19-0176; Rev 0; 6/94
it
K
tion
lua able
a
v
E
il
Ava
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ DC-DC Inverters
____________________________Features
♦ High Efficiency for a Wide Range of Load Currents
________________________Applications
LCD-Bias Generators
Portable Instruments
LAN Adapters
Remote Data-Acquisition Systems
Battery-Powered Applications
♦ 250mA Output Current
♦ 120µA Max Supply Current
♦ 5µA Max Shutdown Current
♦ 3V to 16V Input Voltage Range
♦ -5V (MAX764), -12V (MAX765), -15V (MAX766),
or Adjustable Output from -1V to -16V
♦ Current-Limited PFM Control Scheme
♦ 300kHz Switching Frequency
♦ Internal, P-Channel Power MOSFET
______________Ordering Information
PART
TEMP. RANGE
PIN-PACKAGE
MAX764CPA
0°C to +70°C
8 Plastic DIP
MAX764CSA
MAX764C/D
MAX764EPA
MAX764ESA
MAX764MJA
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
8 SO
Dice*
8 Plastic DIP
8 SO
8 CERDIP**
MAX765CPA
0°C to +70°C
8 Plastic DIP
MAX765CSA
MAX765C/D
MAX765EPA
MAX765ESA
MAX765MJA
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
8 SO
Dice*
8 Plastic DIP
8 SO
8 CERDIP**
Ordering Information continued on last page.
* Dice are tested at TA = +25°C, DC parameters only.
**Contact factory for availability and processing to MIL-STD-883.
__________Typical Operating Circuit
INPUT
3V TO 15V
__________________Pin Configuration
TOP VIEW
V+
OUTPUT
-5V
LX
MAX764
ON/OFF
47µH
SHDN
OUT
1
8
LX
FB
2
7
V+
6
V+
5
GND
SHDN 3
REF 4
MAX764
MAX765
MAX766
OUT
REF
FB
GND
DIP/SO
________________________________________________________________ Maxim Integrated Products
Call toll free 1-800-998-8800 for free samples or literature.
1
MAX764/MAX765/MAX766
_______________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 120µA 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 47µH 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
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ DC-DC Inverters
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 = +70°C)
Plastic DIP (derate 9.09mW/°C above +70°C) ............727mW
SO (derate 5.88mW/°C above +70°C) .........................471mW
CERDIP (derate 8.00mW/°C above +70°C) .................640mW
Operating Temperature Ranges
MAX76_C_A ........................................................0°C to +70°C
MAX76_E_A .....................................................-40°C to +85°C
MAX76_MJA ..................................................-55°C to +125°C
Maximum Junction Temperatures
MAX76_C_A/E_A ..........................................................+150°C
MAX76_MJA .................................................................+175°C
