19-1075; Rev 0; 6/96 NUAL KIT MA ATION SHEET A EVALU T A D WS FOLLO 5V/3.3V or Adjustable, 100% Duty-Cycle, High-Efficiency, Step-Down DC-DC Controllers ________________________Applications PCMCIA Power Supplies Personal Digital Assistants Hand-Held Computers Portable Terminals Low-Cost Notebook Computer Supplies 5V to 3.3V Green PC Applications High-Efficiency Step-Down Regulation Minimum-Component DC-DC Converters Battery-Powered Applications ____________________________Features ♦ Low Dropout Voltage ♦ 100% Maximum Duty Cycle ♦ Soft-Start Limits Start-Up Current ♦ Efficiency >90% (3mA to 2A Loads) ♦ Output Power >12.5W ♦ 90µA Max Quiescent Current ♦ 1µA Max Shutdown Current ♦ Up to 300kHz Switching Frequency ♦ 16.5V Max Input Voltage ♦ Output Voltage: 5V/3.3V (MAX1626) Adjustable (MAX1627) ♦ Current-Limited Control Scheme ______________Ordering Information TEMP. RANGE PART PIN-PACKAGE MAX1626C/D 0°C to +70°C Dice* MAX1626ESA MAX1627C/D MAX1627ESA -40°C to +85°C 0°C to +70°C -40°C to +85°C 8 SO Dice* 8 SO * Dice are tested at TA = +25°C. __________Typical Operating Circuit INPUT 3.3V to 16.5V __________________Pin Configuration V+ TOP VIEW MAX1626 OUT 1 3/5 (FB) 2 SHDN 3 MAX1626 MAX1627 REF 4 8 GND 7 EXT 6 CS 5 V+ ON/OFF SHDN CS EXT 3/5 P OUTPUT 3.3V OUT REF GND SO ( ) ARE FOR MAX1627 ________________________________________________________________ Maxim Integrated Products 1 For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800 MAX1626/MAX1627 _______________General Description The MAX1626/MAX1627 step-down DC-DC switching controllers provide high efficiency over loads ranging from 1mA to more than 2A. A unique current-limited, pulse-frequency-modulated (PFM) control scheme operates with up to a 100% duty cycle, resulting in very low dropout voltages. This control scheme eliminates minimum load requirements and reduces the supply current under light loads to 90µA (versus 2mA to 10mA for common pulse-width modulation controllers). These step-down controllers drive an external P-channel MOSFET, allowing design flexibility for applications to 12W or higher. Soft-start reduces current surges during start-up. A high switching frequency (up to 300kHz) and operation in continuous-conduction mode allow the use of tiny surface-mount inductors. Output capacitor requirements are also reduced, minimizing PC board area and system costs. The output voltage is preset at 5V or 3.3V for the MAX1626 and adjustable for the MAX1627. Input voltages can be up to 16.5V. The MAX1626/MAX1627 are functional upgrades for the MAX1649/MAX1651. MAX1626/MAX1627 5V/3.3V or Adjustable, 100% Duty-Cycle, High-Efficiency, Step-Down DC-DC Controllers ABSOLUTE MAXIMUM RATINGS Supply Voltage, V+ to GND.......................................-0.3V, +17V OUT, FB, 3/5, SHDN, REF, CS, EXT to GND ...-0.3V, (V+ + 0.3V) Maximum Current at REF (IREF) ..........................................15mA Maximum Current at EXT (IEXT) ..........................................50mA Continuous Power Dissipation (TA = +70°C) SO (derate 5.88mW/°C above +70°C) ..........................471mW Operating Temperature Range MAX1626ESA/MAX1627ESA ............................-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+ = +3V to +16.5V, SHDN = 3/5 = 0V, TA = 0°C to +85°C, unless otherwise noted.) PARAMETER Input Voltage Range Supply Current into V+ SYMBOL CONDITIONS V+ I+ MIN Operating, no load 70 V+ = SHDN = 16.5V (shutdown) VOUT OUT Input Current IOUT V 90 2.7 2.8 4.85 5.00 5.15 Circuit of Figure 1, 3/5 = 0V (Note 1) 3.20 3.30 3.40 MAX1626, 3/5 = V+, output forced to 5V MAX1627, includes hysteresis FB Leakage Current MAX1627 CS Input Current 37 50 1.33 V 35 nA 0 100 10 µA mV V SHDN = 0V or V+ 0.4 V ±1 µA V+ - 0.5 V 3/5 Input Voltage Low 3/5 = 0V or V+ EXT Resistance V+ = 5V Minimum EXT Off Time 0.5 V ±1 µA Ω 10 Output forced to 0V 8 10 12 Output in regulation 1.5 2.0 2.5 EXT Duty-Cycle Limit 100 Line Regulation 6.0V < V+ < 12.0V, ILOAD = 1A Load Regulation ILOAD = 0µA mV/V 15 1.27 µs % 5 30mA < ILOAD < 2.0A, V+ = 8V VREF µA 115 1.6 3/5 Leakage Current V 1.30 85 3/5 Input Voltage High V 24 SHDN Input Voltage Low SHDN Input Current µA 1.27 0 VCS SHDN Input Voltage High Reference Voltage UNITS 16.5 Circuit of Figure 1, 3/5 = V+ (Note 1) FB Threshold Voltage CS Threshold Voltage MAX 1 Undervoltage Lockout Output Voltage TYP 3.0 mV/A 1.30 1.33 V REF Load Regulation 0µA ≤ IREF ≤ 100µA 4 10 mV REF Line Regulation V+ = 3V to 16.5V, ILOAD = 0µA 10 100 µV/V 2 _______________________________________________________________________________________ 5V/3.3V or Adjustable, 100% Duty-Cycle, High-Efficiency, Step-Down DC-DC Controllers MAX1626/MAX1627 ELECTRICAL CHARACTERISTICS (V+ = +3V to +16.5V, SHDN = 3/5 = 0V, TA = -40°C to +85°C, unless otherwise noted.) (Note 