19-0191; Rev 1; 3/94 TION KIT EVALUA LE AVAILAB -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers ________________________Applications LCD-Bias Generators ____________________________Features ♦ 85% Efficiency for 5mA to 1A Load Currents ♦ Up to 5W Output Power ♦ 100µA Max Supply Current ♦ 5µA Max Shutdown Current ♦ 3V to 16.5V Input Range ♦ -5V (MAX774), -12V (MAX775), -15V (MAX776), or Adjustable Output Voltage ♦ Current-Limited PFM Control Scheme ♦ 300kHz Switching Frequency ______________Ordering Information TEMP. RANGE PIN-PACKAGE MAX774CPA PART 0°C to +70°C 8 Plastic DIP MAX774CSA MAX774C/D MAX774EPA MAX774ESA MAX774MJA 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. * Contact factory for dice specifications. High-Efficiency DC-DC Converters Battery-Powered Applications Data Communicators __________Typical Operating Circuit INPUT 3V TO 16V __________________Pin Configuration TOP VIEW V+ MAX774 ON/OFF CS OUT 1 FB 2 SHDN 3 REF 4 8 GND 7 EXT 6 CS 5 V+ SHDN EXT P OUTPUT -5V FB REF MAX774 MAX775 MAX776 DIP/SO GND OUT ________________________________________________________________ Maxim Integrated Products Call toll free 1-800-998-8800 for free samples or literature. 1 MAX774/MAX775/MAX776 _______________General Description The MAX774/MAX775/MAX776 inverting switching regulators deliver high efficiency over three decades of load current. A unique current-limited, pulsefrequency-modulated (PFM) control scheme provides the benefits of pulse-width modulation (high efficiency with heavy loads), while using less than 100µA of supply current (vs. 2mA to 10mA for PWM converters). The result is high efficiency over a wide range of loads. These ICs also use tiny external components; their high switching frequency (up to 300kHz) allows for less than 5mm diameter surface-mount magnetics. The MAX774/MAX775/MAX776 accept input voltages from 3V to 16.5V, and have preset output voltages of -5V, -12V, and -15V, respectively. Or, the output voltage can be user-adjusted with two resistors. Maximum VIN - VOUT differential voltage is limited only by the breakdown voltage of the chosen external switch transistor. These inverters use external P-channel MOSFET switches, allowing them to power loads up to 5W. If less power is required, use the MAX764/MAX765/MAX766 inverting switching regulators with on-board MOSFETs. MAX774/MAX775/MAX776 -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers ABSOLUTE MAXIMUM RATINGS Supply Voltages V+ to OUT ...........................................................................21V V+ to GND ..............................................................-0.3V, +17V OUT to GND ........................................................-0.3V, to -17V REF, SHDN, FB, CS...................................-0.3V to (V+ + 0.3V) EXT ...............................................(VOUT - 0.3V) to (V+ + 0.3V) 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: MAX77_C_ _ .........................................................0°C to +70°C MAX77_E_ _ ......................................................-40°C to +85°C MAX77_MJA ...................................................-55°C to +125°C Maximum Junction Temperatures: MAX77_C_ _/E_ _ ...........................................................+150°C MAX77_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 V+ Input Voltage Range SYMBOL CONDITIONS MIN V+ TYP 3.0 V+ = 16.5V, SHDN ≤ 0.4V (operating) Supply Current FB Input Current Output Voltage Reference Voltage 2 V+ = 16.5V, SHDN ≥ 1.6V (shutdown) 4 VOUT VREF -10 5 µA 10 mV ±50 MAX77_E ±70 MAX77_M ±90 MAX774 -4.80 -5 -5.20 MAX775 -11.52 -12 -12.48 MAX776 -14.40 -15 -15.60 IREF = 0µA 0µA ≤ IREF ≤ 100µA REF Line Regulation 3V ≤ V+ ≤ 16.5V Output Voltage Load Regulation (Circuit of Figure 2— Bootstrapped) V MAX77_C REF Load Regulation Output Voltage Line Regulation (Circuit of Figure 2— Bootstrapped) 2 IFB UNITS 16.5 100 V+ = 10V, SHDN ≥ 1.6V (shutdown) 3V ≤ V+ ≤ 16.5V FB Trip Point MAX MAX77_C 1.4700 1.5 1.5300 MAX77_E 1.4625 1.5 1.5375 MAX77_M 1.4550 1.5 1.5450 MAX77_C/E 4 10 MAX77_M 4 15 40 100 MAX774, 4V ≤ V+ ≤ 15V, ILOAD = 0.5A 0.035 MAX775, 4V ≤ V+ ≤ 8V, ILOAD = 0.2A 0.088 MAX776, 