oe MAKiM -5V/-12V/-15V or Adjustable, High-Efficiency, Low Ig inverting DC-DC Controllers General Description Features The MAX774/MAX775/MAX776 inverting switching # 85% Efficiency for 5mA to 1A Load Currents regulators deliver high efficiency over three decades of load current. A unique current-limited, pulse- * Upto 5W Output Power frequency-modulated (PFM) control scheme provides # 100,A Max Supply Current the benefits of pulse-width modulation (high efficiency with heavy loads), while using less than 100pA of sup- 5A Max Shutdown Current ply current (vs. 2mA to 10mA for PWM converters). The 3V to 16.5V Input Range result is high efficiency over a wide range of loads. 5), -15V (MAXT776 These |Cs also use tiny external components; their high * -5V (MAX774), 12V (MAX775), -15V ( ) switching frequency (up to 300kHz) allows for less than or Adjustable Output Voltage 5mm diameter surface-mount magnetics. @ Current-Limited PFM Control Scheme 3V to 16.5V, and have preset output voltages of -8V, -12V, and -15V, respectively. Or, the output voltage can be user-adjusted with two resistors. Maximum a OLLXVW/SLLXVW/PLLXVIN Vin - Vout differential voltage is limited only by the break. ________ Ordering Information down voltage of the chosen external switch transistor. PART TEMP. RANGE PIN-PACKAGE These inverters use external P-channel MOSFET switch- MAX774CPA 0C to +70C 8 Plastic DIP es, allowing them to power loads up to 5W. If less MAX774CSA 0C to +70C 850 power is required, use the MAX764/MAX765/MAX766 MAX774C/D OC to 470C Dice inverting switching regulators with on-board MOSFETs. MAXT74EPA FOC to 48C 3 Plastic DIP Applications MAX774ESA -40C to +85C 8 SO LCD-Bias Generators MAX774MJA -55C to +125C 8 CERDIP High-Efficiency DC-DC Converters Ordering Information continued on last page. er " Contact factory for dice specifications. Battery-Powered Applications Data Communicators Typical Operating Circuit Pin Configuration INPUT TOP VIEW 3V TO 16V a Ve = MAXIM we Kore MAX774 cs out [7] a} GNo ON/OFF SHDN MAXIMA ra[2) waxz7za = LO con | HEE Fac rer [4] [5] vs DIP/SO EXT FB REF NH it-4 MAAXIAA Maxim Integrated Products 4-161 Cail toll free 1-800-998-8800 for free samples or literature.MAX774/MAX775/MAX776 5V/-12V/-15V or Adjustable, High-Efficiency, Low Ig inverting DC-DC Controllers ABSOLUTE MAXIMUM RATINGS Supply Voltages V$ tO OUT cece ce tsecceeeeeeseecenteseenecesterenstenseesenees 21V V+ to GND...... OUT to GND ..... ae REF, SHDN, FB, CS.. EXT beste -0.3V to (V+ + 0.3V) ..{VouT - 0.3V) to (V+ + 0.3V) Continuous Power Dissipation (Ta = +70C) Plastic DIP (derate 9.09mW/C above +70C) ............. 727mW SO (derate 5.88mW/C above +70C) CERDIP (derate 8.00mW/C above +70C)...........006/ 640mWw Operating Temperature Ranges: MAX77 Co ccccccccccscsesscseecersesstepetnecessecesecenseeenes 0C to +70C MAX77_E__.. -40C to +85C MAX77_ MUA oor cereeenenecsteneeeveereeaes 55C to +125C Maximum Junction Temperatures: MAX77_C__/E__ MAX77_MJA Storage Temperature Range .......0... ee -65C to +160C Lead Temperature (soldering, 10SC) 00.0... +300C 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 = OmA, Crer = 0.1p0F, Ta = TMIN to TMAX, Unless otherwise noted. Typical values are at Ta = +25C.