19-0201; Rev 0; 11/93 it K tion lua able a v E il Ava 12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Converters _________________________Applications Flash Memory Programming PCMCIA Cards Battery-Powered Applications High-Efficiency DC-DC Converters High Efficiency for a Wide Range of Load Currents 12V/150mA Flash Memory Programming Supply 110µA Max Supply Current 5µA Max Shutdown Supply Current 2V to 16.5V Input Voltage Range 12V (MAX761), 15V (MAX762) or Adjustable Output Current-Limited PFM Control Scheme 300kHz Switching Frequency Internal, 1A, N-Channel Power FET LBI/LBO Low-Battery Comparator ______________Ordering Information PART TEMP. RANGE PIN-PACKAGE MAX761CPA 0°C to +70°C 8 Plastic DIP MAX761CSA MAX761C/D MAX761EPA MAX761ESA MAX761MJA MAX762CPA 0°C to +70°C 0°C to +70°C -40°C to +85°C -40°C to +85°C -55°C to +125°C 0°C to +70°C 8 SO Dice* 8 Plastic DIP 8 SO 8 CERDIP** 8 Plastic DIP MAX762CSA MAX762C/D MAX762EPA MAX762ESA MAX762MJA 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-883. __________Typical Operating Circuit INPUT 4.75V TO 12V ____________________________Features ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ __________________Pin Configuration TOP VIEW 33µF 18µH LX ON/OFF MAX761 SHDN 33µF V+ LBI LOW-BATTERY DETECTOR INPUT OUTPUT 12V 150mA LBO LOW-BATTERY DETECTOR OUTPUT LBO 1 8 V+ LBI 2 7 LX FB 3 6 GND 5 REF MAX761 MAX762 SHDN 4 REF FB GND DIP/SO ________________________________________________________________ Maxim Integrated Products Call toll free 1-800-998-8800 for free samples or literature. 1 MAX761/MAX762 _______________General Description The MAX761/MAX762 step-up switching regulators provide high efficiency over a wide range of load currents, delivering up to 150mA. A unique, current-limited pulse-frequency-modulated (PFM) control scheme gives the devices the benefits of pulse-width-modulated (PWM) converters (high efficiency with heavy loads), while using less than 110µA of supply current (vs. 2mA to 10mA for PWM converters). The result is high efficiency over a wide range of loads. The MAX761/MAX762 input voltage range is 2V to 16.5V. Output voltages are preset to 12V (MAX761) and 15V (MAX762), or they can be set with two external resistors. With a 5V input, the MAX761 guarantees a 12V, 150mA output. Its high efficiency, low supply current, fast start-up time, SHDN controlling capability, and small size make the MAX761 ideal for powering flash memory. The MAX761/MAX762 have an internal 1A power MOSFET, making them ideal for minimum-component, low- and medium-power applications. These devices use tiny external components, and their high switching frequencies (up to 300kHz) allow for small surface-mount magnetics. For increased output drive capability or higher output voltages, use the MAX770–MAX773, which are similar in design to the MAX761/MAX762, but drive external power MOSFETs. For stepping up to 5V, see the MAX756/ MAX757 and MAX856-MAX859 data sheets. MAX761/MAX762 12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Converters ABSOLUTE MAXIMUM RATINGS Supply Voltage V+ to GND .......................................-0.3V to 17V REF, LBO, LBI, SHDN, FB ............................-0.3V to (V+ + 0.3V) LX..............................................................................-0.3V to 17V LX Peak Current ....................................................................1.5A LBO Current ..........................................................................5mA 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: MAX76_C_A ........................................................0°C to +70°C MAX76_E_A .....................................................-40°C to +85°C MAX76_MJA ..................................................