Maxim MAX766EPA, MAX766ESA, MAX766MJA, MAX765MJA, MAX766C-D Datasheet

19-0176; Rev 0; 6/94

Evaluation Kit

Available

High-Efficiency, Low IQDC-DC Inverters

-5V/-12V/-15V or Adjustable,

_______________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- V

differential is 20V.

OUT

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.

________________________Applications

LCD-Bias Generators
Portable Instruments
LAN Adapters
Remote Data-Acquisition Systems
Battery-Powered Applications

__________Typical Operating Circuit

____________________________Features

♦ High Efficiency for a Wide Range of Load Currents
♦ 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

MAX764CPA

MAX764CSA
MAX764C/D 0°C to +70°C
MAX764EPA
MAX764ESA -40°C to +85°C
MAX764MJA -55°C to +125°C 8 CERDIP**
MAX765CPA
MAX765CSA
MAX765C/D 0°C to +70°C
MAX765EPA
MAX765ESA -40°C to +85°C
MAX765MJA -55°C to +125°C 8 CERDIP**

Ordering Information continued on last page.

* Dice are tested at T
**Contact factory for availability and processing to MIL-STD-883.

TEMP. RANGE PIN-PACKAGE

0°C to +70°C
0°C to +70°C

-40°C to +85°C 8 Plastic DIP

0°C to +70°C 8 Plastic DIP
0°C to +70°C

-40°C to +85°C 8 Plastic DIP

= +25°C, DC parameters only.

A

8 Plastic DIP
8 SO
Dice*

8 SO

8 SO
Dice*

8 SO

__________________Pin Configuration

MAX764/MAX765/MAX766

INPUT

3V TO 15V

ON/OFF

V+

LX

MAX764

SHDN

REF

FB

________________________________________________________________

OUT

GND

47µH

OUTPUT

-5V

TOP VIEW

OUT

SHDN

REF

1

FB

2

MAX764
MAX765

3

MAX766

4

DIP/SO

Maxim Integrated Products

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

LX

8

V+

7

V+

6

GND

5

1

-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQDC-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 (T

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

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.

= +70°C)

A

ELECTRICAL CHARACTERISTICS

(V+ = 5V, I

V+ Input Voltage Range
Supply Current
Shutdown Current

MAX764/MAX765/MAX766

FB Input Current

Output Current and Voltage
(Note 1)

Reference Voltage

REF Load Regulation

Efficiency (Note 2)

SHDN Input Voltage High
SHDN Input Voltage Low

LOAD

= 0mA, C

= 0.1µF, TA= T

REF

to T

MIN

MAX76_C/E

V+ V

MAX76_M
V+ = 16V, SHDN < 0.4V

S

I

SHDN

I

V

V+ = 16V, SHDN > 1.6V
V+ = 10V, SHDN > 1.6V
3V ≤ V+ ≤ 16V
MAX76_C
MAX76_E

I

FB

MAX76_M
MAX764, -4.8V ≤ V
MAX765C/E, -11.52V ≤ V

OUT

MAX765M, -11.52V ≤ V
MAX766, -14.40V ≤ V
MAX76_C
MAX76_E

REF

MAX76_M
0µA ≤ I
3V ≤ V+ ≤ 16V

0mA ≤ I
4V ≤ V+ ≤ 6V

10mA ≤ I
VIN= 5V

V+ = 16V, SHDN = 0V or V+
3V ≤ V+ ≤ 16V

IH

3V ≤ V+ ≤ 16V V0.4V

IL

, unless otherwise noted. Typical values are at TA= +25°C.)

MAX

≤ 100µA

REF

LOAD

LOAD

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

CONDITIONS

3.0 16.0

3.5
90 120I

2
15

OUT

≤ 100mA

≤ 100mA,

≤ 5.2V

OUT

OUT

≤ -15.60V

OUT

MAX76_C/E
MAX76_M

V

≤ 12.48V

≤ 12.48V

= -5V

OUT

= -15V

OUT

150 260

68 120
50 120
35 105

1.4700 1.5 1.5300

1.4625 1.5 1.5375 V

1.4550 1.5 1.5450
410
415

80
82V

±50
±70
±90

UNITSMIN TYP MAXSYMBOLPARAMETER

µA

mV-10 10FB Trip Point

nA

mA

mV

µV/V40 100REF Line Regulation

%/mA0.008Load Regulation (Note 2)

