19-0176; Rev 0; 6/94 it K tion lua able a v E il Ava -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ DC-DC Inverters ____________________________Features ♦ High Efficiency for a Wide Range of Load Currents ________________________Applications LCD-Bias Generators Portable Instruments LAN Adapters Remote Data-Acquisition Systems Battery-Powered Applications ♦ 250mA Output Current ♦ 120µA Max Supply Current ♦ 5µA Max Shutdown Current ♦ 3V to 16V Input Voltage Range ♦ -5V (MAX764), -12V (MAX765), -15V (MAX766), or Adjustable Output from -1V to -16V ♦ Current-Limited PFM Control Scheme ♦ 300kHz Switching Frequency ♦ Internal, P-Channel Power MOSFET ______________Ordering Information PART TEMP. RANGE PIN-PACKAGE MAX764CPA 0°C to +70°C 8 Plastic DIP MAX764CSA MAX764C/D MAX764EPA MAX764ESA MAX764MJA 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** MAX765CPA 0°C to +70°C 8 Plastic DIP MAX765CSA MAX765C/D MAX765EPA MAX765ESA MAX765MJA 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** Ordering Information continued on last page. * Dice are tested at TA = +25°C, DC parameters only. **Contact factory for availability and processing to MIL-STD-883. __________Typical Operating Circuit INPUT 3V TO 15V __________________Pin Configuration TOP VIEW V+ OUTPUT -5V LX MAX764 ON/OFF 47µH SHDN OUT 1 8 LX FB 2 7 V+ 6 V+ 5 GND SHDN 3 REF 4 MAX764 MAX765 MAX766 OUT REF FB GND DIP/SO ________________________________________________________________ Maxim Integrated Products Call toll free 1-800-998-8800 for free samples or literature. 1 MAX764/MAX765/MAX766 _______________General Description The MAX764/MAX765/MAX766 inverting switching regulators are highly efficient over a wide range of load currents, delivering up to 1.5W. A unique, current-limited, pulse-frequency-modulated (PFM) control scheme combines the benefits of traditional PFM converters with the benefits of pulse-width-modulated (PWM) converters. Like PWM converters, the MAX764/MAX765/MAX766 are highly efficient at heavy loads. Yet because they are PFM devices, they use less than 120µA of supply current (vs. 2mA to 10mA for a PWM device). The input voltage range is 3V to 16V. The output voltage is preset at -5V (MAX764), -12V (MAX765), or -15V (MAX766); it can also be adjusted from -1V to -16V using two external resistors (Dual ModeTM). The maximum operating VIN - VOUT differential is 20V. These devices use miniature external components; their high switching frequencies (up to 300kHz) allow for less than 5mm diameter surface-mount magnetics. A standard 47µH inductor is ideal for most applications, so no magnetics design is necessary. An internal power MOSFET makes the MAX764/MAX765/ MAX766 ideal for minimum component count, low- and medium-power applications. For increased output drive capability or higher output voltages, use the MAX774/MAX775/MAX776 or MAX1774, which drive an external power P-channel MOSFET for loads up to 5W. MAX764/MAX765/MAX766 -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ DC-DC Inverters ABSOLUTE MAXIMUM RATINGS V+ to GND ..............................................................-0.3V to +17V OUT to GND ...........................................................+0.5V to -17V Maximum Differential (V+ to OUT) ......................................+21V REF, SHDN, FB to GND ...............................-0.3V to (V+ + 0.3V) LX to V+..................................................................