19-0305; Rev 3; 3/09 KIT ATION EVALU E L B A AVAIL 5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC Controllers The MAX1649/MAX1651 BiCMOS, step-down, DC-DC switching controllers provide high efficiency over loads ranging from 1mA to more than 2.5A. A unique, currentlimited pulse-frequency-modulated (PFM) control scheme gives these devices the benefits of pulse-width-modulation (PWM) converters (high efficiency at heavy loads), while using only 100µA of supply current (vs. 2mA to 10mA for PWM converters). Dropout performance down to 300mV is provided by a high switch duty cycle (96.5%) and a low current-sense threshold (110mV). A high switching frequency (up to 300kHz) allows these devices to use miniature external components. The MAX1649/MAX1651 have dropout voltages less than 0.3V at 500mA and accept input voltages up to 16V. Output voltages are preset at 5V (MAX1649), or 3.3V (MAX1651). They can also be adjusted to any voltage from 1.5V to the input voltage by using two resistors. These step-down controllers drive external P-channel MOSFETs at loads greater than 12.5W. If less power is required, use the MAX639/MAX640/MAX653 step-down converters with on-chip FETs, which allow up to a 225mA load current. ________________________Applications PDAs High-Efficiency Step-Down Regulation 5V-to-3.3V Green PC Applications Battery-Powered Applications ____________________________Features ♦ More than 90% Efficiency (10mA to 1.5A Loads) ♦ More than 12.5W Output Power ♦ Less than 0.3V Dropout Voltage at 500mA ♦ ♦ ♦ ♦ 100µA Max Quiescent Supply Current 5µA Max Shutdown Supply Current 16V Max Input Voltage 5V (MAX1649), 3.3V (MAX1651), or Adjustable Output Voltage ♦ Current-Limited Control Scheme ♦ Up to 300kHz Switching Frequency ♦ Up to 96.5% Duty Cycle ______________Ordering Information PART TEMP RANGE MAX1649CPA 0°C to +70°C 8 Plastic DIP PIN-PACKAGE MAX1649CSA MAX1649C/D MAX1649EPA MAX1649ESA MAX1651CPA 0°C to +70°C 0°C to +70°C -40°C to +85°C -40°C to +85°C 0°C to +70°C 8 SO Dice* 8 Plastic DIP 8 SO 8 Plastic DIP MAX1651CSA MAX1651C/D MAX1651EPA MAX1651ESA 0°C to +70°C 0°C to +70°C -40°C to +85°C -40°C to +85°C 8 SO Dice* 8 Plastic DIP 8 SO *Dice are tested at TA = +25°C. __________Typical Operating Circuit INPUT 3.6V TO 16V __________________Pin Configuration TOP VIEW V+ MAX1651 ON/OFF SHDN CS EXT OUT REF FB GND P OUT 1 8 GND FB 2 7 EXT 6 CS 5 V+ SHDN 3 OUTPUT 3.3V MAX1649 MAX1651 REF 4 DIP/SO ________________________________________________________________ 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 MAX1649/MAX1651 _______________General Description MAX1649/MAX1651 5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC Controllers ABSOLUTE MAXIMUM RATINGS Supply Voltage, V+ to GND.......................................-0.3V, +17V REF, SHDN, FB, CS, EXT, OUT .......................-0.3V, (V+ + 0.3V) Continuous Power Dissipation (TA = +70°C) Plastic DIP (derate 9.09mW/°C above +70°C) .............727mW SO (derate 5.88mW/°C above +70°C) ..........................471mW Operating Temperature Ranges MAX1649C_A, MAX1651C_A ..............................0°C to +70°C MAX1649E_A, MAX1651E_A ............................-40°C to +85°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 (V+ = 5V, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL V+ Input Voltage Range V+ Supply Current I+ CONDITIONS VOUT < V+ MIN V+ = 16V, SHDN ≤ 0.4V (operating, switch off) 78 V+ = 16V, SHDN ≥ 1.6V (shutdown) 2 V+ = 10V, SHDN ≥ 1.6V (shutdown) FB Trip Point FB Input Current IFB Output Voltage VOUT Reference Voltage VREF TYP 3.0 MAX UNITS 16 V 100 µA 1 5 MAX1649C, MAX1651C 1.470 1.5 1.530 MAX1649E, MAX1651E 1.4625 1.5 1.5375 MAX1649C, MAX1651C ±50 MAX1649E, MAX1651E ±70 MAX1649, V+ = 5.5V to 16V 4.80 5.0 5.20 MAX1651, V+ = 3.6V to 16V 3.17 3.3 3.43 MAX1649C, MAX1651C, IREF = 0μA 1.470 1.5 1.530 