 100yA for external loads. 3 5 ss Soft-Start. Capacitor between SS and GND provides soft-start and short-circuit protection. 4 6 cc Compensation Capacitor Input. Externally compensates the outer feedback loop. 5 7 GND Ground Switch Ground - ground of the output power FET. Both pins must be separately tied to 8,9 GND (SW) ground because they are not internally connected. 6 11, 12, 13 LX Drain of internal N-channel power MOSFET 7 44 Vour Output-Voltage Sense Input (MAX73 1) N.C. No Connect - no internal connection (MAX752) 8 16 V+ Supply Voltage Input Vin T I C2 OtyF y- V+ SHON Your MAXIM 4 BIAS 170kH2 MAX731 C5 22uH O.15F 4~ GEN ERROR AMP O8 ce PWM D1 or COMPARATOR Lx | 1NS817 Vout O15 uF - 1.23V BANDGAP t+ 1500? Se LT VREF = ane | CURRENT L SENSE AMP 8S SLOPE S| CLAMP COMPENSATION OVERCURRENT COMPARATOR - Rs GND Figure 1a. MAX731 Detailed Block Diagram with External Components, Bootstrap Mode MAXIMA oSZLXVW/FLEZLXVINMAX731/MAX752 +5V/Adjustable Step-Up Current-Mode DC-DC Converters C3 0.1yF Tr 150uF TT wet alt c2 + 425% ~ SS [ + O.4pF Va $s SLOPE CLAMP COMPENSATION SHON Ve 05 BIAS T70KHz Su 0.15pF RI GEN ERROR AMP osc MAAXIAA ce PWM MAX752 D1 > COMPARATOR ac | N87 VouT C7 R2 2200pF T Tov BANDGAP + c4 I+ = > 300uF [> VREF = 6 Dot CURRENT L = SENSE AMP = 1MQ. : OVERCURRENT COMPARATOR Figure 1b. MAX752 Detailed Block Diagram with External Components, Optimized for 12V Output. Detailed Description The input voltage range has three important components: no-load starting voltage, full-load starting voltage and minimum operating voltage. The no-load starting voltage is usually less than 2.0V, but if a load is added, the circuit may not start. At a slightly higher voltage, more current can be drawn, and the output voltage will rise to the regulated value. With a 2.5V input voltage, the MAX731 will start up and regulate with a 200mA load. The MAX752 will start up and regulate at 12V at 150mA from a minimum input voltage of 4.5V. The MAX731 has a "bootstrapped" output, which means it operates from the output that it generates. Once it generates 5V, it then operates from this 5V, and subse- quently can furnish 200mA from an input as low as 2.0V (the holding voltage). The holding voltage is typically 1.4V for 100mA loads. This capability is very important in battery-operated equipment, because it indicates the voltage to which the battery can discharge without losing output regulation. Input voltages as high as 16V can be applied without damage, but regulation is lost when the input exceeds the normal regulated output. This happens because a DC path through the inductor and diode produces an output voltage one diode drop (0.3-0.6V) less than the input voltage. (The MAX731/MAX752 sense this high output and stop switching.) This path exists even with the IC removed from the circuit. MAXIM+5V/Adjustable Step-Up Current-Mode DC-DC Converters Operating Principle The MAX731/MAX752 switch-mode regulators use a current- mode pulse-width modulation (PWM) controller coupled with a simple boost regulator topography to step up an unregu- lated DC voltage. The MAX731 converts a voltage ranging from 1.4V to 5.25V to 5V. The MAX752 has an adjustable output. The current-mode PWM architecture provides cycle- by-cycle current limiting and excellent load-transient re- sponse characteristics. The controller consists of two feedback loops: an inner (current) loop that monitors the switch current through the current-sense resistor (RS) 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 tran- sistor on-time when the switch current reaches a thresh- old 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. Programmable Soft Start A capacitor between 0.1nF and 5uF is required on the Soft-Start (SS) pin to ensure an orderly power-up. The charging capacitors voltage slowly raises the clamp on the error-amplifier output voltage, limiting surge currents at power-up by slowly increasing the cycle-by-cycle cur- rent-limit threshold. SS timing is controllable from the SS pin by capacitor choice. A typical value is 0.1uF. Table 1 lists timing characteristics for selected capacitor values and circuit conditions. The output voltage sags if more than the maximum load current is drawn. The overcurrent comparator trips if the load exceeds approximately 1.5A. An SS cycle is actively initiated when an overcurrent fault condition triggers an internal transistor to discharge the SS capacitor to ground. Overcurrent Limiting When the load current exceeds approximately 1.5A, the output stage is turned off by the inner loop cycle-by-cycle current-limiting action, and the overcurrent comparator sig- nals the control logic to initiate an SS cycle. On each clock cycle, the output FET turns on again and attempts to deliver current until cycle-by-cycle or overcurrent limits are ex- ceeded. Note that the SS capacitor must be at least 0.01pF for overcurrent protection to function properly. Shutdown Keeping the Shutdown (SHDN) pin at ground holds the