+0.3V to -21V LX Peak Current ...................................................................1.5A 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 Maximum 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, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL V+ Input Voltage Range V+ Supply Current IS Shutdown Current ISHDN Output Current and Voltage (Note 1) IFB IOUT MAX76_M 3.5 VREF TYP MAX 16.0 V+ = 16V, SHDN < 0.4V 90 V+ = 16V, SHDN > 1.6V 2 V+ = 10V, SHDN > 1.6V 1 -10 µA 10 MAX76_E ±70 MAX76_M ±90 150 260 MAX765C/E, -11.52V ≤ VOUT ≤ 12.48V 68 120 MAX765M, -11.52V ≤ VOUT ≤ 12.48V 50 120 V 5 ±50 MAX764, -4.8V ≤ VOUT ≤ 5.2V UNITS 120 MAX76_C MAX766, -14.40V ≤ VOUT ≤ -15.60V Reference Voltage MIN 3.0 3V ≤ V+ ≤ 16V FB Trip Point FB Input Current CONDITIONS MAX76_C/E mV nA mA 35 105 MAX76_C 1.4700 1.5 1.5300 MAX76_E 1.4625 1.5 1.5375 MAX76_M 1.4550 1.5 1.5450 MAX76_C/E 4 10 MAX76_M 4 15 40 100 V REF Load Regulation 0µA ≤ IREF ≤ 100µA REF Line Regulation 3V ≤ V+ ≤ 16V Load Regulation (Note 2) 0mA ≤ ILOAD ≤ 100mA 0.008 %/mA Line Regulation (Note 2) 4V ≤ V+ ≤ 6V 0.12 %/V Efficiency (Note 2) 10mA ≤ ILOAD ≤ 100mA, VOUT = -5V VIN = 5V VOUT = -15V SHDN Leakage Current VIH 3V ≤ V+ ≤ 16V SHDN Input Voltage Low VIL 3V ≤ V+ ≤ 16V ±1 1.6 _______________________________________________________________________________________ µV/V % 82 V+ = 16V, SHDN = 0V or V+ SHDN Input Voltage High 2 80 mV µA V 0.4 V -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ DC-DC Inverters (V+ = 5V, ILOAD = 0mA, CREF = 0.1µF, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL CONDITIONS ILXI + (V+) ≤ 20V LX Leakage Current LX On-Resistance Peak Current at LX IPEAK MIN TYP MAX MAX76_C ±5 MAX76_E ±10 MAX76_M ±30 IVOUTI + (V+) ≥ 10V IVOUTI + (V+) ≥ 10V 1.4 0.5 0.75 UNITS µA 2.5 Ω A Maximum Switch On-Time tON 12 16 20 µs Minimum Switch Off-Time tOFF 1.8 2.3 2.8 µs Note 1: See Maximum Output Current vs. Supply Voltage graph in the Typical Operating Characteristics. Guarantees are based on correlation to switch on-time, switch off-time, on-resistance, and peak current rating. Note 2: Circuit of Figure 2. __________________________________________Typical Operating Characteristics (V+ = 5V, VOUT = -5V, TA = +25°C, unless otherwise noted.) MAX765 EFFICIENCY vs. LOAD CURRENT 100 80 70 70 V+ = 10V 50 40 V+ = 15V 40 20 20 CIRCUIT OF FIGURE 2 VOUT = -5V ±4% 0 V+ = 5V 50 30 10 80 60 30 1 10 100 LOAD CURRENT (mA) 1000 70 V+ = 5V 60 50 40 30 20 CIRCUIT OF FIGURE 2 VOUT = -12V ±4% 10 0 0.1 90 EFFICIENCY (%) 80 60 V+ = 8V 90 100 MAX764-02 V+ = 5V EFFICIENCY (%) EFFICIENCY (%) 90 MAX764-01 100 MAX766 EFFICIENCY vs. LOAD CURRENT MAX764-03 MAX764 EFFICIENCY vs. LOAD CURRENT CIRCUIT OF FIGURE 2 VOUT = -15V ±4% 10 0 0.1 1 10 100 LOAD CURRENT (mA) 1000 0.1 1 10 100 1000 LOAD CURRENT (mA) _______________________________________________________________________________________ 3 MAX764/MAX765/MAX766 ELECTRICAL CHARACTERISTICS (continued) ____________________________Typical Operating Characteristics (continued) (V+ = 5V, VOUT = -5V, TA = +25°C, unless otherwise noted.) NO-LOAD SUPPLY CURRENT vs. SUPPLY VOLTAGE 400 300 200 VOUT = -12V 100 VOUT = -15V 0 80 75 70 65 MAX764 -06 90 85 80 75 70 V+ = 5V 65 60 55 50 3 4 5 6 7 8 9 10 11 12 13 14 15 16 SUPPLY VOLTAGE (V) TEMPERATURE (°C) SHUTDOWN CURRENT vs. TEMPERATURE MAXIMUM SWITCH ON-TIME vs. TEMPERATURE