4V ≤ V+ ≤ 6V, ILOAD = 0.1A 0.137 MAX774, 0A ≤ ILOAD ≤ 1A, V+ = 5V 1.5 MAX775, 0mA ≤ ILOAD ≤ 500mA, V+ = 5V 1.5 MAX776, 0mA ≤ ILOAD ≤ 400mA, V+ = 5V 1.0 _______________________________________________________________________________________ nA V V mV µV/V mV/V mV/A -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers PARAMETER SYMBOL Efficiency (Circuit of Figure 2— Bootstrapped) SHDN Input Current SHDN Input Voltage High SHDN Input Voltage Low VIH VIL Current-Limit Trip Level (V+ – CS) VCS CONDITIONS MAX774, V+ = 5V, ILOAD = 1A MIN TYP 82 MAX775, V+ = 5V, ILOAD = 500mA MAX776, V+ = 5V, ILOAD = 400mA V+ = 16.5V, SHDN = 0V or V+ 3V ≤ V+ ≤ 16.5V 3V ≤ V+ ≤ 16.5V MAX77_C/E 3V ≤ V+ ≤ 16.5V MAX77_M 180 160 210 210 MAX UNITS % 88 87 ±1 1.6 0.4 240 260 µA V V mV ±1 µA Switch Maximum On-Time tON (max) V+ = 12V 12 16 20 µs Switch Minimum Off-Time tOFF (max) V+ = 12V 1.8 2.3 2.8 µs CS Input Current EXT Rise Time EXT Fall Time CEXT = 1nF, V+ = 12V CEXT = 1nF, V+ = 12V 50 50 MAX774/MAX775/MAX776 ELECTRICAL CHARACTERISTICS (continued) ns ns _______________________________________________________________________________________ 3 __________________________________________Typical Operating Characteristics (TA = +25°C, unless otherwise noted.) MAX774 EFFICIENCY vs. LOAD CURRENT VOUT = -5V (NON-BOOTSTRAPPED) VIN = 15V 70 ILOAD = 100mA 80 80 EFFICIENCY (%) VIN = 3V VIN = 5V VIN = 4V EFFICIENCY (%) 80 90 MA774/5/6--1b VIN = 5V EFFICIENCY (%) 90 MAX1774/5/6-01a 90 MAX774 EFFICIENCY vs. TEMPERATURE MAX774/5/6-2 MAX774 EFFICIENCY vs. LOAD CURRENT VOUT = -5V (BOOTSTRAPPED) VIN = 15V 70 ILOAD = 600mA ILOAD = 1A 70 60 60 60 50 50 50 VIN = 5V BOOTSTRAPPED 100 10 1000 -40 0 20 40 60 80 TEMPERATURE (°C) MAX776 EFFICIENCY vs. LOAD CURRENT VOUT = -15V (BOOTSTRAPPED) MAX776 EFFICIENCY vs. LOAD CURRENT VOUT = -15V (NON-BOOTSTRAPPED) MAX775 EFFICIENCY vs. OUTPUT CURRENT VOUT = -12V (BOOTSTRAPPED) 80 VIN = 3V 70 60 VIN = 15V VIN = 6V VIN = 4V 70 60 50 100 70 50 1 1000 VIN = 8V VIN = 4V 60 50 10 VIN = 5V 80 VIN = 5V EFFICIENCY (%) EFFICIENCY (%) VIN = 4V 100 10 1 1000 100 10 1000 LOAD CURRENT (mA) LOAD CURRENT (mA) OUTPUT CURRENT (mA) MAX774/MAX775/MAX776 EFFICIENCY vs. LOAD CURRENT VOUT = -24V (NON-BOOTSTRAPPED) MAX774/MAX775/MAX776 EFFICIENCY vs. LOAD CURRENT VOUT = -24V OUTPUT (ZENER CONNECTION) MAX774 EFFICIENCY vs. INPUT VOLTAGE VOUT = -5V AT 100mA VIN = 5V VIN = 6V 88 86 EFFICIENCY (%) 80 VIN = 4V 70 EFFICIENCY (%) VIN = 5V 80 90 MA774/5/6--1g VIN = 6V MA774/5/6--1f 90 VIN = 4V 70 60 60 50 50 100 90 MA774/5/6-1d 90 VIN = 5V 1 -20 LOAD CURRENT (mA) MA774/5/6--1e 1 1000 VIN = 6V 80 EFFICIENCY (%) 100 MA774/5/6-1c 90 10 LOAD CURRENT (mA) BOOTSTRAPPED MAX774/5/6-3 1 EFFICIENCY (%) MAX774/MAX775/MAX776 -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers 84 82 80 NON-BOOTSTRAPPED 78 76 VOUT = -5V AT 100mA 1 10 100 LOAD CURRENT (mA) 4 1000 74 1 10 100 LOAD CURRENT (mA) 1000 2 4 6 8 10 12 INPUT VOLTAGE (V) _______________________________________________________________________________________ 14 16 -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers 4.5 3.5 VOUT = -5V 3.0 4.0 VOUT = -24V 3.5 3.0 2.5 1000 100 10 LOAD CURRENT (mA) EXT RISE AND FALL TIMES vs. TEMPERATURE EXT RISE AND FALL TIMES vs. TEMPERATURE 500 110 80 5V FALL 70 60 12V RISE 50 CEXT = 5nF 4 6 8 10 12 14 16 INPUT VOLTAGE (V) SWITCH ON-TIME vs. TEMPERATURE 17 V+ = 5V 5V RISE 350 300 250 16 5V FALL 200 12V RISE 150 40 12V FALL 30 100 12V FALL 50 20 -60 -40 -20 0 20 40 60 80 100 120 140 15 -60 -40 -20 0 20 40 60 -60 80 100 120 140 60 0 TEMPERATURE (°C) TEMPERATURE (°C) TEMPERATURE (°C) SWITCH OFF-TIME vs. TEMPERATURE SWITCH ON-TIME/OFF-TIME RATIO SHUTDOWN CURRENT vs. TEMPERATURE 7.8 V+ = 5V 3.5 3.0 7.4 7.2 ICC (µA) tON/tOFF RATIO (µs/µs) 7.6 2.0 4.0 120 MAX774/5/6-7 V+ = 5V MAX774/5/6-6 8.0 MAX761-13 2.5 tOFF (µs) 2 ton (µs) 90 tRISE & tFALL (ns) 5V RISE 1200 1000 450 400 100 1400 1000 MAX774/5/6-10 120 1 LOAD CURRENT (mA) CEXT = 1nF BOOTSTRAPPED 1600 800 0.1 MAX774/5/6-9 130 100 10 1800 NON-BOOTSTRAPPED VOUT = -5V 2.5 1 tRISE & tFALL (ns) VOUT = -15V VOUT = -5V 2000 LOAD CURRENT (mA) VOUT = -12V MAX774/5/6-16 VOUT = -12V START-UP VOLTAGE (V) START-UP VOLTAGE (V) 4.5 