-55°C to +125°C Junction Temperatures: MAX76_C_A/E_A..........................................................+150°C MAX76_MJA.................................................................+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, CREF = 0.1µF, TA = TMIN to TMAX, typical values are at TA = +25°C, unless otherwise noted.) PARAMETER Supply Voltage SYMBOL V+ CONDITIONS Figure 2, bootstrapped Figure 3 or 5 with external resistors. MAX76_C/E MAX76_M MIN 2 TYP MAX 16.5 UNITS 3 16.5 V 3.1 16.5 Minimum Operating Voltage Figure 2, bootstrapped 1.7 Minimum Start-Up Voltage Figure 2, bootstrapped 1.7 2.0 V V+ = 16.5V, normal operation, SHDN = 0V, non-bootstrapped 88 110 Figure 2, MAX761, VIN = 5V, SHDN = 0V, normal operation 300 Supply Current µA Shutdown Current V+ = 10.0V, shutdown mode, SHDN = V+ Figure 2, MAX761, bootstrapped Output Voltage (Note 1) VOUT Peak Current at LX IPEAK Figure 2, MAX762, bootstrapped Maximum Switch-On Time tON Minimum Switch-Off Time tOFF 0mA ≤ ILOAD ≤ 75mA, 3V ≤ V+ ≤ 12V 0mA ≤ ILOAD ≤ 150mA, 4.75V ≤ V+ ≤ 12V 0mA ≤ ILOAD ≤ 50mA, 3V ≤ V+ ≤ 15V 0mA ≤ ILOAD ≤ 100mA, 4.75V ≤ V+ ≤ 15V See Figure 4b Figure 2, 0mA ≤ ILOAD ≤ 200mA, bootstrapped Line Regulation Efficiency 2 VREF 1 5 11.52 12.0 12.48 11.52 12.0 12.48 14.4 15.0 15.6 µA 14.4 15.0 15.6 0.75 1.0 1.25 A 6 8 10 µs 1.3 1.6 V 1.0 Load Regulation Reference Voltage V µs 0.0042 %/mA Figure 2, 4V ≤ VIN ≤ 6V, bootstrapped 0.08 %/V Figure 2, bootstrapped, VOUT = 12V, 60mA ≤ ILOAD ≤ 120mA 86 % MAX76_C 1.4700 1.50 1.5300 MAX76_E 1.4625 1.50 1.5375 MAX76_M 1.4550 1.50 1.5450 _______________________________________________________________________________________ V 12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Converters (V+ = 5V, ILOAD = 0mA, CREF = 0.1µF, TA = TMIN to TMAX, typical values are at TA = +25°C, unless otherwise noted.) PARAMETER SYMBOL CONDITIONS Reference Load Regulation 0µA ≤ ILOAD ≤ 100µA Reference Line Regulation 3.0V ≤ V+ ≤ 16.5V FB Leakage Current Voltage Trip Point IFB VFB LX On Resistance TYP MAX 10 MAX76_M 15 30 100 MAX76_C -5 5 MAX76_E -10 10 MAX76_M -30 30 MAX76_C -20 20 MAX76_E -40 40 MAX76_M -60 60 MAX76_C 1.4700 1.50 MAX76_E 1.4625 1.50 1.5375 MAX76_M 1.4550 1.50 1.5450 1.0 2.2 V+ = 16.5V, LX = 17V LX Leakage Current MIN MAX76_C/E V+ > 5.0V SHDN Input High Voltage VIH 2.0V ≤ V+ ≤ 16.5V SHDN Input Low Voltage VIL 2.0V ≤ V+ ≤ 16.5V V+ = 16.5V, SHDN = 0V or V+ LBI Threshold Voltage LBI falling µV/V µA nA V Ω V -1 0.4 V 1 µA MAX76_C 1.4700 1.50 MAX76_E 1.4625 1.50 1.5375 MAX76_M 1.4550 1.50 1.5450 LBI Hysteresis mV 1.5300 1.6 SHDN Leakage Current UNITS 1.5300 20 V mV LBI Leakage Current V+ = 16.5V, VLBI = 1.5V -20 20 nA LBO Leakage Current V+ = 16.5V, VLBO = 16.5V -1 1 µA LBO Voltage LBI to LBO Delay Note 1: VOL V+ = 5.0V, ISINK = 1mA Overdrive = 5mV 0.4 2.5 V µs See Typical Operating Characteristics for output current capability versus input voltage. Guarantees based on correlation to switching on and off times, on-resistance, and peak-current ratings. _______________________________________________________________________________________ 3 MAX761/MAX762 ELECTRICAL CHARACTERISTICS (continued) __________________________________________Typical Operating Characteristics (Circuit of Figure 2, TA = +25°C, unless otherwise noted.) 