19-0202; Rev 2; 11/96 L MANUA ION KIT HEET T A U L EVA TA S WS DA FOLLO 5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers ________________________Applications Palmtops/Handy-Terminals High-Efficiency DC-DC Converters Battery-Powered Applications ____________________________Features ♦ 90% Efficiency for 10mA to 1A Load Currents ♦ Up to 15W Output Power ♦ 110µA Max Supply Current ♦ 5µA Max Shutdown Current ♦ 2V to 16.5V Input Range (MAX770/MAX771/MAX772) ♦ Internal Shunt Regulator for High Input Voltages (MAX773) ♦ Preset or Adjustable Output Voltages MAX770: 5V or Adjustable MAX771: 12V or Adjustable MAX772: 15V or Adjustable MAX773: 5V, 12V, 15V, or Adjustable ♦ Current-Limited PFM Control Scheme ♦ 300kHz Switching Frequency ______________Ordering Information PART TEMP. RANGE MAX770CPA 0°C to +70°C MAX770CSA MAX770C/D MAX770EPA MAX770ESA MAX770MJA 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 Plastic DIP 8 SO Dice* 8 Plastic DIP 8 SO 8 CERDIP** Ordering Information continued at end of data sheet. *Contact factory for dice specifications. **Contact factory for availability and processing to MIL-STD-883B. Positive LCD-Bias Generators Portable Communicators Flash Memory Programmers _________________Pin Configurations __________Typical Operating Circuit TOP VIEW INPUT 2V TO VOUT OUTPUT 12V MAX771 ON/OFF EXT SHDN CS REF N EXT 1 8 CS V+ 2 7 GND 6 AGND 5 REF FB 3 SHDN 4 MAX770 MAX771 MAX772 DIP/SO FB AGND GND V+ Pin Configurations continued at end of data sheet. ________________________________________________________________ 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 408-737-7600 ext. 3468. MAX770–MAX773 _______________General Description The MAX770–MAX773 step-up switching controllers provide 90% efficiency over a 10mA to 1A load. A unique current-limited pulse-frequency-modulation (PFM) control scheme gives these devices the benefits of pulse-width-modulation (PWM) converters (high efficiency at heavy loads), while using less than 110µA of supply current (vs. 2mA to 10mA for PWM converters). These ICs use tiny external components. Their high switching frequencies (up to 300kHz) allow surfacemount magnetics of 5mm height and 9mm diameter. The MAX770/MAX771/MAX772 accept input voltages from 2V to 16.5V. Output voltages are preset at 5V, (MAX770), 12V (MAX771), and 15V (MAX772); they can also be adjusted using two resistors. The MAX773 accepts inputs from 3V to 16.5V. For a wider input range, it features an internal shunt regulator that allows unlimited higher input voltages. The MAX773’s output can be set to 5V, 12V, or 15V, or it can be adjusted with two resistors. The MAX770–MAX773 drive external N-channel MOSFET switches, allowing them to power loads up to 15W. If less power is required, use the MAX756/MAX757 or MAX761/MAX762 step-up switching regulators with onboard MOSFETs. MAX770–MAX773 5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers ABSOLUTE MAXIMUM RATINGS Supply Voltages V+ to GND.............................................................-0.3V to 17V V+ to SGND.............................................................-0.3V to 7V SGND........................................................-0.3V to (V+ + 0.3V) EXT, CS, REF, LBO, LBI, SHDN, FB.............-0.3V to (V+ + 0.3V) EXTH, EXTL ..................................................-0.3V to (V+ + 0.3V) V5, V12, V15 .............................................................-0.3V to 17V GND to AGND .........................................................0.1V to -0.1V ISGND ..................................................................................50mA Continuous Power Dissipation (TA = +70°C) 8-Pin Plastic DIP (derate 9.09mW/°C above +70°C)....727mW 8-Pin SO (derate 5.88mW/°C above +70°C) ................471mW 8-Pin CERDIP (derate 8.00mW/°C above +70°C) ........640mW 14-Pin Plastic DIP (derate 10.00mW/°C above +70°C) .............................800mW 14-Pin SO (derate 8.33mW/°C above +70°C) ..............667mW 14-Pin CERDIP (derate 9.09mW/°C above +70°C) ......727mW Operating Temperature Ranges MAX77_C_ _ ........................................................0°C to +70°C MAX77_E_ _......................................................-40°C to +85°C MAX77_MJ_ ...................................................-55°C to +125°C Junction Temperatures MAX77_C_ _/E_ _ ..........................................................+150°C MAX77_MJ_..................................................................