General Description The MAX764 MAX765 MAX766 inverting switching regu lators are highly efficient over a wide range of load cur rents delivering up to 5W A unique current limited pulse frequency modulated PFM control scheme com bines 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 A of supply current vs 2mA to 10mA for a PWM device The input voltage range is 3V to 16V The output volt age 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 maxi mum 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 stan dard H 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 capabil i ty or higher output voltages use the MAX774 MAX775 MAX776 or MAX1774 which drive an external power P channel MOSFET for loads up to 5W

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Description

Niveau: Supérieur, Doctorat, Bac+8
_______________General Description The MAX764/MAX765/MAX766 inverting switching regu- lators are highly efficient over a wide range of load cur- rents, delivering up to 1.5W. A unique, current-limited, pulse-frequency-modulated (PFM) control scheme com- bines 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 volt- age 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 maxi- mum 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 stan- dard 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.

  • load current

  • limited

  • off time

  • supply voltage

  • dc-dc inverters

  • pfm converters

  • toffminimum switch


Sujets

Informations

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19-0176; Rev 0; 6/94

-5V/-12V/-15V or Adjustable,
High-Efficiency, Low I
Q
DC-DC Inverters
_______________General Description____________________________Features
The MAX764/MAX765/MAX766 inverting switching regu-
©
High Efficiency for a Wide Range of Load Currents
lators are highly efficient over a wide range of load cur-
rents, delivering up to 1.5W. A unique, current-limited,
©
250mA Output Current
pulse-frequency-modulated (PFM) control scheme com-
©
120µA Max Supply Current
bines the benefits of traditional PFM converters with the
benefits of pulse-width-modulated (PWM) converters.
©
5µA Max Shutdown Current
Like PWM converters, the MAX764/MAX765/MAX766 are
©
3V to 16V Input Voltage Range
highly efficient at heavy loads. Yet because they are PFM
devices, they use less than 120µA of supply current (vs.
©
-5V (MAX764), -12V (MAX765), -15V (MAX766),
2mA to 10mA for a PWM device).
or Adjustable Output from -1V to -16V
The input voltage range is 3V to 16V. The output volt-
©
Current-Limited PFM Control Scheme
age is preset at -5V (MAX764), -12V (MAX765), or -15V
(MAX766); it can also be adjusted from -1V to -16V
©
300kHz Switching Frequency
using two external resistors (Dual Mode
TM
). The maxi-
©
Internal, P-Channel Power MOSFET
mum operating V
IN
- V
OUT
differential is 20V.
These devices use miniature external components; their
______________Ordering Information
high switching frequencies (up to 300kHz) allow for less
than 5mm diameter surface-mount magnetics. A stan-
PARTTEMP. RANGEPIN-PACKAGE
dard 47µH inductor is ideal for most applications, so no
MAX764
CPA0°C to +70°C8 Plastic DIP
magnetics design is necessary.
MAX764CSA0°C to +70°C8 SO
An internal power MOSFET makes the MAX764/MAX765/
MAX764C/D0°C to +70°CDice*
MAX766ideal for minimum component count, low- and
MAX764EPA-40°C to +85°C8 Plastic DIP
medium-power applications. For increased output drive
MAX764ESA-40°C to +85°C8 SO
capability or higher output voltages, use the
MAX764MJA-55°C to +125°C8 CERDIP**
MAX774/MAX775/MAX776 or MAX1774, which drive an
external power P-channel MOSFET for loads up to 5W.
MAX765
CPA0°C to +70°C8 Plastic DIP
MAX765CSA0°C to +70°C8 SO
________________________Applications
MAX765C/D0°C to +70°CDice*
LCD-Bias Generators
MAX765EPA-40°C to +85°C8 Plastic DIP
Portable Instruments
MAX765ESA-40°C to +85°C8 SO
LAN Adapters
MAX765MJA-55°C to +125°C8 CERDIP**
Remote Data-Acquisition Systems
Ordering Information continued on last page.
Battery-Powered Applications
*** CDoicntea acrt ef atectsoteryd faotr Ta
A
va=il a+b2ili5t°y Ca, nDd Cp rpoacreasmsientegr tso oMnlIyL.-STD-883.
__________Typical Operating Circuit__________________Pin Configuration
3VI NTPOU 1T 5V
TOP VIEW
V+
OUTPUT
LX
-5V
OUT
18
LX
MAX764
FB
27
V+
ON/OFFSHDN47µHSHDN
3
MMAAXX776654
6
V+
REF
4
MAX766
5
GND
REFOUT
FBGND
DIP/SO

________________________________________________________________Maxim Integrated Products1
Call toll free 1-800-998-8800 for free samples or literature.