MINIMUM SWITCH OFF-TIME vs. TEMPERATURE 2.0 1.5 V+ = 8V 1.0 16.6 16.4 V+ = 15V 16.2 16.0 15.8 15.6 V+ = 5V 15.4 2.60 15.2 V+ = 4V 0 20 40 60 80 100 120 140 -60 -40 -20 TEMPERATURE (°C) SWITCH ON/OFF-TIME RATIO vs. TEMPERATURE 6.9 6.8 6.7 6.6 V+ = 5V 6.4 6.3 6.2 0 20 40 60 80 100 120 140 TEMPERATURE (°C) 2.40 V+ = 5V 2.35 2.30 2.25 0 20 40 60 80 100 120 140 TEMPERATURE (°C) START-UP SUPPLY VOLTAGE vs. OUTPUT CURRENT LX LEAKAGE CURRENT vs. TEMPERATURE 8 CIRCUIT OF FIGURE 2 7 6 5 4 3 2 10,000 IVOUTI + (V+) = 20V 1000 100 10 1 1 0 -60 -40 -20 V+ = 15V 2.45 TEMPERATURE (°C) LX LEAKAGE CURRENT (nA) 7.0 2.50 -60 -40 -20 0 20 40 60 80 100 120 140 MAX764 -11 7.1 START-UP SUPPLY VOLTAGE (V) MAX764 -10 7.2 2.55 2.20 15.0 -60 -40 -20 0 20 40 60 80 100 120 140 MAX764 -09 16.8 -60 -40 -20 MINIMUM SWITCH OFF-TIME (µs) V+ = 15V 17.0 MAXIMUM SWITCH ON-TIME (µs) MAX764 -07 2.5 0.5 4 V+ = 15V 95 SUPPLY VOLTAGE (V) 3.0 6.5 100 3 4 5 6 7 8 9 10 11 12 13 14 15 16 3.5 SHUTDOWN CURRENT (µA) 85 60 4.0 0 90 110 105 MAX764-12 VOUT = -5V 95 MAX764 -08 500 100 MAX764 -05 CIRCUIT OF FIGURE 2 NO-LOAD SUPPLY CURRENT (µA) MAX764 -04 MAXIMUM OUTPUT CURRENT (mA) 600 NO-LOAD SUPPLY CURRENT vs. TEMPERATURE NO-LOAD SUPPLY CURRENT (µA) MAXIMUM OUTPUT CURRENT vs. SUPPLY VOLTAGE SWITCH ON/OFF-TIME RATIO (µs/µs) MAX764/MAX765/MAX766 -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ DC-DC Inverters 0 50 100 150 200 OUTPUT CURRENT (mA) 250 300 20 30 40 50 60 70 80 90 100 110 120 130 TEMPERATURE (°C) _______________________________________________________________________________________ -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ DC-DC Inverters PEAK CURRENT AT LX vs. TEMPERATURE 0.90 CURRENT AT LX (A) IVOUTI + (V+) = 10V 1.8 1.6 IVOUTI + (V+) = 15V 1.4 1.2 IVOUTI + (V+) = 20V 0.85 IVOUTI + (V+) = 15V 0.80 0.75 0.70 1.0 IVOUTI + (V+) = 10V IVOUTI + (V+) = 20V 250 200 0 20 40 60 80 100 120 140 IREF = 50µA 100 50 IREF = 100µA 0 -60 -40 -20 TEMPERATURE (°C) 0 20 40 60 80 100 120 140 -60 -40 -20 TEMPERATURE (°C) SUPPLY CURRENT vs. SUPPLY VOLTAGE SUPPLY CURRENT (mA) 1.504 1.502 1.500 1.498 1.496 MAX764-17 1000 MAX764 -16 1.506 0 20 40 60 80 100 120 140 TEMPERATURE (°C) REFERENCE OUTPUT vs. TEMPERATURE REFERENCE OUTPUT (V) -60 -40 -20 IREF = 10µA 150 0.65 0.8 MAX764 -15 MAX764 -14 2.0 LX ON-RESISTANCE (Ω) 0.95 MAX764 -13 2.2 REFERENCE OUTPUT RESISTANCE vs. TEMPERATURE REFERENCE OUTPUT RESISTANCE (Ω) LX ON-RESISTANCE vs. TEMPERATURE ILOAD = 100mA 100 10 1 ILOAD = 0mA 0.1 1.494 CIRCUIT OF FIGURE 2 1.492 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) 0.01 0 2 4 6 8 10 12 14 16 SUPPLY VOLTAGE (V) _______________________________________________________________________________________ 5 MAX764/MAX765/MAX766 ____________________________Typical Operating Characteristics (continued) (V+ = 5V, VOUT = -5V, TA = +25°C, unless otherwise noted.) MAX764/MAX765/MAX766 -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ DC-DC Inverters ____________________________Typical Operating Characteristics (continued) (V+ = 5V, VOUT = -5V, TA = +25°C, unless otherwise noted.) TIME TO ENTER/EXIT SHUTDOWN LOAD-TRANSIENT RESPONSE 0V A A B 0V B 0mA 2ms/div CIRCUIT OF FIGURE 2, V+ = 5V, ILOAD = 100mA, VOUT = -5V A: VOUT, 2V/div B: SHUTDOWN PULSE, 0V TO 5V, 5V/div 5ms/div CIRCUIT OF FIGURE 2, V+ = 5V, VOUT = -5V A: VOUT, 50mV/div, AC-COUPLED B: ILOAD, 0mA TO 100mA, 100mA/div DISCONTINUOUS