MAX1771 12V or Adjustable, High-Efficiency, Low I , Step-Up DC-DC Controller
19-0263; Rev 2; 3/02 NUAL KIT MA ATION U L A E V E BL AVAILA 12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller The MAX1771 step-up switching controller provides 90% efficiency over a 30mA to 2A load. A unique current-limited pulse-frequency-modulation (PFM) control scheme gives this device the benefits of pulse-widthmodulation (PWM) converters (high efficiency at heavy loads), while using less than 110µA of supply current (vs. 2mA to 10mA for PWM converters). This controller uses miniature external components. Its high switching frequency (up to 300kHz) allows surface-mount magnetics of 5mm height and 9mm diameter. It accepts input voltages from 2V to 16.5V. The output voltage is preset at 12V, or can be adjusted using two resistors. The MAX1771 optimizes efficiency at low input voltages and reduces noise by using a single 100mV current-limit threshold under all load conditions. A family of similar devices, the MAX770–MAX773, trades some full-load efficiency for greater current-limit accuracy; they provide a 200mV current limit at full load, and switch to 100mV for light loads. The MAX1771 drives an external N-channel MOSFET switch, allowing it to power loads up to 24W. If less power is required, use the MAX756/MAX757 or MAX761/MAX762 step-up switching regulators with on-board MOSFETs. An evaluation kit is available. Applications Features ♦ 90% Efficiency for 30mA to 2A Load Currents ♦ Up to 24W Output Power ♦ 110µA (max) Supply Current ♦ 5µA (max) Shutdown Current ♦ 2V to 16.5V Input Range ♦ Preset 12V or Adjustable Output Voltage ♦ Current-Limited PFM Control Scheme ♦ Up to 300kHz Switching Frequency ♦ Evaluation Kit Available Ordering Information PART TEMP RANGE PIN-PACKAGE MAX1771CPA 0°C to +70°C 8 Plastic DIP MAX1771CSA MAX1771C/D MAX1771EPA MAX1771ESA MAX1771MJA 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** * Contact factory for dice specifications. ** Contact factory for availability and processing to MIL-STD-883B. Positive LCD-Bias Generators Flash Memory Programmers High-Power RF Power-Amplifier Supply Palmtops/Hand-Held Terminals Battery-Powered Applications Pin Configuration Portable Communicators Typical Operating Circuit TOP VIEW INPUT 2V TO VOUT OUTPUT 12V MAX1771 EXT ON/OFF SHDN CS REF FB AGND GND V+ N EXT 1 8 CS V+ 2 7 GND FB 3 6 AGND SHDN 4 5 REF MAX1771 DIP/SO ________________________________________________________________ Maxim Integrated Products For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. 1 MAX1771 General Description MAX1771 12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller ABSOLUTE MAXIMUM RATINGS Supply Voltage V+ to GND ...............................................................-0.3V, 17V EXT, CS, REF, SHDN, FB to GND ...................-0.3V, (V+ + 0.3V) GND to AGND.............................................................0.1V, -0.1V 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 MAX1771C_ A .....................................................0°C to +70°C MAX1771E_ A ..................................................-40°C to +85°C MAX1771MJA ................................................-55°C to +125°C Junction Temperatures MAX1771C_ A/E_ A.......................................................+150°C MAX1771MJA ..............................................................+175°C Storage Temperature Range .............................-65°C to +160°C Lead Temperature (soldering, 10sec) .............................+300°C Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ELECTRICAL CHARACTERISTICS (V+ = 5V, ILOAD = 0mA, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL Input Voltage Range CONDITIONS MIN Standby Current 12.5 MAX1771C/E (external resistors) 3.0 16.5 MAX1771MJA (external resistors) 3.1 16.5 UNITS V 1.8 2.0 V V+ = 16.5V, SHDN = 0V (normal operation) 85 110 µA V+ = 10V, SHDN ≥ 1.6V (shutdown) 2 5 V+ = 16.5V, SHDN ≥ 1.6V (shutdown) 4 µA Output Voltage (Note 1) V+ = 2V to 12V, over full load range, Circuit of Figure 2a Output Voltage Line Regulation (Note 2) V+ = 5V to 7V, VOUT = 12V ILOAD = 700mA, Circuit of Figure 2a 5 mV/V Output Voltage Load Regulation (Note 2) V+ = 6V, VOUT = 12V, ILOAD = 0mA to 500mA, Circuit of Figure 2a 20 mV/A 11.52 12.0 12.48 V Maximum Switch On-Time tON(max) 12 16 20 µs Minimum Switch Off-Time tOFF(min) 1.8 2.3 2.8 µs V+ = 5V, VOUT = 12V, ILOAD = 500mA, Circuit of Figure 2a Efficiency Reference Voltage VREF IREF = 0µA REF Load Regulation 0µA ≤ IREF ≤ 100µA REF Line Regulation 3V ≤ V+ ≤ 16.5V FB Trip Point Voltage 2 MAX 2.0 Minimum Start-Up Voltage