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS V+ Input Voltage Range V+ 3.0 16.5 Vv V+ = 16.5V, SHDN < 0.4V (operating) 100 Supply Current V+ = 10V, SHDN 2 1.6V (shutdown) 2 5 pA V+ = 16.5V, SHDN 2 1.6V (shutdown) 4 FB Trip Point 3V < V+ < 16.5V -10 10 mv MAX77_C +50 FB Input Current IFB MAX77_E +70 nA MAX77_M +90 MAX774 -4.80 5 -5.20 Output Voltage Vout MAX775 -11.52 -12 -12.48 Vv MAX776 -14.40 ~15 -15.60 MAX77_C 1.4700 1.5 1.5300 Reference Voltage VREF IREF = QnA MAX77_E 1.4625 1.5 1.5375 Vv MAX77_M 1.4550 1.5 1.5450 REF Load Regulation OA s IREF s 100nA MAX77_C/E 4 10 mV MAX77_M 4 15 REF Line Regulation 3V $ V+ $ 16.5V 40 100 wv Output Voltage Line Regulation MAX774, 4V < V+ < 15V, ILOAD = 0.5A 0.035 (Circuit of Figure 2 MAX775, 4V < V+ < BV, ILOAD = 0.2A 0.088 mv/V Bootstrapped) MAX776, 4V 5 V+ $6V, ILOAD = 0.1A 0.137 Output Voltage Load Regulation MAX774, OA $ ILoaD $ 1A, V+ = 5V 1.5 (Circuit of Figure 2 MAX775, OmA s ILOAD < 500mA, V+ = 5V 15 mV/A Bootstrapped) MAX776, OMA <ILOAD < 400mA, V+ = 5V 1.0 4-162 MAAXILAA5V/-12V/-15V or Adjustable, High-Efficiency, Low Ig inverting DC-DC Controllers ELECTRICAL CHARACTERISTICS (continued) PARAMETER SYMBOL CONDITIONS MIN TYP MAX | UNITS Efficiency MAX774, V+ = 5V, ILoaD = 1A 82 (Circuit of Figure 2 MAX775, V+ = 5V, ILoaD = 500mMA 88 % Bootstrapped) MAX776, V+ = 5V, ILOAD = 400mA 87 SHDN fnput Current V+ = 16.5V, SHDN = OV or V+ +1 HA SHDN Input Voltage High ViH 3V <V+ s 16.5V 1.6 v SHDN Input Voltage Low VIL 3V s V+ < 16.5V 0.4 v urrent-Limit Trip Level MAX77_C/E 180 210 240 va : cS) rimere Ves | SVsV+s16.5V MAX77_M 160. 210 260 | CS Input Current +1 pA Switch Maximum On-Time ton (max) | V+ = 12V 12 16 20 ys Switch Minimum Off-Time torr (max) | V+ = 12V 1.8 23 2.8 ps EXT Rise Time Cext = 1nF, V+ = 12V 50 ns EXT Fall Time Cext = inF, V+ = 12V 50 ns MAAXLAA 4-163 i 9ZLXVW/SLLXUW/PZLLXUWMAX774/MAX775/MAX776 -5V/-12V/-15V or Adjustable, High-Efficiency, Low Iq Inverting DC-DC Controllers Typical Operating Characteristics (Ta = +25C, unless otherwise noted.) MAX774 MAX774 EFFICIENCY vs. LOAD CURRENT EFFICIENCY vs. LOAD CURRENT MAX774 Vout = -5V (BOGTSTRAPPED) Vout = -5V (NON-BOOTSTRAPPED) EFFICIENCY vs. TEMPERATURE 90 . 0 2 90 ~ 3 c Pf $ LITT vay | | : met hous Um Is At Why pe Wwe 8h MN S & b ~ A 80 N 80 7 & Vin = 3V Lt Ms # yn Vin = 4V / = log = 600mA 5 Vin = 15V 3 Wn ht 3 hoap= tA = 7 II =z 70 f Vin= 15 = 70 eM BY | Z 5 V & & 60 60 60 Vin=5V (| BOOTSTRAPPED 50 50 4 50 ps 1 10 100 1000 1 10 100 1000 40-20 0 2 4 6 80 10 LOAD CURRENT (mA) LOAD CURRENT (mA) TEMPERATURE (C} MAN776 MAX776 MAX775 EFFICIENCY vs. LOAD CURRENT EFFICIENCY vs. LOAD CURRENT EFFICIENCY vs. OUTPUT CURRENT Vout = -15V (BOOTSTRAPPED) Vout = -15V (NON-BOOTSTRAPPED) Vour = -12V (BOOTSTRAPPED) 90 ams 90 : Z te mM: Md a ! | 3 pr Nn hy li 40 rie MN Sv 4 2 80 Vin= 5V 3 - _ Vin= SV rl _ Vin = av Viv 8 e & vw= 18) {I [l|{_/| vw=6v = z Zz 7% WV 2 10 = 2 = & a 60 80 50 50 1 10 100 1000 1 10 100 1000 1 10 100 1000 LOAD CURRENT (mA} LOAD CURRENT (mA) QUTPUT CURRENT (mA) MAX774/MAX775/MAX776 MAX774/MAX775/MAX776 MAX774 EFFICIENCY vs. LOAD CURRENT EFFICIENCY vs. LOAD CURRENT EFFICIENCY vs. INPUT