-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IDC-DC Inverters
QABSOLUTE MAXIMUM RATINGS
V+ to GND..............................................................-0.3V to +17VOperating Temperature Ranges
OUT to GND...........................................................+0.5V to -17VMAX76_C_A ........................................................0°C to +70°C
Maximum Differential (V+ to OUT) ......................................+21VMAX76_E_A .....................................................-40°C to +85°C
REF, SHDN, FB to GND ...............................-0.3V to (V+ + 0.3V)MAX76_MJA ..................................................-55°C to +125°C
LX to V+..................................................................+0.3V to -21VMaximum Junction Temperatures
LX Peak Current ...................................................................1.5AMAX76_C_A/E_A ..........................................................+150°C
Continuous Power Dissipation (T= +70°C)MAX76_MJA .................................................................+175°C
APlastic DIP (derate 9.09mW/°C above +70°C) ............727mWStorage Temperature Range ............................-65°C to +160°C
SO (derate 5.88mW/°C above +70°C) .........................471mWLead Temperature (soldering, 10sec) ............................+300°C
CERDIP (derate 8.00mW/°C above +70°C) .................640mW
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, I
LOAD
= 0mA, C
REF
= 0.1µF, T
A
= T
MIN
to T
MAX
, unless otherwise noted. Typical values are at T
A
= +25°C.)
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITS
V+ Input Voltage RangeV+MAX76_C/E3.016.0V
MAX76_M3.5
Supply CurrentI
S
V+ = 16V, SHDN < 0.4V90120
V+ = 16V, SHDN > 1.6V2µA
Shutdown CurrentI
SHDN
V+ = 10V, SHDN > 1.6V15
FB Trip Point3V
²
V+
²
16V-1010mV
MAX76_C±50
FB Input CurrentI
FB
MAX76_E±70nA
MAX76_M±90
MAX764, -4.8V
²
V
OUT
²
5.2V150260
Output Current and Voltage I
OUT
MAX765C/E, -11.52V
²
V
OUT
²
12.48V68120mA
(Note 1)MAX765M, -11.52V
²
V
OUT
²
12.48V50120
MAX766, -14.40V
²
V
OUT
²
-15.60V35105
MAX76_C1.47001.51.5300
Reference VoltageV
REF
MAX76_E1.46251.51.5375V
MAX76_M1.45501.51.5450
REF Load Regulation0µA
²
I
REF
²
100µAMAX76_C/E410mV
MAX76_M415
REF Line Regulation3V
²
V+
²
16V40100µV/V
Load Regulation (Note 2)0mA
²
I
LOAD
²
100mA0.008%/mA
Line Regulation (Note 2)4V
²
V+
²
6V0.12%/V
Efficiency (Note 2)10mA
²
I
LOAD
²
100mA,V
OUT
= -5V80%
V
IN
= 5VV
OUT
= -15V82
SHDN Leakage CurrentV+ = 16V, SHDN = 0V or V+±1µA
SHDN Input Voltage HighV
IH
3V
²
V+
²
16V1.6V
SHDN Input Voltage LowV
IL
3V
²
V+
²
16V0.4V

2_______________________________________________________________________________________

-5V/-12V/-15V or Adjustable,
High-Efficiency, Low I
Q
DC-DC Inverters
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 5V, I
LOAD
= 0mA, C
REF
= 0.1µF, T
A
= T
MIN
to T
MAX
, unless otherwise noted. Typical values are at T
A
= +25°C.)
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITS
MAX76_C±5
LX Leakage Current
I
LX
I
+ (V+)
²
20VMAX76_E±10µA
MAX76_M±30
LX On-Resistance
I
V
OUT
I
+ (V+)
³
10V1.42.5
½
Peak Current at LXI
PEAK
I
V
OUT
I
+ (V+)
³
10V0.50.75A
Maximum Switch On-Timet
ON
121620µs
Minimum Switch Off-Timet
OFF
1.82.32.8µs
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, V= -5V, T= +25°C, unless otherwise noted.)
ATUO

