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19-0176; Rev 0; 6/94
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-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ DC-DC Inverters
____________________________Features
o High Efficiency for a Wide Range of Load Currents o 250mA Output Current o 120A Max Supply Current o 5A Max Shutdown Current o 3V to 16V Input Voltage Range o -5V (MAX764), -12V (MAX765), -15V (MAX766), or Adjustable Output from -1V to -16V o Current-Limited PFM Control Scheme o 300kHz Switching Frequency o Internal, P-Channel Power MOSFET
_______________General Description
The MAX764/MAX765/MAX766 inverting switching regulators are highly efficient over a wide range of load currents, delivering up to 1.5W. A unique, current-limited, pulse-frequency-modulated (PFM) control scheme combines the benefits of traditional PFM converters with the benefits of pulse-width-modulated (PWM) converters. Like PWM converters, the MAX764/MAX765/MAX766 are highly efficient at heavy loads. Yet because they are PFM devices, they use less than 120A of supply current (vs. 2mA to 10mA for a PWM device). The input voltage range is 3V to 16V. The output voltage is preset at -5V (MAX764), -12V (MAX765), or -15V (MAX766); it can also be adjusted from -1V to -16V using two external resistors (Dual ModeTM). The maximum operating VIN - VOUT differential is 20V. These devices use miniature external components; their high switching frequencies (up to 300kHz) allow for less than 5mm diameter surface-mount magnetics. A standard 47H inductor is ideal for most applications, so no magnetics design is necessary. An internal power MOSFET makes the MAX764/MAX765/ MAX766 ideal for minimum component count, low- and medium-power applications. For increased output drive capability or higher output voltages, use the MAX774/MAX775/MAX776 or MAX1774, which drive an external power P-channel MOSFET for loads up to 5W.
MAX764/MAX765/MAX766
______________Ordering Information
PART MAX764CPA MAX764CSA MAX764C/D MAX764EPA MAX764ESA MAX764MJA MAX765CPA MAX765CSA MAX765C/D MAX765EPA MAX765ESA MAX765MJA TEMP. RANGE 0C to +70C 0C to +70C 0C to +70C -40C to +85C -40C to +85C -55C to +125C 0C to +70C 0C to +70C 0C to +70C -40C to +85C -40C to +85C -55C to +125C PIN-PACKAGE 8 Plastic DIP 8 SO Dice* 8 Plastic DIP 8 SO 8 CERDIP** 8 Plastic DIP 8 SO Dice* 8 Plastic DIP 8 SO 8 CERDIP**
________________________Applications
LCD-Bias Generators Portable Instruments LAN Adapters Remote Data-Acquisition Systems Battery-Powered Applications
Ordering Information continued on last page. * Dice are tested at TA = +25C, DC parameters only. **Contact factory for availability and processing to MIL-STD-883.
__________Typical Operating Circuit
INPUT 3V TO 15V
V+ LX
__________________Pin Configuration
TOP VIEW
OUTPUT -5V
OUT FB
1 2
8
LX V+ V+ GND
MAX764
ON/OFF SHDN 47H
SHDN 3 REF 4
MAX764 MAX765 MAX766
7 6 5
REF FB GND
OUT
DIP/SO
________________________________________________________________ Maxim Integrated Products
1
Call toll free 1-800-998-8800 for free samples or literature.
