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19-3105; Rev 2; 8/96
NUAL KIT MA ATION ET EVALU TA SHE WS DA FOLLO
Switch-Mode Regulator with +5V to 12V or 15V Dual Output
____________________________Features
o Specs Guaranteed for In-Circuit Performance o Load Currents to 2A o 4.2V to 10V Input-Voltage Range o Switches From 15V to 12V Under Logic Control o 4% Output Tolerance Max Over Temp, Line, and Load o 90% Typ Efficiency o o o o Low-Noise, Current-Mode Feedback Cycle-by-Cycle Current Limiting Undervoltage Lockout and Soft-Start 100kHz or 200kHz Operation
_______________General Description
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.
MAX742
______________Ordering Information
PART MAX742CPP MAX742CWP MAX742C/D MAX742EPP MAX742EWP MAX742MJP TEMP. RANGE 0C to +70C 0C to +70C 0C to +70C -40C to +85C -40C to +85C -55C to +125C PIN-PACKAGE 20 Plastic DIP 20 Wide SO Dice* 20 Plastic DIP 20 Wide SO 20 CERDIP
________________________Applications
DC-DC Converter Module Replacement Distributed Power Systems Computer Peripherals
* Contact factory for dice specifications
__________Simplified Block Diagram
MAX742
CC+5V INPUT
__________________Pin Configuration
TOP VIEW
FB+ 1 CC+ 2 AGND 3 AV 4 100/200 5 12/15 6 VREF 7 SS 8 CC- 9 FB- 10 20 CSH+ 19 CSL+ 18 GND 17 EXT+
R -SENSE PWM S -DRIVE
P -VO
MAX742
16 PUMP 15 PDRV 14 EXT13 V+ 12 CSH11 CSL-
+2.0V VREF
OSC +VO S +DRIVE PWM R +SENSE 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
Switch-Mode Regulator with +5V to 12V or 15V Dual Output MAX742
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 +70C .....................................................................500mW derate above +70C by ..........................................100mW/C Operating Temperature Ranges MAX742C_ _ .......................................................0C to +70C MAX742E_ _ ....................................................-40C to +85C MAX742MJP ..................................................-55C to +125C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10sec) .............................+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
(Circuit of Figure 2, +4.5V < V+ < +5.5V.) PARAMETER Output Voltage, 15V Mode (Notes 1, 2) Output Voltage, 12V Mode (Notes 1, 2) SYMBOL CONDITIONS 0mA < IL < 100mA, 12/15 = 0V 0mA < IL < 125mA, 12/15 = V+ TA = +25C TA = TMIN to TMAX TA = +25C TA = TMIN to TMAX MIN 14.55 14.40 11.64 11.52 TYP MAX 15.45 15.60 12.36 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 Line Regulation Load Regulation (Note 2) No-Load Supply Current Undervoltage Lockout Undervoltage Lockout Hysteresis Reference Output Voltage Oscillator Frequency PUMP Frequency Duty-Cycle Limit (Note 3) Positive Current-Limit Threshold (CSH+ to CSL+) Negative Current-Limit Threshold (CSH- to CSL-) EXT+ or EXTCSL+ = 0V, FB+ = open circuit CSH- = V+, FB- = open circuit 85 150 150 fOSC 100/200 = 0V 100/200 = V+ 170 75 UVLO SYMBOL CONDITIONS V+ = 4.5V to 5.5V, PDRV from PUMP ILOAD = 0mA to 100mA V+ = 5V No EXT- or PUMP load, FB+ = FB- = open circuit V+ = 10V 3.8 0.2 2.0 200 100 fOSC/2 90 225 225 300 300 230 125 MIN TYP 0.01 30 MAX 0.05 100 3 10 4.2 UNITS %/% mV mA V V V kHz kHz % mV mV
2
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Switch-Mode Regulator with +5V to 12V or 15V Dual Output
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 2, V+ = 5V, 100/200 = 12/15 = 0V; TA = TMIN to TMAX, unless otherwise noted.) PARAMETER Output Voltage High Output Voltage Low Output Sink Current Output Source Current Output Rise/Fall Time PUMP Output Voltage (Note 4) Compensation Pin Impedance Thermal-Shutdown Threshold Soft-Start Source Current Soft-Start Sink Current SS = 0V V+ = 3.8V, SS = 2V 3 -2 SYMBOL VOH VOL CONDITIONS EXT+, EXT-, IL = 1mA, V+ = 4.5V, PDRV= -3V EXT+, EXT-, IL = -1mA, V+ = 4.5V, PDRV= -3V V+ = 4.5V, PDRV = -3V, EXT+ = 4.5V TA = +25C EXT- = 4.5V V+ = 4.5V, PDRV = -3V, EXT+ = 0V TA = +25C EXT- = -3V EXT+, CLOAD = 2nF EXT-, CLOAD = 4nF, PDRV = -3V V+ = 4.5V, IL = -5mA, TA = +25C CC+, CC10 190 7 -0.5 100 200 200 350 -200 -350 70 100 -3 -100 -200 MIN 4.3 -2.8 TYP MAX UNITS V V mA mA ns V k C A mA
MAX742
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
Switch-Mode Regulator with +5V to 12V or 15V Dual Output MAX742
__________________________________________Typical Operating Characteristics
(Circuit of Figure 2, V+ = 5V, TA = +25C, unless otherwise noted.)