MAX731/MAX782 in shutdown mode. In shutdown mode, the output power FET is off, but there is still an external path from V+ to the load through the inductor and diode, and another path from V+ to GND through the inductor, diode, and external feedback resistors. For the MAX731, the an O.ApF L u 22uH INPUT | SHDN 6 LX o_ pF Dt MNAXILAA 8 1N5817 MAX731. V+ OUTPUT 7 Vout | G5 4 ec 0.15pF c4 SS GND VREF 150uF 7] cite 3 /5 et ce c7 O15pF | 47uF 0.15uF dit NOTE: PIN NUMBERS REFER TO 8-PIN PACKAGES. OPTIONAL LOWPASS OUTPUT FILTER FILTER OUTPUT 12 + og OUTPUT 25H TT 2apr Figure 2a. MAX731 Standard Boost Application Circuit feedback resistors are approximately 80kQ. The internal reference turns off, which causes the SS capacitor to discharge. Typical device standby current in shutdown mode is 35pA. For normal operation, connect SHDN to V+. An SS cycle brings the MAX731 out of shutdown mode. The +1.23V bandgap reference supplies up to 100A at VREF. Abypass capacitor from VREF to GND is required: 4.7uF for the MAX731 and 0.01uF for the MAX752. Output Adjustment - MAX752 The output voltage for the MAX752 is set by two resistors, R1 and R2 (Figures 1b and 2b), which form a voltage divider between the output and the Compensation Ca- pacitor (CC) pin. The regulator adjusts the output so the voltage at the junction of R1 and R2 is equal to the +1.23V bandgap reference voltage. Since CC is a CMOS input, its input impedance is nearly an open circuit, which will not load the voltage divider. R2 can be any value be- tween 10kQ to 30kQ. R11 is given by the formula: VOUT _ 1.23V Capacitors C5 and C7 furnish loop compensation. Smaller values are not recommended because they may produce instability. R1 = R2 ( 1) MAXLAA oSZXVW/ILEZLXVNMAX731/MAX752 +5V/Adjustable Step-Up Current-Mode DC-DC Converters neuT gto OF Lt 8 lH Lt Tom Sul _ Vv 8 1 SHON Lx Thal Ptegi7 | OUTPUT MAAXIAA MAX752 P no. 4 cc C4 SS GND _VREF R2 = 300uF cit 3 [5 2 ce oA 0.01pF NOTE: PIN NUMBERS REFER TO 8-PIN PACKAGES. OPTIONAL LOWPASS OUTPUT FILTER FILTER OUTPUT 12 | te gg OUTPUT 25yH To 22ur Figure 2b. MAX752 Standard Boost Application Circuit Modes Continuous-Current Mode: The MAX731/MAX752 nor- mally operate in continuous-current mode, which means current always flows in the inductor, and the control circuit adjusts the switchs duty cycle on a cycle-by- cycle basis to maintain regulation without exceeding the switch current capability. This mode provides excellent load-transient response. During start-up conditions and under very light loads, this method cannot adjust the duty cycle to the correct value without exceeding the switch current capability, so the controller changes to discon- tinuous-current mode. Discontinuous-Current Mode: {n discontinuous-cur- rent mode, current through the inductor starts at zero, rises to a peak value, then ramps down to zero on each cycle. Although efficiency is still excellent, the output ripple increases slightly and the switch waveforms dis- play ringing (the inductor's self-resonant frequency). This ringing may seem disconcerting at first, but it does not indicate problems. Pulse-Skipping Mode: At load currents under a few milliamperes, even discontinuous-current mode tends to put more energy into the coil than the load requires, so the controller changes to pulse-skipping mode, in which 10 regulation is achieved by skipping entire cycles. Effi- ciency is still good, typically 70% to 80%, reduced in part because the MAX731/MAX752 quiescent supply current becomes a significantly larger fraction of the total current when load currents are low. Pulse-skipping switch wave- forms can be irregular, and the output ripple contains a low-frequency component that may exceed 50mV. Larger, low-ESR filter capacitors can help reduce the ripple voltage in critical applications. The MAX731/MAX752 controller normally operates in continuous-current mode and reverts to discontinuous- current mode or pulse-skipping mode during extreme conditions. Continuous-current mode operation gives a cleaner output than discontinuous or pulse-skipping modes, because peak-to-peak ripple amplitude is mini- mized and the ripple frequency is fixed at the oscillator frequency, making the output easy to filter. It is possible to design circuits around the MAX731 that use discontinuous-current mode as the primary means of regulation, eliminating the compensation capacitor shown in Figure 2. This is not normally recommended for several reasons. First, the peak currents in the switch and the inductor become much higher, reducing the output current. Second, the coils inductance, peak current rating, and resistance values become critical; its physical size increases as well. Finally, the output filter require- ments demand larger components. Application information For fixed outputs of 12V or 15V, the MAX732 or MAX733 can be used. These devices are fully characterized at these voltages at output currents up to 200mA (125mA for MAX733), and do not