-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ DC-DC Inverters MAX764/MAX765/MAX766
ABSOLUTE MAXIMUM RATINGS
V+ to GND ..............................................................-0.3V to +17V OUT to GND ...........................................................+0.5V to -17V Maximum Differential (V+ to OUT) ......................................+21V REF, SHDN, FB to GND ...............................-0.3V to (V+ + 0.3V) LX to V+..................................................................+0.3V to -21V LX Peak Current ...................................................................1.5A Continuous Power Dissipation (TA = +70C) Plastic DIP (derate 9.09mW/C above +70C) ............727mW SO (derate 5.88mW/C above +70C) .........................471mW CERDIP (derate 8.00mW/C above +70C) .................640mW Operating Temperature Ranges MAX76_C_A ........................................................0C to +70C MAX76_E_A .....................................................-40C to +85C MAX76_MJA ..................................................-55C to +125C Maximum Junction Temperatures MAX76_C_A/E_A ..........................................................+150C MAX76_MJA .................................................................+175C Storage Temperature Range ............................-65C to +160C 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
(V+ = 5V, ILOAD = 0mA, CREF = 0.1F, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER V+ Input Voltage Range Supply Current Shutdown Current FB Trip Point FB Input Current IFB SYMBOL V+ IS ISHDN MAX76_C/E MAX76_M V+ = 16V, SHDN < 0.4V V+ = 16V, SHDN > 1.6V V+ = 10V, SHDN > 1.6V 3V V+ 16V MAX76_C MAX76_E MAX76_M MAX764, -4.8V VOUT 5.2V Output Current and Voltage (Note 1) IOUT MAX765C/E, -11.52V VOUT 12.48V MAX765M, -11.52V VOUT 12.48V MAX766, -14.40V VOUT -15.60V MAX76_C Reference Voltage VREF MAX76_E MAX76_M REF Load Regulation REF Line Regulation Load Regulation (Note 2) Line Regulation (Note 2) Efficiency (Note 2) SHDN Leakage Current SHDN Input Voltage High SHDN Input Voltage Low VIH VIL 0A IREF 100A 3V V+ 16V 0mA ILOAD 100mA 4V V+ 6V 10mA ILOAD 100mA, VOUT = -5V VIN = 5V VOUT = -15V V+ = 16V, SHDN = 0V or V+ 3V V+ 16V 3V V+ 16V 1.6 0.4 MAX76_C/E MAX76_M 150 68 50 35 1.4700 1.4625 1.4550 260 120 120 105 1.5 1.5 1.5 4 4 40 0.008 0.12 80 82 1 1.5300 1.5375 1.5450 10 15 100 mV V/V %/mA %/V % A V V V mA -10 CONDITIONS MIN 3.0 3.5 90 2 1 5 10 50 70 90 nA mV 120 A TYP MAX 16.0 UNITS V
2
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-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ DC-DC Inverters
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 5V, ILOAD = 0mA, CREF = 0.1F, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER LX Leakage Current LX On-Resistance Peak Current at LX Maximum Switch On-Time Minimum Switch Off-Time IPEAK tON tOFF SYMBOL CONDITIONS MAX76_C MIN TYP MAX 5 10 30 1.4 0.5 12 1.8 0.75 16 2.3 20 2.8 2.5 A s s A UNITS
MAX764/MAX765/MAX766
ILXI + (V+) 20V IVOUTI + (V+) 10V IVOUTI + (V+) 10V
MAX76_E MAX76_M
Note 1: See Maximum Output Current vs. Supply Voltage graph in the Typical Operating Characteristics. Guarantees are based on correlation to switch on-time, switch off-time, on-resistance, and peak current rating. Note 2: Circuit of Figure 2.
__________________________________________Typical Operating Characteristics
(V+ = 5V, VOUT = -5V, TA = +25C, unless otherwise noted.)
MAX764 EFFICIENCY vs. LOAD CURRENT
MAX764-01
MAX765 EFFICIENCY vs. LOAD CURRENT
MAX764-02
MAX766 EFFICIENCY vs. LOAD CURRENT
90 80
MAX764-03
100 90 80 EFFICIENCY (%) V+ = 5V
100 90 80 EFFICIENCY (%) V+ = 5V V+ = 8V
100
EFFICIENCY (%)
70 60 50 40 30 20 10 0 0.1 1 10 100 1000 LOAD CURRENT (mA) CIRCUIT OF FIGURE 2 VOUT = -5V 4% V+ = 15V V+ = 10V
70 60 50 40 30 20 10 0 0.1 1 10
70 60 50 40 30 20
V+ = 5V
CIRCUIT OF FIGURE 2 VOUT = -12V 4% 100 1000
10 0 0.1 1
CIRCUIT OF FIGURE 2 VOUT = -15V 4% 10 100 1000
LOAD CURRENT (mA)
LOAD CURRENT (mA)
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-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ DC-DC Inverters MAX764/MAX765/MAX766
____________________________Typical Operating Characteristics (continued)
(V+ = 5V, VOUT = -5V, TA = +25C, unless otherwise noted.)
