19-3105; Rev 2; 8/96 NUAL KIT MA ATION EET H S A EVALU T WS DA FOLLO Switch-Mode Regulator with +5V to ±12V or ±15V Dual Output The MAX742 DC-DC converter is a controller for dual-output power supplies in the 3W to 60W range. Relying on simple two-terminal inductors rather than transformers, the MAX742 regulates both outputs independently to within ±4% over all conditions of line voltage, temperature, and load current. The MAX742 has high efficiency (up to 92%) over a wide range of output loading. Two independent PWM currentmode feedback loops provide tight regulation and operation free from subharmonic noise. The MAX742 can operate at 100kHz or 200kHz, so it can be used with small and lightweight external components. Also ripple and noise are easy to filter. The MAX742 provides a regulated output for inputs ranging from 4.2V to 10V (and higher with additional components). External power MOSFETs driven directly from the MAX742 are protected by cycle-by-cycle overcurrent sensing. The MAX742 also features undervoltage lockout, thermal shutdown, and programmable soft-start. If 3W of load power or less is needed, refer to the MAX743 data sheet for a device with internal power MOSFETs. ________________________Applications DC-DC Converter Module Replacement ____________________________Features ♦ Specs Guaranteed for In-Circuit Performance ♦ Load Currents to ±2A ♦ 4.2V to 10V Input-Voltage Range ♦ Switches From ±15V to ±12V Under Logic Control ♦ ±4% Output Tolerance Max Over Temp, Line, and Load ♦ 90% Typ Efficiency ♦ ♦ ♦ ♦ Low-Noise, Current-Mode Feedback Cycle-by-Cycle Current Limiting Undervoltage Lockout and Soft-Start 100kHz or 200kHz Operation ______________Ordering Information PART TEMP. RANGE PIN-PACKAGE MAX742CPP 0°C to +70°C 20 Plastic DIP MAX742CWP MAX742C/D MAX742EPP MAX742EWP MAX742MJP 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 20 Wide SO Dice* 20 Plastic DIP 20 Wide SO 20 CERDIP * Contact factory for dice specifications __________Simplified Block Diagram Distributed Power Systems Computer Peripherals +5V INPUT MAX742 __________________Pin Configuration CC- TOP VIEW FB+ 1 20 CSH+ CC+ 2 19 CSL+ AGND 3 18 GND AV 4 100/200 5 17 EXT+ MAX742 16 PUMP 12/15 6 15 PDRV VREF 7 14 EXT- SS 8 R -SENSE PWM S -DRIVE -VO +2.0V VREF 12 CSH- FB- 10 11 CSL- OSC +VO S +DRIVE PWM R +SENSE 13 V+ CC- 9 P N CC+ DIP/SO ________________________________________________________________ Maxim Integrated Products 1 For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800 MAX742 _______________General Description MAX742 Switch-Mode Regulator with +5V to ±12V or ±15V Dual Output ABSOLUTE MAXIMUM RATINGS V+, AV+ to AGND, GND.........................................-0.3V to +12V PDRV to V+.............................................................+0.3V to -14V FB+, FB- to GND..................................................................±25V Input Voltage to GND (CC+, CC-, CSH+, CSL+, CSH-, CSL-, SS, 100/200, 12/15) ..................................-0.3V to (V+ + 0.3V) Output Voltage to GND (EXT+, PUMP) ..........................................-0.3V to (V+ + 0.3V) EXT- to PDRV................................................-0.3V to (V+ + 0.3V) Continuous Power Dissipation (any package) up to +70°C .....................................................................500mW derate above +70°C by ..........................................100mW/°C Operating Temperature Ranges MAX742C_ _ .......................................................0°C to +70°C MAX742E_ _ ....................................................