19-0918; Rev 0, 5/91 MA AAI/VI Preset/Adjustable Output CMOS inverting Switching Regulators General Description Features The MAX635/MAX636/MAX637 inverting switching regu- lators are designed for minimum component DC-DC conversion in the 5mW to 500mW range. Low power applications require only a diode, output filter capacitor, and a low-cost inductor. An additional MOSFET and driver are needed for higher power appli- cations. Low battery detection circuitry is included on chip. The MAX635/636/637 are preset for -5V, -12V, and -15V Ordering Information outputs, respectively. However, the regulators can be set to other levels by adding 2 resistors. PART" TEMP. RANGE PIN-PACKAGE Maxim manufactures a broad line of step-up, step-down, MAX635XCPA oe to +70 8 Plastic DIP and inverting DC-DC converters, with features such as MAX635XCSA C to +70C 8 Narrow SO logic-leve! shutdown, adjustable oscillator frequency, MAX635XC/D 0C to +70C Dice and external MOSFET drive. MAX635XEPA _--40C. 0 +85C 8 Plastic DIP MAX635XESA -40C to +85C 8 Narrow SO MAX635XEJA -40C to +85C 8 CERDIP A pplications MAX635XMJA -65C to +125C 8 CERDIP a . MAX636XCPA 0C to +70C 8 Plastic DIP Minimum Component, High-Efficiency ; : DC-DC Converters MAX636XCSA 0"C to +70C 8 Narrow SO Portable Instruments MAX636XC/D 0C to +70C Dice Batt p Cc . MAX636XEPA -40C to +85C 8 Plastic DIP allery rower Monversion MAX636XESA_ __-40C to +85C 8 Narrow SO Board Level DC-DG Conversion MAX636XEJA -40C to +85C 8 CERDIP MAX636XMJA -55C to +125C 8 CERDIP @ Preset -5V, -12V, -15V Output Voltages @ Adjustable Output with 2 Resistors @ 85% Typ Efficiency @ Only 3 External Components @ 80.A Typ Operating Current # Low Battery Detector LE9/9E9/SE9OXVN *X = A for 5% Output Accuracy, X = B for 10% Output Accuracy. Ordering information continued on iast page. Pin Configuration Typical Operating Circuit Top View 10uF +OV >+| {_ 6 = aS ee 8 . VFB -VOUT -vour [1 [3 | VFB SAMOA RMAAXLAA wo [2] waxe3s [7] REF MAX637 MAX636 vl[s| axes? tol 8 1) vrer Lx GNo [4] 5] LX LBI GND ar) _ DIP/SO 4 45V TO -15V CONVERTER SA AXLMM Maxim integrated Products 1 For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800MAX635/636/637 Preset/Adjustable Output CMOS Inverting Switching Regulators ABSOLUTE MAXIMUM RATINGS Supply Voltage, +Vs (Note 1) ..... 6... eee ee +18V Input Voltage, LBO, LBI, VFB.......... -0.3V to {+Vs + 0.3V) LX Output Current .......... 66-2 525mA Peak LBO Output Current .......0... 0.02 50mA Power Dissipation Plastic DIP (derate 8.33mW/C above +50C) ...... 625mW Small Outline (derate 6mW/"C above +50C) ....... 450mW CERDIP (derate 8&mW/"C above +50C) ........... 800mW Stresses be Operating Temperature Range MAX63__C Storage Temperature Lead Temperature (Soldering, 10 sec.) operation of the device at these or any other conditions beyond those indicated in the operational sections of the s absolute maximum rating conditions for extended periods may affect device reliability. ELECTRICAL CHARACTERISITICS (TA = +25C, unless otherwise noted.) 0C to +70C -40C to +85C -55C to +125C -65C to +160C +300C nd those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only and functional ecifications is not implied. Exposure to PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS TA = +25C 2.3 16.5 Supply Voltage (Note 1) +Vs Over Temperature 26 16.5 Vv , No Load, LX Off, Supply Current Is Ove romey are 80 150 yA +Vs = +15V 260 500 Ta = +25C 1.24 1.31 1.38 Reference Voltage VREF | Gver Temperature 1.20 1.42 V No Load, VFB = VREF, +Vs = +5V Over Temperature MAXESEA \ Output A a BOB & Output Accuracy -11. -12.0 -12.6 VOUT