MAXIMUM OUTPUT CURRENT vs. SUPPLY VOLTAGE
MAX764 -04
NO-LOAD SUPPLY CURRENT vs. SUPPLY VOLTAGE
MAX764 -05
NO-LOAD SUPPLY CURRENT vs. TEMPERATURE
NO-LOAD SUPPLY CURRENT (A)
MAX764 -06
600 MAXIMUM OUTPUT CURRENT (mA) CIRCUIT OF FIGURE 2 500 400 300 200 VOUT = -12V 100 VOUT = -15V 0 VOUT = -5V
100 NO-LOAD SUPPLY CURRENT (A) 95 90 85 80 75 70 65 60
110 105 100 95 90 85 80 75 70 65 60 55 50 V+ = 15V
V+ = 5V
3 4 5 6 7 8 9 10 11 12 13 14 15 16 SUPPLY VOLTAGE (V)
3 4 5 6 7 8 9 10 11 12 13 14 15 16 SUPPLY VOLTAGE (V)
-60 -40 -20
0 20 40 60 80 100 120 140 TEMPERATURE (C)
SHUTDOWN CURRENT vs. TEMPERATURE
MAX764 -07
MAXIMUM SWITCH ON-TIME vs. TEMPERATURE
MINIMUM SWITCH OFF-TIME (s) MAXIMUM SWITCH ON-TIME (s) 16.8 16.6 16.4 16.2 16.0 15.8 15.6 15.4 15.2 15.0 V+ = 5V V+ = 15V
MAX764 -08
MINIMUM SWITCH OFF-TIME vs. TEMPERATURE
2.55 2.50 2.45 2.40 2.35 2.30 2.25 2.20 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C) V+ = 5V V+ = 15V
MAX764 -09
4.0 3.5 SHUTDOWN CURRENT (A) 3.0 2.5 V+ = 15V 2.0 1.5 V+ = 8V 1.0 0.5 0 V+ = 4V -60 -40 -20
17.0
2.60
0 20 40 60 80 100 120 140 TEMPERATURE (C)
-60 -40 -20
0 20 40 60 80 100 120 140 TEMPERATURE (C)
SWITCH ON/OFF-TIME RATIO vs. TEMPERATURE
MAX764 -10
START-UP SUPPLY VOLTAGE vs. OUTPUT CURRENT
CIRCUIT OF FIGURE 2 START-UP SUPPLY VOLTAGE (V) 7 6 5 4 3 2 1 0 1 0 50 100 150 200 250 300
MAX764 -11
LX LEAKAGE CURRENT vs. TEMPERATURE IVOUTI + (V+) = 20V
LX LEAKAGE CURRENT (nA) 1000
MAX764-12
7.2 SWITCH ON/OFF-TIME RATIO (s/s) 7.1 7.0 6.9 6.8 6.7 6.6 6.5 6.4 6.3 6.2 -60 -40 -20 V+ = 5V
8
10,000
100
10
0 20 40 60 80 100 120 140 TEMPERATURE (C)
20 30 40 50 60 70 80 90 100 110 120 130 TEMPERATURE (C)
OUTPUT CURRENT (mA)
4
_______________________________________________________________________________________
-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ DC-DC Inverters
____________________________Typical Operating Characteristics (continued)
(V+ = 5V, VOUT = -5V, TA = +25C, unless otherwise noted.)
MAX764/MAX765/MAX766
LX ON-RESISTANCE vs. TEMPERATURE
MAX764 -13
PEAK CURRENT AT LX vs. TEMPERATURE
MAX764 -14
REFERENCE OUTPUT RESISTANCE vs. TEMPERATURE
REFERENCE OUTPUT RESISTANCE ()
MAX764 -15
2.2 2.0 LX ON-RESISTANCE ()
0.95 0.90 CURRENT AT LX (A)
250
IVOUTI + (V+) = 10V
1.8 1.6
200
IVOUTI + (V+) = 20V
0.85
IREF = 10A
IVOUTI + (V+) = 15V
1.4 1.2 1.0
IVOUTI + (V+) = 15V
0.80 0.75 0.70
150 IREF = 50A
100
50 IREF = 100A 0 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C)
IVOUTI + (V+) = 20V
0.8 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C) 0.65
IVOUTI + (V+) = 10V
-60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C)
REFERENCE OUTPUT vs. TEMPERATURE
MAX764 -16
SUPPLY CURRENT vs. SUPPLY VOLTAGE
MAX764-17
1.506 1.504 REFERENCE OUTPUT (V) 1.502 1.500 1.498 1.496 1.494 1.492 -60 -40 -20
1000 ILOAD = 100mA
SUPPLY CURRENT (mA)
100
10
1 ILOAD = 0mA 0.1 CIRCUIT OF FIGURE 2
0 20 40 60 80 100 120 140 TEMPERATURE (C)
0.01 0 2 4 6 8 10 12 14 16 SUPPLY VOLTAGE (V)
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-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ DC-DC Inverters MAX764/MAX765/MAX766
____________________________Typical Operating Characteristics (continued)
(V+ = 5V, VOUT = -5V, TA = +25C, unless otherwise noted.)