%/V0.12Line Regulation (Note 2)

%

µA±1SHDN Leakage Current

V1.6V

2 _______________________________________________________________________________________

-5V/-12V/-15V or Adjustable,

High-Efficiency, Low IQDC-DC Inverters

ELECTRICAL CHARACTERISTICS (continued)

(V+ = 5V, I

LX Leakage Current

Peak Current at LX
Maximum Switch On-Time
Minimum Switch Off-Time

Note 1: See Maximum Output Current vs. Supply Voltage graph in the
Note 2: Circuit of Figure 2.

LOAD

= 0mA, C

= 0.1µF, TA= T

REF

MIN

to T

, unless otherwise noted. Typical values are at TA= +25°C.)

MAX

CONDITIONS

MAX76_C

+ (V+) ≤ 20V

ILXI

MAX76_E
MAX76_M

V

+ (V+) ≥ 10V

OUT

I

I

V

PEAK

ON

OFF

OUT

I

+ (V+) ≥ 10V

I

Typical Operating Characteristics

correlation to switch on-time, switch off-time, on-resistance, and peak current rating.

UNITSMIN TYP MAXSYMBOLPARAMETER

±5
±10 µA
±30

Ω1.4 2.5LX On-Resistance

A0.5 0.75I
µs12 16 20t
µs1.8 2.3 2.8t

. Guarantees are based on

__________________________________________Typical Operating Characteristics

(V+ = 5V, V

100

90
80
70
60
50
40

EFFICIENCY (%)

30
20
10

0

= -5V, TA= +25°C, unless otherwise noted.)

OUT

EFFICIENCY vs. LOAD CURRENT

V+ = 5V

0.1 10 1000

MAX764

V+ = 10V

V+ = 15V

CIRCUIT OF FIGURE 2

= -5V ±4%

V

OUT

1 100

LOAD CURRENT (mA)

MAX764-01

EFFICIENCY vs. LOAD CURRENT

100

V+ = 8V

90
80
70
60
50
40

EFFICIENCY (%)

30
20
10

0

0.1 10 1000

MAX765

V+ = 5V

CIRCUIT OF FIGURE 2

= -12V ±4%

V

OUT

1 100

LOAD CURRENT (mA)

MAX764-02

EFFICIENCY vs. LOAD CURRENT

100

90
80
70
60
50
40

EFFICIENCY (%)

30
20
10

0

0.1 10 1000

MAX766

V+ = 5V

CIRCUIT OF FIGURE 2

= -15V ±4%

V

OUT

1 100

LOAD CURRENT (mA)

MAX764/MAX765/MAX766

MAX764-03

_______________________________________________________________________________________ 3

-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQDC-DC Inverters

____________________________Typical Operating Characteristics (continued)

(V+ = 5V, V

= -5V, TA= +25°C, unless otherwise noted.)

OUT

MAXIMUM OUTPUT CURRENT

600

500

400

300

200

100

MAXIMUM OUTPUT CURRENT (mA)

0

4.0

3.5

MAX764/MAX765/MAX766

3.0

2.5

2.0

1.5

1.0

SHUTDOWN CURRENT (µA)

0.5
0

7.2

7.1

7.0

6.9

6.8

6.7

6.6

6.5

6.4

SWITCH ON/OFF-TIME RATIO (µs/µs)

6.3

6.2

vs. SUPPLY VOLTAGE

CIRCUIT OF FIGURE 2

V

= -5V

OUT

V

= -12V

OUT

V

= -15V

OUT

3

4 5 6 7 8 9 10 11 12 13 14 15 16

SUPPLY VOLTAGE (V)

SHUTDOWN CURRENT

vs. TEMPERATURE

V+ = 15V

V+ = 8V

V+ = 4V

-60

-40 -20 0 20 40 60 80 100 120 140
TEMPERATURE (°C)

SWITCH ON/OFF-TIME RATIO

vs. TEMPERATURE

V+ = 5V

-60

-40 -20 0 20 40 60 80 100 120 140
TEMPERATURE (°C)

MAX764 -04

NO-LOAD SUPPLY CURRENT (µA)

MAX764 -07

MAXIMUM SWITCH ON-TIME (µs)