2200 MA744/5/6-15 VOUT = -15V 4.0 5.0 MA744/5/6-14 5.0 MAX774 MAXIMUM LOAD vs. INPUT VOLTAGE START-UP VOLTAGE vs. LOAD CURRENT (NON-BOOTSTRAPPED) MAX761-13 START-UP VOLTAGE vs. LOAD CURRENT (BOOTSTRAPPED) 7.0 6.8 2.5 V+ = 15V 2.0 1.5 6.6 V+ = 8V 1.0 6.4 1.5 -60 0 60 TEMPERATURE (°C) 120 6.2 0.5 6.0 0 V+ = 4V -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) _______________________________________________________________________________________ 5 MAX774/MAX775/MAX776 _____________________________Typical Operating Characteristics (continued) (TA = +25°C, unless otherwise noted.) _____________________________Typical Operating Characteristics (continued) (TA = +25°C, unless otherwise noted.) OPERATING SUPPLY CURRENT vs. TEMPERATURE REFERENCE TEMPERATURE COEFFICIENT 76 V+ = 10V 74 72 V+ = 3V 68 MAX774/5/6-12 1.500 1.498 1.496 1.494 1.492 66 0 20 40 60 80 100 120 140 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) TEMPERATURE (°C) CS TRIP LEVEL REFERENCE OUTPUT RESISTANCE MAX774/5/6-11 235 230 225 220 215 210 205 200 195 190 250 REFERENCE OUTPUT RESISTANCE (Ω) -60 -40 -20 200 IREF = 10µA 150 IREF = 50µA 100 50 IREF = 100µA 0 185 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) 6 1.502 MAX774/5/6-13 ICC (µA) 1.504 REFERENCE OUTPUT (V) V+ = 16.5V 78 70 1.506 MAX774/5/6-8 80 CS TRIP LEVEL (mV) MAX774/MAX775/MAX776 -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (°C) _______________________________________________________________________________________ -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers INDUCTOR CURRENT NEAR FULL LOAD OPERATING WAVEFORMS A 1A/div B 0A C 20µs/div 10µs/div CIRCUIT OF FIGURE 2 VOUT = -5V, V+ = 4.7V ILOAD = 1.05A (1A/div) CIRCUIT OF FIGURE 2 V+ = 6.5V, ILOAD = 1A, VOUT = -5V A: OUTPUT RIPPLE, 200mV/div B: EXT WAVEFORM, 10V/div C: INDUCTOR CURRENT, 2A/div CONTINUOUS CONDUCTION AT ONE-HALF CURRENT LIMIT ENTRY/EXIT FROM SHUTDOWN A 1A/div B 0A 20µs/div CIRCUIT OF FIGURE 2 ILOAD = 300mA, VOUT = -5V V+ = 8V, L = 22µH 2ms/div CIRCUIT OF FIGURE 2 V+ = 6V, ILOAD = 1A, VOUT = -5V A: SHUTDOWN PULSE, 0V TO V+, 5V/div B: VOUT, 2V/div _______________________________________________________________________________________ 7 MAX774/MAX775/MAX776 __________________________________________Typical Operating Characteristics (TA = +25°C, unless otherwise noted.) MAX774/MAX775/MAX776 -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers ____________________________Typical Operating Characteristics (continued) (TA = +25°C, unless otherwise noted.) LOAD-TRANSIENT RESPONSE LINE-TRANSIENT RESPONSE A A B B 100µs/div 2ms/div CIRCUIT OF FIGURE 2 V+ = 6V, VOUT = -5V A: ILOAD, 30mA TO 1A, 1A/div B: VOUT, 100mV/div, AC-COUPLED CIRCUIT OF FIGURE 2 VOUT = -5V, ILOAD = 1A A: V+, 3V TO 8V, 5V/div B: VOUT, 100mV/div, AC-COUPLED ______________________________________________________________Pin Description 8 PIN NAME FUNCTION 1 OUT The sense input for fixed-output operation (VFB = VREF). OUT is connected to the internal voltage divider, and it is the negative supply input for the EXT driver. 2 FB 3 SHDN 4 REF 1.5V reference output that can source 100µA. Bypass to ground with 0.1µF. 5 V+ Positive power-supply input 6 CS Noninverting input to the current-sense comparator. Typical trip level is 210mV (relative to V+). 7 EXT The gate-drive output for an external P-channel power MOSFET. EXT swings from OUT to V+. 8 GND Ground Feedback input. When VFB = VREF, the output will be the factory preset value. For adjustable operation, use an external voltage divider, as described in the Adjustable Output section. Active-high shutdown input. With SHDN high, the part is in shutdown mode and the supply current is less than 5µA. Connect to GND for normal operation. _______________________________________________________________________________________ -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers MAX774/MAX775/MAX776 V+ FB MODE COMPARATOR REF MAX774 MAX775 MAX776 50mV SHDN ERROR COMPARATOR OUT N 1.5V REFERENCE Q TRIG ONE-SHOT FROM V+ S TRIG Q EXT Q R ONE-SHOT FROM OUT CURRENT COMPARATOR CS 0.2V (FULL CURRENT) CURRENT CONTROL CIRCUITS 0.1V (HALF CURRENT) FROM V+ GND Figure 1. Block Diagram _______________Detailed Description The MAX774/MAX775/MAX776 are negative-output, inverting power controllers that can be configured to drive an external P-channel MOSFET. The output voltages are preset to -5V (MAX774), -12V (MAX775), or -15V (MAX776). Additionally, all three parts can be set to any desired output voltage using an external resistor divider. The MAX774/MAX775/MAX776 have a unique control scheme (Figure 1) that combines the advantage of pulse-skipping, pulse-frequency-modulation (PFM) converters (ultra-low supply current) with the advantage of pulse-width-modulation (PWM) converters (high efficiency with heavy loads). This control scheme allows the devices to achieve 85% efficiency with loads from 5mA to 1A. As with traditional PFM converters, the external P-channel MOSFET power transistor is turned on when the voltage comparator senses that the output is below the reference voltage. However, unlike traditional PFM converters, switching is controlled by the combination of a switch current limit (210mV/R SENSE ) and on-time/off-time limits set by one-shots. Once turned on, the MOSFET stays on until: 1) the 16µs maximum on-time limit is reached or 2) the switch current reaches its limit (as set by the current-sense resistor). Once off, the switch is typically held off for a minimum of 2.3µs. It will stay off until the output drops below the level determined by VREF and the feedback divider network. _______________________________________________________________________________________ 9 MAX774/MAX775/MAX776 -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers VIN VIN C2 0.1µF C1 150µF 1 OUT 3 2 4 V+ 1 5 R1 0.07Ω MAX774 SHDN MAX775 CS 6 MAX776 FB R2 EXT C1 150µF Q1 Si9435 P 7 VOUT GND 8 1N5822/ MBR340 L1 22µH C4* MAX775, MAX776 = 120µF, 20V OUTPUT VOLTAGE (V) INPUT VOLTAGE (V) MAX774 -5 3 to 15 1 MAX775 -12 3 to 8 0.5 MAX776 -15 3 to 5 0.4 NOTE: Si9435 HAS VGS OF 20V MAX Figure 2. Bootstrapped Connection Using Fixed Output Voltages VIN 1 OUT C2 0.1µF V+ 5 R3 0.07Ω R1 C3 0.1µF 3 SHDN MAX774 2 MAX775 CS 6 FB MAX776 EXT 4 REF GND 8 7 L1 22µH Q1 Si9435 P VOUT 1N5822/ MBR340 C4* * MAX774 = 330µF, 10V MAX775, MAX776 = 120µF, 20V Figure 3. Bootstrapped Connection Using External Feedback Resistors With light loads, the MOSFET switches on for one or more cycles and then switches off, much like in traditional PFM converters. 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 voltage comparator keeps 10 5 R3 0.07Ω SHDN MAX774 FB MAX775 CS 6 MAX776 7 EXT R1 4 C3 0.1µF REF GND 8 L1 22µH Q1 Si9435 P VOUT 1N5822/ MBR340 C4* MAX775, MAX776 = 120µF, 20V Figure 4. Non-Bootstrapped Operation (VIN > 4.5V) OUTPUT CURRENT (A) R2 V+ * MAX774 = 330µF, 10V * MAX774 = 330µF, 10V C1 150µF 2 C2 0.1µF REF C3 0.1µF PRODUCT 3 OUT the MOSFET off, and the current limit remains at one-half the peak current limit. If the output voltage is out of regulation after two consecutive pulses, the current limit for the next pulse will equal the full current limit. With heavy loads, the MOSFET first switches twice at one-half the peak current value. Subsequently, it stays on until the switch current reaches the full current limit, and then turns off. After it is off for 2.3µs, the MOSFET switches on once more, and remains on until the switch current again reaches its limit. This cycle repeats until the output is in regulation. A benefit of this control scheme is that it is highly efficient over a wide range of input/output ratios and load currents. Additionally, PFM converters do not operate with constant-frequency switching, and have relaxed stability criterion (unlike PWM converters). As a result, their external components require smaller values. With PFM converters, the output voltage ripple is not concentrated at the oscillator frequency (as it is with PWM converters). So for applications where the ripple frequency is important, the PWM control scheme must be used. However, for many other applications, the smaller capacitors and lower supply current of the PFM control scheme make it the better choice. The output voltage ripple with the MAX774/MAX775/MAX776 can be held quite low. For example, using the circuit of Figure 2, only 100mV of