+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 Minimum Start-Up Voltage MIN 2.0 16.5 MAX770–772C/E (external resistors) 3.0 16.5 MAX770–772MJA (external resistors) 3.1 16.5 MAX773C/E 3.0 16.5 MAX773MJD 3.1 MAX770/MAX771/MAX772 Supply Current Standby Current Output Voltage (Note 1) TYP MAX UNITS V 16.5 1.8 2.0 V V+ = 16.5V, SHDN = 0V (normal operation) 85 110 µA V+ = 10.0V, SHDN ≥ 1.6V (shutdown) 2 5 V+ = 16.5V, SHDN ≥ 1.6V (shutdown) 4 µA V+ = 2.0V to 5.0V, over full load range 4.80 5.0 5.20 V+ = 2.0V to 12.0V, over full load range 11.52 12.0 12.48 V+ = 2.0V to 15.0V, over full load range 14.40 15.0 15.60 V Output Voltage Line Regulation (Note 2) Figure 2a, V+ = 2.7V to 4.5V, ILOAD = 700mA, VOUT = 5V 5 mV/V Output Voltage Load Regulation (Note 2) Figure 2a, V+ = 3V, ILOAD = 30mA to 1A, VOUT = 5V 20 mV/A Maximum Switch On-Time tON(max) 12 16 20 µs Minimum Switch Off-Time tOFF(min) 1.8 2.3 2.8 µs MAX77_C 1.4700 1.5 1.5300 MAX77_E 1.4625 1.5 1.5375 MAX77_M 1.4550 1.5 1.5450 Efficiency Reference Voltage 2 CONDITIONS MAX770–772 (internal feedback resistors) V+ = 4V, ILOAD = 500mA, VOUT = 5V VREF IREF = 0µA 87 _______________________________________________________________________________________ % V 5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers MAX770–MAX773 ELECTRICAL CHARACTERISTICS (continued) (V+ = 5V, ILOAD = 0mA, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETERS SYMBOL CONDITIONS REF Load Regulation 0µA ≤ IREF ≤ 100µA REF Line Regulation 3V ≤ V+ ≤ 16.5V FB Trip-Point Voltage FB Input Current VFB IFB SHDN Input High Voltage VIH SHDN Input Low Voltage VIL TYP MAX MAX77_C/E MIN 4 10 MAX77_M 4 15 40 100 MAX77_C 1.4700 1.50 1.5300 MAX77_E 1.4625 1.50 1.5375 MAX77_M 1.4550 1.50 1.5450 MAX77_C ±20 MAX77_E ±40 MAX77_M ±60 V+ = 2.0V to 16.5V 1.6 UNITS mV µV/V V nA V MAX77_C/E, V+ = 2.0V to 16.5V 0.4 MAX77_M, V+ = 2.0V to 16.5V 0.2 SHDN Input Current V+ = 16.5V, SHDN = 0V or V+ ±1 µA LBI Input Current MAX773, V+ = 16.5V, LBI = 1.5V ±20 nA LBI Hysteresis MAX773 LBI Delay 5mV overdrive 20 V mV 2.5 µs MAX77_C 1.4700 1.50 1.5300 MAX77_E 1.4625 1.50 1.5375 MAX77_M 1.4550 LBI Threshold Voltage MAX773, LBI falling 1.50 1.5450 LBO Leakage Current MAX773, V+ = 16.5V, VLBO = 16.5V 0.01 1.00 µA 0.1 0.4 V 200 230 mV 0.01 ±1 µA LBO Output Voltage Low VOL MAX773, V+ = 5V, LBO sinking 1mA Current-Limit Trip Level VCS V+ = 5V to 16.5V 170 CS Input Current V EXT Rise Time V+ = 5V, 1nF from EXT to ground (Note 3) 55 ns EXT Fall Time V+ = 5V, 1nF from EXT to ground (Note 3) 55 ns Supply Voltage in Shunt Mode VSHUNT MAX773, ISHUNT = 1mA to 20mA, SGND = 0V, CSHUNT = 0.1µF 5.5 6.3 V Note 1: Output voltage guaranteed using preset voltages. See Figures 7a–7d for output current capability versus input voltage. Note 2: Output voltage line and load regulation depend on external circuit components. Note 3: For the MAX773, EXT is EXTH and EXTL shorted together. _______________________________________________________________________________________ 3 __________________________________________Typical Operating Characteristics (TA = +25°C, unless otherwise noted.) MAX771 EFFICIENCY vs. OUTPUT CURRENT (BOOTSTRAPPED) 50 0.1 1 EFFICIENCY (%) 60 VIN = 3V 50 0.001 0.01 MAX771 EFFICIENCY vs. OUTPUT CURRENT (NON-BOOTSTRAPPED) MAX770–3-04 VIN = 9V 90 VIN = 6V VIN = 5V 80 VOUT = 12V CIRCUIT OF FIGURE 2b 50 0.1 1 0.1 VOUT = 5V CIRCUIT OF FIGURE 2a 500 400 300 ABOVE 3.4V, THE CIRCUIT STARTS UP UNDER MAXIMUM LOAD CONDITIONS 200 1.0 2 SCHOTTKY DIODE LEAKAGE EXCLUDED 50 75 100 125 TEMPERATURE (°C) 4 ABOVE 3.5V THE CIRCUIT STARTS UP UNDER MAXIMUM LOAD CONDITIONS 200 0 0.4 0.2 2.5 2.0 3.5 BOOTSTRAPPED CIRCUIT OF FIGURE 2b 0.6 3.0 3.5 4.0 MINIMUM START-UP INPUT VOLTAGE (V) EXT RISE/FALL TIME vs. SUPPLY VOLTAGE 250 200 CEXT = 2200pF CEXT = 1000pF 150 CEXT = 446pF CEXT = 100pF 100 NON-BOOTSTRAPPED CIRCUIT OF FIGURE 2c 0 25 300 50 0 0 3.0 VOUT = 12V SUPPLY CURRENT (mA) ENTIRE CIRCUIT -75 -50 -25 2.5 SUPPLY CURRENT vs. SUPPLY VOLTAGE 3 1 2.0 VOUT = 12V CIRCUIT OF FIGURE 2b 400 100 0.8 MAX770–3-07 VOUT = 12V, VIN = 5V CIRCUIT OF FIGURE 2b BOOTSTRAPPED MODE 1.5 500 MINIMUM START-UP INPUT VOLTAGE (V) SUPPLY CURRENT vs. TEMPERATURE 1 MAX771 LOAD CURRENT vs. MINIMUM START-UP INPUT VOLTAGE OUTPUT CURRENT (A) 4 0.1 MAX770 LOAD CURRENT vs. MINIMUM START-UP INPUT VOLTAGE 600 10 0.01 OUTPUT CURRENT (A) 0 0.01 0.001 1 OUTPUT CURRENT (A) 100 70 0.001 VIN = 12V VIN = 9V VIN = 6V VIN = 5V VIN = 3V 60 700 LOAD CURRENT (mA) EFFICIENCY (%) VOUT = 12V CIRCUIT OF FIGURE 2c 70 VIN = 5V OUTPUT CURRENT (A) 100 80 MAX770–3-09 0.01 70 EXT RISE/FALL TIME (ns) 0.001 80 MAX770–3-06 VOUT = 5V CIRCUIT OF FIGURE 2a 60 90 LOAD CURRENT (mA) VIN = 3V MAX770–3-02 90 VOUT = 15V, CIRCUIT OF FIGURE 2b MAX772 SUBSTITUTED FOR MAX771 MAX770–3-05 VIN = 3.5V 70 100 MAX770–3-08 EFFICIENCY (%) 80 VIN = 6V VIN = 9V EFFICIENCY (%) VIN = 4V 90 100 MAX770–3-01 100 MAX772 EFFICIENCY vs. OUTPUT CURRENT (BOOTSTRAPPED) MAX770–3-03 MAX770 EFFICIENCY vs. OUTPUT CURRENT (BOOTSTRAPPED) SUPPLY CURRENT (mA) MAX770–MAX773 5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers 0 2 4 6 8 SUPPLY VOLTAGE (V) 10 12 2 4 6 8 V+ (V) _______________________________________________________________________________________ 10 12 5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers REFERENCE OUTPUT RESISTANCE vs. TEMPERATURE REFERENCE vs. TEMPERATURE 200 1.504 10µA REFERENCE (V) 1.502 150 100 50µA 1.500 1.498 1.496 100µA 50 MAX770–3-11 1.506 MAX770–3-10 REFERENCE OUTPUT RESISTANCE (Ω) 250 1.494 1.492 0 -60 -40 -20 -60 -40 -20 0 20 40 60 80 100 120 140 0 20 40 60 80 100 120 140 TEMPERATURE (°C) TEMPERATURE (°C) MAXIMUM SWITCH ON-TIME vs. TEMPERATURE SHUTDOWN CURRENT vs. TEMPERATURE MAX770–3-12 4.0 MAX770–3-13 16.5 3.5 ICC (µA) tON(MAX) (µs) 3.0 16.0 2.5 2.0 V+ = 15V 1.5 V+ = 8V 1.0 0.5 15.5 -60 -30 0 30 60 90 120 150 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) TEMPERATURE (°C) MINIMUM SWITCH OFF-TIME vs. TEMPERATURE MAXIMUM SWITCH ON-TIME/ MINIMUM SWITCH OFF-TIME RATIO vs. TEMPERATURE 2.25 2.20 MAX770–3-15 8.0 tON(MAX)/tOFF(MIN) RATIO MAX770–3-14 2.30 tOFF(MIN) (µs) V+ = 4V 0 7.5 7.0 6.5 6.0 -60 -30 0 30 60 90 TEMPERATURE (°C) 120 150 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) _______________________________________________________________________________________ 5 MAX770–MAX773 ____________________________Typical Operating Characteristics (continued) (TA = +25°C, unless otherwise noted.) MAX770–MAX773 5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers ____________________________Typical Operating Characteristics (continued) (Circuit of Figure 2a, TA = +25°C, unless otherwise noted.) MAX770 LIGHT-LOAD SWITCHING WAVEFORMS MAX770 HEAVY-LOAD SWITCHNG WAVEFORMS VOUT 0 ILIM A A ILIM ILIM 2 B 2 B 0 0 C C 20µs/div V+ = 3V, IOUT = 165mA A: EXT VOLTAGE, 5V/div B: INDUCTOR CURRENT, 1A/div C: VOUT RIPPLE 100mV/div, AC-COUPLED 20µs/div VIN = 2.9V, IOUT = 0.9A A: EXT VOLTAGE, 5V/div B: INDUCTOR CURRENT 1A/div C: VOUT RIPPLE 100mV/div, AC-COUPLED MAX770 LINE-TRANSIENT RESPONSE MAX770 LOAD-TRANSIENT RESPONSE A 4.5V 2.7V A 0 0 B B 2ms/div IOUT = 0.7A A: VIN, 2.7V TO 4.5V, 2V/div B: VOUT RIPPLE, 100mV/div, AC-COUPLED 6 2ms/div VIN = 3V A: LOAD CURRENT 0.5A/div (0A to 1A) B: VOUT RIPPLE, 100mV/div, AC-COUPLED _______________________________________________________________________________________ 5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers MAX770 EXITING SHUTDOWN A 0 B 0 200µs/div VIN = 3V, IOUT = 0.5A A: SHDN, 2V/div B: VOUT, 2V/div ______________________________________________________________Pin Description PIN MAX770 MAX771 MAX772 MAX773 1 NAME FUNCTION — EXT 2 3 V+ 3 6 FB 4 7 SHDN 5 8 REF 6 — AGND Gate drive for external N-channel power transistor Power-supply input. Also acts as a voltage-sense point when in bootstrapped mode for the MAX770/MAX771/MAX772, or as a shunt regulator when SGND is connected to ground for the MAX773. Bypass to SGND with 0.1µF when using the shunt regulator. 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. 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 7 9 GND 8 11 CS — 1 V12 — 2 V5 — 4 LBO — 5 LBI — 10 SGND High-current ground return for the output driver Positive input to the current-sense amplifier. Connect the current-sense resistor between CS and GND. Input sense point for 12V-output operation. Connect VOUT to V12 for 12V-output operation. Leave unconnected for adjustable-output operation. Input sense point for 5V-output operation. Connect VOUT to V5 for 5V-output operation. Leave unconnected for adjustable-output operation. Low-battery output is an open-drain output that goes low when LBI is less than 1.5V. Connect to V+ through a pull-up resistor. Leave floating if not used. LBO is high impedance in shutdown mode. Input to the internal low-battery comparator. Tie to GND or V+ if not used. Shunt regulator ground. Leave unconnected if the shunt regulator is not used. _______________________________________________________________________________________ 7 MAX770–MAX773 ____________________________Typical Operating Characteristics (continued) (Circuit of Figure 2a, TA = +25°C, unless otherwise noted.) MAX770–MAX773 5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers _________________________________________________Pin Description (continued) PIN MAX770 MAX771 MAX772 MAX773 — 12 EXTL — 13 EXTH — 14 V15 NAME FUNCTION Low-level gate/base drive for external power transistor. Connect to the gate of an external N-channel MOSFET or to the base of an external NPN transistor. High-level gate/base drive for external power transistor. Connect to EXTL when using an external N-channel MOSFET. When using an external NPN transistor, connect a resistor RBASE from EXTH to the base of the NPN to set the maximum base-drive current. Input sense point for 15V-output operation. Connect VOUT to V15 for 15V-output operation. Leave unconnected for adjustable-output operation _______________Detailed Description The MAX770–MAX773 are BiCMOS, step-up, switchmode power-supply controllers that provide preset 5V, 12V, and 15V output voltages, in addition to adjustableoutput operation. Their 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 MAX770–MAX773 block diagram. The MAX770–MAX773 offer three main improvements over prior pulse-skipping control solutions: 1) the converters operate with tiny (5mm height and less than 9mm diameter) surface-mount inductors due to their 300kHz switching frequency; 2) the current-limited PFM control scheme allows 87% efficiencies over a wide range of load currents; and 3) the maximum supply current is only 110µA. The MAX773 can be configured to operate from an internal 6V shunt regulator, allowing very high input/output voltages. Its output can be configured for an adjustable voltage or for one of three fixed voltages (5V, 12V, or 15V), and it has a power-fail comparator for low-battery detection. All devices have shutdown capability, reducing the supply current to 5µA max. Bootstrapped/Non-Bootstrapped Modes Figures 2 and 3 show 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 