MAX764 MAX765 MAX766
EFFICIENCY vs. LOAD CURRENTEFFICIENCY vs. LOAD CURRENTEFFICIENCY vs. LOAD CURRENT
100100100
90V+ = 5V90V+ = 8V90
808080
7070V+ = 5V70V+ = 5V
60V+ = 10V6060
505050
404040
30V+ = 15V3030
202020
10CIRCUIT OF FIGURE 2 10CIRCUIT OF FIGURE 2 10CIRCUIT OF FIGURE 2
V
OUT
= -5V ±4%V
OUT
= -12V ±4%V
OUT
= -15V ±4%
0000.111010010000.111010010000.11101001000
LOAD CURRENT (mA)LOAD CURRENT (mA)LOAD CURRENT (mA)

_______________________________________________________________________________________3

-5V/-12V/-15V or Adjustable,
High-Efficiency, Low I
Q
DC-DC Inverters
____________________________Typical Operating Characteristics (continued)
(V+ = 5V, V
OUT
= -5V, T
A
= +25°C, unless otherwise noted.)
MAXIMUM OUTPUT CURRENT NO-LOAD SUPPLY CURRENT NO-LOAD SUPPLY CURRENT
vs. SUPPLY VOLTAGEvs. SUPPLY VOLTAGEvs. TEMPERATURE
600100110
CIRCUIT OF FIGURE 2105
50095100
V
OUT
= -5V9095V+ = 15V
4008590
583008080
7575V+ = 5V
07002V
OUT
= -12V7065
06001V
OUT
= -15V6555
05060345678910111213141516345678910111213141516-60-40-20020406080100120140
SUPPLY VOLTAGE (V)SUPPLY VOLTAGE (V)TEMPERATURE (°C)
SHUTDOWN CURRENT MAXIMUM SWITCH ON-TIME MINIMUM SWITCH OFF-TIME
vs. TEMPERATUREvs. TEMPERATUREvs. TEMPERATURE
4.017.02.60
3.516.82.55
3.016.62.50
2.516.4V+ = 15V2.45V+ = 15V
2.612.0V+ = 15V16.02.40
1.515.82.35V+ = 5V
V+ = 8V
1.015.6V+ = 5V2.30
4.510.515.22.25
0V+ = 4V15.02.20
-60-40-20020406080100120140-60-40-20020406080100120140-60-40-20020406080100120140
TEMPERATURE (°C)TEMPERATURE (°C)TEMPERATURE (°C)
SWITCH ON/OFF-TIME RATIO START-UP SUPPLY VOLTAGE LX LEAKAGE CURRENT
vs. TEMPERATUREvs. OUTPUT CURRENTvs. TEMPERATURE
7.2810,000
7.17CIRCUIT OF FIGURE 2
I
V
OUT
I
+ (V+) = 20V
7.061000
9.658.66.74100
6.636.5V+ = 5V
6.4210
13.6102.6-60-40-200204060801001201400501001502002503002030405060708090100110120130
TEMPERATURE (°C)OUTPUT CURRENT (mA)TEMPERATURE (°C)
4_______________________________________________________________________________________

-5V/-12V/-15V or Adjustable,
High-Efficiency, Low I
Q
DC-DC Inverters
____________________________Typical Operating Characteristics (continued)
(V+ = 5V, V
OUT
= -5V, T
A
= +25°C, unless otherwise noted.)

LX ON-RESISTANCE PEAK CURRENT AT LX REFERENCE OUTPUT RESISTANCE
vs. TEMPERATUREvs. TEMPERATUREvs. TEMPERATURE
2.20.95250
2.00.90200
I
V
OUT
I
+ (V+) = 10V
I
V
OUT
I
+ (V+) = 20V I
REF
= 10
m
A
1.80.85
1.6
I
V
OUT
I
+ (V+) = 15V150
I
V
OUT
I
+ (V+) = 15V0.80
1.4100 I
REF
= 50
m
A
1.20.75
1.00.7050
I
V
OUT
I
+ (V+) = 20V
I
V
OUT
I
+ (V+) = 10V I
REF
= 100
m
A
0.80.650
-60-40-20020406080100120140-60-40-20020406080100120140-60-40-20020406080100120140
TEMPERATURE (°C)TEMPERATURE (°C)TEMPERATURE (°C)
REFERENCE OUTPUT SUPPLY CURRENT
vs. TEMPERATUREvs. SUPPLY VOLTAGE
1.5061000
405.1100I
LOAD
= 100mA
205.11.50010
1.4981
1.496I
LOAD
= 0mA
1.4940.1
1.492CIRCUIT OF FIGURE 2
10.00246810121416
SUPPLY VOLTAGE (V)

-60-40-20020406080100120140
TEMPERATURE (°C)

_______________________________________________________________________________________5

-5V/-12V/-15V or Adjustable,
High-Efficiency, Low I
Q
DC-DC Inverters
____________________________Typical Operating Characteristics (continued)
(V+ = 5V, V
OUT
= -5V, T
A
= +25°C, unless otherwise noted.)

