19-0225; Rev 3; 9/97 IT K ATION EVALU BLE AVAILA 5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers ________________________Applications 5V-to-3.3V Green PC Applications High-Efficiency Step-Down Regulation Minimum-Component DC-DC Converters ____________________________Features ♦ ♦ ♦ ♦ ♦ ♦ ♦ More than 90% Efficiency (10mA to 1.5A Loads) More than 12.5W Output Power 100µA Max Quiescent Supply Current 5µA Max Shutdown Supply Current Less than 1.0V Dropout Voltage 16.5V Max Input Voltage 5V (MAX649), 3.3V (MAX651), 3V (MAX652), or Adjustable Output Voltage ♦ Current-Limited Control Scheme ♦ Up to 300kHz Switching Frequency ______________Ordering Information PART MAX649CPA TEMP. RANGE 0°C to +70°C MAX649CSA MAX649C/D MAX649EPA MAX649ESA MAX649MJA 0°C to +70°C 0°C to +70°C -40°C to +85°C -40°C to +85°C -55°C to +125°C PIN-PACKAGE 8 Plastic DIP 8 SO Dice* 8 Plastic DIP 8 SO 8 CERDIP** Ordering Information continued at end of data sheet. * Dice are tested at TA = +25°C. **Contact factory for availability and processing to MIL-STD-883. Battery-Powered Applications __________Typical Operating Circuit INPUT 4V TO 16.5V __________________Pin Configuration TOP VIEW V+ MAX651 ON/OFF SHDN CS EXT OUT REF FB OUT 1 8 GND FB 2 7 EXT 6 CS 5 V+ SHDN 3 P OUTPUT 3.3V REF 4 MAX649 MAX651 MAX652 DIP/SO GND ________________________________________________________________ Maxim Integrated Products 1 For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800. For small orders, phone 1-800-835-8769. MAX649/MAX651/MAX652 _______________General Description The MAX649/MAX651/MAX652 BiCMOS, step-down DCDC switching controllers provide high efficiency over three decades of load current. A unique, current-limited 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). The result is high efficiency over loads ranging from 10mA to more than 2.5A. These devices use miniature external components. Their high switching frequency (up to 300kHz) allows for less than 9mm diameter surface-mount inductors. The MAX649/MAX651/MAX652 have dropout voltages less than 1V and accept input voltages up to 16.5V. Output voltages are preset at 5V (MAX649), 3.3V (MAX651), and 3V (MAX652). These controllers 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 10W. 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. MAX649/MAX651/MAX652 5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, 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 CERDIP (derate 8.00mW/°C above +70°C) ..................640mW Operating Temperature Ranges MAX649C_A, MAX65_C_A ..................................0°C to +70°C MAX649E_A, MAX65_E_A ................................-40°C to +85°C MAX649MJA, MAX65_MJA ............................-55°C to +125°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 IQ FB Trip Point FB Input Current Output Voltage Reference Voltage IFB VOUT VREF CONDITIONS V+ = 16.5V, SHDN ≤ 0.4V (operating, switch off) 80 V+ = 16.5V, SHDN ≥ 1.6V (shutdown) 4 V+ = 10V, SHDN ≥ 1.6V (shutdown) 2 MAX UNITS 16.5 V 100 µA 5 MAX649C, MAX65_C 1.470 1.5 1.530 MAX649E, MAX65_E 1.4625 1.5 1.5375 MAX649M, MAX65_M 1.455 1.5 1.545 MAX649C, MAX65_C ±50 MAX649E, MAX65_E ±70 MAX649M, MAX65_M ±90 MAX649, V+ = 6V to 16.5V 4.80 5.0 5.20 MAX651, V+ = 4V to 16.5V 3.17 3.3 3.43 MAX652, V+ = 4V to 16.5V 2.88 3.0 3.12 MAX649C, MAX65_C, IREF = 0 1.470 1.5 1.530 MAX649E, MAX65_E, IREF = 0 1.4625 1.5 1.5375 MAX649M, MAX65_M, IREF = 0 1.455 1.5 1.545 MAX649C/E, MAX65_C/E 4 10 