-40°C to +85°C MAX742MJP ..................................................-55°C to +125°C Storage Temperature Range .............................-65°C to +150°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 (Circuit of Figure 2, +4.5V < V+ < +5.5V.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX Output Voltage, ±15V Mode (Notes 1, 2) 0mA < IL < 100mA, 12/15 = 0V TA = +25°C 14.55 15.45 TA = TMIN to TMAX 14.40 15.60 Output Voltage, ±12V Mode (Notes 1, 2) 0mA < IL < 125mA, 12/15 = V+ TA = +25°C 11.64 12.36 TA = TMIN to TMAX 11.52 12.48 UNITS V V ELECTRICAL CHARACTERISTICS (Circuit of Figure 2, V+ = 5V, 100/200 = 12/15 = 0V; TA = TMIN to TMAX, unless otherwise noted.) PARAMETER SYMBOL CONDITIONS Line Regulation V+ = 4.5V to 5.5V, PDRV from PUMP Load Regulation (Note 2) ILOAD = 0mA to 100mA No-Load Supply Current V+ = 5V No EXT- or PUMP load, FB+ = FB- = open circuit V+ = 10V Undervoltage Lockout UVLO MIN TYP MAX UNITS 0.01 0.05 %/% 30 100 mV 3 10 3.8 4.2 Undervoltage Lockout Hysteresis 0.2 Reference Output Voltage 2.0 Oscillator Frequency fOSC 2 V V V 100/200 = 0V 170 200 230 100/200 = V+ 75 100 125 fOSC/2 kHz EXT+ or EXT- 85 90 % CSL+ = 0V, FB+ = open circuit 150 225 300 mV CSH- = V+, FB- = open circuit 150 225 300 mV PUMP Frequency Duty-Cycle Limit (Note 3) Positive Current-Limit Threshold (CSH+ to CSL+) Negative Current-Limit Threshold (CSH- to CSL-) mA _______________________________________________________________________________________ kHz Switch-Mode Regulator with +5V to ±12V or ±15V Dual Output (Circuit of Figure 2, V+ = 5V, 100/200 = 12/15 = 0V; TA = TMIN to TMAX, unless otherwise noted.) PARAMETER SYMBOL CONDITIONS Output Voltage High VOH EXT+, EXT-, IL = 1mA, V+ = 4.5V, PDRV= -3V Output Voltage Low VOL EXT+, EXT-, IL = -1mA, V+ = 4.5V, PDRV= -3V Output Sink Current V+ = 4.5V, PDRV = -3V, EXT+ = 4.5V TA = +25°C EXT- = 4.5V Output Source Current V+ = 4.5V, PDRV = -3V, EXT+ = 0V TA = +25°C EXT- = -3V Output Rise/Fall Time MIN TYP V -2.8 100 200 200 350 -100 -350 -200 70 100 Compensation Pin Impedance CC+, CC- V mA -200 EXT-, CLOAD = 4nF, PDRV = -3V V+ = 4.5V, IL = -5mA, TA = +25°C UNITS 4.3 EXT+, CLOAD = 2nF PUMP Output Voltage (Note 4) MAX mA ns -3 V 10 Thermal-Shutdown Threshold kΩ 190 Soft-Start Source Current SS = 0V Soft-Start Sink Current V+ = 3.8V, SS = 2V 3 -2 °C 7 µA -0.5 mA Note 1: Devices are 100% tested to these limits under 0mA to 100mA and to 125mA conditions using automatic test equipment. The ability to drive loads up to 1A is guaranteed by the current-limit threshold, output swing, and the output current source/sink tests. See Figures 2 and 3. Note 2: Actual load capability of the circuit of Figure 2 is ±200mA in ±15V mode and ±250mA in ±12V mode. Load regulation is tested at lower limits due to test equipment limitations. Note 3: Guaranteed by design. Note 4: Measured at Point A, circuit of Figure 2, with PDRV disconnected. _______________________________________________________________________________________ 3 MAX742 ELECTRICAL CHARACTERISTICS (continued) __________________________________________Typical Operating Characteristics (Circuit of Figure 2, V+ = 5V, TA = +25°C, unless otherwise noted.) 