8 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 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 for the MAX770–MAX773 is 3V when using external feedback resistors. With supply voltages below 5V, bootstrapped mode is recommended. Note: When using the MAX770/MAX771/MAX772 in non-bootstrapped mode, there is no preset output operation because V+ is also the output voltage sense point 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 2c, 2d, 3b, 3d, 3e) to achieve an overall output voltage accuracy of ±5%. The MAX773 can be operated in non-bootstrapped mode without using external feedback resistors because V+ does not act as the output voltage sense point with preset-output operation. To achieve highest efficiency, operate in bootstrapped mode whenever possible. MAX773 Shunt-Regulator Operation The MAX773 has an internal 6V shunt regulator that allows the device to step up from very high input voltages (Figure 4). _______________________________________________________________________________________ 5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers V12 V5 MAX770–MAX773 LBO V15 FB V+ DUAL-MODE COMPARATOR MAX773 ONLY LBI N MAX770–MAX773 N SHDN 200mV BIAS CIRCUITRY 1.5V REFERENCE REF ERROR COMPARATOR ONE-SHOT Q TRIG N MAX770 MAX771 MAX772 V+ 6V SGND F/F S Q EXT CONTROL R EXTH LOW-VOLTAGE OSCILLATOR 2.5V ONE-SHOT TRIG MAX773 ONLY EXTL Q CURRENT-SENSE AMPLIFIER 0.2V 0.1V EXT MAX770 MAX771 MAX772 CS Figure 1. Block Diagram Floating the shunt-regulator ground (SGND) disables the shunt regulator. To enable it, connect SGND to GND. The shunt regulator requires 1mA minimum current for proper operation; the maximum current must not exceed 20mA. The MAX773 operates in non-bootstrapped mode when the shunt regulator is used, and EXT swings between the 6V shunt-regulator voltage and GND. When using the shunt regulator, use an N-channel power FET instead of an NPN power transistor as the power switch. Otherwise, excessive base drive will collapse the shunt regulator. External Power-Transistor Control Circuitry PFM Control Scheme The MAX770–MAX773 use 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 MAX770– MAX773 use a sense resistor to control the peak inductor current. They also operate with high switching _______________________________________________________________________________________ 9 MAX770–MAX773 5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers VIN = 5V VIN = 3V C2 0.1µF V+ 5 REF C3 0.1µF C2 0.1µF 2 4 SHDN L1 22µH MAX770 3 FB C1 100µF EXT 1 2 V+ 5 REF D1 1N5817 VOUT = 5V @ 1A C3 0.1µF 4 SHDN MAX771 3 FB N EXT 1 CS 6 AGND GND C4 300µF CS 8 RSENSE 100mΩ GND 7 Figure 2b. 12V Preset Output, Bootstrapped VIN = 4V VIN = 5V C1 68µF C2 0.1µF C2 0.1µF 2 L1 22µH V+ 5 REF 4 SHDN MAX770 MAX771 MAX772 6 AGND EXT CS FB 1 2 D1 1N5817 VOUT = 12V @ 0.5A C4 200µF N 5 REF C3 0.1µF 3 4 SHDN MAX770 6 RSENSE 100mΩ 7 L1 20µH V+ MAX771 MAX772 8 GND R2 = (R1) C4 200µF 7 Figure 2a. 5V Preset Output, Bootstrapped C3 0.1µF VOUT = 12V @ 0.5A Si9410DY 8 RSENSE 75mΩ D1 1N5817 N MTP3055EL 6 AGND C1 68µF L1 22µH EXT CS AGND FB VREF = 1.5V N Si9410DY R2 = (R1) VOUT = 9V C4 100µF 8 3 GND 7 R1 18k REF D1 1N5817 RSENSE R2 127k ( VVOUT -1) 1 C1 47µF R2 140k R1 28k ( VVOUT -1) REF VREF = 1.5V Figure 2c. 12V Output, Non-Bootstrapped Figure 2d. 9V Output, Bootstrapped frequencies (up to 300kHz), allowing the use of tiny external components. As with traditional PFM converters, the power transistor is not turned on until the voltage comparator senses that the output is out of regulation. However, unlike traditional PFM converters, the MAX770–MAX773 switch using the combination of a peak current limit and a pair of one-shots that set the maximum on-time (16µs) and minimum off-time (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. 10 The control circuitry allows the ICs to operate in continuous-conduction mode (CCM) while maintaining high efficiency with heavy loads. When the power switch is ______________________________________________________________________________________ 5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers 1 V12 14 V15 SGND 8 REF MAX773 C3 0.1µF R3 10k (1%) C1 47µF VIN 10 C1 100k L1 22µH D1 1N5817 LBO 4 VOUT = 12V Si9410DY 5 LBI EXTH 13 10 SGND 4 LBO C3 0.1µF N C4 CS 11 7 FB LBI RSENSE GND 9 ZTX694B CS 11 V15 14 V12 1 SHDN FB R4 = R3 RSENSE 0.4Ω 6 R1 34k GND 9 NOMINAL MAX 11.0 11.4 C4 150µF 12 V5 2 VTRIP (V) MIN 10.6 D1 VOUT = 24V 1N5818 @ 30mA 910Ω EXTH 13 EXTL 8 REF MAX773 5 EXTL 12 7 SHDN 6 L1 150µH C2 0.1µF 3 V+ 2 V5 R4 63.4k (1%) C2 0.1µF 3 V+ V R2 510k ( VVOUT -1) R2 = (R1) -1) ( VTRIP REF REF VREF = 1.5V VREF = 1.5V Figure 3a. 12V Preset Output, Bootstrapped, N-Channel Power MOSFET Figure 3b. 24V Output, Non-Bootstrapped, NPN Power Transistor VIN VIN = 5V C1 C1 C2 0.1µF 10 SGND 3 V+ 4 LBO 7 6 5 D1 1N5817 VOUT = 15V EXTH 13 N EXTL 12 Si9410DY 4 LBO C4 MAX773 RSENSE SHDN V15 14 FB V12 1 V5 2 LBI GND 9 7 10 EXTH 13 EXTL 12 VOUT = 16V D1 1N5817 N Si9410DY C4 CS 11 8 REF C3 0.1µF 5 L1 20µH C2 0.1µF 3 V+ CS 11 8 REF C3 0.1µF L1 22µH RSENSE MAX773 V15 14 V12 1 LBI SHDN V5 2 R2 133k FB 6 SGND GND 9 R1 13.7k R2 = (R1) ( VVOUT -1) VREF = 