2) PARAMETER SYMBOL Input Voltage CONDITIONS MIN V+ Supply Current into V+ TYP 3.0 IOUT Operating, no load MAX UNITS 16.5 V 100 V+ = SHDN = 16.5V (shutdown) µA 2 Undervoltage Lockout 2.9 Output Voltage VOUT OUT Input Current IOUT Circuit of Figure 1, 3/5 = V+ 4.80 5.20 Circuit of Figure 1, 3/5 = 0V 3.16 3.44 MAX1626, 3/5 = V+, output forced to 5V FB Threshold Voltage MAX1627, includes hysteresis FB Leakage Current MAX1627 ILOAD = 0µA V 24 50 1.25 1.35 V 0 50 nA 80 120 mV 1.25 1.35 V CS Threshold Voltage Reference V µA Note 1: V+ must exceed VOUT to maintain regulation. Note 2: Specifications from 0°C to -40°C are guaranteed by design, not production tested. __________________________________________Typical Operating Characteristics (Circuit of Figure 1, TA = +25°C, unless otherwise noted.) 5V SETTING VOUT = +4.8V 0.15 MAX1626-05 A 70 D E F 60 50 40 0.10 20 0.05 10 0 0 0.5 1.0 1.5 LOAD (A) 90 2.0 2.5 A B C 80 30 0 100 EFFICIENCY (%) 0.25 0.20 C B 80 3.3V SETTING VOUT = +3.17V 0.30 90 EFFICIENCY (%) DROPOUT VOLTAGE (V) 100 MAX1626-11 0.45 0.40 0.35 EFFICIENCY vs. LOAD CURRENT (VOUT = +5V) EFFICIENCY vs. LOAD CURRENT (VOUT = +3.3V) MAX1626-03 DROPOUT VOLTAGE vs. LOAD CURRENT CIRCUIT OF FIGURE 1 0.1m 1m 10m A: V+ = +4.3V B: V+ = +5V C: V+ = +8V D: V+ = +10V E: V+ = +12V F: V+ = +15V 70 50 40 LOAD CURRENT (A) 1 A: V+ = +6V B: V+ = +8V C: V+ = +10V D: V+ = +12V E: V+ = +15V 30 20 10 0 100m D E 60 10 CIRCUIT OF FIGURE 1 0.1m 1m 10m 100m 1 10 LOAD CURRENT (A) _______________________________________________________________________________________ 3 ____________________________Typical Operating Characteristics (continued) (Circuit of Figure 1, TA = +25°C, unless otherwise noted.) MAX1626 EXT OFF TIME vs. OUTPUT VOLTAGE 0.4 0.3 B C D MAX1626-02 10 6 3/5 = V+ 4 8 6 4 3/5 = GND 2 0.1 0 2 0 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) 0 0 1 2 3 4 5 tRISE AND tFALL (ns) 35 tRISE, V+ = +5V 30 25 20 tRISE, tFALL, V+ = +15V 15 10 5 MAX1626-10 400 CEXT = 1nF 3/5 = 0V OUT = 50kHz, 0.3Vp-p, 3.3VDC 300 250 tRISE, V+ = +15V 200 tFALL, V+ = +5V 150 tRISE, V+ = +5V 50 tFALL, V+ = +15V 0 0 0 20 40 60 80 100 120 140 2000 MAX1626 V+ QUIESCENT CURRENT vs. TEMPERATURE CS TRIP LEVEL vs. TEMPERATURE IQ (µA) V+ = +10V 66 62 60 OUT = 0V 110 CS TRIP LEVEL (mV) V+ = +16V 68 64 115 MAX1626-01 70 4000 CAPACITANCE (pF) TEMPERATURE (°C) 72 0.6 V+ = +4V 105 100 95 90 3/5 = 0V OUT FORCED TO 3.4V -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) 0.8 1.0 FB PIN VOLTAGE (V) 100 -60 -40 -20 0 0.4 350 tRISE AND tFALL (ns) tFALL, V+ = +5V 40 0.2 EXT RISE AND FALL TIMES vs. CAPACITANCE MAX1626-09 50 45 0 OUTPUT VOLTAGE (V) EXT RISE AND FALL TIMES vs. TEMPERATURE 4 V+ = +5V 10 8 E 0.2 12 MAX1626-12 0.5 V+ = +5V EXT OFF TIME (µs) 0.6 12 EXT OFF TIME (µs) 0.7 A APPLICATION CIRCUIT SHUTDOWN CURRENT: A: V+ = +15V B: V+ = +10V C: V+ = +4V MAX1626 SHUTDOWN CURRENT: D: V+ = +16V E: V+ = +4V MAX1626-04 0.8 MAX1627 EXT OFF TIME vs. FB PIN VOLTAGE MAX1626-03 MAX1626 SHUTDOWN CURRENT vs. TEMPERATURE SHUTDOWN CURRENT (µA) MAX1626/MAX1627 5V/3.3V or Adjustable, 100% Duty-Cycle, High-Efficiency, Step-Down DC-DC Controllers 85 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) _______________________________________________________________________________________ 1.2 1.4 5V/3.3V or Adjustable, 100% Duty-Cycle, High-Efficiency, Step-Down DC-DC Controllers REFERENCE OUTPUT VOLTAGE vs. TEMPERATURE MAX1626-14 1.305 MAX1626-13 REFERENCE OUTPUT VOLTAGE (V) 1.310 MAX1626 SHUTDOWN RESPONSE TIME AND SUPPLY CURRENT A IREF = 0µA IREF = 10µA 1.300 1.295 B IREF = 50µA 1.290 IREF = 100µA 1.285 C 1.280 20 40 60 80 100 120 140 500µs/div TEMPERATURE (°C) V+ = 8V, VOUT = 5V, LOAD = 1A A: OUT, 2V/div B: SUPPLY CURRENT, 1A/div C: SHDN, 5V/div LINE-TRANSIENT RESPONSE FROM 100% DUTY CYCLE MAX1626 LINE-TRANSIENT RESPONSE MAX1626-15 MAX1626-16 MAX1626 LOAD-TRANSIENT RESPONSE A MAX1626-17 -60 -40 -20 0 A A B B B 100µs/div V+ = 8V, VOUT = 3.3V, LOAD = 30mA to 2A A: OUT, 50mV/div, 3.3V DC OFFSET B: LOAD CURRENT, 1A/div 5ms/div VOUT = 5V, LOAD = 1A, CIN = 33µF A: OUT, 100mV/div, 5V DC OFFSET B: V+ 6V to 12V, 2V/div 5ms/div VOUT = 3.3V, LOAD = 1A, CIN = 47µF A: OUT, 100mV/div, 3.3V DC OFFSET B: V+ 3.3V to 15V, 5V/div _______________________________________________________________________________________ 5 MAX1626/MAX1627 ____________________________Typical Operating Characteristics (continued) (Circuit of Figure 1, TA = +25°C, unless otherwise noted.) MAX1626/MAX1627 5V/3.3V or Adjustable, 100% Duty-Cycle, High-Efficiency, 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 an on-chip voltage divider (MAX1626). It does not supply current. Leave OUT unconnected during adjustable-output operation (MAX1627). — 2 FB Feedback Input for adjustable-output operation. Connect to an external voltage divider between the output and GND (see the Setting the Output Voltage section). 