VOLTAGE Vour = -24 (NON-BOOTSTRAPPED) Vour = -24V OUTPUT (ZENER CONNECTION) Vour = -5V AT 100mA TT 11h LN SOT oak He II an) il 0 KZ Viy=5V 80 tT BOOTSTRAPPED a rT 2 ENA TI 2 "| arc z 3 3 1 5 & 70 & 70 7 a So oo = th & % NON-BOOTSTRAPPED 60 60 Vout =-5V AT 100mA 50 50 1 10 100 1000 1 10 100 1000 2 4 6 8 WM 2 4 16 LOAD CURRENT (mA) LOAD CURRENT (mA) INPUT VOLTAGE (V) 4-164 MAXIMA(Ta = +25C, unless otherwise noted.) START-UP VOLTAGE vs. LOAD CURRENT (BOOTSTRAPPED) 50 45 = 8 = 40 3 s = 35 oc = wn 30 25 1 10 100 ~=-1000 LOAD CURRENT (mA) EXT RISE AND FALL TIMES vs. TEMPERATURE 130 Cext = nF 120 110 100 2 9 z 80 =n 5V FALL = 60 50 40 30 20 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C) SWITCH OFF-TIME vs. TEMPERATURE 25 Ve = 5V e 20 & 15 60 0 60 120 TEMPERATURE (C) START-UP VOLTAGE (V) taise a traLL (ns) ton/torr RATIO (us/us) -5V/-12V/-15V or Adjustable, High-Efficiency, Low Ig inverting DC-DC Controllers Typical Operating Characteristics (continued) START-UP VOLTAGE vs. LOAD CURRENT (NON-BOOTSTRAPPED) Od 1 10 100 LOAD CURRENT (mA) 1000 EXT RISE AND FALL TIMES vs. TEMPERATURE Cext = S0F FALL -60 40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C) SWITCH ON-TIME/OFF-TIME RATIO V+ = 5V "60-40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C) MAX774 MAXIMUM LOAD vs. INPUT VOLTAGE 2200 Vout = -5V 2000 < 1800 _ BOOTSTRAPPED z 1600 og s 2 1400 2 3 3 NON-BOOTSTRAPPED 8 s 4 6 8 10 12 14 16 INPUT VOLTAGE (V) SWITCH ON-TIME vs. TEMPERATURE 7 V+ = 5V MAXTB 1-13. i OLLXVW/SLLXUW/PZLLXVIN ton (us) -60 0 60 120 TEMPERATURE (C) SHUTDOWN CURRENT vs. TEMPERATURE Iec (uA) Ve=4V 0 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C} 4-165 MAAKUMMAX774/MAX775/MAX776 5 V/-12V/-15V or Adjustable, High-Efficiency, Low Ig inverting DC-DC Controllers Typical Operating Characteristics (continued) (Ta = +25C, unless otherwise noted.) OPERATING SUPPLY CURRENT REFERENCE vs. TEMPERATURE TEMPERATURE COEFFICIENT 1.506 = 16.5 1.504 = 1502 2 = B 1500 8 4 ~ % 1498 & = 1496 1.494 1.492 ~60 -40 -20 0 20 40 60 80 100 120 140 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C) TEMPERATURE (C) REFERENCE CS TRIP LEVEL OUTPUT RESISTANCE i g | 2 200 = = lage = 10pA a Z 150 Lt g > LT tner = 50uA = i | bo e 3 10 ay S oO o a # 50 Za La a a TTT her = toy , Pi -60 40 -20 0 20 40 60 80 100 120 140 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C) TEMPERATURE (C) 4-166 MAAXIAN-5V/-12V/-15V or Adjustable, High-Efficiency, Low Ig Inverting DC-DC Controllers Typical Operating Characteristics (Ta = +25C, unless otherwise noted.) OPERATING WAVEFORMS 1Ons/div CIRCUIT OF FIGURE 2 V+ = 6.5V, ILoap = 1A, Vour =-5V A: OUTPUT RIPPLE, 200mV/div B: EXT WAVEFORM, 10V/div C: INDUCTOR CURRENT, 2A/div CONTINUOUS CONDUCTION AT ONE-HALF CURRENT LIMIT 20us/div CIRCUIT OF FIGURE 2 ILoap = 300mA, Vout = -5V V+ = BV, L = 22uH MAAXILSMA INDUCTOR CURRENT NEAR FULL LOAD EAH AOE q j 0A CIRCUIT OF FIGURE 2 Vout = -5V, V+=4.7V ILoap = 1.054 (1A/div} ENTRY/EXIT FROM SHUTDOWN ams/div CIRCUIT OF FIGURE 2 V+ = 6V, ILoap = 1A, Vout = -5V A: SHUTDOWN PULSE, OV TO V+, 5V/div B: Vour, 2V/div 4-167 a OLLXVW/SLLXVW/PLLXVINMAX774/MAX775/MAX776 =5V/-12V/-15V or Adjustable, High-Efficiency, Low Ig