Supply Current TYP MAX1771 (internal feedback resistors) VFB 92 % MAX1771C 1.4700 1.5 1.5300 MAX1771E 1.4625 1.5 1.5375 MAX1771M 1.4550 1.5 1.5450 MAX1771C/E 4 10 MAX1771M 4 15 40 100 MAX1771C 1.4700 1.5 1.5300 MAX1771E 1.4625 1.5 1.5375 MAX1771M 1.4550 1.5 1.5450 _______________________________________________________________________________________ V mV µV/V V 12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller MAX1771 ELECTRICAL CHARACTERISTICS (continued) (V+ = 5V, ILOAD = 0mA, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETERS SYMBOL CONDITIONS MIN TYP MAX1771C ±20 MAX1771E ±40 FB Input Current IFB SHDN Input High Voltage VIH V+ = 2V to 16.5V SHDN Input Low Voltage VIL V+ = 2V to 16.5V MAX1771M SHDN Input Current VCS UNITS nA ±60 1.6 V V+ = 16.5V, SHDN = 0V or V+ Current-Limit Trip Level MAX V+ = 5V to 16V 0.4 V ±1 µA MAX1771C/E 85 100 115 MAX1771M 75 100 125 0.01 ±1 CS Input Current mV µA EXT Rise Time V+ = 5V, 1nF from EXT to ground 55 ns EXT Fall Time V+ = 5V, 1nF from EXT to ground 55 ns Note 1: Output voltage guaranteed using preset voltages. See Figures 4a–4d for output current capability versus input voltage. Note 2: Output voltage line and load regulation depend on external circuit components. Typical Operating Characteristics (TA = +25°C, unless otherwise noted.) 85 VIN = 3V 80 75 VIN = 5V 70 VOUT = 12V CIRCUIT OF FIGURE 2a 65 VIN = 8V 85 80 VIN = 5V 75 70 VOUT = 12V CIRCUIT OF FIGURE 2b 65 60 60 1 10 100 1000 LOAD CURRENT (mA) 10,000 1 10 100 1000 LOAD CURRENT (mA) 10,000 MAX1771-TOC3 VIN =10V 90 VIN = 8V EFFICIENCY (%) EFFICIENCY (%) 90 95 VOUT = 12V, CIRCUIT OF FIGURE 2a EXTERNAL FET THRESHOLD LIMITS FULL-LOAD START-UP BELOW 3.5V 600 LOAD CURRENT (mA) VIN = 10V 700 MAX1771–02 95 100 MAX1771–01 100 LOAD CURRENT vs. MINIMUM START-UP INPUT VOLTAGE EFFICIENCY vs. LOAD CURRENT (NON-BOOTSTRAPED MODE) EFFICIENCY vs. LOAD CURRENT (BOOTSTRAPED MODE) 500 400 300 200 100 0 2.00 2.25 2.50 2.75 3.00 3.25 3.50 MINIMUM START-UP INPUT VOLTAGE (V) _______________________________________________________________________________________ 3 Typical Operating Characteristics (continued) (TA = +25°C, unless otherwise noted.) ENTIRE CIRCUIT 2 SCHOTTKY DIODE LEAKAGE EXCLUDED 0.2 0 25 50 CEXT = 2200pF CEXT = 1000pF 150 CEXT = 446pF CEXT = 100pF 100 NON-BOOTSTRAPPED CIRCUIT OF FIGURE 2b 0 -75 -50 -25 75 100 125 0 2 6 4 12 10 8 2 6 4 12 10 8 TEMPERATURE (°C) SUPPLY VOLTAGE (V) SUPPLY VOLTAGE (V) REFERENCE OUTPUT RESISTANCE vs. TEMPERATURE REFERENCE vs. TEMPERATURE MAXIMUM SWITCH ON-TIME vs. TEMPERATURE 16.5 MAX1771-08 1.506 MAX1771-07 250 1.504 200 10µA 150 100 50µA 1.500 1.498 16.0 1.496 100µA 50 tON(MAX) (µs) 1.502 REFERENCE (V) 1.494 15.5 1.492 0 -60 -40 -20 0 20 40 60 80 100 120 140 -60 -40 -20 0 30 60 90 120 150 TEMPERATURE (°C) TEMPERATURE (°C) SHUTDOWN CURRENT vs. TEMPERATURE MINIMUM SWITCH OFF-TIME vs. TEMPERATURE MAXIMUM SWITCH ON-TIME/ MINIMUM SWITCH OFF-TIME RATIO vs. TEMPERATURE tON(MAX)/tOFF(MIN) RATIO 3.0 2.5 2.0 V+ = 15V 1.5 2.25 V+ = 8V 1.0 MAX1771-12 3.5 8.0 MAX1771-11 2.30 MAX1771-10 4.0 0.5 -60 -30 0 20 40 60 80 100 120 140 TEMPERATURE (°C) tOFF(MIN) (µs) REFERENCE OUTPUT RESISTANCE (Ω) 0.4 200 50 0 7.5 7.0 6.5 V+ = 4V 2.20 0 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) 4 BOOTSTRAPPED CIRCUIT OF FIGURE 2a 0.6 MAX1771-09 1 EXT RISE/FALL TIME (ns) 3 250 MAX1771-05 MAX1771-04 VOUT = 12V SUPPLY CURRENT (mA) SUPPLY CURRENT (mA) VOUT = 12V, VIN = 5V CIRCUIT OF FIGURE 2a BOOTSTRAPPED MODE EXT RISE/FALL TIME vs. SUPPLY VOLTAGE SUPPLY CURRENT vs. SUPPLY VOLTAGE 0.8 MAX1771-06 SUPPLY CURRENT vs. TEMPERATURE 4 SHUTDOWN CURRENT (µA) MAX1771 12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller 6.0 -60 -30 0 30 60 90 TEMPERATURE (°C) 120 150 -60 -30 0 30 60 90 TEMPERATURE (°C) _______________________________________________________________________________________ 120 150 12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller HEAVY-LOAD SWITCHING WAVEFORMS MEDIUM-LOAD SWITCHING WAVEFORMS VOUT VOUT A A 0V 0V ILIM ILIM B B 0A 0A C C 2µs/div VIN = 5V, IOUT = 900mA, VOUT = 12V A: EXT VOLTAGE, 10V/div B: INDUCTOR CURRENT, 1A/div C: VOUT RIPPLE, 50mV/div, AC-COUPLED 10µs/div VIN = 5V, IOUT = 500mA, VOUT = 12V A: EXT VOLTAGE, 10V/div B: INDUCTOR CURRENT, 1A/div C: VOUT RIPPLE, 50mV/div, AC-COUPLED LINE-TRANSIENT RESPONSE A LOAD-TRANSIENT RESPONSE 7V 500mA 5V A 0A 0V B B 5ms/div IOUT = 700mA, VOUT = 12V A: VIN, 5V to 7V, 2V/div B: VOUT RIPPLE, 100mV/div, AC-COUPLED 5ms/div VIN = 6V, VOUT = 12V A: LOAD CURRENT, 0mA to 500mA, 500mA/div B: VOUT RIPPLE, 100mV/div, AC-COUPLED _______________________________________________________________________________________ 