80 40 30 60 50 40 30 20 20 VOUT = 12V 10 1000 100 250 200 150 NON-BOOTSTRAPPED 100 VOUT = 12V 50 10 0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 1000 100 OUTPUT CURRENT (mA) INPUT VOLTAGE (V) REFERENCE OUTPUT RESISTANCE vs. TEMPERATURE REFERENCE vs.TEMPERATURE COEFFICIENT 1.506 MAX761-05 MAX761-04 BOOTSTRAPPED 300 1 250 REFERENCE OUTPUT RESISTANCE (Ω) 400 350 200 10µA 150 100 50µA 100µA 50 1.504 3.5 4.0 4.5 5.0 5.5 6.0 1.502 1.500 1.498 1.496 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 SUPPLY VOLTAGE (V) TEMPERATURE (°C) TEMPERATURE (°C) NO-LOAD START-UP VOLTAGE MAX761 START-UP VOLTAGE vs. RLOAD LX ON-RESISTANCE vs. TEMPERATURE 2.5 NON-BOOTSTRAPPED (EXTERNAL RESISTORS) 2.0 1.5 BOOTSTRAPPED (INTERNAL RESISTORS) 1.6 2.0 1.9 1.8 1.7 1.6 MAX761-09 VOUT = 12V BOOTSTRAPPED INTERNAL RESISTORS 2.1 1.4 LX ON-RESISTANCE (Ω) BOOTSTRAPPED (EXTERNAL RESISTORS) 3.0 2.2 MAX761-08 VOUT = 12V START-UP VOLTAGE (V) 3.5 MAX761-07 3.0 NON-BOOTSTRAPPED 0 0.1 MAXIMUM OUTPUT CURRENT vs. INPUT VOLTAGE MAXIMUM OUTPUT CURRENT (mA) 0.50 0.25 REFERENCE OUTPUT (V) 10 1 OUTPUT CURRENT (mA) 1.0 0.75 0 0.1 BOOTSTRAPPED (EXTERNAL RESISTORS) 1.00 VOUT = 12V 10 0 1.50 1.25 MAX761-06 EFFICIENCY (%) VIN = 2V 50 VIN = 5V 70 VOUT = 12V BOOTSTRAPPED (INTERNAL RESISTORS) 1.75 QUIESCENT CURRENT (mA) VIN = 5V 70 60 VIN = 10V 90 2.00 MAX761-02 VIN = 10V 80 EFFICIENCY (%) 100 MAX761-01 100 90 QUIESCENT CURRENT vs. INPUT VOLTAGE EFFICIENCY vs. OUTPUT CURRENT NON-BOOTSTRAPPED MAX761-03 EFFICIENCY vs. OUTPUT CURRENT BOOTSTRAPPED NO-LOAD START-UP VOLTAGE (V) MAX761/MAX762 12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Converters V+ = 5V 1.2 1.0 0.8 V+ = 12V 1.5 0.6 1.4 0.5 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) 4 0.4 1.3 0.1 1 10 RLOAD (kΩ) 100 1000 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) _______________________________________________________________________________________ 12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Converters PEAK CURRENT AT LX vs. TEMPERATURE 1.4 1.3 3.5 V+ = 12V 3.0 1.2 10 1 1.1 V+ = 5V 1.0 2.5 ICC (µA) IPEAK (A) 0.9 2.0 V+ = 8V 0.8 0.1 0.01 20 40 60 80 100 120 TEMPERATURE (°C) 0.6 0.5 0.5 0 -60 -40 -20 0 20 40 60 80 100 120 140 140 SWITCH-ON TIME vs. TEMPERATURE V+ = 4V -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) TEMPERATURE (°C) SWITCH-OFF TIME vs. TEMPERATURE POWER-SUPPLY CURRENT vs. TEMPERATURE 2.0 MAX761-13 8.5 1.0 0.7 VLX = 16.5V V+ = 15V 1.5 100 MAX761-14 LX LEAKAGE (nA) 100 4.0 MAX761-12 V+ = 15V MAX761-11 1.5 MAX761-10 1000 SHUTDOWN CURRENT vs. TEMPERATURE MAX761-15 LX LEAKAGE vs. TEMPERATURE V+ = 16.5V V+ = 5V V+ = 5V ICC (µA) toff (µs) 8.0 1.5 1.0 7.5 60 0 80 -60 120 90 0 60 120 -60 TEMPERATURE (°C) TEMPERATURE (°C) SWITCH-ON/SWITCH-OFF TIME RATIO vs.TEMPERATURE 7 60 0 120 TEMPERATURE (°C) SHDN RESPONSE TIME MAX761-16 -60 ton/toff RATIO (µs/µs) ton (µs) V+ = 3V 12V V+ = 5V 6 5V 4V 0V 5 -60 0 60 TEMPERATURE (°C) 2ms/div 120 ILOAD = 100mA, VIN = 5V A: VOUT, 2V/div B: SHDN (0V to 4V) _______________________________________________________________________________________ 5 MAX761/MAX762 ____________________________Typical Operating Characteristics (continued) (Circuit of Figure 2, TA = +25°C, unless otherwise noted.) MAX761/MAX762 12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Converters _____________________________Typical Operating Characteristics (continued) (Circuit of Figure 2, TA = +25°C, unless otherwise noted.) LOAD–TRANSIENT RESPONSE LINE–TRANSIENT RESPONSE 200mA 6V A A 0mA 4V B B 5µs/div 5ms/div A: ILOAD, (0mA to 200mA) B: VOUT , AC COUPLED, 100mV/div VIN = 5V, VOUT = 12V A: VIN (4V to 6V) B: VOUT, AC COUPLED, 20mV/div IOUT = 50mA, VOUT = 12V ______________________________________________________________Pin Description 6 PIN NAME FUNCTION 1 LBO 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 LBO floating if not used. 2 LBI Input to the internal low-battery comparator. Tie to GND or V+ if not used. 3 FB Feedback input. For fixed-output bootstrapped operation, connect FB to GND. For adjustable-output bootstrapped operation, connect a resistor divider between V+, FB and GND. For non-bootstrapped operation, there is no fixed-output option. Connect a resistor divider network between VOUT, FB and GND. See Bootstrapped/Non-Bootstrapped Modes section. 