inverting DC-DC Controllers Typical Operating Characteristics (continued) (Ta = +25C, unless otherwise noted.) LOAD-TRANSIENT RESPONSE LINE-TRANSIENT RESPONSE 100ps/div ams/div CIRCUIT OF FIGURE 2 CIRCUIT OF FIGURE 2 V+ = BV, Vout = -5V Vout = -5V, Ioan = 1A A itoan, 30mA TO 1A, 1A/div A. V+, 3V TO BV, SV/div B: Vour, 100mV/div, AC-COUPLED B: Vout, 100mV/dlv, AC-COUPLED Pin Description PIN NAME FUNCTION The sense input for fixed-output operation (VrB = VREF). OUT is connected to the internal voltage divider, 1 OUT es : . . and it is the negative supply input for the EXT driver. 2 FB Feedback input. When Veg = Vrer, the output will be the factory preset value. For adjustable operation, use an external voitage divider, as described in the Adjustable Output section. 3 SHDN Active-high shutdown input. With SHDN high, the part is in shutdown mode and the supply current is less than 5pA. Connect to GND for normal operation. 4 REF 1.5V reference output that can source 100uA. Bypass to ground with 0. 1pF. 5 V+ Positive power-supply input 6 cs Noninverting input to the current-sense comparator. Typical trip level is 210mvV (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 4-168 MAAXIIA~5V/-12V/-15V or Adjustable, High-Efficiency, Low Ig Inverting DC-DC Controllers FB REF 4 (so) we) SHON | ERROR COMPARATOR MODE eo - COMPARATOR MAAXLIA + MAX774 MAX775 MAX776 our 15V ro tw} e - = REFERENCE ONE-SHOT FROM Vs |) Ss a Pa EXT btiaic a , FROM OUT CURRENT ONE-SHOT COMPARATOR cs \ 0.1V CURRENT . 02V (HALF CONTROL CIRCUITS (FULL CURRENT) Ve t 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 advan- tage 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. MAXUM 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/RSENSE) and on-time/off-time limits set by one-shots. Once turned on, the MOSFET stays on until: 1) the 164s 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 2.3ys. It will stay off until the output drops below the level determined by VReF and the feedback divider network. 4-169 a 9ZLLXVW/SLLXVW/PLLXVWNMAX774/MAX775/MAX776 5 V/-12V/-15V or Adjustable, High-Efficiency, Low Ig inverting DC-DC Controllers vn. _t [life wpe 150K Our shart co SHON MAX775 cs}6 =] MAN76 FB EXT AN REE GND 3 8 O.1pF TT t + > * MAX774 = 330uF, 10V MAX775, MAX776 = 120,F, 20V OUTPUT INPUT OUTPUT PRODUCT | vocTaGE (v) | VOLTAGE (V) | CURRENT (A) MAX774 5 31015 1 MAX775 12 3108 05 MAXT76 45 3105 04 NOTE: Si9435 HAS Vgg OF 20V MAX Figure 2. Bootstrapped Connection Using Fixed Output Voltages Vin our ve 5 MAXLAA sHon MAX774 sat MAx775 cs} FB MAX776 EXT | REF GND * MAX774 = 330uF, 10V MAX775, MAX?76 = 120uF, 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 tradi- tional 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 4-170 Vin aut vee MAAXIAN R3 SHDN MAX774 or MAX775 CS FB MAX776 1S0uF EXT Vout REF Gno 4 7 22H MERSED C4 FF * MAX774 = 330uF, 10V MAX775, MAX?776 = 120uF, 20V Figure 4. Non-Bootstrapped Operation (Vin > 4.5V) 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 2.3ns, the MOSFET switches on once more, and remains on until the switch current again reaches its limit. This cycle repeats unti! the output is in regulation. A benefit of this control