UNDERVOLTAGE LOCKOUT HYSTERESIS
MAX742 -1
CHARGE-PUMP LOAD REGULATION
MAX742 -2
PDRV CURRENT vs. CEXTPDRV FORCED TO -4V PUMP DISCONNECTED
MAX742 -3
25 QUIESCENT SUPPLY CURRENT (mA) 15V MODE, 200kHz MODE 20
-5.0 CHARGE-PUMP OUTPUT VOLTAGE (V) MEASURED AT POINT A -4.5
6 5 PDRV CURRENT (mA) 4
15
-4.0
V+ = 5V
200kHz 3 2 1 100kHz
10 LOCKOUT ENABLED
-3.5
5
-3.0
V+ = 4.5V
-2.5 0 1 2 3 4 5 6 0 1 2 34 5 67 8 9 10 0 1 2 3 4 SUPPLY VOLTAGE (V) CHARGE-PUMP LOAD CURRENT (mA) CAPACITANCE AT EXT- (nF)
EFFICIENCY vs. LOAD CURRENT, 22W CIRCUIT, 15V MODE
MAX742 -4
EFFICIENCY vs. LOAD CURRENT, 6W CIRCUIT, 15V MODE
MAX742 -5
EFFICIENCY vs. LOAD CURRENT, 6W CIRCUIT, 12V MODE
MAX742 -6
90 EFFICIENCY (%) EFFICIENCY (%) 100kHz 80 200kHz 70 CIRCUIT OF FIGURE 3, INDUCTORS = GOWANDA 121-AT2502 (MPP CORE), Q2 = TWO IRF9Z30 IN PARALLEL 15V MODE 0 200 400 600 800 1000
90
100kHz EFFICIENCY (%)
90 200kHz
100kHz
80
200kHz
80
70
70
60
60 INDUCTORS = GOWANDA 050-AT1003 (MPP CORE) 50 0 50 100 150 200 250
60 INDUCTORS = GOWANDA 050-AT1003 (MPP CORE) 50 0 75 150 225 300 LOAD CURRENT (mA) LOAD CURRENT (mA)
50 LOAD CURRENT (mA)
PEAK INDUCTOR CURRENT vs. LOAD CURRENT
MAX742 -7
CURRENT-LIMIT THRESHOLD vs. SOFT-START VOLTAGE
CURRENT-LIMIT THRESHOLD (mV)
MAX742 -8
1200 1100 1000 900 800 700 600 500 400 300 200 100 0
PEAK INDUCTOR CURRENT (mA)
100kHz
200
200kHz
150
100
MEASURED AT LX-, 15V MODE 50 100 150 200
50
0
1
2
3
LOAD CURRENT (mA)
SOFT-START VOLTAGE (V)
4
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Switch-Mode Regulator with +5V to 12V or 15V Dual Output
_____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 2, ILOAD = 100mA, unless otherwise noted.)
SWITCHING WAVEFORMS, INVERTING SECTION SWITCHING WAVEFORMS, STEP-UP SECTION
MAX742
A A
B
B
C 2s/div A = GATE DRIVE, 5V/div B = SWITCH VOLTAGE, 10V/div C = SWITCH CURRENT, 0.2A/div 2s/div A = GATE DRIVE, 5V/div B = SWITCH VOLTAGE, 10V/div C = SWITCH CURRENT, 0.2A/div
C
OUTPUT-VOLTAGE NOISE, FILTERED AND UNFILTERED
LOAD-TRANSIENT RESPONSE
A A
B
B
2s/div A = NOISE WITH i FILTER, 1mV/div B = NOISE WITHOUT FILTER, 20mV/div MEASURED AT -VOUT V+ = 5V BW = 5MHz
200s/div A = +VO, 20mV/div B = -VO, 50mV/div
_______________________________________________________________________________________
5
Switch-Mode Regulator with +5V to 12V or 15V Dual Output MAX742
______________________________________________________________Pin Description
PIN 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 NAME FB+ CC+ AGND AV+ 100/200 12/15 VREF SS CCFBCSLCSHV+ EXTPDRV PUMP EXT+ GND CSL+ CSH+ Step-Up Feedback Input Step-Up Compensation Capacitor Analog Ground Analog Supply Voltage Input (+5V) Selects oscillator frequency. Ground for 200kHz, or tie to V+ for 100kHz. Selects VOUT. Ground for 15V, or tie to V+ for 12V. Reference Voltage Output (+2.00V). Force to GND or V+ to disable chip. Soft-Start Timing Capacitor (sources 5A) Inverting Compensation Capacitor Inverting Section Feedback Input Current-Sense Low (inverting section) Current-Sense High (inverting section) Supply Voltage Input (+5V) Push-Pull Output--drives external P-channel MOSFET. Voltage Input--negative supply for P-channel MOSFET driver. Charge-Pump Driver--clock output at 1/2 oscillator frequency. Push-Pull Output--drives external logic-level N-channel MOSFET. High-Current Ground Current-Sense Low (step-up section) Current-Sense High (step-up section) FUNCTION
________________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
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.