4 SHDN 5 REF 1.5V reference output that can source 100µA for external loads. Bypass with 0.1µF or larger capacitor. 6 GND Ground 7 LX Drain of the internal N-channel FET. LX has an output resistance of 1Ω and a peak current limit of 1A. 8 V+ Power-supply input. In bootstrapped mode, V+ is also the output voltage sense input. Active-high TTL/CMOS logic-level input. In shutdown mode (SHDN = V+), the internal switch is turned off and the output voltage equals V+ minus a diode drop (due to the DC path from the input to the output). Tie to GND for normal operation. _______________________________________________________________________________________ 12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Converters V+ MAX761/MAX762 LBO FB DUAL-MODE COMPARATOR LBI MAX761 MAX762 N LBI ERROR COMPARATOR 100mV REF V+ 1.5V REFERENCE Q TRIG ONE-SHOT N Q S R Q TRIG ONE-SHOT LOW INPUT VOLTAGE OSCILLATOR UNDER VOLTAGE COMPARATOR 2.5V LX CURRENT COMPARATOR N 0.2V 0.1V CURRENT CONTROL CIRCUITRY GND Figure 1. Simple Block Diagram ________________Detailed Description Operating Principle The MAX761/MAX762 BiCMOS step-up switch-mode power supplies provide fixed outputs of 12V and 15V, respectively. They have a unique control scheme that combines the advantages of pulse-frequency modulation (low supply current) and pulse-width modulation (high efficiency at high loads). The internal N-channel power MOSFET allows 1A peak currents, increasing the output current capability over previous pulse-frequency-modulation (PFM) devices. Figure 1 shows the MAX761/ MAX762 block diagram. The MAX761/MAX762 offer three main improvements over prior solutions: (1) the converters operate with tiny surface-mount inductors (less than 5mm diameter) because of their 300kHz switching frequency, (2) the current-limited PFM control scheme allows 86% efficiencies over a wide range of load currents, and (3) the maximum supply current is only 110µA. Bootstrapped/Non-Bootstrapped Modes Figures 2 and 3 show the standard application circuits for bootstrapped and non-bootstrapped modes. In bootstrapped mode, the IC is powered from the output (VOUT). In other words, the current needed to power the bootstrapped circuit is different from the V+ current the chip consumes. The voltage applied to the gate of the internal N-channel FET is switched from VOUT to ground, providing more switch-gate drive and increasing the efficiency of the DC-DC converter compared with non-bootstrapped operation. _______________________________________________________________________________________ 7 MAX761/MAX762 12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Converters L1 18µH VIN = +5V D1 1N5817 +12V at 150mA C1 33µF L1 18µH VIN D1 1N5817 C4 C4 33µF R2 = R1 7 5 C3 0.1µF 4 R4 2 8 REF C1 R4 MAX761 V+ 3 2 C2 0.1µF 5 100k LBI FB LBO V+ 6 4 LOW-BATTERY OUTPUT LOW-BATTERY DETECT VTRIP - VREF R4 = R3 VREF VREF = 1.5V NOMINAL C1 = 33µF C2 = 0.1µF C3 = 0.1µF C4 = 33µF ( Figure 2. Bootstrapped Operating Circuit In non-bootstrapped mode, the IC is powered from the supply voltage, VIN, and operates with minimum supply current. Since the voltage applied to the gate of the internal FET is reduced, efficiency declines with low input voltages. Note: In non-bootstrapped mode, there is no fixed-output operation; external resistors must be used to set the output voltage. Use 1% external feedback resistors when operating in non-bootstrapped mode (Figure 3). Use bootstrapped mode when VIN is below approximately 4V. For VIN between 4V and 6V, the trade-off is lower supply current in non-bootstrapped mode versus higher output current in bootstrapped mode (see Typical Operating Characteristics). Pulse-Frequency Modulation (PFM) Control Scheme The MAX761/MAX762 