2 — 3/5 3.3V or 5V Selection. Output voltage is set to 3.3V when this pin is low or 5V when it is high. 3 3 SHDN Active-High Shutdown Input. Device 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. 4 4 REF 1.3V Reference Output. Can source 100µA. Bypass with 0.1µF. 5 5 V+ Positive Supply Input. Bypass with 0.47µF. 6 6 CS Current-Sense Input. Connect current-sense resistor between V+ and CS. External MOSFET is turned off when the voltage across the resistor equals the current-limit trip level (around 100mV). 7 7 EXT Gate Drive for External P-Channel MOSFET. EXT swings between V+ and GND. 8 8 GND Ground MAX1626 MAX1627 1 EXT INPUT C2 68µF LOW-ESR TANTALUM C3 68µF LOW-ESR TANTALUM REF 1.5V C5 0.47µF MAX1626 MAX1627 REF ERROR COMPARATOR OUT MINIMUM ON-TIME ONE-SHOT TRIG Q R2 V+ 3/5 CS SHDN REF C4 0.1µF R1 U1 LOGIC-LEVEL MOSFET EXT GND (FB) Q TRIG MINIMUM OFF-TIME ONE-SHOT RSENSE 0.04Ω MAX1626 P L1 22µH, 3A OUT 3/5 OUTPUT S Q R C1 220µF LOW-ESR TANTALUM D1 CURRENT-SENSE COMPARATOR L1: SUMIDA CDRH125-220 D1: NIHON NSQ03A03 U1: MORTOLA MMSF3PO2HD Figure 1. MAX1626 Typical Operating Circuit 6 R3 SHDN V+ CS ( ) MAX1627 ONLY MAX1626 ONLY Figure 2. Simplified Functional Diagram _______________________________________________________________________________________ 5V/3.3V or Adjustable, 100% Duty-Cycle, High-Efficiency, Step-Down DC-DC Controllers PFM Control Scheme The MAX1626/MAX1627 use a proprietary, third-generation, current-limited PFM control scheme. Improvements include a reduced current-sense threshold and operation to a 100% duty cycle. These devices pulse only as needed to maintain regulation, resulting in a variable switching frequency that increases with the load. This eliminates the current drain associated with constant-frequency pulsewidth-modulation (PWM) controllers, caused by switching the MOSFET unnecessarily. When the output voltage is too low, the error comparator sets a flip-flop, which turns on the external P-channel MOSFET and begins a switching cycle (Figures 1 and 2). As shown in Figure 3, current through the inductor ramps up linearly, storing energy in a magnetic field while dumping charge into an output capacitor and servicing the load. When the MOSFET is turned off, the magnetic field collapses, diode D1 turns on, and the current through the inductor ramps back down, transferring the stored energy to the output capacitor and load. The output capacitor stores energy when the inductor current is high and releases it when the inductor current is low. The MAX1626/MAX1627 use a unique feedback and control system to govern each pulse. When the output voltage is too low, the error comparator sets a flip-flop, which turns on the external P-channel MOSFET. The MOSFET turns off when the current-sense threshold is exceeded or when the output voltage is in regulation. A one-shot enforces a 2µs minimum on-time, except in current limit. The flip-flop resets when the MOSFET turns off. Otherwise the MOSFET remains on, allowing a duty cycle of up to 100%. This feature ensures the lowest possible dropout. Once the MOSFET is turned off, the minimum off-time comparator keeps it off. The minimum off-time is normally 2µs, except when the output is significantly out of regulation. If the output is low by 30% or more, the minimum off-time increases, allowing soft-start. The error comparator has 0.5% hysteresis for improved noise immunity. In the MAX1626, the 3/5 pin selects the output voltage (Figure 2). In the MAX1627, external feedback resistors at FB adjust the output. Operating Modes When delivering low and medium output currents, the MAX1626/MAX1627 operate in discontinuous-conduction mode. Current through the inductor starts at zero, rises as high as the peak current limit set by the current- sense resistor, then ramps down to zero during each cycle (Figure 3). Although efficiency is still excellent, output ripple increases and the switch waveform exhibits ringing. This ringing occurs at the resonant frequency of the inductor and stray capacitance, due to residual energy trapped in the core when the commutation diode (D1 in Figure 1) turns off. It is normal and poses no operational problems. When delivering high output currents, the MAX1626/ MAX1627 operate in continuous-conduction mode (Figure 4). In this mode, current always flows through the inductor and never ramps to zero. The control circuit adjusts the switch duty cycle to maintain regulation without exceeding the peak switching current set by the current-sense resistor. This provides reduced output ripple and high efficiency. 