Voltage (Note 2) MAX637A 1425 150 -1675 | MAX635B -4.5 -5.0 -5.5 MAX636B } 10% Output Accuracy -10.8 -12.0 -13.2 MAX637B -13.5 -15.0 -16.5 Efficiency 85 % Line Regulation (Note 2) +5V<4+V5<4+15V 0.5 %VOUT Load Regulation (Note 2) Pout = OmW to 150mWw 0.2 %VOUT Oscillator Frequency fo +VS = +5V Maxeee ic 25 8 kHz Oscillator Duty Cycle +Vs = +5V 40 50 60 % : Ix = 100mA, +V5 = +5V 9 16 LX On Resistance RON = 415V 4 8 Q +VS = +16.5V LX Leakage Current xt Ta = +25C 0.01 1.0 LA Over Temperature 30 VEB Input Bias Current IFB 0.01 10 nA Low Battery Threshold VLBI 1.31 V Low Battery Input Bias Current ILBI 0.01 10 nA V2 = +0.4V, V3 = +1.1V Low Battery Output Current ILBO Ta = 25C 1.0 mA Over Temperature 0.5 Lon eaeCurent IuBoL | V2 = +16.5V, V3 = +1.4V 0.01 3.0 HA Note 1: In addition to the Absolute Maximum Rating of +18V, the input voltage also must not exceed 24V -| -VOUTI. Note 2: Guaranteed by correlation with DC pulse measurements. PAAXILAAPreset/Adjustable Output CMOS inverting Switching Regulators Pin Description PIN NAME FUNCTION PIN NAME FUNCTION The sense INPUT for fixed output op- 5 LX This pin drives the external inductor 1 VOUT eration, -VOUT, is internally con- with an internal P-channel power nected to-the on-chip voltage divider. MOSFET. LX has an output resis- Although it is connected to the output tance.of typically 6Q and a peak cur- ofthe. oc0C converter (Figure 2. rent rating of 525mA. joes not supply current, ae 6 +Vs The positive Supply Voltage, from +2V does. to +16.5V. The total difference be- > | '8_ | Spa dri hvchennel MOSFET unin Ne poe aa most beans Ra open drain N-channel sinks Current when the voltage at LBI 24V. is below 1.31V. 7 VREF The Voltage Reference output is 3 LBI Low Batiery Detector Input. When the +1.31V, generated by an on-chip voltage at Blis lower oer ety) bandgap reference. attery Detector threshold (+1. , a 8 VFB When VFB is tied to VREF, the DC-DC LBO sinks current. converter output will be the factory 4 GND Ground preset value. When an external voit- age divider is connected to VFB and VRer, this pin becomes the feedback input for adjustable output operation. Typical Operating Characteristics Lx ON RESISTANCE SUPPLY CURRENT SUPPLY CURRENT vs. vs. TEMPERATURE vs. TEMPERATURE SUPPLY VOLTAGE 2 300 | Vs [ sash wa - yt cosy ws Tae $28C H A= 8 > IL - 4 0 e - = z > = 10 3 150 = *5 = +5V Ld 2 x LL - 4 Le 100 Ps 5 } al Vs = +15] 50 0 0 80 -2% 0 2 SO 75 WO 126 50-5 0 2% SO 75 100 125 2 4 6 8 DW RM 1 TEMPERATURE (C] TEMPERATURE (C) os [) OSCILLATOR FREQUENCY va. SUPPLY VOLTAGE Th = +28C fa [kHz] 6 & 8 & & LES/SED/SECOXVNMAX635/636/637 Preset/Adjustable Output CMOS Inverting Switching Regulators Detailed Description Principle of Operation Figure 1 shows a simplified inverting converter. When the switch is closed, a charging current flows through the inductor, creating a magnetic field. When the switch opens, the current continues to flow through the inductor in the same direction as the charging current. Since the switch is now open, the current must flow through the diode, thereby charging the capacitor with a negative voltage. As the energy stored in the inductor is trans- ferred to the output filter capacitor, the current linearly decays to zero, and the magnetic field collapses. The MAX635/636/637 controls the magnitude of the neg- ative output voltage by turning the switch on and off only when the output voltage has become more positive than the desired value. Basic Circuit Operation Figure 2 shows the standard circuit for converting a positive voltage into a negative one. When the output becomes more