CONDUCTION AT HALF AND FULL CURRENT LIMIT LINE-TRANSIENT RESPONSE A A B 0A B 0V C 0V 5ms/div CIRCUIT OF FIGURE 2, VOUT = -5V, ILOAD = 100mA A: VOUT, 50mV/div, AC-COUPLED B: V+, 5V TO 10V, 5V/div 6 5µs/div CIRCUIT OF FIGURE 2, V+ = 5V, VOUT = -5V, ILOAD = 140mA A: OUTPUT RIPPLE, 100mV/div B: INDUCTOR CURRENT, 500mA/div C: LX WAVEFORM, 10V/div _______________________________________________________________________________________ -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ DC-DC Inverters DISCONTINUOUS CONDUCTION AT HALF CURRENT LIMIT CONTINUOUS CONDUCTION AT FULL CURRENT LIMIT A A B B 0A 0A 0V C 5µs/div CIRCUIT OF FIGURE 2, V+ = 5V, VOUT = -5V, ILOAD = 80mA A: OUTPUT RIPPLE, 100mV/div B: INDUCTOR CURRENT, 500mA/div C: LX WAVEFORM, 10V/div 0V C 5µs/div CIRCUIT OF FIGURE 2, V+ = 5V, VOUT = -5V, ILOAD = 240mA A: OUTPUT RIPPLE, 100mV/div B: INDUCTOR CURRENT, 500mA/div C: LX WAVEFORM, 10V/div ______________________________________________________________Pin Description PIN NAME FUNCTION 1 OUT 2 FB Feedback Input. Connect FB to REF to use the internal voltage divider for a preset output. For adjustableoutput operation, use an external voltage divider, as described in the section Setting the Output Voltage. 3 SHDN Active-High Shutdown Input. With SHDN high, the part is in shutdown mode and the supply current is less than 5µA. Connect to ground for normal operation. 4 REF 1.5V Reference Output that can source 100µA for external loads. Bypass to ground with a 0.1µF capacitor. 5 GND Ground 6, 7 V+ Positive Power-Supply Input. Must be tied together. Place a 0.1µF input bypass capacitor as close to the V+ and GND pins as possible. 8 LX Drain of the Internal P-Channel Power MOSFET. LX has a peak current limit of 0.75A. Sense Input for Fixed-Output Operation (VFB = VREF). OUT must be connected to VOUT. _______________________________________________________________________________________ 7 MAX764/MAX765/MAX766 ____________________________Typical Operating Characteristics (continued) (V+ = 5V, VOUT = -5V, TA = +25°C, unless otherwise noted.) MAX764/MAX765/MAX766 -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ DC-DC Inverters FB COMPARATOR MAX764 MAX765 MAX766 REF SHDN ERROR COMPARATOR OUT V+ N V+ 1.5V REFERENCE Q TRIG ONE-SHOT FROM V+ S TRIG Q Q R P CURRENT COMPARATOR FROM OUT LX ONE-SHOT 0.2V (FULL CURRENT) CURRENT CONTROL CIRCUITS 0.1V (HALF CURRENT) FROM V+ GND Figure 1. Block Diagram _______________Detailed Description Operating Principle The MAX764/MAX765/MAX766 are BiCMOS, inverting, switch-mode power supplies that provide fixed outputs of -5V, -12V, and -15V, respectively; they can also be set to any desired output voltage using an external resistor divider. Their unique control scheme combines the advantages of pulse-frequency modulation (pulse skipping) and pulse-width modulation (continuous pulsing). The internal P-channel power MOSFET allows peak currents of 0.75A, increasing the output current capability over previous pulse-frequency-modulation (PFM) devices. Figure 1 shows the MAX764/MAX765/ MAX766 block diagram. The MAX764/MAX765/MAX766 offer three main improvements over prior solutions: 8 1) They can operate with miniature (less than 5mm diameter) surface-mount inductors, because of their 300kHz switching frequency. 2) The current-limited PFM control scheme allows efficiencies exceeding 80% over a wide range of load currents. 