100% Duty Cycle and Dropout The MAX1626/MAX1627 operate with a duty cycle up to 100%. This feature extends usable battery life by turning the MOSFET on continuously when the supply voltage approaches the output voltage. This services the load when conventional switching regulators with less than 100% duty cycle would fail. Dropout voltage is defined as the difference between the input and output voltages when the input is low enough for the output to drop out of regulation. Dropout depends on the MOSFET drain-to-source on-resistance, current-sense resistor, and inductor series resistance, and is proportional to the load current: Dropout Voltage = [ IOUT x RDS(ON) + RSENSE + RINDUCTOR ] _______________________________________________________________________________________ 7 MAX1626/MAX1627 _______________Detailed Description The MAX1626/MAX1627 are step-down DC-DC controllers designed primarily for use in portable computers and battery-powered devices. Using an external MOSFET and current-sense resistor allows design flexibility and the improved efficiencies associated with high-performance P-channel MOSFETs. A unique, current-limited, pulse-frequency-modulated (PFM) control scheme gives these devices excellent efficiency over load ranges up to three decades, while drawing around 90µA under no load. This wide dynamic range optimizes the MAX1626/MAX1627 for battery-powered applications, where load currents can vary considerably as individual circuit blocks are turned on and off to conserve energy. Operation to a 100% duty cycle allows the lowest possible dropout voltage, extending battery life. High switching frequencies and a simple circuit topology minimize PC board area and component costs. Figure 1 shows a typical operating circuit for the MAX1626. MAX1626/MAX1627 5V/3.3V or Adjustable, 100% Duty-Cycle, High-Efficiency, Step-Down DC-DC Controllers A A B B C 0A C 0A 10µs/div 10µs/div CIRCUIT OF FIGURE 1, V+ = 8V, VOUT = 5V, LOAD = 100mA A: MOSFET DRAIN, 5V/div B: OUT, 50mV/div, 5V DC OFFSET C: INDUCTOR CURRENT, 1A/div Figure 3. Discontinuous-Conduction Mode, Light-Load-Current Waveform EXT Drive Voltage Range EXT swings from V+ to GND and provides the gate drive for an external P-channel power MOSFET. A higher supply voltage increases the gate drive to the MOSFET and reduces on-resistance (R DS(ON)). See External Switching Transistor section. Quiescent Current The device’s typical quiescent current is 70µA. However, actual applications draw additional current to supply MOSFET switching currents, OUT pin current, or external feedback resistors (if used), and both the diode and capacitor leakage currents. For example, in the circuit of Figure 1, with V+ at 7V and VOUT at 5V, typical no-load supply current for the entire circuit is 84µA. When designing a circuit for high-temperature operation, select a Schottky diode with low reverse leakage. Shutdown Mode When SHDN is high, the device enters shutdown mode. In this mode, the feedback and control circuit, reference, and internal biasing circuitry are turned off. EXT goes high, turning off the external MOSFET. The shutdown supply current drops to less than 1µA. SHDN is a logiclevel input. Connect SHDN to GND for normal operation. Reference The 1.3V reference is suitable for driving external loads, such as an analog-to-digital converter. It has a guaranteed 10mV maximum load regulation while sourcing load currents up to 100µA. The reference is turned off during 8 CIRCUIT OF FIGURE 1, V+ = 8V, VOUT = 5V, LOAD = 1.5A A: MOSFET DRAIN, 5V/div B: OUT, 50mV/div, 5V DC OFFSET C: INDUCTOR CURRENT, 1A/div Figure 4. Continuous-Conduction Mode, Heavy-Load-Current Waveform shutdown. Bypass the reference with 0.1µF for normal operation. Place the bypass capacitor within 0.2 inches (5mm) of REF, with a direct trace to GND (Figure 7). Soft-Start Soft-start reduces stress and transient voltage slumps on the power source. When the output voltage is near ground, the minimum off-time is lengthened to limit peak switching current. This compensates for reduced negative inductor current slope due to low output voltages. ________________Design Information Setting the Output Voltage The MAX1626’s output voltage can be selected to 3.3V or 5V under logic control by using the 3/5 pin. The 3/5 pin requires less than 0.5V to ensure a 3.3V output, or more than (V+ - 0.5)V to guarantee a 5V output. The voltage sense pin (OUT) must be connected to the output for the MAX1626. The MAX1627’s output voltage is set using two resistors, R2 and R3 (Figure 5), which form a voltage divider between the output and GND. R2 is given by: V R2 = R3 x OUT − 1 V REF where VREF = 1.3V. Since the input bias current at FB has a maximum value of 50nA, large values (10kΩ to 200kΩ) can be used for R3 with no significant accuracy loss. For 1% error, the current through R2 should be at _______________________________________________________________________________________ 5V/3.3V or Adjustable, 100% Duty-Cycle, High-Efficiency, Step-Down DC-DC Controllers FROM OUTPUT R2 CR2 Current-Sense-Resistor Selection The current-sense comparator limits the peak switching current to VCS/RSENSE, where RSENSE is the value of the current-sense resistor and VCS is the current-sense threshold. VCS is typically 100mV, but can range from 85mV to 115mV. Minimizing the peak switching current will increase efficiency and reduce the size and cost of external components. However, since available output current is a function of the peak switching current, the peak current limit must not be set too low. Set the peak current limit above 1.3 times the maximum load current by setting the current-sense resistor to: RCS = VCS(MIN) TO FB R3 Figure 5. Adjustable-Output Operation Using the MAX1627 sense resistors. Power metal-strip resistors feature 1/2W and 1W power dissipation, 1% tolerance, and inductance below 5nH. Resistance values between 10mΩ and 500mΩ are available. 1.3 x IOUT(MAX) Inductor Selection Alternatively, select the current-sense resistor for 5V and 3.3V output applications using the current-sense resistor graphs in Figures 6a and 6b. The current-sense resistor’s power rating should be 20% higher than: RPOWER RATING (W) = V2CS(MAX) The essential parameters for inductor selection are inductance and current rating. The MAX1626/MAX1627 operate with a wide range of inductance values. In many applications, values between 10µH and 68µH take best advantage of the controller’s high switching frequency. Calculate the minimum inductance value as follows: RCS Standard wire-wound resistors have an inductance high enough to degrade performance, and are not recommended. Surface-mount (chip) resistors have very little inductance and are well suited for use as current- L(MIN) = - VOUT ) x 2µs VCS(MIN) RCS where 2µs is the minimum on-time. Inductor values between two and six times L(MIN) are recommended. 3.5 3.5 VOUT = 5V VOUT = 3.3V RSENSE = 0.03Ω MAXIMUM OUTPUT CURRENT (A) MAXIMUM OUTPUT CURRENT (A) (V+(MAX) 3.0 2.5 RSENSE = 0.04Ω 2.0 RSENSE = 0.05Ω 1.5 1.0 RSENSE = 0.1Ω 0.5 0 RSENSE = 0.03Ω 3.0 RSENSE = 0.04Ω 2.5 RSENSE = 0.05Ω 2.0 1.5 RSENSE = 0.1Ω 1.0 0.5 0 4.5 5.0 5.5 6.0 10 12 14 16 INPUT VOLTAGE (V) Figure 6a. MAX1626 5V-Operation Current-Sense Resistor Graph 3.0 3.5 4.0 4.5 10 12 14 16 INPUT VOLTAGE (V) Figure 6b. MAX1626 3.3V-Operation Current-Sense Resistor Graph _______________________________________________________________________________________ 9 MAX1626/MAX1627 least 100 times FB’s input bias current. Capacitor CR2 is used to compensate the MAX1627 for even switching. Values between 0pF and 330pF work for many applications. See the Stability and MAX1627 Feedback Compensation section for details. MAX1626/MAX1627 5V/3.3V or Adjustable, 100% Duty-Cycle, High-Efficiency, Step-Down DC-DC Controllers With high inductor values, the MAX1626/MAX1627 will begin continuous-conduction operation at a lower fraction of the full load (see Detailed Description). Low-value inductors may be smaller and less expensive, but they result in greater peak current overshoot due to currentsense comparator propagation delay. Peak-current overshoot reduces efficiency and could cause the external components’ current ratings to be exceeded. The inductor’s saturation and heating current ratings must be greater than the peak switching current to prevent overheating and core saturation. Saturation occurs when the inductor’s magnetic flux density reaches the maximum level the core can support, and inductance starts to fall. The heating current rating is the maximum DC current the inductor can sustain without overheating. The peak switching current is the sum of the current limit set by the current-sense resistor and overshoot during current-sense comparator propagation delay. IPEAK = or shielded-core inductor to minimize radiated noise. Table 1 lists inductor types and suppliers for various applications. External Switching Transistor The MAX1626/MAX1627 drive P-channel enhancementmode MOSFETs. The EXT output swings from GND to the voltage at V+. To ensure the MOSFET is fully on, use logic-level or low-threshold MOSFETs when the input voltage is less than 8V. Tables 1 and 2 list recommended suppliers of switching transistors. Four important parameters for selecting a P-channel MOSFET are drain-to-source breakdown voltage, current rating, total gate charge (Qg), and RDS(ON). The drain-to-source breakdown voltage rating should be at least a few volts higher than V+. Choose a MOSFET with a maximum continuous drain current rating higher than the peak current limit: (V + − VOUT ) × 1µs VCS + RCS L 1µs is the worst-case current-sense comparator