positive than the preset level, the Error Comparator switches low, and the MOSFET at LX is toggled on and off at the clock frequency. During the low-going period of the oscillator, P1 is on, and current is delivered to the external inductor through the LX pin. Figure 1. Simplified Inverting Converter When the oscillator output goes high, the MOSFET turns off, but current continues to flow through the inductor. Diode D1 thus conducts, and the output filter capacitor, C1, is charged negatively. Basic Step-Down Circuit Table 1 lists some coil manufacturers and typical part numbers. Table 2 shows nominal inductor parameters for a variety of input and output voltages. The data refers to the circuit of Figure 2. When noise is not critical, a LOW BATTERY +VIN OUTPUT Rl - 3 R2 N - 1 700k = VOUT + -VOUT = 100uF = LOW BATTERY Tt ol COMPARATOR ~~ = . VEB [8 ERRORS | +500mV COMPARATOR = DI MODE * 1N5817 SELECT s6 +H31V VREF [7 BANDGAP ~ a 4 + 0:tuF REFERENCE GND Pp ____- 50kHz . +5 16 - OSC. + +VIN "uF Pt ux]5 } L Figure 2. 4 MAX635/636/637 Block Diagram and Typical Circuit (Table 2) MAAXIAAPreset/Adjustable Output CMOS Inverting Switching Regulators low-cost bobbin inductor will suffice. For higher power circuits, or when low noise and EMI are required, pot cores or toroids should be used. If more output power is desired, see the Medium Power Inverters section. Table 1. Coil and Core Manufacturers (Note 3) MANUFACTURER| = [YFICAL DESCRIPTION ASIA TDK Corporation 13-1, Nihonbashi 1-chome Chuo-ku Tokyo 103 Japan EUROPE Richard Jahre GmbH Luetzowstrasse 90 1000 Berlin 30 Germany BOBBIN INDUCTORS Dale IHA-104 500nH, 0.52 Caddell-Burns 7070-29 220uH, 0.552 Gowanda 1B253 250pH, 0.440 TRW LL-500 500pH, 0.752 POTTED TOROIDAL INDUCTORS Dale TE-3Q4TA {mH, 0.822 TRW MH-1 600uH, 1.9 Gowanda O50AT 1003 100HH, 0.05 FERRITE CORES AND TOROIDS (Note 4) Allen Bradley T0451S100A one Siemens B64290-K38-X38 | Tor. Core, 4uH/T? Magnetics 555.130 gt Core. Stackpole 57-3215 Pot Core, an Magnetics G-41408-25 Tee SbonH/r2 Note 3: This list does not constitute an endorsement by Maxim Integrated Products and is not intended to be a compre- hensive list of all manufacturers of these components. Note 4: Permag Corp. is a distributor for many of the listed core and toroid manufacturers (516) 822-3311. MA AXLAA Table 2. Inductor Selection for Common Designs (Figure 2) INDUCTOR Ws YRUT Ay | parte, TT +3 5 5 7070-27 150uH 0.43 +5 5 25 7070-27 150uH 0.43 +9 5 40 7070-31 330uH 0.72 +12 5 45 7070-33 470uH 0.88 +15 5 50 7070-35 680HH 1.5 +5 -12 12 7070-26 120pH 0.32 +9 -12 30 7070-31 330uH 0.72 +12 -12 40 7070-33 470uH 0.88 +3 -15 2 7070-27 150uH 0.43 +5 15 8 7070-27 150uH 0.43 +9 -15 25 7070-31 330uH 0.72 Note 5: Caddell-Burns N.Y. (616) 746-2310. Low Battery Detector The Low Battery Outout, LBO, sinks current whenever the input voltage at Low Battery Input, LBI, is less than +1.31V. LBI is a high impedance CMOS input, with less than 10nA leakage current. LBO is an open drain N- channel MOSFET with about 5002 of output resistance. The trip voltage of the Low Battery Detector can be adjusted using an external voltage divider as shown in Figure 2. If hysteresis is desired, add a resistor between LBO and LBI. Let R2 be any resistance in the 10kQ to 10MQ range, typically 100kQ, then: VLB 1.31V (VLB is the desired Low Battery detection voltage.) Ri =Re2 ( 1) +5VIN y 10, 2 6 >= RB 300k02 WV your *s 8 VFB ane ~10VOUT MAAXIAA R4> 39kQ 100pF MAX635 7 * LBI MAY636 VREF O14 ry = MAX637 up w LX GND T 20H Figure 3. Adjustable Output Operation LZE9/99/SE9XVINMAX635/636/637 Preset/Adjustable