1.5V Figure 3c. 15V Preset Output, Non-Bootstrapped N-Channel Power MOSFET REF Figure 3d. 16V Output, Bootstrapped, N-Channel Power MOSFET ______________________________________________________________________________________ 11 MAX770–MAX773 VIN = 5V MAX770–MAX773 5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers VIN = 24V TO 28V C1 47µF C2 0.1µF RSHUNT 3k D1 MUR115 3 10 4 LBO RSHUNT MAX773 EXTH 13 EXTL 12 N Si9420DY 3 V+ C4 100µF CS 11 8 REF MAX773 V15 14 C2 0.1µF RSENSE 1.0Ω 6V (typ) V12 1 5 LBI 7 VOUT = 100V @ 10mA v+ SGND C3 0.1µF VIN L1 250µH V5 2 6 FB SHDN GND 9 10 SGND R2 732k (1%) R1 11.3k (1%) RSHUNT = R2 = (R1) ( VREF = 1.5V VOUT -1 VREF ) VIN (MIN) - VSHUNT (MAX) I SHUNT * * SEE TEXT FOR ISHUNT CALCULATION Figure 3e. 100V Output, Shunt Regulator, N-Channel Power MOSFET Figure 4. MAX773 Shunt Regulator 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. Low-Voltage Start-Up Oscillator The MAX770/MAX771/MAX772 feature 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 MAX770/MAX771/MAX772 disregard the error-comparator output when the supply voltage is less than 2.5V. Above 2.5V, the error-comparator and normal oneshot timing circuitry are used. The low voltage start-up circuitry is disabled if non-bootstrapped mode is selected (FB is not tied to ground). To increase light-load efficiency, the current limit for the first two pulses is set to one-half the peak current limit. If those pulses bring the output voltage into regulation, the error comparator holds the MOSFET off and the current limit remains at one-half the peak current limit. If the output voltage is still out of regulation after two pulses, the current limit for the next pulse is raised to the peak current limit set by the external sense resistor (see inductor current waveforms in the Typical Operating Characteristics). The MAX770–MAX773 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 180mV of output ripple occurs using the circuit of Figure 2b. 12 The MAX773 does not provide the low-voltage 50% duty-cycle oscillator. Its minimum start-up voltage is 3V for all modes. External Transistor An N-FET power switch is recommended for the MAX770/MAX771/MAX772. The MAX773 can drive either an N-channel MOSFET (N-FET) or an NPN because it provides two separate ______________________________________________________________________________________ 5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers Shutdown Mode When SHDN is high, the MAX770–MAX773 enter shutdown mode. In this mode, the internal biasing circuitry is turned off (including the reference) and V OUT falls to a diode drop below VIN (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. The MAX773’s shunt regulator is not disabled in shutdown mode. Low-Battery Detector The MAX773 provides a low-battery comparator that compares the voltage on LBI to the reference voltage. When the LBI voltage is below VREF, LBO (an opendrain output) goes low. The low-battery comparator’s 20mV of hysteresis adds noise immunity, preventing repeated triggering of LBO. Use a resistor-divider network between V+, LBI, and GND to set the desired trip voltage VTRIP. LBO is high impedance in shutdown mode. __________________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 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. Use the MAX770/MAX771/MAX772 unless one or more of the following conditions applies. If one or more of the following is true, use the MAX773: 1) An NPN power transistor will be used as the power switch 2) The LBI/LBO function is required 3) The shunt regulator must accommodate a high input voltage 4) Preset-output non-bootstrapped operation is desired—for example, to reduce the no-load supply current in a 5V to 12V application. R2 VOUT FB R1 MAX770 MAX771 MAX772 MAX773 R1 = 10k TO 500k GND R2 = R1 V -1) ( VOUT REF VREF = 1.5V Figure 5. Adjustable Output Circuit See Table 1 for a summary of operating characteristics and requirements for the ICs in bootstrapped and nonbootstrapped modes. The MAX770–MAX773’s output voltage can be adjusted from very high voltages down to 3V, using external resistors R1 and R2 configured as shown in Figure 5. For adjustable-output operation, select feedback resistor R1 in the range of 10kΩ to 500kΩ. 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 for the MAX770/MAX771/MAX772). Configure the MAX773 for a preset voltage of 5V, 12V, or 15V by connecting the output to the corresponding sense input pin (i.e., V5, V12, or V15). FB must be tied to ground for preset-output operation. Leave all unused sense input pins unconnected. Failure to do so will cause an incorrect output voltage. The MAX773 can provide a preset output voltage in both bootstrapped and nonbootstrapped modes. Figures 2 and 3 show various circuit configurations for bootstrapped/non-bootstrapped, preset/adjustable operation. Shunt-Regulator Operation When using the shunt regulator, connect SGND to ground and place a 0.1µF capacitor between V+ and SGND, as close to the IC as possible. Increase C2 to 1.0µF to improve shunt regulators performance with heavy loads. Select RSHUNT such that 1mA ≤ ISHUNT ≤ 20mA. ______________________________________________________________________________________ 