propagation delay. Inductors with a core of ferrite, Kool Mu™, METGLAS™, or equivalent, are recommended. Powder iron cores are not recommended for use with high switching frequencies. For optimum efficiency, the inductor windings’ resistance should be on the order of the currentsense resistance. If necessary, use a toroid, pot-core, KOOL Mu is a trademark of Magnetics. METGLAS is a trademark of Allied Signal. ID(MAX) ≥ ILIM (MAX) = VCS(MAX) RSENSE The Qg specification should be less than 100nC to ensure fast drain voltage rise and fall times, and reduce power losses during transition through the linear region. Qg specifies all of the capacitances associated with charging the MOSFET gate. EXT pin rise and fall times vary with different capacitive loads, as shown in the Typical Operating Characteristics. RDS(ON) should be as low as practical to reduce power losses while the MOSFET is on. It should be equal to or less than the current-sense resistor. Table 1. Component Selection Guide PRODUCTION METHOD INDUCTORS Sumida CDRH125-470 (1.8A) CDRH125-220 (2.2A) Surface Mount Miniature Through-Hole Low-Cost Through-Hole CAPACITORS DIODES CURRENT-SENSE RESISTORS MOSFETS Siliconix Little Foot series AVX TPS series Motorola MBRS340T3 Dale WSL series CoilCraft DO3316-473 (1.6A) DO3340-473 (3.8A) Sprague 595D series Nihon NSQ series IRC LRC series Motorola medium-power surface-mount products Sumida RCH875-470M (1.3A) Sanyo OS-CON series low-ESR organic semiconductor IRC OAR series Motorola CoilCraft PCH-45-473 (3.4A) Nichicon PL series Motorola low-ESR electrolytics 1N5817 to 1N5823 United Chemi-Con Motorola TMOS power MOSFETs LXF series 10 ______________________________________________________________________________________ 5V/3.3V or Adjustable, 100% Duty-Cycle, High-Efficiency, Step-Down DC-DC Controllers COMPANY PHONE FAX AVX USA Coilcraft Coiltronics Dale International Rectifier IRC Motorola Nichicon USA USA USA (803) 946-0690 or (800) 282-4975 (847) 639-6400 (516) 241-7876 (605) 668-4131 USA (310) 322-3331 (310) 322-3332 USA USA USA Japan USA Japan USA Japan (512) 992-3377 (602) 994-6430 (847) 843-2798 81-7-5256-4158 (805) 867-2698 81-3-3494-7414 (619) 661-1055 81-7-2070-1174 (714) 255-9400 Siliconix USA Sprague Sumida USA USA Japan (512) 992-7900 (602) 303-5454 (847) 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 (847) 956-0666 81-3-3607-5111 USA (714) 255-9500 Nihon Sanyo United Chemi-Con (803) 626-3123 (847) 639-1469 (516) 241-9339 (605) 665-1627 (408) 970-3950 (603) 224-1430 (847) 956-0702 81-3-3607-5144 Diode Selection The MAX1626/MAX1627’s high switching frequency demands a high-speed rectifier. Schottky diodes, such as the 1N5817–1N5822 family or surface-mount equivalents, are recommended. Ultra-high-speed rectifiers with reverse recovery times around 50ns or faster, such as the MUR series, are acceptable. Make sure that the diode’s peak current rating exceeds the peak current limit set by RSENSE, and that its breakdown voltage exceeds V+. Schottky diodes are preferred for heavy loads due to their low forward voltage, especially in low-voltage applications. For high-temperature applications, some Schottky diodes may be inadequate due to their high leakage currents. In such cases, ultra-highspeed rectifiers are recommended, although a Schottky diode with a higher reverse voltage rating can often provide acceptable performance. Capacitor Selection Choose filter capacitors to service input and output peak currents with acceptable voltage ripple. Equivalent series resistance (ESR) in the capacitor is a major contributor to output ripple, so low-ESR capacitors are recommended. Sanyo OS-CON capacitors are best, and low-ESR tantalum capacitors are second best. Low-ESR aluminum electrolytic capacitors are tolerable, but do not use standard aluminum electrolytic capacitors. Voltage ripple is the sum of contributions from ESR and the capacitor value: VRIPPLE ≈ VRIPPLE,ESR + VRIPPLE,C To simplify selection, assume initially that two-thirds of the ripple results from ESR and one-third results from capacitor value. Voltage ripple as a consequence of ESR is approximated by: VRIPPLE,ESR ≈ (RESR )(IPEAK ) Estimate input and output capacitor values for given voltage ripple as follows: CIN = 1 LI2 2 ∆L VRIPPLE,CINVIN COUT = 1 LI2 2 ∆L VIN VRIPPLE,COUT VOUT VIN − VOUT where I∆L is the change in inductor current (around 0.5IPEAK under moderate loads). These equations are suitable for initial capacitor selection; final values should be set by testing a prototype or evaluation kit. When using tantalum capacitors, use good soldering practices to prevent excessive heat from damaging the devices and increasing their ESR. Also, ensure that the tantalum capacitors’ surge-current ratings exceed the start-up inrush