Output CMOS inverting Switching Regulators Fixed or Adjustable Output For operation at one of the preset output voltages (-5V for the MAX635, -12V for the MAX636, and -15V for the MAX637), VFB is connected to VREF, and no external resistors are required. Other output voltages are selected by connecting an external voltage divider to VFB as shown in Figure 3. The output is set by R3 and R4 as follows: Let R4 be any resistance in the 10kQ to 10MQ range, typically 100kQ, then: R3 VOUT =-1.31V x Ra External Components What Value of inductor? A General Discussion The converters in this data sheet operate by charging an inductor from a DC input and then discharging the induc- tor to generate a DC output that is opposite in polarity to the input. Inductor selection for any DC-DC converter depends on three things: the desired output power, the input voltage (or input voltage range), and the converter's oscillator frequency and duty cycle. The oscillator timing is impor- tant because it determines how long the coil will be charged during each cycle. This, along with the input voltage, determines how much energy will be stored in the coil. The maximum amount of energy (EL) in the coil each cycle is a function of the peak current (Ipk) and the inductance of the coil (L): The inductor must meet four electrical criteria: [ ] Value low enough inductance so it stores ade- quate energy at the worst-case, low input voltage. High enough so excessive and potentially destructive currents are avoided under worst-case high conditions for power-switch transistor on time and high input voit- age. [ ] Saturation - The coil must deliver the correct induc- tance value at the worst-case, high peak operating cur- rent. [ ] EMI Electromagnetic interference must not upset nearby circuitry or the regulator IC. Ferrite bobbin types work well for digital circuits; toroid or pot core types work well for EMl-sensitive analog circuits. [ ] DC resistance Winding resistance must be ade- quately low so efficiency is not affected and self-heating does not occur. Values less than 0.5 are usually more than adequate. Other inductor parameters, such as core loss or self-res- onant frequency, are not a factor at the relatively low MAX635/636/637 operating frequency. inductor Vaiue - Low Enough? The problem that bites designs most often, especially in the production or pre-production phase, happens when the inductor value is too high. These units fail to deliver enough load current and exhibit poor load regulation. The worst case is: { ] Maximum load current [ ] Minimum supply voltage [ ] Maximum inductor value, including tolerance [ ] Maximum on resistance of the switch because it reduces the excitation voltage across the inductor [ ] Worst-case low on time Inductor Value High Enough? The inductor value must aiso be high enough so peak currents do not stress the transistor or cause the inductor core to saturate. Odd symptoms can be traced to exces- sive inductor currents: low efficiency, rattling heat sinks, whining coils, and increased output ripple. Very low inductor values can result in damaged power transistors. The slope of the inductor current, and therefore the peak value that it reaches in a given on time, is determined by the supply voltage and the inductor value. The worst case occurs at: [ ] Maximum supply voltage [ ] Minimum inductor value, including tolerance { ] Minimum on resistance of the switch [ ] Low switching frequency (or maximum switch on time) inductor Selection The inducter equations below must be calculated for both worst-case sets of conditions. The final