output ripple is produced when generating a -5V at 1A output from a +5V input. Bootstrapped vs. Non-Bootstrapped Operation Figures 2 and 3 are the standard application circuits for bootstrapped mode, and Figure 4 is the circuit for nonbootstrapped mode. Since EXT is powered by OUT, ______________________________________________________________________________________ -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers RZ tions, you should see a short current pulse at half the peak current approximately every 100ms (the exact period depends on actual circuit leakages). EXT Drive Voltages 1 OUT GND 8 R2 2 FB R1 4 MAX774 MAX775 MAX776 REF 0.1µF 6V ≤ VZ + VIN ≤ 10V VOUT – VZ > IZ RZ IZ = ZENER BREAKDOWN CURRENT VZ = ZENER BREAKDOWN VOLTAGE VIN = INPUT SUPPLY VOLTAGE Figure 5. Connection Using Zener Diode to Boost Base Drive EXT swings from OUT to V+ and provides the drive output for an external power MOSFET. When using the onchip feedback resistors for the preset output voltages, the voltage at OUT equals the output voltage. When using external feedback resistors, OUT may be tied to GND or some other potential between VOUT and GND. Always observe the V+ to OUT absolute maximum rating of 21V. For V+ to output differentials greater than 21V, OUT must be tied to a potential more positive than the output and, therefore, the output voltage must be set with an external resistor divider. In non-bootstrapped operation with low input voltages (<4V), tie OUT to a negative voltage to fully enhance the external MOSFET. Accomplish this by creating an intermediate voltage for VOUT with a zener diode (Figure 5). __________________Design Procedure Setting the Output Voltage using bootstrapped or non-bootstrapped mode will directly affect the gate drive to the FET. EXT swings from V+ to VOUT. In bootstrapped operation, OUT is connected to the output voltage (-5V, -12V, -15V). In non-bootstrapped operation, OUT is connected to ground, and EXT now swings from V+ to ground. At high input-to-output differentials, it may be necessary to use non-bootstrapped mode to avoid the 21V V+ to VOUT maximum rating. Also, observe the V GS maximum rating of the external transistor. At intermediate voltages and currents, the advantages of bootstrapped vs. non-bootstrapped operation are slight. When input voltages are less than about 4V, always use the bootstrapped circuit. Shutdown and Quiescent Current The MAX774/MAX775/MAX776 are designed to save power in battery-powered applications. A TTL/CMOS logic-level shutdown input (SHDN) has been provided for the lowest-power applications. When shut down (SHDN = V+), most internal bias current sources and the reference are turned off so that less than 5µA of current is drawn. In normal operation, the quiescent current will be less than 100µA. However, this current is measured by forcing the external switch transistor off. Even with no load, in an actual application, additional current will be drawn to supply the feedback resistors’ and the diode’s and capacitor’s leakage current. Under no-load condi- The MAX774/MAX775/MAX776 are preset for -5V, -12V, and -15V output voltages, respectively; however, they may also be adjusted to other values with an external voltage divider. For the preset output voltage, connect FB to REF and connect OUT to the output (Figure 3). In this case, the output voltage is sensed by OUT. For an adjustable output (Figures 3 and 4), connect an external resistor divider from the output voltage to FB, and from FB to REF. In this case, the divided-down output voltage is sensed via the FB pin. There are three reasons to use the external resistor divider: 1) You desire an output voltage other than a preset value 2) The input-to-output differential exceeds 21V or 3) The output voltage (VOUT to GND) exceeds -15V. For adjustable operation, refer to Figures 3 and 4. The impedance of the feedback network should be low enough that the input bias current of FB is not a factor. For best efficiency and precision, allow 10µA to flow through the network. Calculate (V REF - VFB) / R1 = 10µA. Since VREF = 1.5V and VFB = 0V, R1 becomes 150kΩ. Then calculate R2 as follows: R2 _______ VOUT ___ = R1 VREF (or, ______ VOUT = 10µA) R2 ______________________________________________________________________________________ 11 MAX774/MAX775/MAX776 VOUT 0.1µF RSENSE = 0.06Ω 2000 1500 1000 RSENSE = 0.07Ω RSENSE = 0.08Ω 500 0 RSENSE = 0.09Ω VOUT = -5V 3 4 5 6 VOUT = -12V 800 RSENSE = 0.05Ω RSENSE = 0.06Ω RSENSE = 0.07Ω 600 400 RSENSE = 0.08Ω RSENSE = 0.09Ω 200 0 7 8 9 10 11 12 13 14 15 INPUT VOLTAGE (V) Figure 6. MAX774 Maximum Output Current vs. Input Voltage (VOUT = -5V) Choosing an Inductor Practical inductor values range from 10µH to 50µH. The maximum inductor value is not particularly critical. For highest current at high VOUT to V+ ratios, the inductor should not be so large that the peak current never reaches the current limit. That is: [V+(min) - VSW(max)] x 12µs L(max) ≤ _______________________________ ILIM(max) This is only important if VIN 1 t OFF(min) = ___________ VOUT 6 t ON(max) More important is that the inductor not be so small that the current rises much faster than the current-limit comparator can respond. This would be wasteful and reduce efficiency. Calculate the minimum inductor value as follows: _______ < — [V+(max) - VSW(min)] x 0.3µs L(min) ≥ _______________________________ δ(I) x ILIM(min) Where L is in µH, 0.3µs is an ample time for the comparator response, ILIM is the current limit (see CurrentSense Resistor section), and δ(I) is the allowable percentage of overshoot. As an example, Figure 2's circuit uses a 3A peak current. If we allow a 15% overshoot and 15V is the maximum input voltage, then L(min) is 16µH. The actual value of L above this limit has minimal effect on this circuit's operation. For highest efficiency, use a coil with low DC resistance. Coils with 30mΩ or lower resistance are available. To 12 1000 MAX775-FIG07 RSENSE = 0.05Ω MAXIMUM OUTPUT CURRENT (mA) MAX775-fig6 2500 MAXIMUM OUTPUT CURRENT (mA) MAX774/MAX775/MAX776 -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers 3 4 5 6 7 INPUT VOLTAGE (V) 8 9 Figure 7. MAX775 Maximum Output Current vs. Input Voltage (VOUT = -12V) minimize radiated noise, use a torroid, pot-core, or shielded-bobbin inductor. Inductors with a ferrite core or equivalent are recommended. Make sure that the inductor’s saturation current rating is greater than ILIM(max). Diode Selection The ICs’ high switching frequencies demand a highspeed rectifier. Schottky diodes such as the 1N5817 to 1N5822 families are recommended. Choose a diode with an average current rating approximately equal to or greater than ILIM (max) and a voltage rating higher than VIN(max) + VOUT. For high-temperature applications, Schottky diodes may be inadequate due to their high leakage currents; instead, high-speed silicon diodes may be used. At heavy loads and high temperature, the benefits of a Schottky diode’s low forward voltage may outweigh the disadvantages of its high leakage current. Current-Sense Resistor The current-sense resistor limits the peak switch current to 210mV/RSENSE, where RSENSE is the value of the current-sense resistor, and 210mV is the currentsense comparator threshold (see Current-Limit Trip Level in the Electrical Characteristics). To maximize efficiency and reduce the size and cost of external components, minimize the peak current. However, since the output current is a function of the peak current, do not set the limit too low. Refer to Figures 6–9 to determine the sense resistor (and, therefore, peak current) for the required load current. ______________________________________________________________________________________ 500 400 300 200 RSENSE = 0.08Ω RSENSE = 0.09Ω 600 MAX776-FIG09 RSENSE = 0.05Ω RSENSE = 0.06Ω RSENSE = 0.07Ω 600 800 MAXIMUM OUTPUT CURRENT (mA) MAXIMUM OUTPUT CURRENT (mA) VOUT = -15V VOUT = -24V RSENSE = 0.05Ω RSENSE = 0.06Ω RSENSE = 0.07Ω 400 200 RSENSE = 0.08Ω RSENSE = 0.09Ω 0 100 3 4 5 6 INPUT VOLTAGE (V) 7 3 4 5 6 7 8 9 10 11 12 13 14 15 INPUT VOLTAGE (V) Figure 8. MAX776 Maximum Output Current vs. Input Voltage (VOUT = -15V) Figure 9. MAX774/MAX775/MAX776 Maximum Output Current vs. Input Voltage (VOUT = -24V) To choose the proper current-sense resistor, simply follow the two-step procedure outlined below. 