Storage Temperature Range ............................-65°C to +160°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
(V+ = 5V, ILOAD = 0mA, CREF = 0.1µF, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
V+ Input Voltage Range
V+
Supply Current
IS
Shutdown Current
ISHDN
Output Current and Voltage
(Note 1)
IFB
IOUT
MAX76_M
3.5
VREF
TYP
MAX
16.0
V+ = 16V, SHDN < 0.4V
90
V+ = 16V, SHDN > 1.6V
2
V+ = 10V, SHDN > 1.6V
1
-10
µA
10
MAX76_E
±70
MAX76_M
±90
150
260
MAX765C/E, -11.52V ≤ VOUT ≤ 12.48V
68
120
MAX765M, -11.52V ≤ VOUT ≤ 12.48V
50
120
V
5
±50
MAX764, -4.8V ≤ VOUT ≤ 5.2V
UNITS
120
MAX76_C
MAX766, -14.40V ≤ VOUT ≤ -15.60V
Reference Voltage
MIN
3.0
3V ≤ V+ ≤ 16V
FB Trip Point
FB Input Current
CONDITIONS
MAX76_C/E
mV
nA
mA
35
105
MAX76_C
1.4700
1.5
1.5300
MAX76_E
1.4625
1.5
1.5375
MAX76_M
1.4550
1.5
1.5450
MAX76_C/E
4
10
MAX76_M
4
15
40
100
V
REF Load Regulation
0µA ≤ IREF ≤ 100µA
REF Line Regulation
3V ≤ V+ ≤ 16V
Load Regulation (Note 2)
0mA ≤ ILOAD ≤ 100mA
0.008
%/mA
Line Regulation (Note 2)
4V ≤ V+ ≤ 6V
0.12
%/V
Efficiency (Note 2)
10mA ≤ ILOAD ≤ 100mA, VOUT = -5V
VIN = 5V
VOUT = -15V
SHDN Leakage Current
VIH
3V ≤ V+ ≤ 16V
SHDN Input Voltage Low
VIL
3V ≤ V+ ≤ 16V
±1
1.6
_______________________________________________________________________________________
µV/V
%
82
V+ = 16V, SHDN = 0V or V+
SHDN Input Voltage High
2
80
mV
µA
V
0.4
V
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ DC-DC Inverters
(V+ = 5V, ILOAD = 0mA, CREF = 0.1µF, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
CONDITIONS
ILXI + (V+) ≤ 20V
LX Leakage Current
LX On-Resistance
Peak Current at LX
IPEAK
MIN
TYP
MAX
MAX76_C
±5
MAX76_E
±10
MAX76_M
±30
IVOUTI + (V+) ≥ 10V
IVOUTI + (V+) ≥ 10V
1.4
0.5
0.75
UNITS
µA
2.5
Ω
A
Maximum Switch On-Time
tON
12
16
20
µs
Minimum Switch Off-Time
tOFF
1.8
2.3
2.8
µs
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 = +25°C, unless otherwise noted.)
MAX765
EFFICIENCY vs. LOAD CURRENT
100
80
70
70
V+ = 10V
50
40
V+ = 15V
40
20
20
CIRCUIT OF FIGURE 2
VOUT = -5V ±4%
0
V+ = 5V
50
30
10
80
60
30
1
10
100
LOAD CURRENT (mA)
1000
70
V+ = 5V
60
50
40
30
20
CIRCUIT OF FIGURE 2
VOUT = -12V ±4%
10
0
0.1
90
EFFICIENCY (%)
80
60
V+ = 8V
90
100
MAX764-02
V+ = 5V
EFFICIENCY (%)
EFFICIENCY (%)
90
MAX764-01
100
MAX766
EFFICIENCY vs. LOAD CURRENT
MAX764-03
MAX764
EFFICIENCY vs. LOAD CURRENT
CIRCUIT OF FIGURE 2
VOUT = -15V ±4%
10
0
0.1
1
10
100
LOAD CURRENT (mA)
1000
0.1
1
10
100
1000
LOAD CURRENT (mA)
_______________________________________________________________________________________
3
MAX764/MAX765/MAX766
ELECTRICAL CHARACTERISTICS (continued)
____________________________Typical Operating Characteristics (continued)
(V+ = 5V, VOUT = -5V, TA = +25°C, unless otherwise noted.)