MAX764 -10

START-UP SUPPLY VOLTAGE (V)

NO-LOAD SUPPLY CURRENT

100

95
90
85
80
75
70
65
60

17.0

16.8

16.6

16.4

16.2

16.0

15.8

15.6

15.4

15.2

15.0

vs. SUPPLY VOLTAGE

3

4 5 6 7 8 9 10 11 12 13 14 15 16

SUPPLY VOLTAGE (V)

MAXIMUM SWITCH ON-TIME

vs. TEMPERATURE

V+ = 5V

-60

-40 -20 0 20 40 60 80 100 120 140
TEMPERATURE (°C)

START-UP SUPPLY VOLTAGE

8
7
6
5
4
3
2
1
0

vs. OUTPUT CURRENT

CIRCUIT OF FIGURE 2

0

50 100 150 200 250 300

OUTPUT CURRENT (mA)

V+ = 15V

110
105

MAX764 -05

100

95
90
85
80
75
70
65
60

NO-LOAD SUPPLY CURRENT (µA)

55
50

-60

-40 -20 0 20 40 60 80 100 120 140

2.60

2.55

MAX764 -08

2.50

2.45

2.40

2.35

2.30

MINIMUM SWITCH OFF-TIME (µs)

2.25

2.20

-60

10,000

MAX764 -11

1000

100

10

LX LEAKAGE CURRENT (nA)

1

20

NO-LOAD SUPPLY CURRENT

vs. TEMPERATURE

V+ = 15V

V+ = 5V

TEMPERATURE (°C)

MINIMUM SWITCH OFF-TIME

vs. TEMPERATURE

V+ = 15V

V+ = 5V

-40 -20 0 20 40 60 80 100 120 140
TEMPERATURE (°C)

LX LEAKAGE CURRENT

vs. TEMPERATURE

IV

I + (V+) = 20V

OUT

30 40 50 60 70 80 90 100 110 120 130

TEMPERATURE (°C)

MAX764 -06

MAX764 -09

MAX764-12

4 _______________________________________________________________________________________

-5V/-12V/-15V or Adjustable,

MAX764

14

0

High-Efficiency, Low IQDC-DC Inverters

____________________________Typical Operating Characteristics (continued)

(V+ = 5V, V

2.2

2.0

1.8

1.6

1.4

1.2

LX ON-RESISTANCE (Ω)

1.0

0.8

= -5V, TA= +25°C, unless otherwise noted.)

OUT

LX ON-RESISTANCE
vs. TEMPERATURE

IV

I + (V+) = 10V

OUT

IV

I + (V+) = 15V

OUT

IV

I + (V+) = 20V

OUT

-60

-40 -20 0 20 40 60 80 100 120 140
TEMPERATURE (°C)

REFERENCE OUTPUT

1.506

1.504

1.502

1.500

1.498

1.496

REFERENCE OUTPUT (V)

1.494

1.492

-60

vs. TEMPERATURE

-40 -20 0 20 40 60 80 100 120 140
TEMPERATURE (°C)

0.95

MAX764 -13

0.90

0.85

0.80

0.75

CURRENT AT LX (A)

0.70

0.65

PEAK CURRENT AT LX

vs. TEMPERATURE

IV

I + (V+) = 20V

OUT

IV

I + (V+) = 15V

OUT

IV

I + (V+) = 10V

OUT

-40 -20 0 20 40 60 80 100 120 14

-60
TEMPERATURE (°C)

MAX764 -16

1000

100

10

1

SUPPLY CURRENT (mA)

0.1

0.01

REFERENCE OUTPUT RESISTANCE

250

-

200

150

100

50

REFERENCE OUTPUT RESISTANCE (Ω)

0

-60

SUPPLY CURRENT

vs. SUPPLY VOLTAGE

0 2 4 6 10 12 14816

SUPPLY VOLTAGE (V)

vs. TEMPERATURE

I

= 10µA

REF

I

= 50µA

REF

I

= 100µA

REF

-40 -20 0 20 40 60 80 100 120 140
TEMPERATURE (°C)

I

= 100mA

LOAD

I

= 0mA

LOAD

CIRCUIT OF FIGURE 2

MAX764-17

MAX764/MAX765/MAX766

MAX764 -15

_______________________________________________________________________________________

5

-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQDC-DC Inverters

____________________________Typical Operating Characteristics (continued)

(V+ = 5V, V

= -5V, TA= +25°C, unless otherwise noted.)