15 10 LOCKOUT ENABLED MEASURED AT POINT A PDRV FORCED TO -4V PUMP DISCONNECTED 5 -4.5 -4.0 PDRV CURRENT (mA) 20 6 MAX742 -2 CHARGE-PUMP OUTPUT VOLTAGE (V) MAX742 -1 QUIESCENT SUPPLY CURRENT (mA) ±15V MODE, 200kHz MODE 5 PDRV CURRENT vs. CEXT- CHARGE-PUMP LOAD REGULATION -5.0 V+ = 5V -3.5 -3.0 V+ = 4.5V MAX742 -3 UNDERVOLTAGE LOCKOUT HYSTERESIS 25 4 200kHz 3 2 100kHz 1 -2.5 1 2 3 6 5 4 0 1 2 3 4 5 6 7 8 9 0 10 EFFICIENCY vs. LOAD CURRENT, 6W CIRCUIT, ±15V MODE EFFICIENCY vs. LOAD CURRENT, 6W CIRCUIT, ±12V MODE 100kHz 80 200kHz 70 CIRCUIT OF FIGURE 3, INDUCTORS = GOWANDA 121-AT2502 (MPP CORE), Q2 = TWO IRF9Z30 IN PARALLEL ±15V MODE 90 EFFICIENCY (%) EFFICIENCY (%) 100kHz 200kHz 80 70 60 ±400 ±600 50 0 ±800 ±1000 ±50 ±100 ±150 ±200 ±250 150 LOAD CURRENT (mA) 200 ±150 ±225 ±300 LOAD CURRENT (mA) MAX742 -8 CURRENT-LIMIT THRESHOLD (mV) MEASURED AT LX-, ±15V MODE ±75 CURRENT-LIMIT THRESHOLD vs. SOFT-START VOLTAGE MAX742 -7 PEAK INDUCTOR CURRENT (mA) 200kHz 100 0 LOAD CURRENT (mA) 100kHz 50 70 INDUCTORS = GOWANDA 050-AT1003 (MPP CORE) PEAK INDUCTOR CURRENT vs. LOAD CURRENT 0 200kHz 80 60 LOAD CURRENT (mA) 1200 1100 1000 900 800 700 600 500 400 300 200 100 100kHz INDUCTORS = GOWANDA 050-AT1003 (MPP CORE) 50 ±200 200 150 100 50 0 4 MAX742 -6 MAX742 -5 EFFICIENCY vs. LOAD CURRENT, 22W CIRCUIT, ±15V MODE 50 4 3 CAPACITANCE AT EXT- (nF) 90 0 2 CHARGE-PUMP LOAD CURRENT (mA) 90 60 1 SUPPLY VOLTAGE (V) MAX742 -4 0 EFFICIENCY (%) MAX742 Switch-Mode Regulator with +5V to ±12V or ±15V Dual Output 1 2 SOFT-START VOLTAGE (V) _______________________________________________________________________________________ 3 Switch-Mode Regulator with +5V to ±12V or ±15V Dual Output (Circuit of Figure 2, ILOAD = 100mA, unless otherwise noted.) SWITCHING WAVEFORMS, INVERTING SECTION SWITCHING WAVEFORMS, STEP-UP SECTION A A B B C 2µs/div A = GATE DRIVE, 5V/div B = SWITCH VOLTAGE, 10V/div C = SWITCH CURRENT, 0.2A/div C 2µs/div A = GATE DRIVE, 5V/div B = SWITCH VOLTAGE, 10V/div C = SWITCH CURRENT, 0.2A/div OUTPUT-VOLTAGE NOISE, FILTERED AND UNFILTERED LOAD-TRANSIENT RESPONSE A A B 2µs/div A = NOISE WITH i FILTER, 1mV/div B = NOISE WITHOUT FILTER, 20mV/div MEASURED AT -VOUT V+ = 5V BW = 5MHz B 200µs/div A = +VO, 20mV/div B = -VO, 50mV/div _______________________________________________________________________________________ 5 MAX742 _____________________________Typical Operating Characteristics (continued) MAX742 Switch-Mode Regulator with +5V to ±12V or ±15V Dual Output ______________________________________________________________Pin Description PIN NAME FUNCTION 1 FB+ Step-Up Feedback Input 2 CC+ Step-Up Compensation Capacitor 3 AGND 4 AV+ 5 100/200 6 12/15 Selects VOUT. Ground for ±15V, or tie to V+ for ±12V. 7 VREF Reference Voltage Output (+2.00V). Force to GND or V+ to disable chip. 8 SS Soft-Start Timing Capacitor (sources 5µA) 9 CC- Inverting Compensation Capacitor 10 FB- Inverting Section Feedback Input 11 CSL- Current-Sense Low (inverting section) 12 CSH- Current-Sense High (inverting section) 13 V+ 14 EXT- Push-Pull Output—drives external P-channel MOSFET. 15 PDRV Voltage Input—negative supply for P-channel MOSFET driver. 16 PUMP Charge-Pump Driver—clock output at 1/2 oscillator frequency. 17 EXT+ Push-Pull Output—drives external logic-level N-channel MOSFET. 18 GND High-Current Ground 19 CSL+ Current-Sense Low (step-up section) 20 CSH+ Current-Sense High (step-up section) Analog Ground Analog Supply Voltage Input (+5V) Selects oscillator frequency. Ground for 200kHz, or tie to V+ for 100kHz. Supply Voltage Input (+5V) ________________Operating Principle Each current-mode controller consists of a summing amplifier that adds three signals: the current waveform from the power switch FET, an output-voltage error signal, and a ramp signal for AC compensation generated by the oscillator. The output of the summing amplifier resets a flip-flop, which in turn activates the power FET driver stage (Figure 1). Both external transistor switches are synchronized to the oscillator and turn on