value chosen should be between the minimum value and maximum value calculated. Within these bounds, the value can be adjusted slightly lower for extra load capability or higher for lowest ripple. VOUT + VDIODE 1 = MT tk (0.25) Win - sw) (OU? VIN-V 2} b= SS con) pk where VSW is the voltage drop across the switch in the on state. Conservatively, the worst case is about 0.75V MA AXIAAMPreset/Adjustable Output CMOS inverting Switching Regulators max, 0.25V min with VIN = +15V and 1.5V max, 0.5V min with VIN = +5V. Example: A +5V 10% input must be convertec to -12V at 12mA. A Schottky diode (1N5817) and a MAX636A are used. Calculate the maximum inductor value allowed: hog = t2V=0.4V Pk = (9 35) (4.5V - 1.5V) | = 45V-1.5V ~~ 498mA Calculate the minimum inductor value allowed: (12mA) = 198mA (Sus) = 136yH lok = 525mA (from table of max ratings; use the power MOSFET max ratings for external transistor circuits.) | _55V-0.5V ~~ 525mA A value of 120HH would be a good choice for this application. Ipk must also be compared to the current rating of the LX switch. If pk exceeds the peak current rating of the switch (525mA), an external MOSFET or transistor with an acequate current rating must be used (see Medium Power Inverters). (11ps) = 105yH The coil resistance has a significant effect on the output current; a coil with a low resistance will increase the output current and overall efficiency. The inductor 100uF =. 5y } I = 6 Vi 6 ; VREF Ns 54 . 1 4 VFB Lx MAX626 IRF9541 pF C1 T MAX635 LG le uN = OR MAX636 a7 = uF | > 27H N.C.o-44 LB0 LBI_ GND -VOUT nee 3 4 7 a - Od7uF 1000uF CERAMIC T+ ]+ Figure 4. Medium Power Inverter MAAXISA should have a powdered iron or ferrite core and should have a resistance less than 0.5. Medium Power inverters In the circuit of Figure 4, the MAX626 MOSFET driver is used to convert the open drain LX output to a signal suitable for driving the gate of an external P-Channel MOSFET. The IRF9541 has a gate threshold voltage of 2V to 4V so it will nave a relatively high resistance if driven with only 5V of gate drive. To increase the gate drive voltage, and thereby increase efficiency, the negative supply pin of the CMOS inverter is connected to the negative output rather than to the ground. Once the circuit is started, the gate drive swings from +5V to -VOUT. At start-up, the voltage at -VOUT is one Schottky diode drop above ground, and the gate drive to the power MOSFET is slightly less than 5V. The output should be only lightly loaded to ensure start-up, since the output power capability of the circuit is very low until -VOUT is a couple of volts negative. (See Table 3 for component values for L2 and !C1.) Table 3. Component Selector for Medium Power Inverters (Figure 4) Vin -VOUT IOUT EFFICIENCY 1c1 Li 5V -5V 400mA 70% MAX635 = 27H 5V -5V 500mA 64% MAX635 = 18H 5V -12V. -150mA 75% MAX636 27H 5V -12V. 200mA 70% MAX636 18uH Notes: 18H Coil = Caddell-Burn's (Mineola, NY) Model 6860-04. 27nH Coil = Caddell-Burns Model 6860-06. External Dicde In mast DC-DC converter circuits, the current in the catch diode (Figure 2, D1} abruptly goes from zero to its peak value each time the MOSFET at LX switches off. To avoid excessive losses, the diode must have a fast turn-on time. For low power circuits with peak currents less than 100mA, signal diodes such as 1N4148s perform well. For higher current circuits, or for maximum effi- ciency at low power, the 1N5817 series of Schottky diodes are recommended. Although 1N4001s and other general purpose rectifiers are rated for high currents, they are unacceptable because their slow turn-on time