6
_______________________________________________________________________________________
Switch-Mode Regulator with +5V to 12V or 15V Dual Output MAX742
FB+ CC+ 100/200 AV+
CSH+
CSL+
MAX742
V+ R S GND AGND PULSE VREF 12/15 VREF 12/15 SELECT RAMP OSC SOFT-START AND THERMAL SHUTDOWN TO V+ S R Q SS PUMP Q EXT+
SQUARE
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.
_______________________________________________________________________________________
EXTPDRV CSH-
CSL-
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
Switch-Mode Regulator with +5V to 12V or 15V Dual Output MAX742
VIN 4.5V to 6V* L1 100H D1 C8 150F C9 150F
R1 100
L3 25H +VO C14 2.2F OPTIONAL
Q1 C1 0.1F C2 3.3nF
1
FB+ CC+ AGND AV+ 100/200
CSH+ CSL+
R2 0.16 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 POINT A
MAX742
GND EXT+ PUMP 1F PDRV EXTV+ CSHCSLQ2 R3 0.1 C10 150F C7 1F C6
D3
D4
J1 12/15 C3 10F C4 C5 3.3nF VREF SS CCFB-
C13 0.1F DISC CERAMIC
L4 25H -VO D2 C11 150F C12 150F C15 2.2F OPTIONAL
L2 100H
* 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 MAX742
VIN 4.5V to 6V*
C13 330F
R1 100
L1 25H
D1 1N5820 +VO Q1 C8 1000F C9 1000F
C1 0.1F C2 6.8nF
1
FB+ CC+ AGND AV+ 100/200
CSH+ CSL+
R2 0.02
MAX742
GND EXT+ PUMP 1F PDRV EXTV+ CSHCSLQ2 R3 0.02 C10 1000F 10V D3 1N914 C7 1F C6 D4 1N914
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)
J1 12/15 C3 10F C4 2.2F C5 6.8nF VREF SS CCFB-
C14 0.1F DISC CERAMIC
-VO L2 25H D2 1N5820 C11 1000F C12 1000F
* FOR HIGHER INPUT VOLTAGE, SEE SUPPLY-VOLTAGE RANGE SECTION.
Figure 3. High-Power 22W Application Circuit
_______________________________________________________________________________________
9
Switch-Mode Regulator with +5V to 12V or 15V Dual Output MAX742
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 40H or 50H. 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.
+5V
MAX742
5A +2V REFERENCE 8 SS EXTERNAL SS CAPACITOR TO CURRENT- LIMIT COMPARATOR N FAULT
Figure 4. Soft-Start Equivalent Circuit
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.
__________________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 () 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)
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 5A 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 1F to 10F.
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
10
______________________________________________________________________________________
Switch-Mode Regulator with +5V to 12V or 15V Dual Output
where: VSW is the voltage drop across the the switch transistor and current-sense resistor in the on state (0.3V typ). 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).
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) 10k
MAX742
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)
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.
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 = 60F per output or the following, whichever is greater: CF = 0.015/RLOAD (in Farads, 15V mode) CF = 0.01/RLOAD (in Farads, 12V mode)
______________________________________________________________________________________
11
Switch-Mode Regulator with +5V to 12V or 15V Dual Output MAX742
Table 1. Trouble-Shooting Chart
SYMPTOM Unstable Output. Noise or jitter on output ripple waveform. Scope may not trigger correctly. CORRECTION 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 10F or larger between AGND and VREF. Use 150F to bypass the input right at AV+. If there is high source resistance, 1000F 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.
AGND CC+ FB+ CSH+ CSL+ GND AV+ EXT+ PUMP
___________________Chip Topography
100/200 12/15
0.135" (3.45mm) PDRV
Noisy Output. Switching is steady, but large inductive spikes are seen at the outputs.
EXTVREF V+ SS CC- FB- CSL- CSH0.080" (2.03mm)
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. A. Check connections. VREF should be +2V. B. When input voltage is less than +4.2V, undervoltage lockout is enabled.
TRANSISTOR COUNT: 375 SUBSTRATE CONNECTED TO V+
No Output. +VO = 5V or less. -VO = 0V.
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-.
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