use a proprietary current-limited PFM control scheme. This control scheme combines the ultra-low supply current of pulse-skipping PFM converters with the high full-load efficiency characteristic of current-mode pulse-width-modulation (PWM) converters. It allows the devices to achieve high efficiency over a wide range of loads, while the current-sense function and high operating frequency allow the use of tiny external components. As with traditional PFM converters, the internal power MOSFET is turned on when the voltage comparator senses the output is out of regulation (Figure 1). However, unlike traditional PFM converters, switching is accomplished through the combination of a peak cur- R2 MAX761 MAX762 FB 3 100k LBI R1 REF C3 1 GND 8 C2 LX 8 R3 R3 ( VVOUT -1) REF 7 LX SHDN ADJUSTABLE OUTPUT (VOUT) SHDN LBO 1 GND ) 6 LOW-BATTERY DETECT OUTPUT Figure 3. Non-Bootstrapped Operating Circuit rent limit and a pair of one-shots that set the maximum on-time (8µs) and minimum off-time (1.3µs) for the switch. Once off, the minimum off-time one-shot holds the switch off for 1.3µs. After this minimum time, the switch either (1) stays off if the output is in regulation, or (2) turns on again if the output is out of regulation. The MAX761/MAX762 also limit the peak inductor current, allowing the devices to run in continuous-conduction mode (CCM) and maintain high efficiency with heavy loads (Figure 4a). This current-limiting feature is a key component of the control circuitry. Once turned on, the switch stays on until either (1) the maximum ontime one-shot turns it off (8µs later), or (2) the current limit is reached. To increase light-load efficiency, the current limit for the first two pulses is set to half the peak current limit. If those pulses bring the output voltage into regulation, the voltage comparator holds the MOSFET off, and the current limit remains at half 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 full current limit of 1A (Figure 4b). Internal vs. External Resistors When external feedback resistors are used, an internal undervoltage lockout system prevents start-up until V+ rises to about 2.7V. When external feedback resistors are _______________________________________________________________________________________ 12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Converters 1A 500mA 500mA MAX761/MAX762 1A 0A Figure 4a. CCM, Heavy Load Current Waveform (500mA/div) Figure 4b. Light/Medium Load Current Waveform (500mA/div) used in a bootstrapped circuit (Figure 5), undervoltage lockout prevents start-up at low input voltages; but once started, operation can continue down to a lower voltage that depends on the load. There is no undervoltage lockout when the internal feedback resistors are used (Figure 2), and special circuitry guarantees start-up at 2.0V. The start-up circuitry fixes the duty cycle at 50% until V+ is driven to 2.5V, above which the normal control system takes over. ringing (the inductor's self-resonant frequency). This ringing is normal and poses no operational problems. Shutdown Mode The MAX761/MAX762 enter shutdown mode when SHDN is high. In this mode, the internal biasing circuitry is turned off (including the reference) and VOUT equals V+ minus a diode drop (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. LBO is high impedance during shutdown. Modes of Operation When delivering high output currents, the MAX761/ MAX762 operate in CCM. In this mode, current always flows in the inductor, and the control circuit adjusts the switch’s duty cycle on a cycle-by-cycle basis to maintain regulation without exceeding the switch-current capability. This provides excellent load-transient response and high efficiency. In discontinuous-conduction mode (DCM), current