results in excessive losses. Output Filter Capacitor The MAX635/636/637's output ripple has 2 components which are 90 out of phase. One component results from the change in the stored charge on the filter capacitor with each LX pulse. The other is the product of the capacitors charge-discharge current and its Equivalent 7 LE9/9E9/SEOXVNMAX635/636/637 Preset/Adjustable Output CMOS Inverting Switching Regulators Series Resistance (ESR). With low-cost aluminum elec- trolytic capacitors, the ESR produced ripple is often larger than that caused by the change in charge. Con- sequently, high quality aluminum or tantalum filter capac- itors will minimize output ripple, even if smaller capacitance values are used. Best results at reasonable cost are typically achieved with a high quality aluminum electrolytic, in the 100yF to SOOuF range, in parallel with aO.1pF ceramic capacitor. Application Hints inductor Saturation When using off-the-shelf inductors, make sure that their peak current rating is observed. When designing your own inductors, observe the core manufacturers Ampere- turns on NI ratings. Failure to observe the peak current or NI ratings may lead to saturation of the inductor, especially in circuits with external boosting transistors. Inductor saturation leads to very high current levels through the power switching device causing excessive power dissipation, poor efficiency, and possible dam- age. Test for saturation by applying the maximum load and the maximum input voltage while monitoring the inductor cur- rent with a current probe. The normal inductor current waveform is a sawtooth with a linear current ramp. Satu- ration creates a nonlinear current waveform with a very rapid increase in current once the inductor saturates. _ Ordering Information (continued) PART* TEMP. RANGE PIN - PACKAGE MAX637XCPA OC to +70C 8 Plastic DIP MAX637XCSA OC to +70C 8 Narrow SO MAX637XCJA CC ta +70C 8 CERDIP MAX637XC/D 0C to +70C Dice MAX637XEPA -40C to +85C 8 Plastic DIP MAX637XESA -40C to +85C 8 Narrow SO MAX637XEJA -40C to +85C 8 CERDIP MAX637XMJA -58C to +125C 8 CERDIP *X = A for 5% Output Accuracy, X = B for 10% Quiput Accuracy. Bypassing and Compensation The high current pulses in the LX output and the external inductor can cause erratic operation unless the MAX635/636/637 is properly bypassed. Connect a 10mF bypass capacitor directly across the device be- tween +VS and GND to minimize the inductance and high frequency impedance of the power source. Also make sure that the high current ground return path of the inductor does not cause a voltage drop in the regulator's ground line. The reference voltage output, VREF, should be bypassed to ground with a 0.1pWF capacitor. Avoid coupling to the high current path that includes the LX output and the inductor ground return. When the value of the voltage setting resistors (R3 and R4, Figure 3) exceed 50kQ, stray capacitance at the VFB input can add a lag" to the feedback response causing output pulses to occur in bursts. This increases low- frequency ripple and lowers efficiency. This problem can often be avoided by minimizing lead lengths and circuit board trace size at the VFB node. Normal operation with evenly distributed output pulses can be restored by adding a lead compensation capacitor ({O0pF to 10nF) in parallel with R3. Chip Topography GND LBI LBO -Vout ee LX 0.070" (1.78mm) 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. 8 Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600 1991 Maxim Integrated Products Printed USA MAAXIAMA is a registered trademark of Maxim integrated Products.