NO-LOAD SUPPLY CURRENT
vs. SUPPLY VOLTAGE
400
300
200
VOUT = -12V
100
VOUT = -15V
0
80
75
70
65
MAX764 -06
90
85
80
75
70
V+ = 5V
65
60
55
50
3 4 5 6 7 8 9 10 11 12 13 14 15 16
SUPPLY VOLTAGE (V)
TEMPERATURE (°C)
SHUTDOWN CURRENT
vs. TEMPERATURE
MAXIMUM SWITCH ON-TIME
vs. TEMPERATURE
MINIMUM SWITCH OFF-TIME
vs. TEMPERATURE
2.0
1.5
V+ = 8V
1.0
16.6
16.4
V+ = 15V
16.2
16.0
15.8
15.6
V+ = 5V
15.4
2.60
15.2
V+ = 4V
0 20 40 60 80 100 120 140
-60 -40 -20
TEMPERATURE (°C)
SWITCH ON/OFF-TIME RATIO
vs. TEMPERATURE
6.9
6.8
6.7
6.6
V+ = 5V
6.4
6.3
6.2
0 20 40 60 80 100 120 140
TEMPERATURE (°C)
2.40
V+ = 5V
2.35
2.30
2.25
0 20 40 60 80 100 120 140
TEMPERATURE (°C)
START-UP SUPPLY VOLTAGE
vs. OUTPUT CURRENT
LX LEAKAGE CURRENT
vs. TEMPERATURE
8
CIRCUIT OF FIGURE 2
7
6
5
4
3
2
10,000
IVOUTI + (V+) = 20V
1000
100
10
1
1
0
-60 -40 -20
V+ = 15V
2.45
TEMPERATURE (°C)
LX LEAKAGE CURRENT (nA)
7.0
2.50
-60 -40 -20
0 20 40 60 80 100 120 140
MAX764 -11
7.1
START-UP SUPPLY VOLTAGE (V)
MAX764 -10
7.2
2.55
2.20
15.0
-60 -40 -20
0 20 40 60 80 100 120 140
MAX764 -09
16.8
-60 -40 -20
MINIMUM SWITCH OFF-TIME (µs)
V+ = 15V
17.0
MAXIMUM SWITCH ON-TIME (µs)
MAX764 -07
2.5
0.5
4
V+ = 15V
95
SUPPLY VOLTAGE (V)
3.0
6.5
100
3 4 5 6 7 8 9 10 11 12 13 14 15 16
3.5
SHUTDOWN CURRENT (µA)
85
60
4.0
0
90
110
105
MAX764-12
VOUT = -5V
95
MAX764 -08
500
100
MAX764 -05
CIRCUIT OF FIGURE 2
NO-LOAD SUPPLY CURRENT (µA)
MAX764 -04
MAXIMUM OUTPUT CURRENT (mA)
600
NO-LOAD SUPPLY CURRENT
vs. TEMPERATURE
NO-LOAD SUPPLY CURRENT (µA)
MAXIMUM OUTPUT CURRENT
vs. SUPPLY VOLTAGE
SWITCH ON/OFF-TIME RATIO (µs/µs)
MAX764/MAX765/MAX766
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ DC-DC Inverters
0
50
100
150
200
OUTPUT CURRENT (mA)
250
300
20 30 40 50 60 70 80 90 100 110 120 130
TEMPERATURE (°C)
_______________________________________________________________________________________
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ DC-DC Inverters
PEAK CURRENT AT LX
vs. TEMPERATURE
0.90
CURRENT AT LX (A)
IVOUTI + (V+) = 10V
1.8
1.6
IVOUTI + (V+) = 15V
1.4
1.2
IVOUTI + (V+) = 20V
0.85
IVOUTI + (V+) = 15V
0.80
0.75
0.70
1.0
IVOUTI + (V+) = 10V
IVOUTI + (V+) = 20V
250
200
0 20 40 60 80 100 120 140
IREF = 50µA
100
50
IREF = 100µA
0
-60 -40 -20
TEMPERATURE (°C)
0 20 40 60 80 100 120 140
-60 -40 -20
TEMPERATURE (°C)
SUPPLY CURRENT
vs. SUPPLY VOLTAGE
SUPPLY CURRENT (mA)
1.504
1.502
1.500
1.498
1.496
MAX764-17
1000
MAX764 -16
1.506
0 20 40 60 80 100 120 140
TEMPERATURE (°C)
REFERENCE OUTPUT
vs. TEMPERATURE
REFERENCE OUTPUT (V)
-60 -40 -20
IREF = 10µA
150
0.65
0.8
MAX764 -15
MAX764 -14
2.0
LX ON-RESISTANCE (Ω)
0.95
MAX764 -13
2.2
REFERENCE OUTPUT RESISTANCE
vs. TEMPERATURE
REFERENCE OUTPUT RESISTANCE (Ω)
LX ON-RESISTANCE
vs. TEMPERATURE
ILOAD = 100mA
100
10
1
ILOAD = 0mA
0.1
1.494
CIRCUIT OF FIGURE 2
1.492
-60 -40 -20
0 20 40 60 80 100 120 140
TEMPERATURE (°C)
0.01
0
2
4
6
8
10
12
14
16
SUPPLY VOLTAGE (V)
_______________________________________________________________________________________
5
MAX764/MAX765/MAX766
____________________________Typical Operating Characteristics (continued)
(V+ = 5V, VOUT = -5V, TA = +25°C, unless otherwise noted.)