13 MAX770–MAX773 drive outputs (EXTH and EXTL) that operate 180° out of phase (Figures 3a and 3b). In Figure 3b, the resistor in series with EXTH limits the base current, and EXTL (which is connected directly to the base) turns the transistor off. MAX770–MAX773 5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers Table 1. Bootstrapped vs. Non-Bootstrapped Operation PARAMETER BOOTSTRAPPED* NON-BOOTSTRAPPED Gate Drive GND to VOUT GND to V+ FET On Resistance Lower Higher Gate-Drive Capacitive Losses Higher Lower No-Load Supply Current Higher 2V to 16.5V (MAX770/MAX771/MAX772), (internal feedback resistors) 3V to 16.5V (MAX770/MAX771/MAX772), (external feedback resistors) 3V to 16.5V (MAX773) Lower Possible Input Voltage Range 3V to 16.5V (MAX770/MAX771/MAX772), 3V and up (MAX773) Normally Recommended Input Voltage Range 2V to 5V (MAX770/MAX771/MAX772), 3V to 5V (MAX773) 5V to 16.5V (MAX770/MAX771/MAX772), 5V and up (MAX773) Fixed Output Available MAX770–MAX773(N) MAX773(N)/MAX773(S) Adjustable Output Available MAX770–MAX773(N) MAX770/MAX771/MAX772/ MAX773(N)/MAX773(S) *MAX773(S) indicates shunt mode; MAX773(N) indicates NOT in shunt mode. Use an N-channel FET as the power switch when using the shunt regulator (see MAX773 Shunt-Regulator Operation in the Detailed Description). The shunt-regulator current powers the MAX773 and also provides the FET gate-drive current, which depends largely on the FET’s total gate charge at VGS = 5V. To determine the shunt-resistor value, first determine the maximum shunt current required. ISHUNT = ISUPP + IGATE See N-Channel MOSFETs in the Power-Transistor Selection section to determine IGATE. Determine the shunt-resistor value using the following equation: VIN C2 0.1µF RSHUNT 3 V+ SGND EXTH EXTL 10 L1 20µH 13 12 NPN 2N2222A 100Ω CS where VSHUNT(max) is 6.3V. The shunt regulator is not disabled in shutdown mode, and continues to draw the calculated shunt current. If the calculated shunt regulator current exceeds 20mA, or if the shunt current exceeds 5mA and less shunt regulator current is desired, use the circuit of Figure 6 to provide increased drive and reduced shunt current when driving N-FETs with large gate capacitances. Select ISHUNT = 3mA. This provides adequate biasing current for this circuit, although higher shunt currents can be used. FB D1 VOUT N PNP 2N2907A MAX773 VIN(min) - VSHUNT(max) RSHUNT(max) = ———————————— ISHUNT 14 C1 R2 C4 11 RSENSE 6 R1 Figure 6. Increased N-FET Gate Drive when Using the Shunt Regulator To prevent the shunt regulator from drawing current in shutdown mode, place a switch in series with the shunt resistor. ______________________________________________________________________________________ 5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers 3.0 MAXIMUM OUTPUT CURRENT (A) MAXIMUM OUTPUT CURRENT (A) 3.5 VOUT = 5V L = 22µH RSENSE = 40mΩ 2.5 RSENSE = 50mΩ 2.0 RSENSE = 75mΩ 1.5 1.0 RSENSE = 100mΩ VOUT = 12V L = 22µH 3.0 RSENSE = 40mΩ RSENSE = 50mΩ 2.5 RSENSE = 75mΩ 2.0 1.5 1.0 RSENSE = 100mΩ 0.5 0.5 RSENSE = 200mΩ RSENSE = 200mΩ 0 0 2 3 4 INPUT VOLTAGE (V) 3.5 2 5 Figure 7a. Maximum Output Current vs. Input Voltage (VOUT = 5V) RSENSE = 40mΩ RSENSE = 75mΩ 2.0 10 12 VOUT = 24V L =150µH RSENSE = 50mΩ 2.5 6 8 INPUT VOLTAGE (V) 0.8 VOUT = 15V L = 22µH 3.0 4 Figure 7b. Maximum Output Current vs. Input Voltage (VOUT = 12V) 1.5 1.0 RSENSE = 100mΩ MAXIMUM OUTPUT CURRENT (A) MAXIMUM OUTPUT CURRENT (A) MAX770–MAX773 3.5 0.6 RSENSE = 100mΩ RSENSE = 200mΩ 0.4 0.2 0.5 RSENSE = 400mΩ RSENSE = 200mΩ 0 0 2 4 6 8 10 12 INPUT VOLTAGE (V) 14 16 2 6 10 14 INPUT VOLTAGE (V) Figure 7c. Maximum Output Current vs. Input Voltage (VOUT = 15V) Figure 7d. Maximum Output Current vs. Input Voltage (VOUT = 24V) Determining RSENSE The Typical Operating Characteristics graphs show the output current capability for various modes, sense resistors, and input/output voltages. Use these graphs, along with the theoretical output current curves shown in Figures 7a-7d, to select RSENSE. These theoretical curves assume that an external N-FET power switch is used. They were derived using the minimum (worstcase) current-limit comparator threshold value, and the inductance value. 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. To use the graphs, locate the graph with the appropriate output voltage or the graph having the nearest output voltage higher than the desired output voltage. On this graph, find the curve for the largest sense-resistor value with an output current that is adequate at the lowest input voltage. Determining the Inductor (L) Practical inductor values range from 10µH to 300µH. 20µH 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 (tON(min)) is approximately 2µs; select an inductor that allows the current to ramp up to I LIM/2 in no less than 2µs. Choosing a value of ILIM/2 allows the half-size current pulses to occur, increasing light-load efficiency and minimizing output ripple. ______________________________________________________________________________________ 15 MAX770–MAX773 5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers IC(PEAK) MAX770 MAX771 MAX772 L MAX773 IB EXT N RBASE EXTH NPN EXTL CS CS RSENSE RSENSE Figure 8a. Use an N-Channel MOSFET with the MAX770/MAX771/MAX772 L MAX773 Figure 8c. Using an NPN Transistor with the MAX773 preferably under 20mΩ. To minimize radiated noise, use a toroid, a pot core, or a shielded coil. Table 2 lists inductor suppliers and specific recommended inductors. Power Transistor Selection EXTH N EXTL CS RSENSE Figure 8b. Using an N-Channel MOSFET with the MAX773 The standard operating circuits use a 22µH inductor. If a different inductance value is desired, select L such that: VIN(max) x tON(min) L ≥ —————————— ILIM / 2 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. 