OUT

TIME TO ENTER/EXIT SHUTDOWN

0V

A

0V

CIRCUIT OF FIGURE 2, V+ = 5V, I
A: V

, 2V/div

OUT

B: SHUTDOWN PULSE, 0V TO 5V, 5V/div

2ms/div

LOAD

= 100mA, V

OUT

B

= -5V

MAX764/MAX765/MAX766

LINE-TRANSIENT RESPONSE

A

B

0V

LOAD-TRANSIENT RESPONSE

0mA

CIRCUIT OF FIGURE 2, V+ = 5V, V
A: V

, 50mV/div, AC-COUPLED

OUT

B: I

, 0mA TO 100mA, 100mA/div

LOAD

DISCONTINUOUS CONDUCTION AT

HALF AND FULL CURRENT LIMIT

0A

0V

5ms/div

A

B

= -5V

OUT

A

B

C

CIRCUIT OF FIGURE 2, V
A: V

, 50mV/div, AC-COUPLED

OUT

B: V+, 5V TO 10V, 5V/div

OUT

5ms/div

= -5V, I

LOAD

= 100mA

CIRCUIT OF FIGURE 2, V+ = 5V, V
A: OUTPUT RIPPLE, 100mV/div
B: INDUCTOR CURRENT, 500mA/div
C: LX WAVEFORM, 10V/div

5µs/div

OUT

= -5V, I

6 _______________________________________________________________________________________

LOAD

= 140mA

-5V/-12V/-15V or Adjustable,

High-Efficiency, Low IQDC-DC Inverters

____________________________Typical Operating Characteristics (continued)

(V+ = 5V, V

0A

0V

= -5V, TA= +25°C, unless otherwise noted.)

OUT

DISCONTINUOUS CONDUCTION AT

HALF CURRENT LIMIT

CIRCUIT OF FIGURE 2, V+ = 5V, V
A: OUTPUT RIPPLE, 100mV/div
B: INDUCTOR CURRENT, 500mA/div
C: LX WAVEFORM, 10V/div

5µs/div

OUT

= -5V, I

LOAD

= 80mA

CONTINUOUS CONDUCTION AT

FULL CURRENT LIMIT

A

B

C

0A

0V

CIRCUIT OF FIGURE 2, V+ = 5V, V
A: OUTPUT RIPPLE, 100mV/div
B: INDUCTOR CURRENT, 500mA/div
C: LX WAVEFORM, 10V/div



5µs/div

= -5V, I

LOAD

= 240mA

OUT

A

B

C

______________________________________________________________Pin Description

PIN

Sense Input for Fixed-Output Operation (VFB= V

FB2

SHDN3

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

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.

1.5V Reference Output that can source 100µA for external loads. Bypass to ground with a 0.1µF capacitor.REF4
GroundGND5

V+6, 7

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.

Drain of the Internal P-Channel Power MOSFET. LX has a peak current limit of 0.75A.LX8

FUNCTIONNAME

). OUT must be connected to V

REF

.OUT1

OUT

Setting the Output Voltage.

MAX764/MAX765/MAX766

_______________________________________________________________________________________ 7

-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQDC-DC Inverters

FB

COMPARATOR

REF

SHDN

MAX764/MAX765/MAX766

Figure 1. Block Diagram

ERROR

COMPARATOR

TRIG Q

ONE-SHOT

ONE-SHOT

TRIGQ

SRQ

CURRENT

CONTROL CIRCUITS

_______________Detailed Description

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:

Operating Principle

MAX764
MAX765
MAX766

N

1.5V

REFERENCE

FROM V+

P

COMPARATOR

GND

CURRENT

FROM OUT

(FULL

CURRENT)

0.2V

0.1V
(HALF
CURRENT)

FROM V+

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 (V

). The principal benefit of

OUT

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 IV

plus a diode drop (when the

OUT

I

OUT

V+

V+

LX

8 _______________________________________________________________________________________

-5V/-12V/-15V or Adjustable,

High-Efficiency, Low IQDC-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.