MAX764/MAX765/MAX766
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ DC-DC Inverters
____________________________Typical Operating Characteristics (continued)
(V+ = 5V, VOUT = -5V, TA = +25°C, unless otherwise noted.)
TIME TO ENTER/EXIT SHUTDOWN
LOAD-TRANSIENT RESPONSE
0V
A
A
B
0V
B
0mA
2ms/div
CIRCUIT OF FIGURE 2, V+ = 5V, ILOAD = 100mA, VOUT = -5V
A: VOUT, 2V/div
B: SHUTDOWN PULSE, 0V TO 5V, 5V/div
5ms/div
CIRCUIT OF FIGURE 2, V+ = 5V, VOUT = -5V
A: VOUT, 50mV/div, AC-COUPLED
B: ILOAD, 0mA TO 100mA, 100mA/div
DISCONTINUOUS CONDUCTION AT
HALF AND FULL CURRENT LIMIT
LINE-TRANSIENT RESPONSE
A
A
B
0A
B
0V
C
0V
5ms/div
CIRCUIT OF FIGURE 2, VOUT = -5V, ILOAD = 100mA
A: VOUT, 50mV/div, AC-COUPLED
B: V+, 5V TO 10V, 5V/div
6
5µs/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
_______________________________________________________________________________________
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ DC-DC Inverters
DISCONTINUOUS CONDUCTION AT
HALF CURRENT LIMIT
CONTINUOUS CONDUCTION AT
FULL CURRENT LIMIT
A
A
B
B
0A
0A
0V
C
5µs/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
0V
C
5µs/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
NAME
FUNCTION
1
OUT
2
FB
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.
3
SHDN
Active-High Shutdown Input. With SHDN high, the part is in shutdown mode and the supply current is less
than 5µA. Connect to ground for normal operation.
4
REF
1.5V Reference Output that can source 100µA for external loads. Bypass to ground with a 0.1µF capacitor.
5
GND
Ground
6, 7
V+
Positive Power-Supply Input. Must be tied together. Place a 0.1µF input bypass capacitor as close to
the V+ and GND pins as possible.
8
LX
Drain of the Internal P-Channel Power MOSFET. LX has a peak current limit of 0.75A.
Sense Input for Fixed-Output Operation (VFB = VREF). OUT must be connected to VOUT.
_______________________________________________________________________________________
7
MAX764/MAX765/MAX766
____________________________Typical Operating Characteristics (continued)
(V+ = 5V, VOUT = -5V, TA = +25°C, unless otherwise noted.)
MAX764/MAX765/MAX766
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ DC-DC Inverters
FB
COMPARATOR
MAX764
MAX765
MAX766
REF
SHDN
ERROR
COMPARATOR
OUT
V+
N
V+
1.5V
REFERENCE
Q
TRIG
ONE-SHOT
FROM V+
S
TRIG
Q
Q
R
P
CURRENT
COMPARATOR
FROM OUT
LX
ONE-SHOT
0.2V
(FULL
CURRENT)
CURRENT
CONTROL CIRCUITS
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 120µA.
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
_______________________________________________________________________________________
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ DC-DC Inverters
VIN
1
C1
120µF
20V
7
V+
OUT
C2
0.1µF
3
2
4
SHDN
MAX764
MAX765
MAX766
FB
6
V+
D1
1N5817
8
LX
VOUT
REF
C4
68µF
20V
GND
C3
0.1µF
L1
47µH
5
PRODUCT
OUTPUT
VOLTAGE (V)
INPUT
VOLTAGE (V)
MAX764
MAX765
MAX766
-5
-12
-15
3 to 15
3 to 8
3 to 5
Figure 2. Fixed Output Voltage Operation
VIN
C1
120µF
20V
C2
0.1µF
R2
1
3
2
V+
OUT
SHDN MAX764
V+
MAX765
MAX766
FB
LX
7
6
8
D1
1N5817
R1
4
REF
GND
C3
0.1µF
5
L1
47µH
Figure 3. Adjustable Output Voltage Operation
C4
68µF
20V
VOUT
-1V to
-16V
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 (16µs) and minimum off-time (2.3µs) for the
switch. Once off, the minimum off-time one-shot holds
the switch off for 2.3µs. 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
(16µs 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.)