5 MAX1771 Typical Operating Characteristics (continued) (Circuit of Figure 2a, TA = +25°C, unless otherwise noted.) MAX1771 12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller Typical Operating Characteristics (continued) (Circuit of Figure 2a, TA = +25°C, unless otherwise noted.) ENTERING/EXITING SHUTDOWN A 0V B 5V 0V 2ms/div IOUT = 500mA, VIN = 5V A: SHDN, 5V/div B: VOUT, 5V/div Pin Description 6 PIN NAME FUNCTION 1 EXT Gate Drive for External N-Channel Power Transistor 2 V+ Power-Supply Input. Also acts as a voltage-sense point when in bootstrapped mode. 3 FB Feedback Input for Adjustable-Output Operation. Connect to ground for fixed-output operation. Use a resistor divider network to adjust the output voltage. See Setting the Output Voltage section. 4 SHDN 5 REF 6 AGND 7 GND 8 CS Active-High TTL/CMOS Logic-Level Shutdown Input. In shutdown mode, VOUT is a diode drop below V+ (due to the DC path from V+ to the output) and the supply current drops to 5µA maximum. Connect to ground for normal operation. 1.5V Reference Output that can source 100µA for external loads. Bypass to GND with 0.1µF. The reference is disabled in shutdown. Analog Ground High-Current Ground Return for the Output Driver Positive Input to the Current-Sense Amplifier. Connect the current-sense resistor between CS and GND. _______________________________________________________________________________________ 12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller The MAX1771 is a BiCMOS, step-up, switch-mode power-supply controller that provides a preset 12V output, in addition to adjustable-output operation. Its unique control scheme combines the advantages of pulse-frequency modulation (low supply current) and pulse-width modulation (high efficiency with heavy loads), providing high efficiency over a wide output current range, as well as increased output current capability over previous PFM devices. In addition, the external sense resistor and power transistor allow the user to tailor the output current capability for each application. Figure 1 shows the MAX1771 functional diagram. Bootstrapped/Non-Bootstrapped Modes Figure 2 shows the standard application circuits for bootstrapped and non-bootstrapped modes. In bootstrapped mode, the IC is powered from the output (VOUT, which is connected to V+) and the input voltage range is 2V to VOUT. The voltage applied to the gate of the external power transistor is switched from VOUT to ground, providing more switch gate drive and thus reducing the transistor’s on-resistance. In non-bootstrapped mode, the IC is powered from the input voltage (V+) and operates with minimum supply current. In this mode, FB is the output voltage sense point. Since the voltage swing applied to the gate of the external power transistor is reduced (the gate swings from V+ to ground), the power transistor’s on-resistance The MAX1771 offers three main improvements over prior pulse-skipping control solutions: 1) the converter operates with miniature (5mm height and less than 9mm diameter) surface-mount inductors due to its 300kHz switching frequency; 2) the current-limited PFM control scheme allows 90% efficiencies over a wide REF FB DUAL-MODE COMPARATOR SHDN MAX1771 50mV BIAS CIRCUITRY 1.5V REFERENCE ERROR COMPARATOR MIN OFF-TIME ONE-SHOT Q TRIG V+ N 2.3µs F/F S Q R MAX ON-TIME ONE-SHOT TRIG Q 16µs LOW-VOLTAGE OSCILLATOR 2.5V CURRENT-SENSE AMPLIFIER EXT 0.1V CS Figure 1. Functional Diagram _______________________________________________________________________________________ 7 MAX1771 range of load currents; and 3) the maximum supply current is only 110µA. The device has a shutdown mode that reduces the supply current to 5µA max. Detailed Description MAX1771 12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller VIN = 5V C1 68µF C2 0.1µF 4 SHDN 2 L1 22µH V+ L1 22µH MAX1771 3 FB 6 C2 0.1µF 2 V+ 5 REF C3 0.1µF VIN = 5V EXT CS AGND 1 5 REF C1 68µF D1 1N5817-22 VOUT = 12V AT 0.5A C3 0.1µF 4 SHDN MAX1771 6 AGND N CS Si9410DY/ MTD20N03HDL 8 FB GND C4 300µF RSENSE 40mΩ GND EXT 7 7 R2 = (R1) VOUT (V REF -1 1 VOUT = 12V AT 0.5A D1 1N5817-22 C4 300µF N MTD20N03HDL 8 3 RSENSE 40mΩ R1 18kΩ ) R2 127kΩ C5 100pF VREF = 1.5V Figure 2a. 12V Preset Output, Bootstrapped Figure 2b. 12V Output, Non-Bootstrapped VIN = 4V C2 0.1µF 2 5 REF C3 0.1µF V+ 4 SHDN MAX1771 6 EXT CS AGND FB GND 7 R2 = (R1) C1 47µF L1 22µH D1 1N5817-22 1 VOUT = 9V N Si9410DY/ MTD20N03HDL 8 RSENSE 40mΩ 3 R1 28kΩ ( VVOUT -1) C4 200µF R2 140kΩ C5 100pF REF VREF = 1.5V Figure 2c. 9V Output, Bootstrapped increases at low input voltages. However, the supply current is also