through the inductor starts at zero, rises to a peak value, then ramps down to zero on each cycle. Although efficiency is still excellent, the switch waveforms contain Low-Battery Detector The MAX761/MAX762 provide a low-battery comparator that compares the voltage on LBI to the 1.5V reference voltage. When the LBI voltage is below VREF, LBO (an open-drain 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 (Figure 3). When SHDN is high, LBI is ignored and LBO is high impedance. The value of resistor R3 should be no larger than 500kΩ to ensure the LBI leakage current does not cause inaccuracies in VTRIP. __________________Design Procedure Setting the Output Voltage The MAX761/MAX762’s output voltage can be adjusted from 5V to 16.5V using external resistors R1 and R2 configured as shown in Figures 3 and 5. For adjustableoutput operation, select feedback resistor R1 in the 10kΩ to 250kΩ range. Higher R1 values within this range give lowest supply current and best light-load efficiency. R2 is given by: R2 = (R1)( VOUT - 1) VREF where VREF = 1.5V. Note: Tie FB to GND for fixed-output operation (bootstrapped mode only). _______________________________________________________________________________________ 9 MAX761/MAX762 12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Converters D1 1N5817 L1 18µH VIN C1 VOUT C4 7 LX 5 REF MAX761 MAX762 V+ 8 C2 C3 2 4 R2 LBI FB SHDN Diode Selection GND C1 = 33µF C2 = 0.1µF C3 = 0.1µF C4 = 33µF 3 R1 6 VREF = 1.5V NOMINAL R2 = R1 ( VVOUT -1) REF Figure 5. Bootstrapped Operation with Adjustable Output Selecting the Inductor (L) In both CCM and DCM, practical inductor values range from 10µH to 50µH. If the inductor value is too low, the current in the coil 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 2.5µs, so select an inductance that allows the current to ramp up to ILIM/2 in no less than 2.5µs. Choosing a value of ILIM/2 allows the half-size pulses to occur, giving higher light-load efficiency and minimizing ripple. Hence, calculate the minimum inductance value as: L ≥ (VIN(max))(tON(min)) ILIM/2 OR L ≥ (VIN(max))(5) where VIN(max) is in volts and L is in microhenries. The coil’s inductance need not satisfy this criterion exactly, as the circuit can tolerate a wide range of values. Larger inductance values tend to produce physically larger coils and increase the start-up time, but are otherwise acceptable. Smaller inductance values allow the coil current to ramp up to higher levels before the switch can turn off, producing higher ripple at light loads. In general, an 18µH inductor is sufficient for most applications (VIN ≤ 5V). An 18µH inductor is appropriate for input voltages up to 3.6V, as calculated above. However, the same 18µH coil can be used with input voltages up to 5V with only small increases in peak current, as shown in Figures 4a and 4b. 10 Inductors with a ferrite core or equivalent are recommended. The inductor’s incremental saturation-current rating should be greater than the 1A peak current limit. 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 100mΩ. To minimize radiated noise, use a toroid, a pot core, or a shielded coil. Table 1 lists inductor types and suppliers for various applications. The listed surface-mount inductors’ efficiencies are nearly equivalent to those of the larger throughhole inductors. The MAX761/MAX762’s high switching frequency demands a high-speed rectifier. Use a Schottky diode with a 1A average current rating, such as a 1N5817. For high-temperature applications, use a high-speed silicon diode, such as the MUR105 or the EC11FS1. These diodes have lower high-temperature leakage than Schottky diodes (Table 1). 