MAX1649E, MAX1651E, IREF = 0μA 1.4625 1.5 1.5375 V nA V V REF Load Regulation 0µA ≤ IREF ≤ 100µA, sourcing only 4 10 mV REF Line Regulation 3V ≤ V+ ≤ 16V 40 100 µV/V Output Voltage Line Regulation Circuit of Figure 1 Output Voltage Load Regulation Circuit of Figure 1 Circuit of Figure 1 Efficiency SHDN Input Current 2.6 MAX1651, 3.6V ≤ V+ ≤ 16V, ILOAD = 1A 1.7 MAX1649, 0A ≤ ILOAD ≤ 1.5A, VIN = 10V -47 MAX1651, 0A ≤ ILOAD ≤ 1.5A, VIN = 5V -45 MAX1649, V+ = 10V, ILOAD = 1A 90 mV/V mV/A % MAX1651, V+ = 5V, ILOAD = 1A 90 V+ = 16V, SHDN = 0V or V+ SHDN Input Voltage High VIH 3V ≤ V+ ≤ 16V SHDN Input Voltage Low VIL 3V ≤ V+ ≤ 16V 2 MAX1649, 5.5V ≤ V+ ≤ 16V, ILOAD = 1A 1 1.6 _______________________________________________________________________________________ µA V 0.4 V 5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC Controllers (V+ = 5V, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL Current-Limit Trip Level (V+ to CS) VCS CONDITIONS 3V ≤ V+ ≤ 16V MIN TYP MAX UNITS 80 110 140 mV CS Input Current 3V ≤ V+ ≤ 16V ±1 µA Switch Maximum On-Time tON (max) V+ = 12V 24 32 40 µs Switch Minimum Off-Time tOFF (min) V+ = 12V 0.8 1.1 1.8 µs EXT Rise Time CEXT = 0.001µF, V+ = 12V 25 ns EXT Fall Time CEXT = 0.001µF, V+ = 12V 25 ns Maximum Duty Cycle tON x 100% tON + tOFF 96.5 % 95 __________________________________________Typical Operating Characteristics (TA = +25°C, unless otherwise noted.) 74 I+ (μA) 2.5 I+ (μA) 50 tRISE & tFALL (ns) V+ = 16V V+ = 10V 72 2.0 1.5 V+ = 8V 70 1.0 0.5 -60 -40 -20 0 20 40 60 80 100 120 140 EXT RISE AND FALL TIMES vs. TEMPERATURE (5nF) 180 160 V+ = 5V, tFALL 140 120 V+ = 15V, tRISE 100 80 60 40 90 VOUT = 5V CIRCUIT OF FIGURE 1 TEMPERATURE (°C) -60 -40 -20 0 20 40 60 80 100 120 140 EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V) 80 70 TOP TO BOTTOM: VIN = 6V VIN = 8V VIN = 10V VIN = 12V VIN = 15V 60 50 V+ = 15V, tFALL TEMPERATURE (°C) V+ = 15V, tFALL -60 -40 -20 0 20 40 60 80 100 120 140 1 100 1k 10 LOAD CURRENT (mA) VOUT = 3.3V CIRCUIT OF FIGURE 1 90 80 TOP TO BOTTOM: 70 VIN = 4.3V VIN = 5V VIN = 8V VIN = 10V VIN = 12V VIN = 15V 60 50 40 0.1 100 EFFICIENCY (%) tRISE & tFALL (ns) 200 20 40 60 80 100 120 140 MAX1649/51-A1 V+ = 5V, tRISE 100 EFFICIENCY (%) CEXT = 5nF V+ = 15V, tRISE EFFICIENCY vs. LOAD CURRENT (VOUT = 5V) MAX1649/51-02 220 30 TEMPERATURE (°C) TEMPERATURE (°C) 240 V+ = 5V, tFALL 35 15 0 -60 -40 -20 0 40 20 V+ = 4V 66 V+ = 5V, tRISE 45 25 V+ = 4V 68 CEXT = 1nF 55 3.0 76 MAX1649/51-01 3.5 10k MAX1649/51-A2 V+ = 16V 60 MAX1649-TOC05 78 4.0 MAX1649-TOC06 80 EXT RISE AND FALL TIMES vs. TEMPERATURE (1nF) SHUTDOWN CURRENT vs. TEMPERATURE SUPPLY CURRENT vs. TEMPERATURE 40 0.1 1 100 1k 10 LOAD CURRENT (mA) _______________________________________________________________________________________ 10k 3 MAX1649/MAX1651 ELECTRICAL CHARACTERISTICS (continued) ____________________________Typical Operating Characteristics (continued) (TA = +25°C, unless otherwise noted.) SWITCH OFF-TIME vs. TEMPERATURE 33.5 1.4 99 1.3 33.0 tOFF (μs) 32.5 32.0 DUTY CYCLE (%) 1.2 1.1 1.0 0.9 31.5 0.8 31.0 98 97 96 95 0.7 30.5 94 0.6 30.0 0.5 93 -60 -40 -20 0 20 40 60 80 100 120 140 -60 -40 -20 0 20 40 60 80 100 120 140 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) TEMPERATURE (°C) TEMPERATURE (°C) 110 105 100 400 VOUT = 4.80V 300 VOUT = 3.17V 200 100 0 95 0 -60 -40 -20 0 20 40 60 80 100 120 140 0.5 1.0 1.5 LOAD CURRENT (A) REFERENCE OUTPUT RESISTANCE vs. TEMPERATURE REFERENCE OUTPUT VOLTAGE vs. TEMPERATURE 150 IREF = 50μA 100 50 IREF = 100μA 2.0 MAX1649-TOC01 1.506 REFERENCE OUTPUT VOLTAGE (V) IREF = 10μA 200 MAX1649-TOC07 TEMPERATURE (°C) 250 REFERENCE OUTPUT RESISTANCE (Ω) CIRCUIT OF FIGURE 1 500 DROPOUT VOLTAGE (mV) 115 CS TRIP LEVEL (mV) 600 MAX1649/51-06 120 MAX1649/51-A3 DROPOUT VOLTAGE vs. LOAD CURRENT CS TRIP LEVEL vs. TEMPERATURE 1.504 1.502 IREF = 10μA 