TIME TO ENTER/EXIT SHUTDOWN
LOAD-TRANSIENT RESPONSE
0V A A
B 0V 2ms/div CIRCUIT OF FIGURE 2, V+ = 5V, ILOAD = 100mA, VOUT = -5V A: VOUT, 2V/div B: SHUTDOWN PULSE, 0V TO 5V, 5V/div
0mA 5ms/div CIRCUIT OF FIGURE 2, V+ = 5V, VOUT = -5V A: VOUT, 50mV/div, AC-COUPLED B: ILOAD, 0mA TO 100mA, 100mA/div
B
LINE-TRANSIENT RESPONSE
DISCONTINUOUS CONDUCTION AT HALF AND FULL CURRENT LIMIT
A A B 0A
B 0V 5ms/div CIRCUIT OF FIGURE 2, VOUT = -5V, ILOAD = 100mA A: VOUT, 50mV/div, AC-COUPLED B: V+, 5V TO 10V, 5V/div
0V
C
5s/div CIRCUIT OF FIGURE 2, V+ = 5V, VOUT = -5V, ILOAD = 140mA A: OUTPUT RIPPLE, 100mV/div B: INDUCTOR CURRENT, 500mA/div C: LX WAVEFORM, 10V/div
6
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-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ DC-DC Inverters
____________________________Typical Operating Characteristics (continued)
(V+ = 5V, VOUT = -5V, TA = +25C, unless otherwise noted.) DISCONTINUOUS CONDUCTION AT HALF CURRENT LIMIT CONTINUOUS CONDUCTION AT FULL CURRENT LIMIT
MAX764/MAX765/MAX766
A
A B
B 0A 0A
0V
C
0V
C
5s/div CIRCUIT OF FIGURE 2, V+ = 5V, VOUT = -5V, ILOAD = 80mA A: OUTPUT RIPPLE, 100mV/div B: INDUCTOR CURRENT, 500mA/div C: LX WAVEFORM, 10V/div
5s/div CIRCUIT OF FIGURE 2, V+ = 5V, VOUT = -5V, ILOAD = 240mA A: OUTPUT RIPPLE, 100mV/div B: INDUCTOR CURRENT, 500mA/div C: LX WAVEFORM, 10V/div
______________________________________________________________Pin Description
PIN 1 2 3 4 5 6, 7 8 NAME OUT FB SHDN REF GND V+ LX FUNCTION Sense Input for Fixed-Output Operation (VFB = VREF). OUT must be connected to VOUT. Feedback Input. Connect FB to REF to use the internal voltage divider for a preset output. For adjustableoutput operation, use an external voltage divider, as described in the section Setting the Output Voltage. Active-High Shutdown Input. With SHDN high, the part is in shutdown mode and the supply current is less than 5A. Connect to ground for normal operation. 1.5V Reference Output that can source 100A for external loads. Bypass to ground with a 0.1F capacitor. Ground Positive Power-Supply Input. Must be tied together. Place a 0.1F input bypass capacitor as close to the V+ and GND pins as possible. Drain of the Internal P-Channel Power MOSFET. LX has a peak current limit of 0.75A.