V

IN

-12

-15

5

-5

GND

V+

V+

LX

47µH

7

6

8

L1

INPUT

VOLTAGE (V)

3 to 15

3 to 8
3 to 5

7

V+

6

V+

8

LX

L1

47µH

D1

1N5817

1N5817

V

OUT

C4

68µF

20V

V

OUT

D1

-1V to

-16V

C4

68µF

20V

1

OUT

C2

C1

120µF

20V

0.1µF
3

MAX764

SHDN

MAX765

MAX766

2

FB

4

REF

GND

5

OUTPUT

VOLTAGE (V)

0.1µF

C3

PRODUCT

MAX764
MAX765

MAX766

Figure 2. Fixed Output Voltage Operation

V

IN

C1

120µF

20V

C2

0.1µF

R2

1

OUT

3

MAX764

SHDN

MAX765

MAX766

2

FB

R1

4

REF

C3

0.1µF

Figure 3. Adjustable Output Voltage Operation

_______________________________________________________________________________________ 9

The MAX764/MAX765/MAX766 use a proprietary, cur-

PFM Control Scheme

rent-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

.)

MAX764/MAX765/MAX766

.)

-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQDC-DC Inverters

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.

__________________Design Procedure

Modes of Operation

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:

MAX764/MAX765/MAX766

where V
For fixed-output operation, tie FB to REF.

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.

REF

= 1.5V.

Setting the Output Voltage

V

R2 = (R1) I———

V

OUT
REF

I

Inductor Selection

The MAX764/MAX765/MAX766’s high switching fre-

Diode Selection

quency 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 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.

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 possi-

ble to the V+ and GND pins.

10 ______________________________________________________________________________________

-5V/-12V/-15V or Adjustable,

High-Efficiency, Low IQDC-DC Inverters

Bypass REF with a 0.1µF capacitor (C3). The REF out-

Reference Capacitor

put can source up to 100µA for external loads.

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 pos-
sible 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 DIODES

Sumida
CD75/105 series

Surface Mount

Miniature Through-Hole

Low-Cost Through-Hole

SUPPLIER PHONE FAX

AVX USA: [phone] [phone]
Coilcraft USA: [phone] [phone]
Coiltronics USA: [phone] [phone]

Matsuo
Motorola USA: [phone] [phone]
Nichicon

Nihon
Renco USA: [phone] [phone]
Sanyo OS-CON
Sprague Electric Co. USA: [phone] [phone]
Sumida

Coiltronics
CTX series

Coilcraft
DT/D03316 series

Sumida
RCH895 series

Renco
RL1284 series

USA: [phone]
Japan: 81-6-337-6450

USA: [phone]
Japan: 81-7-5231-8461

USA: [phone]
Japan: 81-3-3494-7411

USA: [phone]
Japan: 81-7-2070-1005

USA: [phone]
Japan: 81-3-3607-5111

Matsuo
267 series

Sprague
595D/293D series

AVX
TPS series

Sanyo
OS-CON series (very low ESR)

Nichicon
PL series

[phone]
81-6-337-6456

[phone]
81-7-5256-4158

[phone]
81-3-3494-7414

[phone]
81-7-2070-1174

[phone]
81-3-3607-5144

Nihon
EC10QS02L (Schottky)

EC11FS1 (high-speed silicon)

Motorola
1N5817, 1N5818, (Schottky)
MUR105 (high-speed silicon)

MAX764/MAX765/MAX766

______________________________________________________________________________________ 11

-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQDC-DC Inverters

_Ordering Information (continued) ___________________Chip Topography

PART

MAX766CPA

MAX766CSA
MAX766C/D 0°C to +70°C
MAX766EPA
MAX766ESA -40°C to +85°C
MAX766MJA -55°C to +125°C 8 CERDIP**

* Dice are tested at TA= +25°C, DC parameters only.
**Contact factory for availability and processing to MIL-STD-883.

TEMP. RANGE PIN-PACKAGE

0°C to +70°C
0°C to +70°C

-40°C to +85°C 8 Plastic DIP

8 Plastic DIP
8 SO
Dice*

8 SO

OUT

LX

0.145"

FB

(3683µm)

V+

MAX764/MAX765/MAX766

SHDN

REF

0.080"

(2032µm)

TRANSISTOR COUNT: 443
SUBSTRATE CONNECTED TO V+

V+

GND

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

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