1) Determine: • Input voltage range, V+ • Maximum (absolute) output voltage, VOUT • Maximum output current, ILOAD resistors. If you want to use through-hole resistors, IRC has a wire resistor that is simply a band of metal shaped as a “U” so that inductance is less than 10nH (an order of magnitude less than metal-film resistors). These are available in resistance values between 5mΩ and 0.1Ω. For example, let V+ range from 4V to 6V, and choose VOUT = -24V and IOUT = 150mA. 2) Next, referring to Figure 9, find the curve with the lowest current limit whose output current (with the lowest input voltage) meets your requirements. In our example, we want a curve where IOUT is >150mA with a 4V input and a -24V output. The RSENSE = 80mΩ (shown in Figure 9) shows only approximately 125mA of output current with a 4V input, so we look next at the RSENSE = 70mΩ line. It shows IOUT >150mA for V+ = 4V and VOUT = -24V. The current limit will be 0.210V / 0.070Ω = 3A. These curves take into account worst-case inductor (±10%) and current-sense trip levels, but not sense-resistor tolerance. The switch on resistance is 70mΩ. Standard wire-wound and metal-film resistors have an inductance high enough to degrade performance. Metal-film resistors are usually deposited on a ceramic rod in a spiral, making their inductances relatively high. Surface-mount (or chip) resistors have very little inductance and are well suited for use as current-sense The MAX774/MAX775/MAX776 are capable of driving P-channel enhancement-mode MOSFET transistors only. The choice of power transistor is dictated by input and output voltage, peak current rating, on resistance, gatesource threshold, and gate capacitance. The drain-tosource rating must be greater than the V+ - V OUT input-to-output voltage differential. The gate-to-source rating must be greater than V+ (the source voltage) plus the absolute value of the most negative swing of EXT. For bootstrapped operation, the most negative swing of EXT is VOUT. In non-bootstrapped operation, this may be ground or some other negative voltage. Gate capacitance is not normally a limiting factor, but values should be less than 1nF for best efficiency. For maximum efficiency, the MOSFET should have a very low on resistance at the peak current and be capable of handling that current. The transistor chosen for the typical operating circuit has a 30V drain-source voltage limit and a 0.07Ω drain-source on resistance at VGS = -10V. Table 1 lists suppliers of switching transistors suitable for use with the MAX774/MAX775/MAX776. External Switching Transistor ______________________________________________________________________________________ 13 MAX774/MAX775/MAX776 700 MAX776-FIG08 -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers MAX774/MAX775/MAX776 -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers Table 1. Component Suppliers SUPPLIER PHONE FAX Coiltronics (407) 241-7876 (407) 241-9339 Gowanda (716) 532-2234 (716) 532-2702 Sumida USA (708) 956-0666 (708) 956-0702 Sumida Japan 81-3-3607-5111 81-3-3607-5144 Kemet (803) 963-6300 (803) 963-6322 Matsuo (714) 969-2491 (714) 960-6492 Nichicon (708) 843-7500 (708) 843-2798 INDUCTORS CAPACITORS Sanyo USA (619) 661-6835 (619) 661-1055 Sanyo Japan 81-7-2070-6306 81-7-2070-1174 Sprague (603) 224-1961 (603) 224-1430 United Chemi-Con (714) 255-9500 (714) 255-9400 Motorola (800) 521-6274 (602) 952-4190 Nihon USA (805) 867-2555 (805) 867-2556 Nihon Japan 81-3-3494-7411 81-3-3494-7414 Harris (407) 724-3729 (407) 724-3937 International Rectifier (310) 322-3331 (310) 322-3332 Siliconix (408) 988-8000 (408) 970-3950 DIODES POWER MOSFETS CURRENT-SENSE RESISTORS IRC (704) 264-8861 When evaluating this equation, be sure to use the capacitance value at the switching frequency. At 200kHz, the 330µF tantalum capacitor of Figures 2, 3, or 4 may degrade by a factor of ten, which will significantly alter the ripple voltage calculation. The ESR of both the bypass and filter capacitors also affects efficiency. Best performance is obtained by doubling up on the filter capacitors or using low-ESR capacitors. Capacitors must have a ripple current rating equal to the peak current. The smallest