MAX649M, MAX65_M 4 15 40 100 Circuit of Figure 1 0 ≤ IREF ≤ 100µA, sourcing only REF Line Regulation 4V ≤ V+ ≤ 16.5V 2 TYP 4.0 REF Load Regulation Output Voltage Line Regulation MIN Circuit of Figure 1 MAX649, 6V ≤ V+ ≤ 16V, ILOAD = 1A 2.6 MAX651, 4.5V ≤ V+ ≤ 16V, ILOAD = 1A 1.7 MAX652, 4V ≤ V+ ≤ 16V, ILOAD = 1A 1.9 _______________________________________________________________________________________ V nA V V mV µV/V mV/V 5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers (V+ = 5V, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL Output Voltage Load Regulation CONDITIONS Circuit of Figure 1 Circuit of Figure 1 Efficiency SHDN Input Current TYP -47 MAX651, 0 ≤ ILOAD ≤ 1.5A, VIN = 5V -45 MAX652, 0 ≤ ILOAD ≤ 1.5A, VIN = 5V -45 MAX649, V+ = 10V, ILOAD = 1A 92 MAX651, V+ = 5V, ILOAD = 1A 89 MAX652, V+ = 5V, ILOAD = 1A 88 V+ = 16.5V, SHDN = 0V or V+ SHDN Input Voltage High VIH 4V ≤ V+ ≤ 16.5V SHDN Input Voltage Low VIL 4V ≤ V+ ≤ 16.5V Current-Limit Trip Level (V+ to CS) MIN MAX649, 0 ≤ ILOAD ≤ 1.5A, VIN = 10V VCS 4V ≤ V+ ≤ 16.5V % 1 MAX649C/E, MAX65_C/E 180 210 240 MAX649M, MAX65_M 160 210 260 tON (max) V+ = 12V 12 Switch Minimum Off-Time tOFF (min) V+ = 12V 1.8 µA V 0.4 Switch Maximum On-Time UNITS mV/A 1.6 4V ≤ V+ ≤ 16.5V CS Input Current MAX V mV ±1 µA 16 20 µs 2.3 2.8 µs EXT Rise Time CEXT = 0.001µF, V+ = 12V 50 ns EXT Fall Time CEXT = 0.001µF, V+ = 12V 50 ns _______________________________________________________________________________________ 3 MAX649/MAX651/MAX652 ELECTRICAL CHARACTERISTICS (continued) __________________________________________Typical Operating Characteristics (TA = +25°C, unless otherwise noted.) SHUTDOWN CURRENT vs. TEMPERATURE 3.5 3.0 V+ = 16.5V 2.5 74 I+ (mA) I+ (mA) 76 V+ = 10V 72 2.0 1.5 V+ = 8V 70 1.0 V+ = 4V 68 0.5 2500 MAX649-A03 V+ = 16.5V MAX649-A02 78 4.0 MAX649-A01 80 MAX649 MAXIMUM LOAD CURRENT vs. SUPPLY VOLTAGE MAXIMUM LOAD CURRENT (mA) SUPPLY CURRENT vs. TEMPERATURE 2000 1500 1000 500 VOUT = 5V CIRCUIT OF FIGURE 1 V+ = 4V 0 20 40 60 80 100 120 140 0 -60 -40 -20 0 TEMPERATURE (°C) MAX649 EFFICIENCY vs. LOAD CURRENT INPUT VOLTAGE (V) MAX652 EFFICIENCY vs. LOAD CURRENT 90 80 100 MAX649-A05 100 MAX649-A04 90 90 80 80 50 40 30 60 50 40 30 20 VOUT = 5V 0 100µ 1m 10m 100m 20 VOUT = 3.3V VOUT = 3V 0 100µ 1m 10m 100m SWITCH ON-TIME vs. TEMPERATURE SWITCH OFF-TIME vs. TEMPERATURE 2.5 MAX649-A07 40 10 LOAD CURRENT (A) V+ = 5V 50 20 LOAD CURRENT (A) 17 60 10 0 1 TOP TO BOTTOM: VIN = 4.3V VIN = 5V VIN = 8V VIN = 10V VIN = 12V VIN = 15V 70 30 V+ = 5V 100µ 1 1m 10m 100m 1 LOAD CURRENT (A) SWITCH ON-TIME/OFF-TIME RATIO vs. TEMPERATURE 8.0 MAX649-A08 60 TOP TO BOTTOM: VIN = 4.3V VIN = 5V VIN = 8V VIN = 10V VIN = 12V VIN = 15V 70 EFFICIENCY (%) TOP TO BOTTOM: VIN = 6V VIN = 8V VIN = 10V VIN = 12V VIN = 15V 70 EFFICIENCY (%) EFFICIENCY (%) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 1415 MAX651 EFFICIENCY vs. LOAD CURRENT 100 10 20 40 60 80 100 120 140 TEMPERATURE (°C) MAX649-A06 -60 -40 -20 0 MAX649-A9 66 V+ = 5V 7.8 7.6 16 tON/tOFF RATIO tOFF (ms) 7.4 tON (ms) MAX649/MAX651/MAX652 5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers 2.0 7.2 7.0 6.8 6.6 6.4 6.2 15 -60 -40 -20 0 20 40 60 TEMPERATURE (°C) 4 6.0 1.5 80 100 120 -60 -40 -20 0 20 40 60 TEMPERATURE (°C) 80 100 120 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) _______________________________________________________________________________________ 5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers 450 110 tRISE & tFALL (ns) 90 80 V+ = 5V, tFALL 70 60 300 V+ = 12V, tRISE V+ = 5V, tFALL 200 V+ = 12V, tRISE 50 350 250 1000 150 40 30 V+ = 12V, tFALL 20 40 60 80 100 120 140 DROPOUT VOLTAGE vs. TEMPERATURE CS TRIP LEVEL vs. TEMPERATURE MAX652 800 MAX651 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 LOAD CURRENT (A) REFERENCE OUTPUT VOLTAGE vs. TEMPERATURE 1.506 1.504 225 700 220 215 210 205 200 195 ILOAD = 1A CIRCUIT OF FIGURE 1 1.502 1.500 1.498 1.496 1.494 190 600 1.492 185 -60 -40 -20 0 0 MAX649-A14 230 CS TRIP LEVEL (mV) 900 235 MAX649-A13 MAX649 400 300 20 40 60 80 100 120 140 TEMPERATURE (°C) 1000 500 0 -60 -40 -20 0 TEMPERATURE (°C) 1100 MAX651, VOUT = 3.3V 600 100 V+ = 12V, tFALL 50 -60 -40 -20 0 800 700 200 100 20 MAX649, VOUT = 5V MAX652, VOUT = 3V 900 REFRENCE OUTPUT (V) tRISE & tFALL (ns) V+ = 5V, t RISE 400 100 20 40 60 80 100 120 140 -60 -40 -20 0 TEMPERATURE (°C) 20 40 60 80 100 120 140 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) TEMPERATURE (°C) REFERENCE OUTPUT RESISTANCE vs. TEMPERATURE 250 REFRENCE OUTPUT RESISTANCE (Ω) DROPOUT VOLTAGE (mV) CEXT = 5nF MAX649-A12 500 MAX649-A15 V+ = 5V, tRISE MAX649-A11 CEXT = 1nF MAX649-A16 120 MAX649-A10 130 DROPOUT VOLTAGE vs. LOAD CURRENT EXT RISE AND FALL TIMES vs. TEMPERATURE (5nF) DROPOUT VOLTAGE (mV) EXT RISE AND FALL TIMES vs. TEMPERATURE (1nF) IREF = 10µA 200 150 IREF = 50µA 100 50 IREF = 100µA 0 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) _______________________________________________________________________________________ 5 MAX649/MAX651/MAX652 ____________________________Typical Operating Characteristics (continued) (TA = +25°C, unless otherwise noted.) MAX649/MAX651/MAX652 5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers _____________________________Typical Operating Characteristics (continued) MAX649 LOAD-TRANSIENT RESPONSE MAX649 LINE-TRANSIENT RESPONSE A A B B 250µs/div 250µs/div ILOAD = 1A A: INPUT VOLTAGE (7V & 12V), 5V/div B: 5V OUT, AC COUPLED, 100mV/div A: LOAD CURRENT (100mA & 1A), 500mA/div B: 5V OUTPUT VOLTAGE, AC COUPLED, 50mV/div MAX649 SHUTDOWN RESPONSE TIME A B 1ms/div ILOAD = 1A A: SHDN INPUT VOLTAGE (0V & 5V), 2V/div B: 5V OUTPUT VOLTAGE, 2V/div ______________________________________________________________Pin Description PIN 6 NAME FUNCTION 1 OUT Sense input for fixed 5V, 3.3V, or 3V output operation. OUT is internally connected to the on-chip voltage divider. Although it is connected to the output of the circuit, the OUT pin does not supply current. 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 Active-high TTL/CMOS logic-level input. Part is placed in shutdown when SHDN is driven high. In shutdown mode, the reference and the external MOSFET are turned off, and OUT = 0V. Connect to GND for normal operation. 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 _______________________________________________________________________________________ 5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers V+ 4 CS SHDN EXT REF OUT FB C3 0.1µF C1 100µF R1 0.1Ω MAX649 MAX651 MAX652 3 C4 0.1µF 5 6 P1 Si9430* 7 OUTPUT @ 1.5A L1 22µH** 1 GND 2 8 D1 NSQ03A02L C2 330µF *SILICONIX SURFACE-MOUNT MOSFET **SUMIDA CDR125-220 Figure 1. Test Circuit _______________Detailed Description The MAX649/MAX651/MAX652 are BiCMOS, stepdown, switch-mode power-supply controllers that provide fixed outputs of 5V, 3.3V, and 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. The MAX649/MAX651/MAX652 offer three main improvements over prior solutions: 1) The converters operate with tiny (less than 9mm diameter) 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 (1.0mA to 1.5A). 