scheme is that it is highly effi- cient 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 100mvV 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 non- bootstrapped mode. Since EXT is powered by OUT, MAXIM-5V/-12V/-15V or Adjustable, High-Efficiency, Low Ig inverting DC-DC Controllers R2 Ri O.1pF tr 6VsV74Vin 5 10V |vout| - vz Az Iz = ZENER BREAKDOWN CURRENT Vz = ZENER BREAKDOWN VOLTAGE Vin = INPUT SUPPLY VOLTAGE >k Figure 5. Connection Using Zener Diode to Boost Base Drive 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 neces- sary to use non-bootstrapped mode to avoid the 21V V+ to VouT maximum rating. Also, observe the Ves maximum rating of the external transistor. At intermedi- ate voltages and currents, the advantages of boot- strapped 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 SpA of Current is drawn. In normal operation, the quiescent current will be less than 100pA. 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- MAAXIIA 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 EXT swings from OUT to V+ and provides the drive out- put for an external power MOSFET. When using the on- chip 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 rat- ing 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 inter- mediate voltage for VouT with a zener diode (Figure 5). Design Procedure Setting the Output Voltage 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 voitage, 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 voitage 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 voitage 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 10pA to flow through the network. Calculate (VREF - VFB) / R1 = 10nA. Since VReF = 1.5V and VeB = OV, R1 becomes 150kQ. Then calculate R2 as follows: R2 Vout R1 = VREFE (or, VouT = 10pA) R2 4-171 a OLLXVW/SLLXVW/PLLXVINMAX774/MAX775/MAX776 5V/-12V/-15V or Adjustable, High-Efficiency, Low Iq Inverting DC-DC Controllers me TY ees 0080 ? SENSE = 0. z _ | rtd Leo? de 2000 + Rsense = 0.060 Z NLT = V4 | 2 1500 i WA Jet 5 Vy Leow | | = 0 (4 Cet Qo Lor"| Ri =0.070 = 500 Aiea N Rsense = 00802 = Rsense = 0.0982 Vout = -5V | 34 56 7 8 9 10 i 12 13 14 15 INPUT VOLTAGE (V) 1000 Vout : -12V MAX775- FIG 800 [- Rsenge = 0.062 sense = 0.0692 600 + Reense = 0.072 TV a y Za Ku = 0.082 __| Rsense = 0.082 \ MMA MAXIMUM OUTPUT CURRENT (mA) IANS 3 4 5 6 7 8 9 INPUT VOLTAGE (V} Figure 6. MAX774 Maximum Output Current vs. Input Voltage (VouT = -5V) Choosing an Inductor Practical inductor values range from 10HH to 50yuH. 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) - Vgy(max)] x 12ys (max) < ILim(max) This is only important if | VIN | 1 toFF(min) VouT 6 ton(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 effi- ciency. Calculate the minimum inductor value as follows: ; [V+(max) - Vswimin)] x 0.3us L(min) 2 -@om {__ 6(1) x ILIM(min) Where L is in WH, 0.3us is an ample time for the com- parator response, ILM is the current limit (see Current- Sense Resistor section), and &(l) is the allowable per- centage 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 16H. 