1.500 1.498 1.496 1.494 1.492 0 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) 4 100 MAX1649/51-04 1.5 MAX1649/51-03 34.0 MAXIMUM DUTY CYCLE vs. TEMPERATURE MAX1649/51-05 SWITCH ON-TIME vs. TEMPERATURE tON (μs) MAX1649/MAX1651 5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC Controllers -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) _______________________________________________________________________________________ 5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC Controllers MAX1649 LOAD-TRANSIENT RESPONSE MAX1649 LINE-TRANSIENT RESPONSE A A 16V 1.6A B B 6V 0A 200μs/div CIRCUIT OF FIGURE 1, V+ = 10V A: VOUT = 5V, 100mV/div, AC-COUPLED B: ILOAD = 30mA TO 1.6A, 1A/div 5ms/div CIRCUIT OF FIGURE 1, ILOAD = 1A A: VOUT = 5V, 100mV/div, AC-COUPLED B: V+ = 6V TO 16V, 5V/div MAX1649 SHDN RESPONSE TIME 5V OUTPUT 0V 4V SHDN INPUT 0V 1ms/div CIRCUIT OF FIGURE 1, V+ = 10V, ILOAD = 1A _______________________________________________________________________________________ 5 MAX1649/MAX1651 ____________________________Typical Operating Characteristics (continued) (TA = +25°C, unless otherwise noted.) MAX1649/MAX1651 5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC Controllers ______________________________________________________________Pin Description PIN NAME FUNCTION 1 OUT Sense Input for fixed 5V or 3.3V output operation. OUT is internally connected to the on-chip voltage divider. Although it is connected to the output of the circuit, OUT does not supply current. Leave OUT unconnected for adjustable-output operation. 2 FB Feedback Input. Connect to GND for fixed-output operation. Connect a resistor divider between OUT, FB, and GND for adjustable-output operation. See Setting the Output Voltage section. 3 SHDN 4 REF 1.5V Reference Output that can source 100µA. Bypass with 0.1µF. 5 V+ Positive Power-Supply Input 6 CS Current-Sense Input. Connect current-sense resistor between V+ and CS. When the voltage across the resistor equals the current-limit trip level, the external MOSFET is turned off. 7 EXT Gate Drive for External P-Channel MOSFET. EXT swings between V+ and GND. 8 GND Ground Active-High Shutdown Input. Part is placed in shutdown when SHDN is driven high. In shutdown mode, the reference, output, and external MOSFET are turned off. Connect to GND for normal operation. VIN V+ MAX1649 MAX1651 CS 3 4 SHDN EXT REF OUT FB C3 0.1μF 2 5 C4 0.1μF C1 100μF R1 0.05Ω 6 7 1 P1 Si9430* OUTPUT @ 1.5A L1 47μH** GND 8 D1 NSQ03A02L C2 330μF *SILICONIX SURFACE-MOUNT MOSFET **SUMIDA CDRH125-470 Figure 1. Typical Application Circuit _______________Detailed Description The MAX1649/MAX1651 are BiCMOS, step-down, switch-mode power-supply controllers that provide adjustable and fixed outputs of 5V and 3.3V, respectively. Their unique control scheme combines the advantages of pulse-frequency-modulation (low supply current) and pulse-width-modulation (high efficiency at high loads). An external P-channel power MOSFET allows peak currents in excess of 3A, increasing the output current capability over previous PFM devices. Figure 2 is the block diagram. 6 The MAX1649/MAX1651 offer four main improvements over prior solutions: 1) The converters operate with miniature surface-mount inductors, due to their 300kHz switching frequency. 2) The current-limited PFM control scheme allows greater than 90% efficiencies over a wide range of load currents (10mA to 1.5A). 3) Dropout voltage has been reduced to less than 300mV for many applications. 