3) The maximum supply current is only 100µA. PFM Control Scheme The MAX649/MAX651/MAX652 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 oneshots that set the maximum switch on-time (16µs) and minimum switch off-time (2.3µs). Once off, the minimum off-time one-shot holds the switch off for 2.3µs. 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 MAX649/MAX651/MAX652 also limit the peak inductor current, which allows them to run in continuous-conduction mode and maintain high efficiency with heavy loads (Figure 3a). 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 (16µs later), or 2) the current limit is reached. To increase light-load efficiency, the current limit for the first two pulses is set to half the peak current limit. If those pulses bring the output voltage into regulation, the voltage comparator holds the MOSFET off and the current limit remains at half its peak. If the output voltage is still out of regulation after two pulses, the current limit for the next pulse is raised to its peak (Figure 3b). Calculate the peak current limit by dividing the Current-Limit Trip Level (see Electrical Characteristics) by the value of the current-sense resistor. Shutdown Mode When SHDN is high, the MAX649/MAX651/MAX652 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 TTL/CMOS logic-level input. Connect SHDN to GND for normal operation. Quiescent Current In normal operation, the quiescent current is less than 100µA. However, this current is measured by forcing the external transistor switch off. 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, the typical quiescent current is 90µA. EXT Drive Voltage Range EXT swings from V+ to GND and provides the drive output for an external P-channel power MOSFET. Modes of Operation When delivering high output currents, the MAX649/ MAX651/MAX652 operate in continuous-conduction mode (CCM). In this mode, current always flows in the _______________________________________________________________________________________ 7 MAX649/MAX651/MAX652 VIN MAX649/MAX651/MAX652 5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers V+ FB DUAL-MODE™ COMPARATOR SHDN MAX649 MAX651 MAX652 ERROR COMPARATOR OUT REF 1.5V REFERENCE N Q MINIMUM OFF-TIME TRIG ONE-SHOT FROM V+ S MAXIMUM TRIG ON-TIME Q ONE-SHOT EXT Q R CURRENT COMPARATOR CS CURRENT CONTROL CIRCUITS 0.2V (FULL CURRENT) 0.1V (HALF CURRENT) FROM V+ GND Figure 2. Block Diagram 8 _______________________________________________________________________________________ 5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers MAX649/MAX651/MAX652 2.5A 1.5A 2.0A 1A 1.5A 1.0A 0A 0.5A 0A 5µs/div 2µs/div V+ = 10V, ILOAD = 1.3A CIRCUIT OF FIGURE 1, R1 = 150mΩ V+ = 10V, ILOAD = 1.4A CIRCUIT OF FIGURE 1, R1 = 100mΩ Figure 3a. MAX649 Continuous-Conduction Mode, Heavy Load-Current Waveform (500mA/div) inductor, and the control circuit adjusts the switch duty cycle to maintain regulation without exceeding the switch current capability (Figure 3a). This provides excellent load-transient response and high efficiency. In discontinuous-conduction mode (DCM), 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 waveforms exhibit ringing (the self-resonant frequency of the inductor). This ringing is to be expected and poses no