and peak switching currents. Pursuing output ripple lower than the error comparator’s hysteresis (0.5% of the output voltage) is not practical, since the MAX1626/MAX1627 will switch as needed, until the output voltage crosses the hysteresis threshold. Choose an output capacitor with a working voltage rating higher than the output voltage. The input filter capacitor reduces peak currents drawn from the power source and reduces noise and voltage ripple on V+ and CS, caused by the circuit’s switching action. Use a low-ESR capacitor. Two smaller-value low-ESR capacitors can be connected in parallel for lower cost. Choose input capacitors with working voltage ratings higher than the maximum input voltage. Place a surface-mount ceramic capacitor very close to V+ and GND, as shown in Figure 7. This capacitor bypasses the MAX1626/MAX1627, and prevents spikes and ringing on the power source from obscuring the ______________________________________________________________________________________ 11 MAX1626/MAX1627 Table 2. Component Suppliers MAX1626/MAX1627 5V/3.3V or Adjustable, 100% Duty-Cycle, High-Efficiency, Step-Down DC-DC Controllers current feedback signal and causing jitter. 0.47µF is recommended. Increase the value as necessary in high-power applications. Bypass REF with 0.1µF. This capacitor should be placed within 0.2 inches (5mm) of the IC, next to REF, with a direct trace to GND (Figure 7). Layout Considerations High-frequency switching regulators are sensitive to PC board layout. Poor layout introduces switching noise into the current and voltage feedback signals, resulting in jitter, instability, or degraded performance. The currentsense resistor must be placed within 0.2 inches (5mm) of the controller IC, directly between V+ and CS. Place voltage feedback resistors (MAX1627) next to the FB pin (no more than 0.2") rather than near the output. Place the 0.47µF input and 0.1µF reference bypass capacitors within 0.2 inches (5mm) of V+ and REF, and route directly to GND. Figure 7 shows the recommended layout and routing for these components. High-power traces, highlighted in the Typical Operating Circuit (Figure 1), should be as short and as wide as possible. The supply-current loop (formed by C2, C3, RSENSE, U1, L1, and C1) and commutation-current loop (D1, L1, and C1) should be as tight as possible to reduce radiated noise. Place the anode of the commutation diode (D1) and the ground pins of the input and output filter capacitors close together, and route them to a common “star-ground” point. Place components and route ground paths so as to prevent high currents from causing large voltage gradients between the ground pin of the output filter capacitor, the controller IC, and the reference bypass capacitor. Keep the extra copper on the component and solder sides of the PC board, rather than etching it away, and connect it to ground for use as a pseudo-ground plane. Refer to the MAX1626 Evaluation Kit manual for a two-layer PC board example. Stability and MAX1627 Feedback Compensation Use proper PC board layout and recommended external components to ensure stable operation. In oneshot, sequenced PFM DC-DC converters, instability is manifested as “Motorboat Instability.” It is usually caused by excessive noise on the current or voltage feedback signals, ground, or reference, due to poor PC board design or external component selection. Motorboat instability is characterized by grouped switching pulses with large gaps and excessive lowfrequency output ripple. It is normal to see some grouped switching pulses during the transition from discontinuous to continuous current mode. This effect is associated with small gaps between pulse groups 12 yyyyy yyyyy yyyyy yyyy yyyyy yyyy yyyyy yyyy yy yyyyy yyyy yy yyyyy yyyy yy yyyyy yyyy yyyyy MAX1626 4x SCALE R SENSE C REF C V+ BYPASS Figure 7. Recommended Placement and Routing of the Current-Sense Resistor, 0.1µF Reference, and 0.47µF Input Bypass Capacitors and output ripple similar to or less than that seen during no-load conditions. Instability can also be caused by excessive stray capacitance on FB when using the MAX1627. Compensate for this by adding a 0pF to 330pF feed-forward capacitor across the upper feedback resistor (R2 in Figure 5). MAX1626/MAX1627 vs. MAX1649/MAX1651 vs. MAX649/MAX651 The MAX1626/MAX1627 are specialized, third-generation upgrades to the MAX649/MAX651 step-down controllers. They feature improved efficiency, a reduced current-sense threshold (100mV), soft-start, and a 100% duty cycle for lowest dropout. The MAX649/ MAX651 have a two-step (210mV/110mV) currentsense threshold. The MAX1649/MAX1651 are secondgeneration upgrades with a 96.5% maximum duty cycle for improved dropout performance and a reduced current-sense threshold (110mV) for higher efficiency, especially at low input voltages. The MAX1649/ MAX1651 are preferable for special applications where a 100% duty cycle is undesirable, such as flyback and SEPIC circuits. Since the MAX1626’s pinout is similar to those of the MAX649 and MAX1649 family parts, the MAX1626 can be substituted (with minor external component value changes) into fixed-output mode applications, provided the PC board layout is adequate. The MAX1627 can also be substituted when MAX649 or MAX1649 family parts are used in adjustable mode, but the feedback resistor values must be changed, since the MAX1627 has a lower reference voltage (1.3V vs. 1.5V). Reduce the current-sense resistor value by 50% when substituting for the MAX649 or MAX651. ______________________________________________________________________________________ 5V/3.3V or Adjustable, 100% Duty-Cycle, High-Efficiency, Step-Down DC-DC Controllers INPUT C2 68µF LOW-ESR TANTALUM C3 68µF LOW-ESR TANTALUM C2 68µF LOW-ESR TANTALUM C5 0.47µF C3 0.47µF V+ V+ RSENSE 0.15Ω MAX1627 N.C. OUT CS SHDN P EXT GND L1 22µH, 3A FB D1 R3 R2 CR2 SHDN ADJUSTABLE OUTPUT C1 220µF LOW-ESR TANTALUM RSENSE 0.15Ω MAX1626 CS U1 LOGIC-LEVEL MOSFET REF C4 0.1µF 3/5 REF U1 LOGIC-LEVEL MOSFET P EXT GND ________________________Applications The MAX1626/MAX1627 typical operating circuits (Figures 1 and 8) are designed to output 2A at a 5V output voltage. The following circuits provide examples and guidance for other applications. C1 100µF LOW-ESR TANTALUM D1 L1: SUMIDA CDR1053-680 D1: MOTOROLA MBRS130T3 U1: MOTOROLA MMSF3P02HD Figure 9. 0.5A Step-Down Converter INPUT 3V TO 6V C5 100µF C4 100µF C6 0.1µF Micropower Step-Down Converter When designing a low-power, battery-based application, choose an external MOSFET with low gate capacitance (to minimize switching losses), and use a low peak current limit to reduce I2R losses. The circuit in Figure 9 is optimized for 0.5A. C7 0.1µF C8 1.0µF C9 1.0µF V+ RCS1, RCS2 0.025Ω MAX1627 N.C. OUT SHDN REF High-Current Step-Down Converter The circuit in Figure 10 outputs 6A at 2.5V from a 5V or 3.3V input. High-current design is difficult, and board layout is critical due to radiated noise, switching transients, and voltage gradients on the PC board traces. Figure 11 is a recommended PC board design. Choose the external MOSFET to minimize RDS(ON). Keep the gate-charge factor below the MAX1626/MAX1627’s drive capability (see Ext Rise and Fall Times vs. Capacitance graph in the Typical Operating Characteristics). Otherwise, increased MOSFET rise and fall times will contribute to efficiency losses. For higher efficiencies, especially at low output voltages, the MAX796 family of step-down controllers with synchronous rectification is recommended. OUTPUT OUT C4 0.1µF L1: SUMIDA CDRH125-220 D1: NIHON NSQ03A03 U1: MOTOROLA MMSF3P02HD Figure 8. MAX1627 Typical Operating Circuit L1 68µH, 0.7A C10 0.1µF GND CS P EXT FB Q1 LOGIC-LEVEL MOSFET L1 2.7µH >8A OUTPUT 2.5V, 6A D1 R3 21.5k, 1% R2 20k, 1% CR2 220pF C1 C2 C3 220µF 220µF 220µF C1–C3: SANYO OS-CON 220µF, 6.3V C4, C5: SANYO OS-CON 100µF, 20V RCS1, RCS2: 0.025Ω DALE WSL-2512 Q1: MOTOROLA MTB50PO3HDL D1: NIEC C10T04Q L1: SUMIDA CDRH127-2R7NC Figure 10. 6A Step-Down Converter ______________________________________________________________________________________ 13 MAX1626/MAX1627 INPUT MAX1626/MAX1627 5V/3.3V or Adjustable, 100% Duty-Cycle, High-Efficiency, Step-Down DC-DC Controllers VIA VIAS VIA COMPONENT PLACEMENT GUIDE—COMPONENT SIDE COPPER ROUTING—FRONT SIDE COPPER ROUTING—BACK SIDE Figure 11. Recommended PC Board Design for 6A Step-Down Converter 14 ______________________________________________________________________________________ 5V/3.3V or Adjustable, 100% Duty-Cycle, High-Efficiency, Step-Down DC-DC Controllers MAX1626/MAX1627 ___________________Chip Topography OUT GND GND EXT 3/5 (FB) 0.105" (2.63mm) CS SHDN VCC REF 0.081" (2.06mm) ( ) ARE FOR MAX1627 TRANSISTOR COUNT: 375 SUBSTRATE CONNECTED TO V+ ______________________________________________________________________________________ 15 MAX1626/MAX1627 5V/3.3V or Adjustable, 100% Duty-Cycle, High-Efficiency, Step-Down DC-DC Controllers ________________________________________________________Package Information DIM D 0°-8° A 0.101mm 0.004in. e B A1 E 16 C H L Narrow SO SMALL-OUTLINE PACKAGE (0.150 in.) A A1 B C E e H L INCHES MAX MIN 0.069 0.053 0.010 0.004 0.019 0.014 0.010 0.007 0.157 0.150 0.050 0.244 0.228 0.050 0.016 DIM PINS D D D 8 14 16 MILLIMETERS MIN MAX 1.35 1.75 0.10 0.25 0.35 0.49 0.19 0.25 3.80 4.00 1.27 5.80 6.20 0.40 1.27 INCHES MILLIMETERS MIN MAX MIN MAX 0.189 0.197 4.80 5.00 0.337 0.344 8.55 8.75 0.386 0.394 9.80 10.00 ______________________________________________________________________________________ 21-0041A