3) Maximum quiescent supply current is only 120µA. Figures 2 and 3 show the standard application circuits for these devices. In these configurations, the IC is powered from the total differential voltage between the input (V+) and output (VOUT). The principal benefit of this arrangement is that it applies the largest available signal to the gate of the internal P-channel power MOSFET. This increased gate drive lowers switch on-resistance and increases DC-DC converter efficiency. Since the voltage on the LX pin swings from V+ (when the switch is ON) to IVOUTI plus a diode drop (when the _______________________________________________________________________________________ -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ DC-DC Inverters VIN 1 C1 120µF 20V 7 V+ OUT C2 0.1µF 3 2 4 SHDN MAX764 MAX765 MAX766 FB 6 V+ D1 1N5817 8 LX VOUT REF C4 68µF 20V GND C3 0.1µF L1 47µH 5 PRODUCT OUTPUT VOLTAGE (V) INPUT VOLTAGE (V) MAX764 MAX765 MAX766 -5 -12 -15 3 to 15 3 to 8 3 to 5 Figure 2. Fixed Output Voltage Operation VIN C1 120µF 20V C2 0.1µF R2 1 3 2 V+ OUT SHDN MAX764 V+ MAX765 MAX766 FB LX 7 6 8 D1 1N5817 R1 4 REF GND C3 0.1µF 5 L1 47µH Figure 3. Adjustable Output Voltage Operation C4 68µF 20V VOUT -1V to -16V PFM Control Scheme The MAX764/MAX765/MAX766 use a proprietary, current-limited PFM control scheme that blends the best features of PFM and PWM devices. It combines the ultra-low supply currents of traditional pulse-skipping PFM converters with the high full-load efficiencies of current-mode pulse-width modulation (PWM) converters. This control scheme allows the devices to achieve high efficiencies over a wide range of loads, while the current-sense function and high operating frequency allow the use of miniature external components. As with traditional PFM converters, the internal power MOSFET is turned on when the voltage comparator senses that the output is out of regulation (Figure 1). However, unlike traditional PFM converters, switching is accomplished through 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) for the switch. Once off, the minimum off-time one-shot holds the switch off for 2.3µs. After this minimum time, the switch either 1) stays off if the output is in regulation, or 2) turns on again if the output is out of regulation. The MAX764/MAX765/MAX766 limit the peak inductor current, which allows them to run in continuous-conduction mode and maintain high efficiency with heavy loads. (See the photo Continuous Conduction at Full Current Limit in the Typical Operating Characteristics.) This current-limiting feature is a key component of the control circuitry. Once turned on, the switch stays on until either 1) the maximum on-time one shot turns it off (16µs later), or 2) the current limit is reached. To increase light-load efficiency, the current limit is set to half the peak current limit for the first two pulses. 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 is raised to its 0.75A peak for the next pulse. (See the photo Discontinuous Conduction at Half and Full Current Limit in the Typical Operating Characteristics.) Shutdown Mode When SHDN is high, the MAX764/MAX765/MAX766 enter a shutdown mode in which the supply current drops to less than 5µA. In this mode, the internal biasing circuitry (including the reference) is turned off and OUT discharges to ground. SHDN is a TTL/CMOS-logic level input. Connect SHDN