4) The quiescent supply current is only 100µA. PFM Control Scheme The MAX1649/MAX1651 use a proprietary, current-limited PFM control scheme. As with traditional PFM converters, the external power MOSFET is turned on when the voltage comparator senses that the output is out of regulation. However, unlike traditional PFM converters, switching is accomplished through the combination of a peak current limit and a pair of one-shots that set the maximum switch on-time (32µs) and minimum switch off-time (1.1µs). Once off, the off-time one-shot holds the switch off for 1.1µs. After this minimum time, the switch either 1) stays off if the output is in regulation, or 2) turns on again if the output is out of regulation. The MAX1649/MAX1651 also limit the peak inductor current, which allows them to run in continuous-conduction mode and maintain high efficiency with heavy loads (Figure 3). This current-limiting feature is a key component of the control circuitry. Once turned on, the switch stays on until either 1) the maximum on-time one-shot turns it off (32µs later), or 2) the current limit is reached. EXT swings from V+ to GND and provides the drive output for an external P-channel power MOSFET. _______________________________________________________________________________________ 5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC Controllers MAX1649/MAX1651 V+ FB DUAL-MODE™ COMPARATOR MAX1649 MAX1651 50mV OUT SHDN ERROR COMPARATOR REF 1.5V REFERENCE N Q MINIMUM OFF-TIME TRIG ONE-SHOT FROM V+ S EXT Q F/F MAXIMUM TRIG ON-TIME Q ONE-SHOT R CURRENT COMPARATOR CS 110mV FROM V+ GND Figure 2. Block Diagram Shutdown Mode Quiescent Current When SHDN is high, the MAX1649/MAX1651 enter shutdown mode. In this mode, the internal biasing circuitry is turned off (including the reference) and the supply current drops to less than 5µA. EXT goes high, turning off the external MOSFET. SHDN is a logic-level input. Connect SHDN to GND for normal operation. In normal operation, the device's typical quiescent current is 78µA. In an actual application, even with no load, additional current is drawn to supply external feedback resistors (if used) and the diode and capacitor leakage currents. In the circuit of Figure 1, with V+ at 5V and VOUT at 3.3V, typical no-load supply current for the entire circuit is 90µA. Dual Mode is a trademark of Maxim Integrated Products, Inc. _______________________________________________________________________________________ 7 MAX1649/MAX1651 5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC Controllers VIN V+ MAX1649 MAX1651 CS 1.5A 1A 3 0A 4 SHDN EXT REF OUT GND ( VOUT R2 = R3 –1 VREF ) C1 100μF R1 0.05Ω 6 P1 Si9430 7 L1 47μH OUTPUT @ 1.5A 1 2 R2 8 C3 0.1μF 2μs/div V+ = 10V, ILOAD = 1.3A CIRCUIT OF FIGURE 1, R1 = 75mΩ FB C4 0.1μF 5 C2 330μF D1 1N5820 R3 150k VREF = 1.5V Figure 3. MAX1649 Continuous-Conduction Mode, Heavy Load-Current Waveform (500mA/div) Modes of Operation When delivering high output currents, the MAX1649/ MAX1651 operate in continuous-conduction mode. In this mode, current always flows in the inductor, and the control circuit adjusts the switch duty cycle to maintain regulation without exceeding the switch current capability (Figure 3). This provides excellent load-transient response and high efficiency. In discontinuous-conduction mode, current through the inductor starts at zero, rises to a peak value, then ramps down to zero. Although efficiency is still excellent, the output ripple increases slightly, and the switch waveform exhibits ringing (at the inductor's self-resonant frequency). This ringing is to be expected and poses no operational problems. Dropout The MAX1649/MAX1651 are in dropout when the input voltage (V+) is low enough that the output drops below the minimum output voltage specification (see Electrical Characteristics). The dropout voltage is the difference between the input and output voltage when dropout occurs. See the Typical Operating Characteristics for the Dropout Voltage vs. Load Current and Dropout Voltage vs. Temperature graphs. 