simultaneously when the flipflop is set. The switches turn off individually when their 6 source currents reach a trip threshold determined by the output-voltage error signal. This creates a dutycycle modulated pulse train at the oscillator frequency, where the on time is proportional to both the outputvoltage error signal and the peak inductor current. Low peak currents or high output-voltage error signals result in a high duty cycle (up to 90% maximum). AC stability is enhanced by the internal ramp signal applied to the error amplifier. This scheme eliminates regenerative “staircasing” of the inductor current, which is otherwise a problem when in continuous current mode with greater than 50% duty cycle. _______________________________________________________________________________________ Switch-Mode Regulator with +5V to ±12V or ±15V Dual Output CC+ 100/200 MAX742 FB+ AV+ CSH+ CSL+ MAX742 ∑ V+ R EXT+ Q S GND AGND PULSE VREF 12/15 SELECT VREF 12/15 RAMP OSC SOFT-START AND THERMAL SHUTDOWN SS PUMP SQUARE ∑ TO V+ S R Q EXTPDRV CSH- CSL- FB- CC- Figure1. MAX742 Detailed Block Diagram _______________Detailed Description 100kHz/200kHz Oscillator The MAX742 oscillator frequency is generated without external components and can be set at 100kHz or 200kHz by pin strapping. Operating the device at 100kHz results in lower supply current and improved efficiency, particularly with light loads. However, component stresses increase and noise becomes more difficult to filter. For a given inductor value, the lower operating frequency results in slightly higher peak currents in the inductor and switch transistor (see Typical Operating Characteristics, Peak Inductor Current vs. Load Current graph). When the lower frequency is used in conjunction with an LC-type output filter (optional components in Figure 2), larger component values are required for equivalent filtering. Charge-Pump Voltage Inverter The charge-pump (PUMP) output is a rail-to-rail square wave at half the oscillator frequency. The square wave drives an external diode-capacitor circuit to generate a negative DC voltage (Point A in Figure 2), which in turn biases the inverting-output drive stage via PDRV. The charge pump thus increases the gate-source voltage applied to the external P-channel FET. The low onresistance resulting from increased gate drive ensures high efficiency and guarantees start-up under heavy loads. If a -5V to -8V supply is already available, it can be tied directly to PDRV and all of the charge-pump components removed. For input voltages greater than 8V, ground PDRV to prevent overvoltage. Observe PDRV absolute maximum ratings. _______________________________________________________________________________________ 7 MAX742 Switch-Mode Regulator with +5V to ±12V or ±15V Dual Output VIN 4.5V to 6V* L1 100µH R1 100Ω L3 25µH D1 +VO C8 150µF Q1 C9 150µF C14 2.2µF OPTIONAL C1 0.1µF 1 C2 3.3nF FB+ CSH+ CC+ CSL+ AGND MAX742 AV+ R2 0.16Ω NOTES: Q1 = Motorola MTP15N05L Q2 = Motorola MTP12P05 L1, L2 = MAXL001 C8–C12 = MAXC001 D1, D2 = 1N5817 D3, D4 = Fuji ERA82-004 or 1N5817 R2, R3 = RCD RSF 1A Metal Film ±3% L3, L4 = Wilco MFB 250 GND EXT+ 100/200 PUMP 12/15 PDRV VREF EXT- J1 C6 D3 D4 1µF C3 10µF C4 C5 3.3nF SS POINT A C7 1µF V+ CC- CSH- FB- CSL- R3 0.1Ω C13 0.1µF DISC CERAMIC C10 150µF L4 25µH Q2 -VO D2 L2 100µH C11 150µF C12 150µF C15 2.2µF OPTIONAL * FOR HIGHER INPUT VOLTAGE, SEE SUPPLY-VOLTAGE RANGE SECTION. Figure 2. Standard 6W Application Circuit 8 _______________________________________________________________________________________ Switch-Mode Regulator with +5V to ±12V or ±15V Dual Output C13 330µF L1 25µH R1 100Ω MAX742 VIN 4.5V to 6V* D1 1N5820 +VO C8 1000µF Q1 C1 0.1µF 1 C2 6.8nF FB+ CSH+ CC+ CSL+ AGND MAX742 AV+ R2 0.02Ω 100/200 PUMP 12/15 PDRV VREF EXT- J1 C6 1µF C3 10µF C4 2.2µF C5 6.8nF SS NOTES: Q1 = Motorola MTP25N06L Q2 = International Rectifier IRF9Z30 L1, L2 = Gowanda 121AT2502VC R2, R3 = KRL LB4-1 ±3% C8–C13 = Nichicon PL Series (25V or 35V) GND EXT+ C9 1000µF D4 1N914 D3 1N914 C7 1µF V+ CC- CSH- FB- CSL- R3 0.02Ω C10 1000µF 10V C14 0.1µF DISC CERAMIC Q2 -VO L2 25µH D2 1N5820 C11 1000µF C12 1000µF * FOR HIGHER INPUT VOLTAGE, SEE SUPPLY-VOLTAGE RANGE SECTION. Figure 3. High-Power 22W Application Circuit _______________________________________________________________________________________ 9 MAX742 Switch-Mode Regulator with +5V to ±12V or ±15V Dual Output Supply-Voltage Range Although designed for operation from a +5V logic supply, the MAX742 works well from 4.2V (the upper limit of the undervoltage lockout threshold) to +10V (absolute maximum rating plus a safety margin). The upper limit can be further increased by limiting the voltage at V+ with a zener shunt or series regulator. To ensure AC stability, the inductor value should be scaled linearly with the nominal input voltage. For example, if Figure 3’s application circuit is powered from a nominal 9V source, the inductor value should be increased to 40µH or 50µH. At high input voltages (>8V), the charge pump can cause overvoltage at PDRV. If the input can exceed 8V, ground PDRV and remove the capacitors and diodes associated with the charge pump. In-Circuit Testing for Guaranteed Performance Figure 2’s circuit has been tested at all extremes of line, load, and temperature. Refer to the Electrical Characteristics table for guaranteed in-circuit specifications. Successful use of this circuit requires no component calculations. Soft-Start A capacitor connected between Soft-Start (SS) and ground limits surge currents at power-up. As shown in the Typical Operating Characteristics, the peak switch current limit is a function of the voltage at SS. SS is internally connected to a 5µA current source and is diode-clamped to 2.6V (Figure 8). Soft-start timing is therefore set by the SS capacitor value. As the SS voltage ramps up, peak inductor currents rise until they reach normal operating levels. Typical values for the SS capacitor, when it is required at all, are in the range of 1µF to 10µF. Fault Conditions Enabling SS Reset In addition to power-up, the soft-start function is enabled by a variety of fault conditions. Any of the following conditions will cause an internal pull-down transistor to discharge the SS capacitor, triggering a soft-start cycle: Undervoltage lockout Thermal shutdown VREF shorted to ground or supply VREF losing regulation +5V MAX742 5µA +2V REFERENCE 8 SS EXTERNAL SS CAPACITOR TO CURRENT– LIMIT COMPARATOR N FAULT Figure 4. Soft-Start Equivalent Circuit __________________Design Procedure Inductor Value An exact inductor value isn’t critical. The inductor value can be varied in order to make tradeoffs between noise, efficiency, and component sizes. Higher inductor values result in continuous-conduction operation, which maximizes efficiency and minimizes noise. Physically smallest inductors (where E = 1/2 LI2 is minimum) are realized when operating at the crossover point between continuous and discontinuous modes. Lowering the inductor value further still results in discontinuous current even at full load, which minimizes the output capacitor size required for AC stability by eliminating the right-half-plane zero