reduced because V+ is at a lower voltage, and because less energy is consumed while charging and discharging the external MOSFET’s gate capacitance. The minimum input voltage is 3V when using external feedback resistors. With supply voltages below 5V, bootstrapped mode is recommended. Note: When using the MAX1771 in non-bootstrapped mode, there is no preset output operation because V+ is also the output voltage sense point 8 for fixed-output operation. External resistors must be used to set the output voltage. Use 1% external feedback resistors when operating in adjustable-output mode (Figures 2b, 2c) to achieve an overall output voltage accuracy of ±5%. To achieve highest efficiency, operate in bootstrapped mode whenever possible. External Power-Transistor Control Circuitry PFM Control Scheme The MAX1771 uses a proprietary current-limited PFM control scheme to provide high efficiency over a wide range of load currents. This control scheme combines the ultra-low supply current of PFM converters (or pulse skippers) with the high full-load efficiency of PWM converters. Unlike traditional PFM converters, the MAX1771 uses a sense resistor to control the peak inductor current. The device also operates with high switching frequencies (up to 300kHz), allowing the use of miniature external components. As with traditional PFM converters, the power transistor is not turned on until the voltage comparator senses the output is out of regulation. However, unlike traditional PFM converters, the MAX1771 switch uses the combination of a peak current limit and a pair of one-shots that set the maximum on-time (16µs) and minimum offtime (2.3µs); there is no oscillator. 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. _______________________________________________________________________________________ 12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller Low-Voltage Start-Up Oscillator The MAX1771 features a low input voltage start-up oscillator that guarantees start-up with no load down to 2V when operating in bootstrapped mode and using internal feedback resistors. At these low voltages, the supply voltage is not large enough for proper error-comparator operation and internal biasing. The start-up oscillator has a fixed 50% duty cycle and the MAX1771 disregards the error-comparator output when the supply voltage is less than 2.5V. Above 2.5V, the error-comparator and normal one-shot timing circuitry are used. The lowvoltage start-up circuitry is disabled if non-bootstrapped mode is selected (FB is not tied to ground). Shutdown Mode When SHDN is high, the MAX1771 enters shutdown mode. In this mode, the internal biasing circuitry is turned off (including the reference) and VOUT falls to a diode drop below V IN (due to the DC path from the input to the output). In shutdown mode, the supply current drops to less than 5µA. SHDN is a TTL/CMOS logic-level input. Connect SHDN to GND for normal operation. Design Procedure Setting the Output Voltage To set the output voltage, first determine the mode of operation, either bootstrapped or non-bootstrapped. Bootstrapped mode provides more output current capability, while non-bootstrapped mode reduces the supply current (see the Typical Operating Characteristics). If a decaying voltage source (such as a battery) is used, see the additional notes in the Low Input Voltage Operation section. The MAX1771’s output voltage can be adjusted from very high voltages down to 3V, using external resistors MAX1771 The control circuitry allows the IC to operate in continuous-conduction mode (CCM) while maintaining high efficiency with heavy loads. When the power switch is turned on, it stays on until either 1) the maximum ontime one-shot turns it off (typically 16µs later), or 2) the switch current reaches the peak current limit set by the current-sense resistor. The MAX1771 switching frequency is variable (depending on load current and input voltage), causing variable switching noise. However, the subharmonic noise generated does not exceed the peak current limit times the filter capacitor equivalent series resistance (ESR). For example, when generating a 12V output at 500mA from a 5V input, only 100mV of output ripple occurs using the circuit of Figure 2a. R2 VOUT FB MAX1771 R1 C5* R1 = 10kΩ TO 500kΩ GND R2 = R1 V -1) ( VOUT REF VREF = 1.5V * SEE TEXT FOR VALUE Figure 3. Adjustable Output Circuit R1 and R2 configured as shown in Figure 3. For adjustable-output operation, select feedback resistor R1 in the 10kΩ to 500kΩ range. R2 is given by: VOUT -1 R2 = (R1) ––––– VREF ( ) where VREF equals 1.5V. For preset-output operation, tie FB