TIME TO ENTER/EXIT SHUTDOWN

V0

V0

V0

A

B2ms/div
CIRCUIT OF FIGURE 2, V+ = 5V, I
LOAD
= 100mA, V
OUT
= -5V
A: V
OUT
, 2V/div
B: SHUTDOWN PULSE, 0V TO 5V, 5V/div

LINE-TRANSIENT RESPONSE

5ms/div
CIRCUIT OF FIGURE 2, V
OUT
= -5V, I
LOAD
= 100mA
BA:: VV+
O
,
U
5
T
,V 5T0Om 1V0/dVi, v5, VA/Cdi-vCOUPLED

AB

LOAD-TRANSIENT RESPONSE

m0A5ms/div
CIRCUIT OF FIGURE 2, V+ = 5V, V
OUT
= -5V
A: V
OUT
, 50mV/div, AC-COUPLED
B: I
LOAD
, 0mA TO 100mA, 100mA/div

DHISACLFO NATNIDN UFOUULLS CCUORNRDEUNCTT ILOINM IATT

0AV0

5
m
s/div
CIRCUIT OF FIGURE 2, V+ = 5V, V
OUT
= -5V, I
LOAD
= 140mA
A: OUTPUT RIPPLE, 100mV/div
B: INDUCTOR CURRENT, 500mA/div
C: LX WAVEFORM, 10V/div

6_______________________________________________________________________________________

A

B

ABC

-5V/-12V/-15V or Adjustable,
High-Efficiency, Low I
Q
DC-DC Inverters
____________________________Typical Operating Characteristics (continued)
(V+ = 5V, V
OUT
= -5V, T
A
= +25°C, unless otherwise noted.)
DISCONTINUOUS CONDUCTION AT CONTINUOUS CONDUCTION AT
HALF CURRENT LIMITFULL CURRENT LIMIT

A0

V0

AB

C

5
m
s/div
CIRCUIT OF FIGURE 2, V+ = 5V, V
OUT
= -5V, I
LOAD
= 80mA
A: OUTPUT RIPPLE, 100mV/div
B: INDUCTOR CURRENT, 500mA/div
C: LX WAVEFORM, 10V/div

A0

V0

5
m
s/div
CIRCUIT OF FIGURE 2, V+ = 5V, V
OUT
= -5V, I
LOAD
= 240mA
A: OUTPUT RIPPLE, 100mV/div
B: INDUCTOR CURRENT, 500mA/div
C: LX WAVEFORM, 10V/div

AB

C

______________________________________________________________Pin Description
PINNAMEFUNCTION
1OUTSense Input for Fixed-Output Operation (V
FB
= V
REF
). OUT must be connected to V
OUT
.
2FBFeedback Input. Connect FB to REF to use the internal voltage divider for a preset output. For adjustable-
output operation, use an external voltage divider, as described in the section Setting the Output Voltage.
3SHDNActive-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.
4REF1.5V Reference Output that can source 100µA for external loads. Bypass to ground with a 0.1µF capacitor.
5GNDGround
6, 7V+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.
8LXDrain of the Internal P-Channel Power MOSFET. LX has a peak current limit of 0.75A.