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 inductor current variation and the output filter capacitor’s ESR determines the amplitude of the high-frequency ripple seen on the output voltage. A 33µF, 16V Sanyo OS-CON capacitor with 100mΩ ESR typically provides 100mV ripple when stepping up from 5V to 12V at 150mA. Because the output filter capacitor’s ESR affects efficiency, use low-ESR capacitors for best performance. The smallest low-ESR SMT tantalum capacitors currently available are the Sprague 595D series. Sanyo OS-CON organic semiconductor through-hole capacitors and Nichicon PL series also exhibit very low ESR. Table 1 lists some suppliers of low-ESR capacitors. 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 MAX761/MAX762’s switching action. 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 250mA, 33µF (C1) is adequate, although smaller bypass capacitors may also be acceptable. Bypass the IC separately with a 0.1µF ceramic capacitor, C2, placed close to the V+ and GND pins. ______________________________________________________________________________________ 12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Converters Connect a pull-up resistor (e.g., 100kΩ) between LBO and VOUT. Tie LBI to GND or V+ and leave LBO floating if the low-battery detector is not used. Setting the Low-Battery Detector Voltage ___________Applications Information To set the low-battery detector’s falling trip voltage (VTRIP), select R3 between 10kΩ and 500kΩ (Figures 2 and 3), and calculate R4 as follows: (VTRIP - VREF) R4 = R3 [ ] VREF where VREF = 1.5V. The rising trip voltage is higher because of the comparator’s hysteresis of approximately 20mV, and can be calculated by: VTRIP(rising) = (VREF + 20mV)(1 + R4/R3). Connect a high-value resistor (larger than R3 + R4) between LBI and LBO if additional hysteresis is required. Layout Considerations Proper PC board layout is essential because of high current levels and fast switching waveforms that radiate noise. Minimize ground noise by connecting GND, the input bypass-capacitor ground lead, and the output filtercapacitor ground lead to a single point (star ground configuration). Also minimize lead lengths to reduce stray capacitance, trace resistance, and radiated noise. The traces connected to FB and LX, in particular, must be short. Place bypass capacitor C2 as close as possible to V+ and GND. Table 1. Component Suppliers PRODUCTION METHOD INDUCTORS Sumida CD54-180 (22µH) Surface Mount CAPACITORS Matsuo 267 series DIODES Nihon EC10 series Coiltronics CTX 100-series Miniature Through-Hole Low-Cost Through-Hole Sumida RCH855-180M Renco RL 1284-18 Sanyo OS-CON series Low-ESR organic semiconductor Nichicon PL series Low-ESR electrolytics Motorola 1N5817, MUR105 United Chemi-Con LXF series Coiltronics Matsuo Matsuo Nichicon Nihon Renco Sanyo Sanyo Sumida Sumida United Chem-Con (USA) (USA) (Japan) (USA) (USA) (USA) (USA) (Japan) (USA) (Japan) (USA) (407) 241-7876 (714) 969-2491 81-6-337-6450 (708) 843-7500 (805) 867-2555 (516) 586-5566 (619) 661-6835 (0720) 70-1005 (708) 956-0666 81-3-607-5111 (714) 255-9500 FAX (407) 241-9339 FAX (714) 960-6492 FAX 81-6-337-6456 FAX (708) 843-2798 FAX (805) 867-2556 FAX (516) 586-5562 FAX (619) 661-1055 FAX (0720) 70-1174 FAX 81-3-607-5144 FAX (714) 255-9400 ______________________________________________________________________________________ 11 MAX761/MAX762 Reference Capacitor Bypass REF with a 0.1µF capacitor. REF can source up to 100µA. MAX761/MAX762 12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Converters ___________________Chip Topography LBO V+ LX LBI 0.142" (3.607mm) FB GND REF SHDN 0.080" (2.030mm) TRANSISTOR COUNT: 492; 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. 12 __________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600 © 1993 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.