to GND (this forces bootstrapped-mode operation. Figure 2 shows various circuit configurations for bootstrapped/non-bootstrapped, preset/adjustable operation. Determining RSENSE Use the theoretical output current curves shown in Figures 4a–4d to select RSENSE. They were derived using the minimum (worst-case) current-limit comparator threshold value over the extended temperature range (-40°C to +85°C). No tolerance was included for R SENSE . The voltage drop across the diode was assumed to be 0.5V, and the drop across the power switch rDS(ON) and coil resistance was assumed to be 0.3V. Determining the Inductor (L) Practical inductor values range from 10µH to 300µH. 22µH is a good choice for most applications. In applications with large input/output differentials, the IC’s output current capability will be much less when the inductance value is too low, because the IC will always operate in discontinuous mode. If the inductor value is too low, the current will ramp up to a high level before the current-limit comparator can turn off the switch. The minimum on-time for the switch (t ON (min)) is _______________________________________________________________________________________ 9 MAXIMUM OUTPUT CURRENT (A) 3.5 3.5 VOUT = 5V L = 22µH 3.0 MAXIMUM OUTPUT CURRENT (A) MAX1771 12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller RSENSE = 20mΩ 2.5 RSENSE = 25mΩ 2.0 RSENSE = 35mΩ 1.5 1.0 RSENSE = 50mΩ 0.5 VOUT = 12V L = 22µH 3.0 RSENSE = 20mΩ RSENSE = 25mΩ 2.5 RSENSE = 35mΩ 2.0 1.5 1.0 RSENSE = 50mΩ 0.5 RSENSE = 100mΩ 0 2 3 4 INPUT VOLTAGE (V) 5 Figure 4a. Maximum Output Current vs. Input Voltage (VOUT = 5V) 3.5 2 RSENSE = 20mΩ RSENSE = 35mΩ 2.0 10 12 VOUT = 24V L =150µH RSENSE = 25mΩ 2.5 6 8 INPUT VOLTAGE (V) 0.8 VOUT = 15V L = 22µH 3.0 4 Figure 4b. Maximum Output Current vs. Input Voltage (VOUT = 12V) 1.5 1.0 RSENSE = 50mΩ MAXIMUM OUTPUT CURRENT (A) MAXIMUM OUTPUT CURRENT (A) RSENSE = 100mΩ 0 0.6 RSENSE = 50mΩ RSENSE = 100mΩ 0.4 0.2 0.5 RSENSE = 200mΩ RSENSE = 100mΩ 0 0 2 4 6 8 10 12 14 16 INPUT VOLTAGE (V) 2 6 10 INPUT VOLTAGE (V) 14 Figure 4c. Maximum Output Current vs. Input Voltage (VOUT = 15V) Figure 4d. Maximum Output Current vs. Input Voltage (VOUT = 24V) approximately 2µs; select an inductor that allows the current to ramp up to I LIM. Inductors with a ferrite core or equivalent are recommended; powder iron cores are not recommended for use with high switching frequencies. Make sure the inductor’s saturation current rating (the current at which the core begins to saturate and the inductance starts to fall) exceeds the peak current rating set by RSENSE. However, it is generally acceptable to bias the inductor into saturation by approximately 20% (the point where the inductance is 20% below the nominal value). For highest efficiency, use a coil with low DC resistance, preferably under 20mΩ. To minimize radiated noise, use a toroid, a pot core, or a shielded coil. Table 1 lists inductor suppliers and specific recommended inductors. The standard operating circuits use a 22µH inductor. If a different inductance value is desired, select L such that: VIN(max) x 2µs L ≥ —————----—-ILIM Larger inductance values tend to increase the start-up time slightly, while smaller inductance values allow the coil current to ramp up to higher levels before the switch turns off, increasing the ripple at light loads. 10 ______________________________________________________________________________________ 12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller Diode Selection The MAX1771’s high switching frequency demands a high-speed rectifier. Schottky diodes such as the 1N5817–1N5822 are recommended. Make sure the Schottky diode’s average current rating exceeds the peak current limit set by RSENSE, and that its breakdown voltage exceeds V OUT. For high-temperature applications, Schottky diodes may be inadequate due to their high leakage currents; high-speed silicon diodes such as the MUR105 or EC11FS1 can be used instead. At heavy loads and high temperatures, the benefits of a Schottky diode’s low forward voltage may outweigh the disadvantages of its high leakage current. The bypass capacitor on V+ (C2) must instantaneously furnish the gate charge without excessive droop (e.g., less than 200mV): Qg ∆V+ = —— C2 Continuing with the example, ∆V+ = 17nC/0.1µF = 170mV. Figure 2a’s application circuit uses an 8-pin Si9410DY surface-mount N-FET that has 50mΩ on-resistance with 4.5V VGS, and a guaranteed VTH of less than 3V. Figure 2b’s application circuit uses an MTD20N03HDL logiclevel N-FET with a guaranteed threshold voltage (VTH) of 2V. Capacitor Selection Output