_______________________________________________________________________________________7

-H5iVg/h--1E2fVfi/-c1ie5nVc oy,r LAodjwu Ist
Q
aDblCe-,DC Inverters
BFCOMPARATOR

FERSNDH

COMPAREARTROORR

MAX764
MAX765
MAX766

NREFE1.R5EVN CE
QGIRTONE-SHOT
FROM V+
QSTRIGQCOMPCAURRARTEONRT FROM OUT
RONE-SHOT

P

TUO

+V+V

XL

0.2V 0.1V
CURRENT (FULL (HALF
CONTROL CIRCUITSCURRENT)CURRENT)
FROM V+
DNGFigure 1. Block Diagram
_______________Detailed Description
1)They can operate with miniature (less than 5mm
diameter) surface-mount inductors, because of their
Operating Principle
300kHz switching frequency.
The MAX764/MAX765/MAX766 are BiCMOS, inverting,2)The current-limited PFM control scheme allows efficien-
switch-mode power supplies that provide fixed outputscies exceeding 80% over a wide range of load currents.
of -5V, -12V, and -15V, respectively; they can also be
set to any desired output voltage using an external3)Maximum quiescent supply current is only 120µA.
resistor divider. Their unique control scheme combinesFigures 2 and 3 show the standard application circuits
the advantages of pulse-frequency modulation (pulsefor these devices. In these configurations, the IC is
skipping) and pulse-width modulation (continuous puls-powered from the total differential voltage between the
ing). The internal P-channel power MOSFET allowsinput (V+) and output (V
OUT
). The principal benefit of
peak currents of 0.75A, increasing the output currentthis arrangement is that it applies the largest available
capability over previous pulse-frequency-modulationsignal to the gate of the internal P-channel power MOS-
(PFM) devices. Figure 1 shows the MAX764/MAX765/FET. This increased gate drive lowers switch on-resis-
MAX766 block diagram.tance and increases DC-DC converter efficiency.
The MAX764/MAX765/MAX766 offer three mainSince the voltage on the LX pin swings from V+ (when the
improvements over prior solutions: switch is ON) to
I
V
OUT
I
plus a diode drop (when the
8_______________________________________________________________________________________

-5V/-12V/-15V or Adjustable,
High-Efficiency, Low I
Q
DC-DC Inverters
switch is OFF), the range of input and output voltages is
PFM Control Scheme
limited to a 21V absolute maximum differential voltage.The MAX764/MAX765/MAX766 use a proprietary, cur-
When output voltages more negative than -16V arerent-limited PFM control scheme that blends the best
required, substitute the MAX764/MAX765/MAX766 withfeatures of PFM and PWM devices. It combines the
Maxim’s MAX774/MAX775/MAX776 or MAX1774, whichultra-low supply currents of traditional pulse-skipping
use an external switch.PFM converters with the high full-load efficiencies of
current-mode pulse-width modulation (PWM) convert-
ers. This control scheme allows the devices to achieve
V
IN
high efficiencies over a wide range of loads, while the
current-sense function and high operating frequency
17
allow the use of miniature external components.
C1 C2 OUTV+
As with traditional PFM converters, the internal power
120
m
F 0.1
m
F
MOSFET is turned on when the voltage comparator
20V3SHDN
MAX764
V+6
senses that the output is out of regulation (Figure 1).
MAX765
However, unlike traditional PFM converters, switching is
MAX766
accomplished through the combination of a peak cur-
2FBLX81N5D81 17
rent limit and a pair of one-shots that set the maximum
V
OUT
on-time (16µs) and minimum off-time (2.3µs) for the
4
switch. Once off, the minimum off-time one-shot holds
REF
the switch off for 2.3µs. After this minimum time, the
C3 GNDL1 68
m
CF4
switch either 1) stays off if the output is in regulation, or
0.1
m
F547
m
H20V
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-con-
OUTPUT INPUT
duction mode and maintain high efficiency with heavy
PRODUCTVOLTAGE (V)VOLTAGE (V)
loads. (See the photo Continuous Conduction at Full
MAX764 -5 3 to 15
Current Limit in the Typical Operating Characteristics.)
MAX765 -12 3 to 8
This current-limiting feature is a key component of the
control circuitry. Once turned on, the switch stays on
MAX766-153 to 5
until either 1) the maximum on-time one shot turns it off
Figure 2. Fixed Output Voltage Operation
(16µs later), or 2) the current limit is reached.
To increase light-load efficiency, the current limit is set to
V
IN
half the peak current limit for the first two pulses. If those
pulses bring the output voltage into regulation, the volt-
C1
age comparator holds the MOSFET off and the current
12200
m
VF 0C.21
m
F1OUTV+7
limit remains at half the peak current limit. If the output
voltage is still out of regulation after two pulses, the cur-
rent limit is raised to its 0.75A peak for the next pulse.
36
(See the photo Discontinuous Conduction at Half and Full
R2SHDN
MMAAXX776654
V+
Current Limit in the Typical Operating Characteristics.)
2
MAX766
8D1 V
OUT
Shutdown Mode
FBLX1N5817--11V6 tVo
When SHDN is high, the MAX764/MAX765/MAX766
enter a shutdown mode in which the supply current
R1
drops to less than 5µA. In this mode, the internal biasing
4REFGNDC4
circuitry (including the reference) is turned off and OUT
C3 547
m
LH1 68
m
F
discharges to ground. SHDN is a TTL/CMOS-logic level
0.1
m
F20V
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 cur-
rent surges. (See the Supply Current vs. Supply Voltage
Figure 3. Adjustable Output Voltage Operation
graph in the Typical Operating Characteristics.)
_______________________________________________________________________________________9