operational problems. Figure 3b. MAX649 Light/Medium Load-Current Waveform (500mA/div) VIN V+ MAX649 MAX651 MAX652 3 4 The MAX649/MAX651/MAX652 are said to be 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. CS EXT REF OUT GND FB 6 7 ( VOUT R2 = R3 –1 VREF ) P1 Si9430 L1 22µH OUTPUT @ 1.5A 1 2 R2 8 C3 0.1µF C1 100µF R1 0.1Ω SHDN Dropout C4 0.1µF 5 C2 330µF D1 1N5820 R3 150k VREF = 1.5V Figure 4. Adjustable-Output Operation _______________________________________________________________________________________ 9 RS = 0.07Ω 2.0 RS = 0.08Ω 1.5 RS = 0.10Ω RS = 0.12Ω 1.0 RS = 0.14Ω 0.5 RS = 0.06Ω MAX649 VOUT = 5V 5 6 7 RS = 0.07Ω 2.0 RS = 0.08Ω 1.5 RS = 0.10Ω 1.0 RS = 0.12Ω RS = 0.14Ω 0.5 MAX651 VOUT = 3.3V 0 0 3 4 2.5 MAX649-A26 3.0 MAXIMUM OUTPUT CURRENT (A) RS = 0.06Ω 2.5 MAX649-A25 3.0 MAXIMUM OUTPUT CURRENT (A) 8 9 10 11 12 13 14 15 16 3 4 5 INPUT VOLTAGE (V) 6 7 8 9 10 11 12 13 14 15 16 INPUT VOLTAGE (V) Figure 5a. MAX649 Current-Sense Resistor Graph Figure 5b. MAX651 Current-Sense Resistor Graph Setting the Output Voltage The MAX649/MAX651/MAX652 are preset for 5V, 3.3V, and 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. 150kΩ is a good value—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 210mV/RSENSE, where RSENSE is the value of the current-sense resistor, and 210mV is the currentlimit trip level (see Electrical Characteristics). 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. 10 3.0 RS = 0.06Ω 2.5 MAX649-A27 __________________Design Procedure MAXIMUM OUTPUT CURRENT (A) MAX649/MAX651/MAX652 5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers RS = 0.07Ω 2.0 RS = 0.08Ω RS = 0.10Ω 1.5 RS = 0.12Ω 1.0 RS = 0.14Ω 0.5 MAX652 VOUT = 3.0V 0 3 4 5 6 7 8 9 10 11 12 13 14 15 16 INPUT VOLTAGE (V) Figure 5c. MAX652 Current-Sense Resistor Graph 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, 5b, or 5c, 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 worst-case values for sense resistor (±5%), inductor (22µH ±10%), diode drop (0.6V), and the IC’s current-sense trip level; an external MOSFET on-resistance of 0.13Ω is assumed for VGS = -4.5V. ______________________________________________________________________________________ 5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers Inductor Selection Practical inductor values range from 10µH to 50µH or more. The circuit operates in discontinuous-conduction mode if: VOUT x (R + 1) VD V + ≤ ———————— + —— + VSW R R R, the switch on-time/off-time ratio, equals 6.7. VD is the diode’s drop, and VSW is the voltage drop across the P-channel FET. To get the full output capability in discontinuous-conduction mode, choose an inductor value no larger than: RSENSE x 12µs x (V+ - VSW - VOUT) L(max) = ————————————————— VCS where VCS is the current-sense voltage. In both the continuous and discontinuous modes, the lower limit of the inductor is more important. With a small inductor value, the current rises faster and overshoots the desired peak current limit because the current-limit comparator cannot respond fast enough. This reduces efficiency slightly and, more importantly, could cause the current rating of the external components to be exceeded. Calculate the minimum inductor value as follows: (V+(max) - VSW - VOUT) x 0.3µs L(min) = ————————————––—— ∆I x ILIM(min) where ∆I is the percentage of inductor-current overshoot, where ILIM = VCS/RSENSE and 0.3µs is the time it takes the comparator to switch. An overshoot of 10% is usually not a problem. Inductance values above the minimum work well if the maximum value defined above is not exceeded. Smaller inductance values cause higher output ripple because of overshoot. Larger values tend to produce physically larger coils. For highest efficiency, use a coil with low DC resistance; a value smaller than 0.1V/I LIM 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 induc- tor’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). 