found in boost and inverting topologies. Ideal current-mode slope compensation where m = 2 x V/L is achieved if L (Henries) = RSENSE (Ω) x 0.001, but again the exact value isn’t critical and the inductor value can be adjusted freely to improve AC performance. The following equations are given for continuous-conduction operation since the MAX742 is mainly intended for low-noise analog power supplies. See Appendix A in Maxim’s Battery Management and DC-DC Converter Circuit Collection for crossover point and discontinuous-mode equations. Boost (positive) output: (VIN - VSW)2 (VOUT + VD - VIN) L = ——————————————— (VOUT + VD)2 (ILOAD)(F)(LIR) Inverting (negative) output: (VIN - VSW)2 L = ————————————— (VOUT + VD)(ILOAD)(F)(LIR) 10 ______________________________________________________________________________________ Switch-Mode Regulator with +5V to ±12V or ±15V Dual Output VD is the rectifier forward voltage drop (0.4V typ). LIR is the ratio of peak-to-peak ripple current to DC offset current in the inductor (0.5 typ). Current-Sense Resistor Value The current-sense resistor values are calculated according to the worst-case-low current-limit threshold voltage from the Electrical Characteristics table and the peak inductor current. The peak inductor current calculations that follow are also useful for sizing the switches and specifying the inductor current saturation ratings. 150mV RSENSE = ———— IPEAK ILOAD (VOUT + VD) +IPEAK (boost) = ————————— + VIN - VSW (VIN - VSW) (VOUT + VD - VIN) ————————————— (2)(F)(L)(VOUT + VD) ILOAD (VOUT + VD+ VIN) +IPEAK (inverting) = ———————————— + VIN - VSW (VIN - VSW) (VOUT + VD + VIN) ————————————— (2)(F)(L) (VOUT + VD) Filter Capacitor Value The output filter capacitor values are generally determined by the effective series resistance (ESR) and voltage rating requirements rather than actual capacitance requirements for loop stability. In other words, the capacitor that meets the ESR requirement for noise purposes nearly always has much more output capacitance than is required for AC stability. Output voltage noise is dominated by ESR and can be roughly calculated by an Ohm’s Law equation: VNOISE (peak-to-peak) = IPEAK x RESR where VNOISE is typically 0.15V. Ensure the output capacitors selected meet the following minimum capacitance requirements: Minimum CF = 60µF per output or the following, whichever is greater: CF = 0.015/RLOAD (in Farads, ±15V mode) CF = 0.01/RLOAD (in Farads, ±12V mode) Compensation Capacitor (CC) Value The compensation capacitors (CC+ and CC-) cancel the zero introduced by the output filter capacitors’ ESR, improving phase margin, and AC stability. The compensation poles set by CC+ and CC- should be set to match the ESR zero frequencies of the output filter capacitors according to the following: RESR x CF CC (in Farads) = —————— (use 1000pF minimum) 10kΩ Standard 6W Application The 6W supply (Figure 2) generates ±200mA at ±15V, or ±250mA at ±12V. Output capability is increased to 10W or more by heatsinking the power FETs, using cores with higher current capability (such as Gowanda #050AT1003), and using higher filter capacitance. Ferrite and MPP inductor cores optimize efficiency and size. Iron-power toroids designed for high frequencies are economical, but larger. Ripple is directly proportional to filter capacitor equivalent series resistance (ESR). In addition, about 250mV transient noise occurs at the LX switch transitions. A very short scope probe ground lead or a shielded enclosure is need for making accurate measurements of transient noise. Extra filtering, as shown in