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-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ DC-DC Inverters MAX764/MAX765/MAX766
FB COMPARATOR
REF SHDN ERROR COMPARATOR
MAX764 MAX765 MAX766
OUT
V+ N 1.5V REFERENCE Q TRIG ONE-SHOT FROM V+ S TRIG ONE-SHOT Q Q CURRENT COMPARATOR FROM OUT LX P V+
R
CURRENT CONTROL CIRCUITS
0.2V (FULL CURRENT)
0.1V (HALF CURRENT) FROM V+
GND
Figure 1. Block Diagram
_______________Detailed Description
Operating Principle
The MAX764/MAX765/MAX766 are BiCMOS, inverting, switch-mode power supplies that provide fixed outputs of -5V, -12V, and -15V, respectively; they can also be set to any desired output voltage using an external resistor divider. Their unique control scheme combines the advantages of pulse-frequency modulation (pulse skipping) and pulse-width modulation (continuous pulsing). The internal P-channel power MOSFET allows peak currents of 0.75A, increasing the output current capability over previous pulse-frequency-modulation (PFM) devices. Figure 1 shows the MAX764/MAX765/ MAX766 block diagram. The MAX764/MAX765/MAX766 offer three main improvements over prior solutions:
8
1) They can operate with miniature (less than 5mm diameter) surface-mount inductors, because of their 300kHz switching frequency. 2) The current-limited PFM control scheme allows efficiencies exceeding 80% over a wide range of load currents. 3) Maximum quiescent supply current is only 120A. Figures 2 and 3 show the standard application circuits for these devices. In these configurations, the IC is powered from the total differential voltage between the input (V+) and output (VOUT). The principal benefit of this arrangement is that it applies the largest available signal to the gate of the internal P-channel power MOSFET. This increased gate drive lowers switch on-resistance and increases DC-DC converter efficiency. Since the voltage on the LX pin swings from V+ (when the switch is ON) to IVOUTI plus a diode drop (when the
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-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ DC-DC Inverters
switch is OFF), the range of input and output voltages is limited to a 21V absolute maximum differential voltage. When output voltages more negative than -16V are required, substitute the MAX764/MAX765/MAX766 with Maxim's MAX774/MAX775/MAX776 or MAX1774, which use an external switch.
VIN
PFM Control Scheme
The MAX764/MAX765/MAX766 use a proprietary, current-limited PFM control scheme that blends the best features of PFM and PWM devices. It combines the ultra-low supply currents of traditional pulse-skipping PFM converters with the high full-load efficiencies of current-mode pulse-width modulation (PWM) converters. This control scheme allows the devices to achieve high efficiencies over a wide range of loads, while the current-sense function and high operating frequency allow the use of miniature external components. As with traditional PFM converters, the internal power MOSFET is turned on when the voltage comparator senses that the output is out of regulation (Figure 1). However, unlike traditional PFM converters, switching is accomplished through the combination of a peak current limit and a pair of one-shots that set the maximum on-time (16s) and minimum off-time (2.3s) for the switch. Once off, the minimum off-time one-shot holds the switch off for 2.3s. After this minimum time, the switch either 1) stays off if the output is in regulation, or 2) turns on again if the output is out of regulation. The MAX764/MAX765/MAX766 limit the peak inductor current, which allows them to run in continuous-conduction mode and maintain high efficiency with heavy loads. (See the photo Continuous Conduction at Full Current Limit in the Typical Operating Characteristics.) This current-limiting feature is a key component of the control circuitry. Once turned on, the switch stays on until either 1) the maximum on-time one shot turns it off (16s later), or 2) the current limit is reached. To increase light-load efficiency, the current limit is set to half the peak current limit for the first two pulses. If those pulses bring the output voltage into regulation, the voltage comparator holds the MOSFET off and the current limit remains at half the peak current limit. If the output voltage is still out of regulation after two pulses, the current limit is raised to its 0.75A peak for the next pulse. (See the photo Discontinuous Conduction at Half and Full Current Limit in the Typical Operating Characteristics.)