Filter Capacitor The primary criterion for selecting the output filter capacitor (C4) is low effective series resistance (ESR). The product of the peak inductor current and the output filter capacitor’s ESR determines the amplitude of the ripple seen on the output voltage. Two OS-CON 150µF, 16V output filter capacitors in parallel with 35mΩ of ESR each typically provide 75mV ripple when stepping up from 5V to 12V at 500mA (Figure 2a). Smaller-value and/or higher-ESR capacitors are acceptable for light loads or in applications that can tolerate higher output ripple. Since the output filter capacitor’s ESR affects efficiency, use low-ESR capacitors for best performance. See Table 1 for component selection. Input Bypass Capacitors The input bypass capacitor (C1) reduces peak currents drawn from the voltage source and also reduces noise at the voltage source caused by the switching action of the MAX1771. 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. For output currents up to 1A, 68µF (C1) is adequate, although smaller bypass capacitors may also be acceptable. Bypass the IC with a 0.1µF ceramic capacitor (C2) placed as close to the V+ and GND pins as possible. Reference Capacitor Bypass REF with a 0.1µF capacitor (C3). REF can source up to 100µA of current for external loads. Feed-Forward Capacitor In adjustable output voltage and non-bootstrapped modes, parallel a 47pF to 220pF capacitor across R2, as shown in Figures 2 and 3. Choose the lowest capacitor value that insures stability; high capacitance values may degrade line regulation. ______________________________________________________________________________________ 11 MAX1771 Power Transistor Selection Use an N-channel MOSFET power transistor with the MAX1771. To ensure the external N-channel MOSFET (N-FET) is turned on hard, use logic-level or low-threshold N-FETs when the input drive voltage is less than 8V. This applies even in bootstrapped mode, to ensure start-up. N-FETs provide the highest efficiency because they do not draw any DC gate-drive current. When selecting an N-FET, three important parameters are the total gate charge (Qg), on-resistance (rDS(ON)), and reverse transfer capacitance (CRSS). Qg takes into account all capacitances associated with charging the gate. Use the typical Qg value for best results; the maximum value is usually grossly overspecified since it is a guaranteed limit and not the measured value. The typical total gate charge should be 50nC or less. With larger numbers, the EXT pins may not be able to adequately drive the gate. The EXT rise/fall time varies with different capacitive loads as shown in the Typical Operating Characteristics. The two most significant losses contributing to the N-FET’s power dissipation are I2R losses and switching losses. Select a transistor with low rDS(ON) and low CRSS to minimize these losses. Determine the maximum required gate-drive current from the Qg specification in the N-FET data sheet. The MAX1771’s maximum allowed switching frequency during normal operation is 300kHz; but at start-up, the maximum frequency can be 500kHz, so the maximum current required to charge the N-FET’s gate is f(max) x Qg(typ). Use the typical Qg number from the transistor data sheet. For example, the Si9410DY has a Qg(typ) of 17nC (at VGS = 5V), therefore the current required to charge the gate is: IGATE (max) = (500kHz) (17nC) = 8.5mA. MAX1771 12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller Table 1. Component Suppliers PRODUCTION Surface Mount Through Hole SUPPLIER INDUCTORS CAPACITORS Sumida CD54 series CDR125 series Coiltronics CTX20 series Coilcraft DO3316 series DO3340 series Matsuo 267 series Sprague 595D series AVX TPS series Sumida RCH855 series RCH110 series Sanyo OS-CON series Nichicon PL series PHONE DIODES Central Semiconductor CMPSH-3 CMPZ5240 Nihon EC11 FS1 series (highspeed silicon) Motorola MBRS1100T3 MMBZ5240BL Motorola 1N5817–1N5822 MUR115 (high voltage) MUR105 (high-speed silicon) FAX AVX USA: (803) 448-9411 (803) 448-1943 Central Semiconductor USA: (516) 435-1110 (516) 435-1824 Coilcraft USA: (708) 639-6400 (708) 639-1469 Coiltronics USA: (516) 241-7876 (516) 241-9339 Matsuo USA: (714) 969-2491 Japan: 81-6-337-6450 (714) 960-6492 81-6-337-6456 Motorola USA: (800) 521-6274 (602) 952-4190 Nichicon USA: (708) 843-7500 (708) 843-2798 Nihon USA: (805) 867-2555 (805) 867-2556 Sanyo USA: (619) 661-6835 Japan: 81-7-2070-1005 (619) 661-1055 81-7-2070-1174 Siliconix USA: (800) 554-5565 (408) 970-3950 Sprague USA: (603) 224-1961 (603) 224-1430 Sumida USA: (708) 956-0666 Japan: 81-3-3607-5111 (708) 956-0702 81-3-3607-5144 12 