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 3.1A. Using a slightly underrated inductor can sometimes reduce size and cost, with only a minor impact on efficiency. The MAX649/MAX651/MAX652 current limit prevents any damage from an underrated inductor’s low inductance at high currents. 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 MAX649/MAX651/MAX652’s high switching frequency demands a high-speed rectifier (commonly called a catch diode when used in switching-regulator circuits). Schottky diodes, such as the 1N5817 through 1N5822 families (and their surface-mount equivalents), are recommended. Choose a diode with an average current rating equal to or greater than ILIM(max) and a voltage rating higher than V+(max). For high-temperature applications, where Schottky diodes can be inadequate because of high leakage currents, use high-speed silicon diodes instead. At heavy loads and high temperatures, the disadvantages of a Schottky diode’s high leakage current may outweigh the benefits of its low forward voltage. Table 1 lists diode types and suppliers for various applications. External Switching Transistor The MAX649/MAX651/MAX652 drive P-channel enhancement-mode MOSFET transistors only. The choice of power transistor is primarily dictated by the input voltage and the peak current. The transistor's on-resistance, gate-source threshold, and gate capacitance must also be appropriately chosen. The drain-to-source and gate-to-source breakdown voltage ratings must be greater than V+. The total gate-charge specification is normally not 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 on-resistance must be low for the minimum available VGS , which equals V+(min). Select a transistor with an on-resistance between 50% and 100% of the current-sense resistor. The Si9430 transistor chosen for the Typical Operating Circuit has ______________________________________________________________________________________ 11 MAX649/MAX651/MAX652 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 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). MAX649/MAX651/MAX652 5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers a drain-to-source rating of -20V and a typical on-resistance of 0.115Ω 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 ESR of the output filter capacitor 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 = 0.15Ωis 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. Use low-ESR capacitors for best performance. The smallest low-ESR SMT tantalum capacitors currently available are from the Sprague 595D series. Sanyo OSCON organic semiconductor through-hole capacitors and the Nichicon PL series also exhibit very low ESR. Table 1 lists some suppliers of low-ESR capacitors. amount of noise at the voltage source caused by the switching action of the MAX649/MAX651/MAX652. 