8 Figure 4. Adjustable-Output Operation __________________Design Procedure Setting the Output Voltage The MAX1649/MAX1651 are preset for 5V and 3.3V output voltages, respectively; tie FB to GND for fixed-output operation. They may also be adjusted from 1.5V (the reference voltage) to the input voltage, using external resistors R2 and R3 configured as shown in Figure 4. For adjustable-output operation, 150kΩ is recommended for resistor R3—high enough to avoid wasting energy, yet low enough to avoid RC delays caused by parasitic capacitance at FB. R2 is given by: VOUT R2 = R3 x ——— -1 VREF ( ) where VREF = 1.5V. When using external resistors, it does no harm to connect OUT and the output together, or to leave OUT unconnected. Current-Sense Resistor Selection The current-sense resistor limits the peak switch current to 110mV/RSENSE, where RSENSE is the value of the current-sense resistor, and 110mV is the currentlimit trip level (see Electrical Characteristics). _______________________________________________________________________________________ 5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC Controllers 2.5 rs = 0.030 2.0 rs = 0.040 rs = 0.050 1.5 rs = 0.060 1.0 rs = 0.080 0.5 rs = 0.100 0 where ΔI is the inductor-current overshoot factor, ILIM = VCS/RSENSE, and 0.3µs is the time it takes the comparator to switch. Set ΔI = 0.1 for an overshoot of 10%. For highest efficiency, use a coil with low DC resistance; a value smaller than 0.1V/ILIM works best. To minimize radiated noise, use a toroid, pot core, or shielded-bobbin inductor. Inductors with a ferrite core or equivalent are recommended. Make sure the inductor’s saturation-current rating is greater than ILIM(max). However, it is generally acceptable to bias the inductor into saturation by about 20% (the point where the inductance is 20% below its nominal value). 3.0 VOUT = 3.3V 2.5 rs = 0.030 2.0 rs = 0.040 1651 Fig05b VOUT = 5V 1649 Fig05a MAXIMUM OUTPUT CURRENT (A) 3.0 (V+(max) - VOUT) x 0.3µs L(min) = ——————————––—— ΔI x ILIM MAXIMUM OUTPUT CURRENT (A) Inductor Selection The MAX1649/MAX1651 operate with a wide range of inductor values, although for most applications coils between 10µH and 68µH take best advantage of the con- trollers’ high switching frequency. With a high inductor value, the MAX1649/MAX1651 will begin continuous-current operation (see Detailed Description) at a lower fraction of full-load current. In general, smaller values produce higher ripple (see below) while larger values require larger size for a given current rating. In both the continuous and discontinuous modes, the lower limit of the inductor is important. With a too-small inductor value, the current rises faster and overshoots the desired peak current limit because the current-limit comparator has a finite response time (300ns). This reduces efficiency and, more importantly, could cause the current rating of the external components to be exceeded. Calculate the minimum inductor value as follows: rs = 0.050 1.5 rs = 0.060 1.0 rs = 0.080 0.5 rs = 0.100 0 5.0 5.4 5.8 6.2 6.6 16.0 INPUT VOLTAGE (V) Figure 5a. MAX1649 Current-Sense Resistor Graph 3.0 3.4 3.8 4.2 4.6 16.0 INPUT VOLTAGE (V) Figure 5b. MAX1651 Current-Sense Resistor Graph _______________________________________________________________________________________ 9 MAX1649/MAX1651 To maximize efficiency and reduce the size and cost of external components, minimize the peak current. However, since the available output current is a function of the peak current, the peak current must not be too low. To choose the proper current-sense resistor for a particular output voltage, determine the minimum input voltage and the maximum load current. Next, referring to Figures 5a or 5b, using the minimum input voltage, find the curve with the largest sense resistor that provides sufficient output current. It is not necessary to perform worst-case calculations. These curves take into account the sense-resistor (±5%) and inductor (47µH ±10%) values, the diode drop (0.4), and the IC’s current-sense trip level (85mV); an external MOSFET on-resistance of 0.07Ω is assumed for VGS = -5V. Standard wire-wound and metal-film resistors have an inductance high enough to degrade performance. Surface-mount (chip) resistors have very little inductance and are well suited for use as current-sense resistors. A U-shaped wire resistor made by IRC works well in through-hole applications. Because this resistor is a band of metal shaped as a “U”, its inductance is less than 10nH (an order of magnitude less than metal film resistors). Resistance values between 5mΩ and 0.1Ω are available (see Table 1). MAX1649/MAX1651 5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC Controllers Table 1. Component Selection Guide PRODUCTION METHOD Surface Mount Miniature Through-Hole Low-Cost Through-Hole INDUCTORS CAPACITORS Sumida CDRH125-470 (1.8A) CDRH125-220 (2.2A) Coilcraft DO3316-473 (1.6A) DO3340-473 (3.8A) CURRENT-SENSE RESISTORS MOSFETS Siliconix Little Foot series AVX TPS series Motorola MBRS340T3 Dale WSL Series Sprague 595D series Nihon NSQ series IRC LRC series Motorola medium-power surface-mount products IRC OAR series Motorola Sumida RCH875-470M (1.3A) Sanyo OS-CON series low-ESR organic semiconductor Coilcraft PCH-45-473 (3.4A) Nichicon PL series Motorola low-ESR electrolytics 1N5817 to 1N5823 United Chemi-Con LXF series The peak current of Figure 1 is 2.35A for a 1.5A output. The inductor used in this circuit is specified to drop by 10% at 2.2A (worst case); a curve provided by the manufacturer shows that the inductance typically drops by 20% at 2.7A. Using a slightly underrated inductor can sometimes reduce size and cost, with only a minor impact on efficiency. Table 1 lists inductor types and suppliers for various applications. The efficiencies of the listed surfacemount inductors are nearly equivalent to those of the larger size through-hole versions. Diode Selection The MAX1649/MAX1651’s high switching frequency demands a high-speed rectifier. Schottky diodes, such as the 1N5817 through 1N5823 (and their surfacemount equivalents), are recommended. Choose a diode with an average current rating equal to or greater than I LIM (max) and a voltage rating higher than V+(max). External Switching Transistor The MAX1649/MAX1651 drive P-channel enhancementmode MOSFET transistors only. The choice of power transistor is primarily dictated by the input voltage and the peak current. The transistor’s on-resistance, gatesource threshold, and gate charge must also be appropriately chosen. The drain-to-source and gate-tosource breakdown voltage ratings must be greater than V+. The total gate-charge specification is normally not 10 DIODES Motorola TMOS power MOSFETs critical, but values should be less than 100nC for best efficiency. The MOSFET should be capable of handling the peak current and, for maximum efficiency, have a very low on-resistance at that current. Also, the onresistance must be low for the minimum available VGS, which equals V+(min). Select a transistor with an onresistance between 50% and 100% of the currentsense resistor. The Si9430 transistor chosen for the Typical Operating Circuit has a drain-to-source rating of -20V and a typical on-resistance of 0.070Ω at 2A with VGS = -4.5V. Tables 1 and 2 list suppliers of switching transistors suitable for use with these devices. Capacitor Selection Output Filter Capacitor The primary criterion for selecting the output filter capacitor is low equivalent series resistance (ESR), rather than high capacitance. An electrolytic capacitor with low enough ESR will automatically have high enough capacitance. The product of the inductor-current variation and the output filter capacitor’s ESR determines the amplitude of the high-frequency ripple seen on the output voltage. When a 330µF, 10V Sprague