-5V/-12V/-15V or Adjustable,
High-Efficiency, Low I
Q
DC-DC Inverters
Modes of OperationDiode Selection
When delivering high output currents, the MAX764/The MAX764/MAX765/MAX766’s high switching fre-
MAX765/MAX766 operate in continuous-conductionquency demands a high-speed rectifier. Use a
mode. In this mode, current always flows in the induc-Schottky diode with a 0.75A average current rating,
tor, and the control circuit adjusts the duty-cycle of thesuch as the 1N5817 or 1N5818. High leakage currents
switch on a cycle-by-cycle basis to maintain regulationmay make Schottky diodes inadequate for high-temper-
without exceeding the switch-current capability. Thisature and light-load applications. In these cases you
provides excellent load-transient response and highcan use high-speed silicon diodes, such as the
efficiency.MUR105 or the EC11FS1. At heavy loads and high
In discontinuous-conduction mode, current through thetemperatures, the benefits of a Schottky diode’s low for-
inductor starts at zero, rises to a peak value, thenward voltage may outweigh the disadvantages of its
ramps down to zero on each cycle. Although efficiencyhigh leakage current.
is still excellent, the output ripple may increase slightly.
Capacitor Selection
__________________Design Procedure
Output Filter Capacitor
The primary criterion for selecting the output filter
Setting the Output Voltage
capacitor (C4) is low effective series resistance (ESR).
The MAX764/MAX765/MAX766’s output voltage can beThe product of the inductor-current variation and the
adjusted from -1.0V to -16V using external resistors R1output filter capacitor’s ESR determines the amplitude
and R2, configured as shown in Figure 3. Forof the high-frequency ripple seen on the output voltage.
adjustable-output operation, select feedback resistorA 68µF, 20V Sanyo OS-CON capacitor with ESR =
R1 = 150k
½
. R2 is given by:45m
½
(SA series) typically provides 50mV ripple when
V
OUT
converting from 5V to -5V at 150mA.
R2 = (R1) ——— Output filter capacitor ESR also affects efficiency. To
V
REF
obtain optimum performance, use a 68µF or larger,
where V
REF
= 1.5V.low-ESR capacitor with a voltage rating of at least
For fixed-output operation, tie FB to REF. 20V. The smallest low-ESR surface-mount tantalum
capacitors currently available are from the Sprague
Inductor Selection
595D series. Sanyo OS-CON series organic semi-
In both continuous- and discontinuous-conductionconductors and AVX TPS series tantalum capacitors
modes, practical inductor values range from 22µH toalso exhibit very low ESR. OS-CON capacitors are
68µH. If the inductor value is too low, the current in theparticularly useful at low temperatures. Table 1 lists
coil will ramp up to a high level before the current-limitsome suppliers of low-ESR capacitors.
comparator can turn off the switch, wasting power andFor best results when using capacitors other than those
reducing efficiency. The maximum inductor value is notsuggested in Table 1 (or their equivalents), increase
critical. A 47µH inductor is ideal for most applications.the output filter capacitor’s size or use capacitators in
For highest efficiency, use a coil with low DC resis-parallel to reduce ESR.
tance, preferably under 100m
½
. To minimize radiated
noise, use a toroid, pot core, or shielded coil.
Input Bypass Capacitor
Inductors with a ferrite core or equivalent are recom-The input bypass capacitor, C1, reduces peak currents
mended. The inductor’s incremental saturation-currentdrawn from the voltage source and reduces the amount
rating should be greater than the 0.75A peak currentof noise at the voltage source caused by the switching
limit. It is generally acceptable to bias the inductor intoaction of the MAX764–MAX766. The input voltage
saturation by approximately 20% (the point where thesource impedance determines the size of the capacitor
inductance is 20% below the nominal value).required at the V+ input. As with the output filter
capacitor, a low-ESR capacitor is highly recommended.
Table 1 lists inductor types and suppliers for variousFor output currents up to 250mA, a 100µF to 120µF
applications. The listed surface-mount inductors’ effi-capacitor with a voltage rating of at least 20V (C1) in
ciencies are nearly equivalent to those of the larger-parallel with a 0.1µF capacitor (C2) is adequate in most
size through-hole inductors.applications.
C2 must be placed as close as possi-
ble to the V+ and GND pins.

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