to GND for normal operation. With a current-limited supply, power-up the device while unloaded or in shutdown mode (hold SHDN high until V+ exceeds 3.0V) to save power and reduce power-up current surges. (See the Supply Current vs. Supply Voltage graph in the Typical Operating Characteristics.) _______________________________________________________________________________________ 9 MAX764/MAX765/MAX766 switch is OFF), the range of input and output voltages is limited to a 21V absolute maximum differential voltage. When output voltages more negative than -16V are required, substitute the MAX764/MAX765/MAX766 with Maxim’s MAX774/MAX775/MAX776 or MAX1774, which use an external switch. MAX764/MAX765/MAX766 -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ DC-DC Inverters Modes of Operation Diode Selection When delivering high output currents, the MAX764/ MAX765/MAX766 operate in continuous-conduction mode. In this mode, current always flows in the inductor, and the control circuit adjusts the duty-cycle of the switch 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, 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 output ripple may increase slightly. The MAX764/MAX765/MAX766’s high switching frequency demands a high-speed rectifier. Use a Schottky diode with a 0.75A average current rating, such as the 1N5817 or 1N5818. High leakage currents may make Schottky diodes inadequate for high-temperature and light-load applications. In these cases you can use high-speed silicon diodes, such as the MUR105 or the EC11FS1. 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. __________________Design Procedure 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 68µF, 20V Sanyo OS-CON capacitor with ESR = 45mΩ (SA series) typically provides 50mV ripple when converting from 5V to -5V at 150mA. Output filter capacitor ESR also affects efficiency. To obtain optimum performance, use a 68µF or larger, low-ESR capacitor with a voltage rating of at least 20V. The smallest low-ESR surface-mount tantalum capacitors currently available are from the Sprague 595D series. Sanyo OS-CON series organic semiconductors and AVX TPS series tantalum capacitors also exhibit very low ESR. OS-CON capacitors are particularly useful at low temperatures. Table 1 lists some suppliers of low-ESR capacitors. For best results when using capacitors other than those suggested in Table 1 (or their equivalents), increase the output filter capacitor’s size or use capacitators in parallel to reduce ESR. Setting the Output Voltage The MAX764/MAX765/MAX766’s output voltage can be adjusted from -1.0V to -16V using external resistors R1 and R2, configured as shown in Figure 3. For adjustable-output operation, select feedback resistor R1 = 150kΩ. R2 is given by: I I VOUT R2 = (R1) ——— VREF where VREF = 1.5V. For fixed-output operation, tie FB to REF. Inductor Selection In both continuous- and discontinuous-conduction modes, practical inductor values range from 22µH to 68µH. 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, wasting power and reducing efficiency. The maximum inductor value is not critical. A 47µH inductor is ideal for most applications. For highest efficiency, use a coil with low DC resistance, preferably under 100mΩ. To minimize radiated noise, use a toroid, pot core, or shielded coil. Inductors with a ferrite core or equivalent are recommended. The inductor’s incremental saturation-current rating should be greater than the 0.75A 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). Table 1 lists inductor types and suppliers for various applications. The listed surface-mount inductors’ efficiencies are nearly equivalent to those of the largersize through-hole inductors. 