low-ESR SMT capacitors currently available are the Sprague 595D series. Sanyo OS-CON organic semiconductor through-hole capacitors also exhibit low ESR and are especially effective at low temperatures. Table 1 lists the phone numbers of these and other manufacturers. PC Layout and Grounding (704) 264-8866 Capacitors Choose the output capacitor (C4 of Figures 2, 3, and 4) to be consistent with your size, ripple, and output voltage requirements. Place capacitors in parallel if the size you want is unobtainable. You will not only increase the capacitance, but also decrease the capacitor’s ESR (a major contributor of ripple). A 330µF tantalum output filter capacitor with 0.07Ω ESR 14 typically maintains 120mV p-p output ripple when generating -5V at 1A from a 5V input. Smaller capacitors are acceptable for lighter loads or in applications that can tolerate higher output ripple. The value of C4 is chosen such that it acquires as small a charge as possible during the switch on-time. The amount of ripple as a function of capacitance is give by: VOUT x IOUT x ESR IOUT x tOFF(min) ∆Vp-p = _____________________ + _________________ C VIN Due to high current levels and fast switching waveforms, proper PC board layout is essential. Use a star ground configuration; connect the ground lead of the input bypass capacitor, the output capacitor, the inductor, and the GND pin of the MAX774/MAX775/MAX776 at a common point very close to the device. Additionally, input capacitor C2 (Figures 3 and 4) should be placed extremely close to the device. If an external resistor divider is used (Figures 3 and 4), the trace from FB to the resistors must be extremely short. ______________________________________________________________________________________ -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers TEMP. RANGE PIN-PACKAGE MAX775CPA PART 0°C to +70°C 8 Plastic DIP MAX775CSA MAX775C/D MAX775EPA MAX775ESA MAX775MJA MAX776CPA MAX776CSA MAX776C/D MAX776EPA MAX776ESA MAX776MJA 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 8 SO Dice* 8 Plastic DIP 8 SO 8 CERDIP 8 Plastic DIP 8 SO Dice* 8 Plastic DIP 8 SO 8 CERDIP ___________________Chip Topography OUT GND EXT FB .109" (2.769mm) CS SHDN * Contact factory for dice specifications. REF V+ 0.080 (2.032mm) TRANSISTOR COUNT: 442; SUBSTRATE CONNECTED TO V+. ________________________________________________________Package Information DIM E A A1 B C D E e H h L α H INCHES MAX MIN 0.069 0.053 0.010 0.004 0.019 0.014 0.010 0.007 0.197 0.189 0.157 0.150 0.050 BSC 0.244 0.228 0.020 0.010 0.050 0.016 8˚ 0˚ MILLIMETERS MIN MAX 1.35 1.75 0.10 0.25 0.35 0.49 0.19 0.25 4.80 5.00 3.80 4.00 1.27 BSC 5.80 6.20 0.25 0.50 0.40 1.27 0˚ 8˚ 21-325A h x 45˚ D α A 0.127mm 0.004in. e A1 C L 8-PIN PLASTIC SMALL-OUTLINE PACKAGE B ______________________________________________________________________________________ 15 MAX774/MAX775/MAX776 _Ordering Information (continued) MAX774/MAX775/MAX776 -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers ________________________________________________________Package Information DIM D1 A A1 A2 A3 B B1 C D D1 E E1 e eA eB L α E E1 D A3 A A2 L A1 INCHES MAX MIN 0.200 – – 0.015 0.175 0.125 0.080 0.055 0.022 0.016 0.065 0.050 0.012 0.008 0.390 0.348 0.035 0.005 0.325 0.300 0.280 0.240 0.100 BSC 0.300 BSC 0.400 – 0.150 0.115 15˚ 0˚ MILLIMETERS MIN MAX – 5.08 0.38 – 3.18 4.45 1.40 2.03 0.41 0.56 1.27 1.65 0.20 0.30 8.84 9.91 0.13 0.89 7.62 8.26 6.10 7.11 2.54 BSC 7.62 BSC – 10.16 2.92 3.81 0˚ 15˚ 21-324A α 8-PIN PLASTIC DUAL-IN-LINE PACKAGE C e B1 eA B eB DIM S S1 E1 D E B2 A MILLIMETERS MIN MAX – 5.08 0.36 0.58 0.97 1.65 0.58 1.14 0.20 0.38 – 10.29 5.59 7.87 7.37 8.13 2.54 BSC 3.18 5.08 3.81 – 0.38 1.52 – 1.40 0.13 – 0˚ 15˚ 21-326D α Q L A B B1 B2 C D E E1 e L L1 Q S S1 α INCHES MAX MIN 0.200 – 0.023 0.014 0.065 0.038 0.045 0.023 0.015 0.008 0.405 – 0.310 0.220 0.320 0.290 0.100 BSC 0.200 0.125 – 0.150 0.060 0.015 0.055 – – 0.005 15˚ 0˚ e L1 B1 B C 8-PIN CERAMIC DUAL-IN-LINE PACKAGE Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time. 16 __________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600 © 1994 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.