MAX764/MAX765/MAX766
1 C1 120F 20V C2 0.1F 3
OUT
V+
7
SHDN
MAX764 MAX765 MAX766
V+
6
2 4 C3 0.1F
FB REF GND 5
LX
8
D1 1N5817
VOUT
L1 47H
C4 68F 20V
PRODUCT MAX764 MAX765 MAX766
OUTPUT VOLTAGE (V) -5 -12 -15
INPUT VOLTAGE (V) 3 to 15 3 to 8 3 to 5
Figure 2. Fixed Output Voltage Operation
VIN C1 120F 20V C2 0.1F
1
OUT
V+
7
R2
3
SHDN MAX764
V+
6
2 R1 4 C3 0.1F
MAX765 MAX766
FB LX
8
D1 1N5817
VOUT -1V to -16V
Shutdown Mode
When SHDN is high, the MAX764/MAX765/MAX766 enter a shutdown mode in which the supply current drops to less than 5A. In this mode, the internal biasing circuitry (including the reference) is turned off and OUT discharges to ground. SHDN is a TTL/CMOS-logic level input. Connect SHDN to GND for normal operation. With a current-limited supply, power-up the device while unloaded or in shutdown mode (hold SHDN high until V+ exceeds 3.0V) to save power and reduce power-up current surges. (See the Supply Current vs. Supply Voltage graph in the Typical Operating Characteristics.)
9
REF GND 5 L1 47H C4 68F 20V
Figure 3. Adjustable Output Voltage Operation
_______________________________________________________________________________________
-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ DC-DC Inverters MAX764/MAX765/MAX766
Modes of Operation
When delivering high output currents, the MAX764/ MAX765/MAX766 operate in continuous-conduction mode. In this mode, current always flows in the inductor, and the control circuit adjusts the duty-cycle of the switch on a cycle-by-cycle basis to maintain regulation without exceeding the switch-current capability. This provides excellent load-transient response and high efficiency. In discontinuous-conduction mode, current through the inductor starts at zero, rises to a peak value, then ramps down to zero on each cycle. Although efficiency is still excellent, the output ripple may increase slightly.
Diode Selection
The MAX764/MAX765/MAX766's high switching frequency demands a high-speed rectifier. Use a Schottky diode with a 0.75A average current rating, such as the 1N5817 or 1N5818. High leakage currents may make Schottky diodes inadequate for high-temperature and light-load applications. In these cases you can use high-speed silicon diodes, such as the MUR105 or the EC11FS1. At heavy loads and high temperatures, the benefits of a Schottky diode's low forward voltage may outweigh the disadvantages of its high leakage current.
Capacitor Selection
Output Filter Capacitor The primary criterion for selecting the output filter capacitor (C4) is low effective series resistance (ESR). The product of the inductor-current variation and the output filter capacitor's ESR determines the amplitude of the high-frequency ripple seen on the output voltage. A 68F, 20V Sanyo OS-CON capacitor with ESR = 45m (SA series) typically provides 50mV ripple when converting from 5V to -5V at 150mA. Output filter capacitor ESR also affects efficiency. To obtain optimum performance, use a 68F or larger, low-ESR capacitor with a voltage rating of at least 20V. The smallest low-ESR surface-mount tantalum capacitors currently available are from the Sprague 595D series. Sanyo OS-CON series organic semiconductors and AVX TPS series tantalum capacitors also exhibit very low ESR. OS-CON capacitors are particularly useful at low temperatures. Table 1 lists some suppliers of low-ESR capacitors. For best results when using capacitors other than those suggested in Table 1 (or their equivalents), increase the output filter capacitor's size or use capacitators in parallel to reduce ESR. Input Bypass Capacitor The input bypass capacitor, C1, reduces peak currents drawn from the voltage source and reduces the amount of noise at the voltage source caused by the switching action of the MAX764-MAX766. The input voltage source impedance determines the size of the capacitor required at the V+ input. As with the output filter capacitor, a low-ESR capacitor is highly recommended. For output currents up to 250mA, a 100F to 120F capacitor with a voltage rating of at least 20V (C1) in parallel with a 0.1F capacitor (C2) is adequate in most applications. C2 must be placed as close as possible to the V+ and GND pins.
__________________Design Procedure
Setting the Output Voltage
The MAX764/MAX765/MAX766's output voltage can be adjusted from -1.0V to -16V using external resistors R1 and R2, configured as shown in Figure 3. For adjustable-output operation, select feedback resistor R1 = 150k. R2 is given by: VOUT R2 = (R1) ------ VREF
where VREF = 1.5V. For fixed-output operation, tie FB to REF.