TRANSISTORS Siliconix Si9410DY Si9420DY (high voltage) Motorola MTP3055EL MTD20N03HDL MMFT3055ELT1 MTD6N1O MMBT8099LT1 MMBT8599LT1 ______________________________________________________________________________________ 12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller VIN* 3V TO 11V D2 1N5817 2 4 V+ SHDN L1 20µH 1 CTX20-4 3V = OFF C4 5 REF 0.1µF MAX1771 EXT CS GND AGND 6 7 C2 Q1** 47µF 16V 1 8 L2 VOUT 5V 500mA C3 220µF 10V R1 0.1Ω FB 3 D1 1N5817 R3† R2† C5 47pF SEE TEXT FOR FURTHER COMPONENT INFO **VIN MAY BE LOWER THAN INDICATED IF THE SUPPLY IS NOT **REQUIRED TO START UNDER FULL LOAD **MOTOROLA MMFT3055ELT1 † FOR 5V: R2 = 200kΩ, R3 = 470kΩ 3.3V: R2 = 100kΩ, R3 = 20kΩ Figure 5. Step-Up/Down for a 5V/3.3V Output Applications Information Low Input Voltage Operation When using a power supply that decays with time (such as a battery), the N-FET transistor will operate in its linear region when the voltage at EXT approaches the threshold voltage of the FET, dissipating excessive power. Prolonged operation in this mode may damage the FET. This effect is much more significant in nonbootstrapped mode than in bootstrapped mode, since bootstrapped mode typically provides much higher VGS voltages. To avoid this condition, make sure VEXT is above the VTH of the FET, or use a voltage detector (such as the MAX8211) to put the IC in shutdown mode once the input supply voltage falls below a predetermined minimum value. Excessive loads with low input voltages can also cause this condition. Starting Up Under Load The Typical Operating Characteristics show the StartUp Voltage vs. Load Current graph for bootstrappedmode operation. This graph depends on the type of power switch used. The MAX1771 is not designed to start up under full load in bootstrapped mode with low input voltages. Due to high current levels and fast switching waveforms, which radiate noise, proper PC board layout is essential. Protect sensitive analog grounds by using a star ground configuration. Minimize ground noise by connecting GND, the input bypass capacitor ground lead, and the output filter capacitor ground lead to a single point (star ground configuration). Also, minimize lead lengths to reduce stray capacitance, trace resistance, and radiated noise. Place input bypass capacitor C2 as close as possible to V+ and GND. Excessive noise at the V+ input may falsely trigger the timing circuitry, resulting in short pulses at EXT. If this occurs it will have a negligible effect on circuit efficiency. If desired, place a 4.7µF directly across the V+ and GND pins (in parallel with the 0.1µF C2 bypass capacitor) to reduce the noise at V+. Other Application Circuits 4 Cells to 5V (or 3 Cells to 3.3V), 500mA Step-Up/Down Converter The circuit shown in Figure 5 generates 5V (or 3.3V) at 500mA with 85% efficiency, from an input voltage that varies above and below the output. The output couples to the switching circuitry via a capacitor. This configuration offers two advantages over flyback-transformer and step-up linear-regulator circuits: smooth regulation as the input passes through the output, and no output current in shutdown. This circuit requires two inductors, which can be wound on one core with no regard to coupling since they do not work as a transformer. L1 and L2 can either be wound together (as with the Coiltronics CTX20-4) or kept as two separate inductors; both methods provide equal performance. Capacitors C2 and C3 should be low-ESR types for best efficiency. A 1µF ceramic capacitor will work at C2, but with about 3% efficiency loss. C2’s voltage rating must be greater than the maximum input voltage. Also note that the LX switch must withstand a voltage equal to the sum of the input and output voltage; for example, when converting 11V to 5V, the switch must withstand 16V. LX switch pulses are captured by Schottky diode D2 to boost V+ to (VOUT + VIN). This improves efficiency with a low input voltage, but also limits the maximum input supply to 11V. If the input voltage does not fall below 4V and if a 3V logic threshold FET is used for Q1, you may omit D2 and connect V+ directly to the input supply. 12V Output Buck/Boost The circuit in Figure 6 generates 12V from a 4.5V to 16V input. Higher input voltages are possible if you ______________________________________________________________________________________ 13 MAX1771 Layout Considerations C1 2.2µF MAX1771 12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller VIN 4.5V TO 15V C1 33µF 2 16V OFF 4 L1 20µH V+ C2* 1µF SHDN ON MAX1771 EXT CS AGND 5 C5 0.1µF REF R3 28kΩ 1% GND FB 3 1 Q1** L2* 20µH 8 6 7 D2* 1N4148 NOTE: KEEP ALL TRACES CONNECTED TO PIN 3 AS SHORT AS POSSIBLE VOUT 12V 250mA D1 1N5819 R1 0.1Ω C4 C3 100µF 100µF 16V 16V NOTE: HIGHCURRENT GND R2 200kΩ 1% *SEE TEXT FOR FURTHER COMPONENT INFORMATION **Q1 = MOTOROLA MMFT3055ELT1 L1 + L2 = ONE COILTRONICS CTX20-4 Figure 6. 