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. Reference Capacitor Bypass REF with a 0.1µF or larger capacitor. REF can source at least 100µA. Layout Considerations 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 catch diode, 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 V+ and GND. Input Bypass Capacitor The input bypass capacitor reduces peak currents drawn from the voltage source, and also reduces the Table 1. Component Selection Guide PRODUCTION METHOD INDUCTORS CAPACITORS Sumida Matsuo CDR125-220 (22µH) 267 series Surface Mount Miniature Through-Hole Low-Cost Through-Hole 12 DIODES CURRENT-SENSE RESISTORS MOSFETS Siliconix Little Foot series Nihon NSQ series Coiltronics CTX 100 series Sprague 595D series Sumida RCH855-220M Sanyo OS-CON series low-ESR organic semiconductor Renco RL 1284-22 Nichicon PL series Motorola low-ESR electrolytics 1N5820, 1N5823 United Chemi-Con LXF series IRC LRC series IRC OAR series Motorola medium-power surface-mount products Motorola Motorola TMOS power MOSFETs ______________________________________________________________________________________ 5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers MAX649/MAX651/MAX652 Table 2. Component Suppliers COMPANY PHONE FAX Coiltronics USA (407) 241-7876 (407) 241-9339 Harris USA (800) 442-7747 (407) 724-3937 International Rectifier USA (310) 322-3331 (310) 322-3332 IRC USA (704) 264-8861 (704) 264-8866 Matsuo USA Japan (714) 969-2491 81-6-337-6450 (714) 960-6492 81-6-337-6456 Motorola USA (800) 521-6274 (602) 244-4015 Nichicon USA Japan (708) 843-7500 81-7-5231-8461 (708) 843-2798 81-7-5256-4158 Nihon USA Japan (805) 867-2555 81-3-3494-7411 (805) 867-2556 81-3-3494-7414 Renco USA (516) 586-5566 (516) 586-5562 Sanyo USA Japan (619) 661-6835 81-7-2070-6306 (619) 661-1055 81-7-2070-1174 Siliconix USA (408) 988-8000 (408) 970-3950 Sprague USA (603) 224-1961 (603) 224-1430 Sumida USA Japan (708) 956-0666 81-3-3607-5111 (708) 956-0702 81-3-3607-5144 United Chemi-Con USA (714) 255-9500 (714) 255-9400 __Ordering Information (continued) TEMP. RANGE PIN-PACKAGE MAX651CPA PART 0°C to +70°C 8 Plastic DIP MAX651CSA MAX651C/D MAX651EPA MAX651ESA MAX651MJA MAX652CPA 0°C to +70°C 0°C to +70°C -40°C to +85°C -40°C to +85°C -55°C to +125°C 0°C to +70°C 8 SO Dice* 8 Plastic DIP 8 SO 8 CERDIP** 8 Plastic DIP MAX652CSA MAX652C/D MAX652EPA MAX652ESA MAX652MJA 0°C to +70°C 0°C to +70°C -40°C to +85°C -40°C to +85°C -55°C to +125°C 8 SO Dice* 8 Plastic DIP 8 SO 8 CERDIP** * Dice are tested at TA = +25°C. **Contact factory for availability and processing to MIL-STD-883. ___________________Chip Topography GND OUT EXT FB 0.109" (2.769mm) CS SHDN REF V+ 0.080" (2.032mm) TRANSISTOR COUNT: 442; SUBSTRATE CONNECTED TO V+. ______________________________________________________________________________________ 13 PDIPN.EPS ________________________________________________________Package Information SOICN.EPS MAX649/MAX651/MAX652 5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers 14 ______________________________________________________________________________________ 5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers CDIPS.EPS ______________________________________________________________________________________ 15 MAX649/MAX651/MAX652 ___________________________________________Package Information (continued) MAX649/MAX651/MAX652 5V/3.3V/3V or Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Controllers NOTES 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. 16 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 © 1997 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.