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, 16 Use an N-channel MOSFET power transistor with the MAX770/MAX771/MAX772 (Figure 8a). Use an N-FET whenever possible with the MAX773. An NPN transistor can be used, but be extremely careful when determining the base current (see NPN Transistors section). An NPN transistor is not recommended when using the shunt regulator. N-Channel MOSFETs 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, but they are typically more expensive than NPN transistors. When using an N-FET with the MAX773, connect EXTH and EXTL to the N-FET’s gate (Figure 8b). 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 with various capacitive loads as shown in the Typical Operating Characteristics. ______________________________________________________________________________________ 5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers Continuing with the example, ∆V+ = 17nC/0.1µF = 170mV. Use I GATE when calculating the appropriate shunt resistor. See the Shunt Regulator Operation section. Figure 2a’s application circuit uses an MTD3055EL logic-level N-FET with a guaranteed threshold voltage (V TH) of 2V. Figure 2b’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. NPN Transistors The MAX773 can drive NPN transistors, but be extremely careful when determining the base-current requirements. Too little base current can cause excessive power dissipation in the transistor; too much base current can cause the base to oversaturate, so the transistor remains on continually. Both conditions can damage the transistor. When using the MAX773 with an NPN transistor, connect EXTL to the transistor’s base, and connect RBASE between EXTH and the base (Figure 8c). To determine the required peak inductor current, IC(PEAK), observe the Typical Operating Characteristics efficiency graphs and the theoretical output current capability vs. input voltage graphs to determine a sense resistor that will allow the desired output current. Divide the 170mV worst-case (smallest) voltage across the current-sense amplifier VCS(max) by the senseresistor value. To determine IB, set the peak inductor current (ILIM) equal to the peak transistor collector cur- rent IC(PEAK). Calculate IB as follows: IB = ILIM/ß Use the worst-case (lowest) value for ß given in the transistor’s electrical specification, where the collector current used for the test is approximately equal to ILIM. It may be necessary to use even higher base currents (e.g., IB = ILIM /10), although excessive IB may impair operation by extending the transistor’s turn-off time. RBASE is determined by: (VEXTH - VBE - VCS(min )) RBASE = ————————————– IB Where V EXTH is the voltage at V+ (in bootstrapped mode VEXTH is the output voltage), VBE is the 0.7V transistor base-emitter voltage, VCS(min) is the voltage drop across the current-sense resistor, and I B is the minimum base current that forces the transistor into saturation. This equation reduces to (V+ - 700mV 170mV) / IB. For maximum efficiency, make RBASE as large as possible, but small enough to ensure the transistor is always driven near saturation. Highest efficiency is obtained with a fast-switching NPN transistor (fT ≥ 150MHz) with a low collector-emitter saturation voltage and a high current gain. A good transistor to use is the Zetex ZTX694B. Diode Selection The MAX770–MAX773’s high switching frequency demands a high-speed rectifier. Schottky diodes such as the 1N5817–1N5822 are recommended. Make sure that the Schottky diode’s average current rating exceeds the peak current limit set by RSENSE, and that its breakdown voltage exceeds VOUT. For high-temperature applications, Schottky diodes may be inadequate due to their high leakage currents; high-speed silicon diodes may 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. Capacitor Selection Output Filter Capacitor The primary criterion for selecting the output filter capacitor (C2) 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. An OS-CON 300µF, 6.3V output filter capacitor has approximately 50mΩ of ESR and typically provides 180mV ripple when stepping up from 3V to 5V at 1A (Figure 2a). ______________________________________________________________________________________ 17 MAX770–MAX773 The two most significant losses contributing to the N-FET’s power dissipation are I2R losses and switching losses. Select a transistor with low r DS(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 MAX773’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. The bypass capacitor on V+ (C2) must instantaneously furnish the gate charge without excessive droop (e.g., less than 200mV): Qg ∆V+ = —— C2 MAX770–MAX773 5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers Smaller 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. The smallest low-ESR surface-mount tantalum capacitors currently available are the Sprague 595D series. Sanyo OS-CON organic semiconductor through-hole capacitors and the Nichicon PL series also exhibit low ESR. See Table 2. 