II
Inductor Selection
In both continuous- and discontinuous-conduction modes, practical inductor values range from 22H to 68H. If the inductor value is too low, the current in the coil will ramp up to a high level before the current-limit comparator can turn off the switch, wasting power and reducing efficiency. The maximum inductor value is not critical. A 47H inductor is ideal for most applications. For highest efficiency, use a coil with low DC resistance, preferably under 100m. To minimize radiated noise, use a toroid, pot core, or shielded coil. Inductors with a ferrite core or equivalent are recommended. The inductor's incremental saturation-current rating should be greater than the 0.75A peak current limit. It is generally acceptable to bias the inductor into saturation by approximately 20% (the point where the inductance is 20% below the nominal value). Table 1 lists inductor types and suppliers for various applications. The listed surface-mount inductors' efficiencies are nearly equivalent to those of the largersize through-hole inductors.
10
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-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ DC-DC Inverters
Reference Capacitor Bypass REF with a 0.1F capacitor (C3). The REF output can source up to 100A for external loads.
output filter capacitor ground lead to a single point (star ground configuration). Also minimize lead lengths to reduce stray capacitance, trace resistance, and radiated noise. In particular, keep the traces connected to FB and LX short. C2 must be placed as close as possible to the V+ and GND pins. If an external resistor divider is used (Figure 3), the trace from FB to the resistors must be extremely short.
MAX764/MAX765/MAX766
Layout Considerations
Proper PC board layout is essential to reduce noise generated by high current levels and fast switching waveforms. Minimize ground noise by connecting GND, the input bypass capacitor ground lead, and the
Table 1. Component Suppliers
PRODUCTION METHOD INDUCTORS Sumida CD75/105 series Surface Mount Coiltronics CTX series Coilcraft DT/D03316 series Miniature Through-Hole Low-Cost Through-Hole SUPPLIER AVX Coilcraft Coiltronics Matsuo Motorola Nichicon Nihon Renco Sanyo OS-CON Sprague Electric Co. Sumida USA: USA: USA: Sumida RCH895 series Renco RL1284 series PHONE (803) 448-9411 (708) 639-6400 (407) 241-7876 CAPACITORS Matsuo 267 series Sprague 595D/293D series AVX TPS series Sanyo OS-CON series (very low ESR) Nichicon PL series FAX (803) 448-1943 (708) 639-1469 (407) 241-9339 (714) 960-6492 81-6-337-6456 (602) 952-4190 (708) 843-2798 81-7-5256-4158 (805) 867-2556 81-3-3494-7414 (516) 586-5562 (619) 661-1055 81-7-2070-1174 (603) 224-1430 (708) 956-0702 81-3-3607-5144 DIODES
Nihon EC10QS02L (Schottky) EC11FS1 (high-speed silicon)
Motorola 1N5817, 1N5818, (Schottky) MUR105 (high-speed silicon)
USA: (714) 969-2491 Japan: 81-6-337-6450 USA: (800) 521-6274
USA: (708) 843-7500 Japan: 81-7-5231-8461 USA: (805) 867-2555 Japan: 81-3-3494-7411 USA: (516) 586-5566
USA: (619) 661-6835 Japan: 81-7-2070-1005 USA: (603) 224-1961
USA: (708) 956-0666 Japan: 81-3-3607-5111
______________________________________________________________________________________
11
-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ DC-DC Inverters MAX764/MAX765/MAX766
_Ordering Information (continued)
PART MAX766CPA MAX766CSA MAX766C/D MAX766EPA MAX766ESA MAX766MJA TEMP. RANGE 0C to +70C 0C to +70C 0C to +70C -40C to +85C -40C to +85C -55C to +125C PIN-PACKAGE 8 Plastic DIP 8 SO Dice* 8 Plastic DIP 8 SO 8 CERDIP**
OUT
___________________Chip Topography
LX
* Dice are tested at TA = +25C, DC parameters only. **Contact factory for availability and processing to MIL-STD-883.
0.145" (3683m) V+
FB
V+ SHDN
REF
GND 0.080" (2032m)
TRANSISTOR COUNT: 443 SUBSTRATE CONNECTED TO V+
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.
12 __________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600 (c) 1994 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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