require external voltage dividers. They accept input voltages above 4.0V. Figure 2a shows the standard step-up application circuit. This circuit will operate with inputs from 2.5V to 5.25V. The output current depends on the input voltage (see Maximum Output Current vs. Supply Voltage, Typical Operating Char- acteristics). Inductor Selection A 22uH inductor is sufficient for most MAX731 designs and a S0uH inductor is sufficient for most MAX752 de- signs. The important specification is the inductors incre- mental saturation current rating, which should be greater than 2.5 times the DC load current (500mA for 12V, 200mA loads). For lower-power applications, smaller inductor values may be used. Table 2 shows recom- mended inductor types and suppliers for various appli- cations. The listed surface-mount inductors efficiencies are nearly equivalent to those of the larger-sized, through- hole inductors. MAAXUM+5V/Adjustable Step-Up Current-Mode DC-DC Converters Table 2. Component Suppliers PRODUCTION METHOD INDUCTORS CAPACITORS Surface Mount Sumida Matsuo For MAX731: CD54-220 (22uH) 267-series For MAX752: CD54-220 (22H) CD54-470 (47pH) for discontinuous mode Coiltronics CTX 100-series Miniature Through-Hole Sumida Sanyo OS-CON For MAX731: OS-CON-series RCH654-220 Low ESR Organic Semiconductor For MAX752: RCH654-470 Low-Cost Through-Hole Renco Maxim For MAX731: MAXC001 RL 1284-22 150uF, Low ESR Electrolytic For MAX752: RL1284-47 Nichicon PL-series Low ESR Electrolytics United Chemi-Con LXF-series Sumida (708) 956-0666 Renco (516) 585-5566 Sanyo OS-CON (619) 661-6835 Output Filter Capacitor Selection The primary criterion for selecting the output filter ca- pacitor is low equivalent series resistance (ESR). The product of the inductor current variation and the output capacitor's ESR determines the high-frequency ampli- tude seen on the output voltage. The capacitors ESR should be less than 0.25Q to keep the output ripple less than 50mVp-p over the entire current range (using the recommended inductor). In addition, the output filter capacitor's ESR should be minimized to maintain AC stability. Refer to Table 2 for suggested capacitor sup- pliers. In the standard application of Figure 2, the output capacitor value should be at least 300uF in order to maintain stability at full loads. 150uF capacitors (MAXC001) are available from Maxim in production quantities. Two of these capacitors can be connected in parallel. Lighter loads require proportionately lower capacitor values. Other Components Use a Schottky diode with a current rating of at least 500mA for full-load (200mA) operation. The 1N5817 is a good choice. The two compensation capacitor values at the CC input are critical because they have been selected to provide the best transient response. Coiltronics (516) 241-7876 Matsuo USA (714) 969-2491 Matsuo Japan (06) 332-0871 United Chemi-Con (708) 696-2000 Nichicon (708) 843-7500 Output-Ripple Filtering An optional lowpass pi-filter (Figure 2) can be added to the output to reduce output ripple to about 5MVp-p. The cutoff frequency of the filter shown is 21kHz. Since the filter inductor is in series with the circuit output, its resis- tance should be minimized to avoid excessive voltage drop. Note that the feedback must be taken before the filter, not after the filter. Printed Circuit Layout Printed circuit board layout is not critical, except to ensure quiet operation. Bypass capacitors should be located as close to the device as possible to prevent instability and noise pickup. The Schottky diode leads should also be kept short to prevent fast rise-time pulses in the output. A ground plane is recommended but not necessary. V+ Bypassing For MAX752 applications where greater than 13V Is generated with more than 100mA load current, capacitor C2 (Fig2b) should be focated less than 1/2 inch from V+ and GND pins of the IC. This capacitor snubs high voltages created by large load transients. 11 MAXUM 2SZLXVW/FEZLXVINMAX731/MAX752 +5V/Adjustable Step-Up Current-Mode DC-DC Converters ____ Pin Configurations (continued) _ Ordering Information (continued) PART TEMP. RANGE PIN-PACKAGE TOP VIEW MAX752CPA 0C to +70C 8 Plastic DIP ne. [4] * He] V+ MAX752CWE OC to +70C 16 Wide SO SHON [2 | i15] NC. MAX752C/D 0C to +70C Dice* MAXLI 5 - VREF [3] MAX731 14] Vout (N.C.) MAX752EPA -40C to +85C 8 Plastic DIP NC. [4] MAX752 fra] x MAX752EWE -40C to +85C 16 Wide SO Ss [5] 12] Lx MAX752MJA -58C to +125C 8 CERDIP* ce [6 fra} ux * Dice are tested at Ta = +25C only. GND [7] HO] N.C. *Contact factory for availability and processing to MIL-STD-883. GND (SWITCH) [8 | [9] GND (SWITCH) so () ARE FOR MAX752 Chip Topographies MAX731 MAX752 0.123" (3.124 mm) 0.114" (2.895 mm) NOTE: CONNECT SUBSTRATE TO V+ NOTE: CONNECT SUBSTRATE TO V+ TRANSISTOR COUNT: 228 TRANSISTOR COUNT: 228 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 1996 Maxim Integrated Products Printed USA MAXIM is a registered trademark of Maxim Integrated Products.