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 MAX770–MAX773. The input voltage source impedance determines the size of the capacitor required at the V+ input. As with the output filter capacitor, a lowESR capacitor is recommended. For output currents up to 1A, 150µF (C1) is adequate, although smaller bypass capacitors may also be acceptable. Bypass the IC with a 0.1µF ceramic capacitor (C2) placed close to the V+ and GND pins. Reference Capacitor Bypass REF with a 0.1µF capacitor (C3). REF can source up to 100µA of current. Setting the Low-Battery-Detector Voltage To set the low-battery detector’s falling trip voltage (VTRIP(falling)), select R3 between 10kΩ and 500kΩ (Figure 9), and calculate R4 as follows: R4 = (R3) V -V (——————— ) V TRIP REF REF where VREF = 1.5V. The rising trip voltage is higher because of the comparator’s approximately 20mV of hysteresis, and is determined by: R4 VTRIP (rising) = (VREF + 20mV) (1 + ——) R3 Connect a high value resistor (larger than R3 + R4) between LBI and LBO if additional hysteresis is required. Connect a pull-up resistor (e.g., 100kΩ) between LBO and V+. Tie LBI to GND and leave LBO floating if the low-battery detector is not used. 18 VIN V+ R4 LBI MAX773 R5 100k LBO R3 LOW-BATTERY OUTPUT GND R4 = R3 V ( VTRIP -1) REF VREF = 1.5V Figure 9. Input Voltage Monitor Circuit __________Applications Information MAX773 Operation with High Input/Output Voltages The MAX773’s shunt regulator input allows high voltages to be converted to very high voltages. Since the MAX773 runs off the 6V shunt (bootstrapped operation is not allowed), the IC will not see the high input voltage. Use an external logic-level N-FET as the power switch, since only 6V of VGS are available. Also, make sure all external components are rated for very high output voltage. Figure 3e shows a circuit that converts 28V to 100V. 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. ______________________________________________________________________________________ 5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers Layout Considerations 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+. Table 2. Component Suppliers PRODUCTION INDUCTORS CAPACITORS TRANSISTORS Surface Mount Sumida CD54 series CDR125 series Coiltronics CTX20 series Matsuo 267 series Sprague 595D series N-FET Siliconix Si9410DY Si9420DY (high voltage) Motorola MTP3055EL MTD20N03HDL Through Hole Sumida RCH855 series RCH110 series Renco RL1284-18 Sanyo OS-CON series Nichicon PL series United Chemi-Con LXF series NPN Zetex ZTX694B SUPPLIER PHONE DIODES Nihon EC10 series Motorola 1N5817–1N5822 MUR115 (high voltage) FAX Coiltronics USA: (561) 241-7876 (561) 241-9339 Matsuo USA: (714) 969-2491 Japan: 81-6-337-6450 (714) 960-6492 81-6-337-6456 Nichicon USA: (847) 843-7500 (847) 843-2798 Nihon USA: (805) 867-2555 (805) 867-2698 Renco USA: (516) 586-5566 (516) 586-5562 Sanyo USA: (619) 661-6835 Japan: 81-7-2070-6306 (619) 661-1055 81-7-2070-1174 Sumida USA: (847) 956-0666 Japan: 81-3-3607-5111 81-3-3607-5144 United Chemi-Con USA: (714) 255-9500 (714) 255-9400 Zetex USA: (516) 543-7100 UK: 44-61-627-4963 (516) 864-7630 44-61-627-5467 ______________________________________________________________________________________ 19 MAX770–MAX773 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 MAX770–MAX773 are not designed to start up under full load in bootstrapped mode with low input voltages. MAX770–MAX773 5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers ___Ordering Information (continued) PART TEMP. RANGE MAX771CPA 0°C to +70°C MAX771CSA MAX771C/D MAX771EPA MAX771ESA MAX771MJA MAX772CPA MAX772CSA MAX772C/D MAX772EPA MAX772ESA MAX772MJA MAX773CPD MAX773CSD MAX773C/D MAX773EPD MAX773ESD MAX773MJD 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 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 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 _________________Chip Topographies MAX770/MAX771/MAX772 PIN-PACKAGE 8 Plastic DIP 8 SO Dice* 8 Plastic DIP 8 SO 8 CERDIP 8 Plastic DIP 8 SO Dice* 8 Plastic DIP 8 SO 8 CERDIP 14 Plastic DIP 14 SO Dice* 14 Plastic DIP 14 Narrow SO 14 CERDIP EXT V+ CS 0.126" (3.200mm) GND AGND FB SHDN 0.080" REF (2.032mm) *Contact factory for dice specifications. ____Pin Configurations (continued) TRANSISTOR COUNT: 501; SUBSTRATE CONNECTED TO V+. MAX773 TOP VIEW V5 V12 V15 EXTH V12 1 14 V15 V5 2 13 EXTH V+ 3 12 EXTL LBO 4 MAX773 LBI 5 EXTL V+ LBO CS SGND LBI 0.126" 11 CS (3.200mm) 10 SGND FB 6 9 SHDN 7 8 GND GND REF GND FB DIP/SO SHDN 0.080" REF (2.032mm) TRANSISTOR COUNT: 501; SUBSTRATE CONNECTED TO V+. 20 ______________________________________________________________________________________