10 Capacitor Selection Input Bypass Capacitor The input bypass capacitor, C1, reduces peak currents drawn from the voltage source and reduces the amount of noise at the voltage source caused by the switching action of the MAX764–MAX766. 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 highly recommended. For output currents up to 250mA, a 100µF to 120µF capacitor with a voltage rating of at least 20V (C1) in parallel with a 0.1µF capacitor (C2) is adequate in most applications. C2 must be placed as close as possible to the V+ and GND pins. ______________________________________________________________________________________ -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ DC-DC Inverters Layout Considerations Proper PC board layout is essential to reduce noise generated by high current levels and fast switching waveforms. 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. In particular, keep the traces connected to FB and LX short. C2 must be placed as close as possible to the V+ and GND pins. If an external resistor divider is used (Figure 3), the trace from FB to the resistors must be extremely short. Table 1. Component Suppliers PRODUCTION METHOD INDUCTORS CAPACITORS Sumida CD75/105 series Matsuo 267 series Coiltronics CTX series Sprague 595D/293D series Coilcraft DT/D03316 series AVX TPS series Miniature Through-Hole Sumida RCH895 series Sanyo OS-CON series (very low ESR) Low-Cost Through-Hole Renco RL1284 series Nichicon PL series Surface Mount SUPPLIER PHONE DIODES Nihon EC10QS02L (Schottky) EC11FS1 (high-speed silicon) Motorola 1N5817, 1N5818, (Schottky) MUR105 (high-speed silicon) FAX AVX USA: (803) 448-9411 (803) 448-1943 Coilcraft USA: (708) 639-6400 (708) 639-1469 Coiltronics USA: (407) 241-7876 (407) 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 Japan: 81-7-5231-8461 (708) 843-2798 81-7-5256-4158 Nihon USA: (805) 867-2555 Japan: 81-3-3494-7411 (805) 867-2556 81-3-3494-7414 Renco USA: (516) 586-5566 (516) 586-5562 Sanyo OS-CON USA: (619) 661-6835 Japan: 81-7-2070-1005 (619) 661-1055 81-7-2070-1174 Sprague Electric Co. USA: (603) 224-1961 (603) 224-1430 Sumida USA: (708) 956-0666 Japan: 81-3-3607-5111 (708) 956-0702 81-3-3607-5144 ______________________________________________________________________________________ 11 MAX764/MAX765/MAX766 Reference Capacitor Bypass REF with a 0.1µF capacitor (C3). The REF output can source up to 100µA for external loads. MAX764/MAX765/MAX766 -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ DC-DC Inverters _Ordering Information (continued) PART TEMP. RANGE ___________________Chip Topography PIN-PACKAGE MAX766CPA 0°C to +70°C 8 Plastic DIP MAX766CSA MAX766C/D MAX766EPA MAX766ESA MAX766MJA 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** LX OUT * Dice are tested at TA = +25°C, DC parameters only. **Contact factory for availability and processing to MIL-STD-883. 0.145" (3683µm) V+ FB V+ SHDN REF GND 0.080" (2032µm) TRANSISTOR COUNT: 443 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 © 1994 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.