Shutdown Mode
When SHDN is high, the MAX764/MAX765/MAX766
enter a shutdown mode in which the supply current
drops to less than 5µA. 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
MAX764/MAX765/MAX766
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.
MAX764/MAX765/MAX766
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ DC-DC Inverters
Modes of Operation
Diode Selection
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.
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.
__________________Design Procedure
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 68µF, 20V Sanyo OS-CON capacitor with ESR =
45mΩ (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 68µF 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.
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 = 150kΩ. R2 is given by:
I I
VOUT
R2 = (R1) ———
VREF
where VREF = 1.5V.
For fixed-output operation, tie FB to REF.
Inductor Selection
In both continuous- and discontinuous-conduction
modes, practical inductor values range from 22µH to
68µH. 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 47µH inductor is ideal for most applications.
For highest efficiency, use a coil with low DC resistance, preferably under 100mΩ. 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
Capacitor Selection
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 100µF to 120µF
capacitor with a voltage rating of at least 20V (C1) in
parallel with a 0.1µF capacitor (C2) is adequate in most
applications. C2 must be placed as close as possible to the V+ and GND pins.
______________________________________________________________________________________
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ DC-DC Inverters
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
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.
Table 1. Component Suppliers
PRODUCTION METHOD
INDUCTORS
CAPACITORS
Sumida
CD75/105 series
Matsuo
267 series
Coiltronics
CTX series
Sprague
595D/293D series
Coilcraft
DT/D03316 series
AVX
TPS series
Miniature Through-Hole
Sumida
RCH895 series
Sanyo
OS-CON series (very low ESR)
Low-Cost Through-Hole
Renco
RL1284 series
Nichicon
PL series
Surface Mount
SUPPLIER
PHONE
DIODES
Nihon
EC10QS02L (Schottky)
EC11FS1 (high-speed silicon)
Motorola
1N5817, 1N5818, (Schottky)
MUR105 (high-speed silicon)
FAX
AVX
USA:
(803) 448-9411
(803) 448-1943
Coilcraft
USA:
(708) 639-6400
(708) 639-1469
Coiltronics
USA:
(407) 241-7876
(407) 241-9339
Matsuo
USA:
(714) 969-2491
Japan: 81-6-337-6450
(714) 960-6492
81-6-337-6456
Motorola
USA:
(800) 521-6274
(602) 952-4190
Nichicon
USA:
(708) 843-7500
Japan: 81-7-5231-8461
(708) 843-2798
81-7-5256-4158
Nihon
USA:
(805) 867-2555
Japan: 81-3-3494-7411
(805) 867-2556
81-3-3494-7414
Renco
USA:
(516) 586-5566
(516) 586-5562
Sanyo OS-CON
USA:
(619) 661-6835
Japan: 81-7-2070-1005
(619) 661-1055
81-7-2070-1174
Sprague Electric Co.
USA:
(603) 224-1961
(603) 224-1430
Sumida
USA:
(708) 956-0666
Japan: 81-3-3607-5111
(708) 956-0702
81-3-3607-5144
______________________________________________________________________________________
11
MAX764/MAX765/MAX766
Reference Capacitor
Bypass REF with a 0.1µF capacitor (C3). The REF output can source up to 100µA for external loads.
MAX764/MAX765/MAX766
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ DC-DC Inverters
_Ordering Information (continued)
PART
TEMP. RANGE
___________________Chip Topography
PIN-PACKAGE
MAX766CPA
0°C to +70°C
8 Plastic DIP
MAX766CSA
MAX766C/D
MAX766EPA
MAX766ESA
MAX766MJA
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
8 SO
Dice*
8 Plastic DIP
8 SO
8 CERDIP**
LX
OUT
* Dice are tested at TA = +25°C, DC parameters only.
**Contact factory for availability and processing to MIL-STD-883.
0.145"
(3683µm)
V+
FB
V+
SHDN
REF
GND
0.080"
(2032µm)
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
© 1994 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
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