12V Buck/Boost from a 4.5V to 15V Input carefully observe the component voltage ratings, since some components must withstand the sum of the input and output voltage (27V in this case). The circuit operates as an AC-coupled boost converter, and does not change operating modes when crossing from buck to boost. There is no instability around a 12V input. Efficiency ranges from 85% at medium loads to about 82% at full load. Also, when shutdown is activated (SHDN high) the output goes to 0V and sources no current. A 1µF ceramic capacitor is used for C2. A larger capacitor value improves efficiency by about 1% to 3%. D2 ensures start-up for this AC-coupled configuration by overriding the MAX1771’s Dual-Mode feature, which allows the use of preset internal or user-set external feedback. When operating in Dual-Mode, the IC first 14 tries to use internal feedback and looks to V+ for its feedback signal. However, since V+ may be greater than the internally set feedback (12V for the MAX1771), the IC may think the output is sufficiently high and not start. D2 ensures start-up by pulling FB above ground and forcing the external feedback mode. In a normal (not AC-coupled) boost circuit, D2 isn’t needed, since the output and FB rise as soon as input power is applied. Transformerless -48V to +5V at 300mA The circuit in Figure 7 uses a transformerless design to supply 5V at 300mA from a -30V to -75V input supply. The MAX1771 is biased such that its ground connections are made to the -48V input. The IC’s supply voltage (at V+) is set to about 9.4V (with respect to -48V) by a zener-biased emitter follower (Q2). An N-channel FET (Q1) is driven in a boost configuration. Output regulation is achieved by a transistor (Q3), which level shifts a feedback signal from the 5V output to the IC’s FB input. Conversion efficiency is typically 82%. When selecting components, be sure that D1, Q1, Q2, Q3, and C6 are rated for the full input voltage plus a reasonable safety margin. Also, if D1 is substituted, it should be a fast-recovery type with a trr less than 30ns. R7, R9, C8, and D3 are optional and may be used to soft start the circuit to prevent excessive current surges at power-up. Battery-Powered LCD Bias Supply The circuit in Figure 8 boosts two cells (2V min) to 24V for LCD bias or other positive output applications. Output power is boosted from the battery input, while V+ voltage for the MAX1771 is supplied by a 5V or 3.3V logic supply. 5V, 1A Boost Converter The circuit in Figure 9 boosts a 2.7V to 5.5V input to a regulated 5V, 1A output for logic, RF power, or PCMCIA applications. Efficiency vs. load current is shown in the adjacent graph. ______________________________________________________________________________________ 12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller 4 C5 0.1µF EXT SHDN 5 3 7 MAX1771 V+ +5V 300mA C8 1µF R9 5.1kΩ 1 C7 220pF R7 200Ω Q2 MMBT8099LT1 -48V 6 R6 200kΩ R5 1kΩ R1 0.15Ω C6 10µF 100V R2 47kΩ 1% C1 220µF 10V C2 220µF 10V C3 0.33µF Q3 MMBT8599LT1 D3 CMPSH-3 2 AGND GND 3 Q1 MTD6N10 2 8 CS REF FB 1 D1 MBRS1100T3 MAX1771 L1 D03340 220µH–680µH R4 100kΩ C4 2.2µF 20V R3 16kΩ 1% D2 CMPZ5240/ MMBZ5240BL Figure 7. -48V Input to 5V Output at 300mA, Without a Transformer BATTERY INPUT 2V TO 12V 3.3V OR 5V LOGIC SUPPLY OFF ON L1 22µH 0.1µF 4 2 V+ EXTL SHDN CS 1 0.1µF N MMFT3055ELT1 47µF 8 MAX1771 REF 5 1N5817 OUTPUT ADJ = 12V TO 24V (AS SHOWN) FB 3 GND 6, 7 R2 150kΩ RSENSE 0.2Ω R3 10kΩ 10kΩ Figure 8. 2V Input to 24V Output LCD Bias ______________________________________________________________________________________ 15 INPUT 2.7V TO 5.5V 22µH EFFICIENCY vs. LOAD CURRENT 150µF 1N5820 EXT OFF 4 CS SHDN 1 100 OUTPUT 5V 1A 90 MTD20N03HDL EFFICIENCY (%) MAX1771 12V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controller 8 ON 330µF 0.04Ω MAX1771 5 V+ REF 2 0.1µF GND 7 AGND FB 6 0.1µF 232kΩ 80 VIN = 4V VIN = 3V 70 60 100pF 3 50 1m 100kΩ 10m 100m 1 LOAD CURRENT (A) Figure 9. 5V/1A Boost Converter ___________________Chip Topography EXT V+ CS 0.126" (3.200mm) GND AGND FB SHDN 0.080" REF (2.032mm) TRANSISTOR COUNT: 501 SUBSTRATE CONNECTED TO V+ 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 © 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
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MAX750A/MAX758A Adjustable, Step-Down, Current-Mode PWM Regulators _______________General Description