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 30mQ or lower resistance are available. To 4-172 Figure 7, MAX775 Maximum Output Current vs. input Voltage (Vour = -12V) minimize radiated noise, use a torroid, pot-core, or shield- ed-bobbin inductor. Inductors with a ferrite core or equiv- alent are recommended. Make sure that the inductors saturation current rating is greater than !Lim(max). Diode Selection The ICs high switching frequencies demand a high- speed 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 applica- tions, Schottky diodes may be inadequate due to their high leakage currents; instead, high-speed silicon diodes may be used. At heavy loads and high temper- ature, the benefits of a Schottky diodes low forward voltage may outweigh the disadvantages of its high leakage current. Current-Sense Resistor The current-sense resistor limits the peak switch cur- rent to 210mV/RSeNSe, where RsensE is the value of the current-sense resistor, and 210mV is the current- sense 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, there- fore, peak current) for the required load current. MAAXLIA5V/-12V/-15V or Adjustable, High-Efficiency, Low Ig inverting DC-DC Controllers 700 Vout = -15V i = Asense = 0.058 LZ BF OF Reese = 0.060 3 Ee = a 500 Asense = 0.072: A s ~~ SZ 5 0} 41 +3 = mM = 200 Asense = 0.08822 4 L oN Rgaige = 0.090 100 l l 3 4 5 6 7 INPUT VOLTAGE (V) 800 Vour = -24V Pit be! LETT I Rsense = nos RseNse = or f| J g "| MT HTL bey Loaneagenra 8 WA N\A LL Le mn ry ) Rsense = 0.082 Rsense = 0.0902 0 a ee 34567 8 9 1011 12 13 14 15 INPUT VOLTAGE (V) va LL Lod MAXIMUM OUTPUT CURRENT (mA) 8 4 Figure 8. MAX776 Maximum Output Current vs. Input Voltage (Vout = -15V) To choose the proper current-sense resistor, simply fol- low the two-step procedure outlined below. 1) Determine: Input voltage range, V+ Maximum (absolute) output voltage, VouT * Maximum output current, ILOAD For example, let V+ range from 4V to 6V, and choose VouT = -24V and louT = 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 lout is >150mA with a 4V input and a -24V output. The RsSENSE = 80mQ (shown in Figure 9) shows only approximately 125mA of output current with a 4V input, so we look next at the RSENSE = 70mQ line. It shows lout >150mA for V+ = 4V and VouT = -24V. The cur- rent limit will be 0.210V / 0.070Q = 3A. These curves take into account worst-case inductor (410%) and current-sense trip levels, but not sense-resistor toler- ance. The switch on resistance is 70mQ. 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 induc- tance and are weil suited for use as current-sense MAAXLM Figure 9. MAX774/MAX775/MAX776 Maximum Output Current vs. Input Voltage (VouT = -24V) 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 5mQ and 0.12. External Switching Transistor 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, gate- source threshold, and gate capacitance. The drain-to- source rating must be greater than the V+ - VOUT 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 capac- itance is not normally a limiting factor, but values should be less than 1nF for best efficiency. For maximum effi- ciency, the MOSFET should have a very low on resis- tance at the peak current and be capable of handling that current. The transistor chosen for the typical oper- ating circuit has a 30V drain-source voltage limit and a 0.07Q drain-source on resistance at VGs = -10V. Table 1 lists suppliers of switching transistors suitable for use with the MAX774/MAX775/MAX7 76. 