surface-mount capacitor (595D series) with ESR = 0.15Ω is used, 40mV of output ripple is typically observed when stepping down from 10V to 5V at 1A. The output filter capacitor's ESR also affects efficiency. Again, low-ESR capacitors perform best. Table 1 lists some suppliers of low-ESR capacitors. ______________________________________________________________________________________ 5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC Controllers COMPANY PHONE FAX AVX USA Coiltronics Coilcraft Dale International Rectifier IRC USA USA USA (207) 282-5111 or (800) 282-4975 (516) 241-7876 (708) 639-6400 (402) 564-3131 USA (310) 322-3331 (310) 322-3332 USA (512) 992-3377 Siliconix USA Sprague USA USA Japan (512) 992-7900 (602) 244-3576 or (602) 244-5303 (708) 843-7500 81-7-5231-8461 (805) 867-2555 81-3-3494-7411 (619) 661-6835 81-7-2070-6306 (408) 988-8000 or (800) 554-5565 (603) 224-1961 (708) 956-0666 81-3-3607-5111 USA (714) 255-9500 Motorola Nichicon Nihon Sanyo Sumida United Chemi-Con USA USA Japan USA Japan USA Japan (207) 283-1941 (516) 241-9339 (708) 639-1469 (402) 563-1841 (602) 244-4015 (708) 843-2798 81-7-5256-4158 (805) 867-2556 81-3-3494-7414 (619) 661-1055 81-7-2070-1174 (408) 970-3950 (603) 224-1430 (708) 956-0702 81-3-3607-5144 Proper PC board layout is essential because of high current levels and fast switching waveforms that radiate noise. Minimize ground noise by connecting the anode of the rectifier, the input bypass capacitor ground lead, and the output filter capacitor ground lead to a single point (“star” ground configuration). A ground plane is recommended. Also minimize lead lengths to reduce stray capacitance, trace resistance, and radiated noise. In particular, the traces connected to FB (if an external resistor divider is used) and EXT must be short. Place the 0.1µF ceramic bypass capacitor as close as possible to the V+ and GND pins. MAX1649/MAX1651 vs. MAX649/MAX651 The MAX1649 and MAX1651 are pin compatible with the MAX649 and MAX651, but have been optimized for improved dropout performance and efficiency—particularly with low input voltages. The MAX1649/MAX1651 feature increased maximum switch duty cycle (96.5%) and reduced current-limit sense voltage (110mV). Their predecessors, the MAX649/MAX651, use a higher two-step (210mV/110mV) current-limit sense voltage to provide tighter current-sense accuracy and reduced inductor peak current at light loads. ___________________Chip Topography (714) 255-9400 GND OUT Input Bypass Capacitor The input bypass capacitor reduces peak currents drawn from the voltage source, and also reduces the amount of noise at the voltage source caused by the switching action of the MAX1649/MAX1651. The input voltage source impedance determines the size of the capacitor required at the V+ input. As with the output filter capacitor, a low-ESR capacitor is recommended. Bypass the IC separately with a 0.1µF ceramic capacitor placed close to the V+ and GND pins. EXT FB 0.106" (2.692mm) CS SHDN Reference Capacitor Bypass REF with a 0.1µF or larger capacitor. REF V+ Package Information For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. PACKAGE TYPE PACKAGE CODE DOCUMENT NO. 8 PDIP P8-2 21-0041 8 SO S8-4 21-0043 0.081" (2.057mm) TRANSISTOR COUNT: 428 SUBSTRATE CONNECTED TO V+ ______________________________________________________________________________________ 11 MAX1649/MAX1651 Layout Considerations Table 2. Component Suppliers MAX1649/MAX1651 5V/3.3V or Adjustable, High-Efficiency, Low-Dropout, Step-Down DC-DC Controllers Revision History REVISION NUMBER REVISION DATE 3 3/09 DESCRIPTION Corrected Output Voltage conditions and Figure 1 title PAGES CHANGED 2, 6 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 © 2009 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.
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