Figure 2, reduces both noise components. High-Power 22W Application The 22W application circuit (Figure 3) generates ±15V at ±750mA or ±12V at ±950mA. Noninductive wirewound resistors with Kelvin current-sensing connections replace the metal-film resistors of the previous (6W) circuit. Gate drive for the P-channel FET is bootstrapped from the negative supply via diode D6. The 2.7V zener (D5) is required in 15V mode to prevent overvoltage. The charge pump (D3, D4, and C6) may not be necessary if the circuit is lightly loaded (<100mA) on start-up. AIE part #415-0963 is a ferrite pot-core inductor that can be used in place of a smaller, more expensive moly-permalloy toroid inductor (L1, L2). Higher efficiencies can be achieved by adding extra MOSFETs in parallel. Load levels above 10W make it necessary to add heatsinks, especially to the Pchannel FET. ______________________________________________________________________________________ 11 MAX742 where: VSW is the voltage drop across the the switch transistor and current-sense resistor in the on state (0.3V typ). MAX742 Switch-Mode Regulator with +5V to ±12V or ±15V Dual Output Table 1. Trouble-Shooting Chart SYMPTOM CORRECTION Unstable Output. Noise or jitter on output ripple waveform. Scope may not trigger correctly. Loop stability problem. A. CC+ or CC- disconnected. B. EMI: Move inductor away from IC or use shielded inductors. Keep noise sources away from CC- and CC+. C. Grounding: Tie AGND directly to the filter capacitor ground lead. Ensure that current spikes from GND do not cause noise at AGND or compensation capacitor or reference bypass ground leads. Use wide PC traces or a ground plane. D. Bypass: Tie 10µF or larger between AGND and VREF. Use 150µF to bypass the input right at AV+. If there is high source resistance, 1000µF or more may be required. E. Current limiting: Reduce load currents. Ensure that inductors are not saturating. F. Slope compensation: Inductor value not matched to sense resistor. A. Ground noise: Probe ground is picking up switching EMI. Reduce probe ground lead length (use probe tip shield) or put circuit in shielded enclosure. B. Poor HF response: Add ceramic or tantalum capacitors in parallel with output filter capacitors. Noisy Output. Switching is steady, but large inductive spikes are seen at the outputs. Self-Destruction. A. Input overvoltage: Never apply more than +12V. Transistors or IC die on power-up. B. FB+ or FB- disconnected or shorted. This causes runaway and output overvoltage. C. CC+ or CC- shorted. D. Output filter capacitor disconnected. Poor Efficiency. Supply current is high. Output will not drive heavy loads. A. Inductor saturation: Peak currents exceed coil ratings. B. MOSFET on-resistance too high. C. Switching losses: Diode is slow or has high forward voltage. Inductor has high DC resistance. Excess capacitance at LX nodes. D. Inductor core losses: Hysteresis losses cause self-heating in some core materials. E. Loop instability: See Unstable Output above. No Output. +VO = 5V or less. -VO = 0V. A. Check connections. VREF should be +2V. B. When input voltage is less than +4.2V, undervoltage lockout is enabled. ___________________Chip Topography AGND CC+ FB+ CSH+ CSL+ GND AV+ EXT+ PUMP 0.135" (3.45mm) 100/200 12/15 PDRV EXTVREF V+ SS CC- FB- CSL- CSH0.080" (2.03mm) TRANSISTOR COUNT: 375 SUBSTRATE CONNECTED TO V+ No Switching. Output is unloaded. Apply ±30mA or greater load to observe waveform. ±VO are correct, but no waveform is seen at LX+ or LX-. 12 ______________________________________________________________________________________