4-173 a 9LLXVIW/SLLXVW/PLLXVINMAX774/MAX775/MAX776 =-5V/-12V/-15V or Adjustable, High-Efficiency, Low Ig Inverting DC-DC Controllers Table 1. Component Suppliers SUPPLIER | PHONE | FAX INDUCTORS 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 CAPACITORS Kemet (803) 963-6300 | (803) 963-6322 Matsuo (714) 969-2491 | (714) 960-6492 Nichicon (708) 843-7500 | (708) 843-2798 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 DIODES 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 POWER MOSFETS Harris (407) 724-3729 | (407) 724-3937 Intemational Rectifier | (310) 322-3331 | (310) 322-3332 Siliconix (408) 988-8000 | (408) 970-3950 CURRENT-SENSE RESISTORS IRC | (704) 264-8861 | (704) 264-8866 Capacitors Choose the output capacitor (C4 of Figures 2, 3, and 4) to be consistent with your size, ripple, and output volt- age requirements. Place capacitors in parallel if the size you want is unobtainable. You will not only increase the capacitance, but also decrease the capacitors ESR (a major contributor of ripple). A 330yF tantalum output filter capacitor with 0.07Q ESR 4-174 typically maintains 120MVp-p output ripple when generating -5V at 1A from a 5V input. Smaller capaci- tors 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 louT x ESR IOUT X tOFF(min) VIN Cc When evaluating this equation, be sure to use the capacitance value at the switching frequency. At 200kHz, the 330uF tantalum capacitor of Figures 2, 3, or 4 may degrade by a factor of ten, which will signifi- cantly 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 rat- ing equal to the peak current. The smallest low-ESR SMT capacitors currently avail- able are the Sprague 595D series. Sanyo OS-CON organic semiconductor through-hoie capacitors also exhibit low ESR and are especially effective at low tem- peratures. Table 1 lists the phone numbers of these and other manufacturers. AVp-p = PC Layout and Grounding Due to high current levels and fast switching wave- forms, proper PC board layout is essential. Use a star ground configuration; connect the ground lead of the input bypass capacitor, the output capacitor, the induc- tor, and the GND pin of the MAX774/MAX775/MAX776 at a common point very close to the device. Addi- tionally, 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. MAXLIA-5V/-12V/-15V or Adjustable, High-Efficiency, Low Ig Inverting DC-DC Controllers _Ordering Information (continued) PART TEMP. RANGE PIN-PACKAGE MAX775CPA 0C to +70C 8 Plastic DIP MAX775CSA aC to +70C 8SO MAX775C/D OC to +70C Dice MAX775EPA -40C to +85C 8 Plastic DIP MAX775ESA -40C to +85C 850 MAX775MJA -56C to +125C 8 CERDIP MAX776CPA OC to +70C 8 Plastic DIP MAX776CSA 0C to +70C 8SO0 MAX776C/D OC to +70C Dice* MAX776EPA -40C to +85C 8 Plastic DIP MAX776ESA -40C to +85C 8S0 MAX776MJA -55C to +125C 8 CERDIP * Contact factory for dice specifications. MAAXLIM Chip Topography OUT GND FB SHON REF (2.032mm) TRANSISTOR COUNT: 442; SUBSTRATE CONNECTED TO V+. a OLLXVW/SLLXVW/PLLXVIN 4-175