Download datasheet for MAX764 by Maxim Integrated

Download datasheet for MAX764 by Maxim Integrated

19-0176; Rev 0; 6/94

Evaluation Kit

Available

-5 V /-1 2 V /-1 5 V or Adjust a ble ,

H igh-Effic ie nc y, Low I

Q

DC-DC I nve rt e rs

_______________Ge ne ra l De sc ript ion

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 Mode TM ). The maximum operating V

IN

- V

OUT 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 c ap ab ility or hig her outp ut voltag es, use the

MAX774/MAX775/MAX776 or MAX1774, which drive an external power P-channel MOSFET for loads up to 5W.

________________________Applic a t ions

LCD-Bias Generators

Portable Instruments

LAN Adapters

Remote Data-Acquisition Systems

Battery-Powered Applications

__________Typic a l Ope ra t ing Circ uit

INPUT

3V TO 15V

V+

LX

OUTPUT

-5V

ON/OFF SHDN

MAX764

47µH

____________________________Fe a t ure s

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

______________Orde ring I nform a t ion

PART

MAX764CPA

MAX764CSA

MAX764C/D

MAX764EPA

MAX764ESA

MAX764MJA

MAX765CPA

MAX765CSA

MAX765C/D

MAX765EPA

MAX765ESA

TEMP. RANGE

0°C to +70°C

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

0°C to +70°C

0°C to +70°C

0°C to +70°C

-40°C to +85°C

-40°C to +85°C

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

MAX765MJA -55°C to +125°C 8 CERDIP**

Ordering Information continued on last page.

* Dice are tested at T

A

= +25°C, DC parameters only.

**Contact factory for availability and processing to MIL-STD-883.

__________________Pin Configura t ion

TOP VIEW

OUT

1

FB

2

SHDN 3

REF

4

M AX764

M AX765

M AX766

6

5

8

7

LX

V+

V+

GND

REF

FB GND

OUT

DIP/SO

________________________________________________________________ Maxim Integrated Products 1

Ca ll t oll fre e 1 -8 0 0 -9 9 8 -8 8 0 0 for fre e sa m ple s or lit e ra t ure .

-5 V /-1 2 V /-1 5 V or Adjust a ble ,

H igh-Effic ie nc y, Low I

Q

DC-DC I nve rt e rs

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

A

= +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, 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.)

PARAMETER TYP

V+ Input Voltage Range

Supply Current

Shutdown Current

FB Trip Point

FB Input Current

Output Current and Voltage

(Note 1)

Reference Voltage

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

SYMBOL

I

I

V+

I

S

SHDN

I

FB

OUT

V

REF

V

V

IH

IL

CONDITIONS

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

V

OUT

5.2V

MAX765C/E, -11.52V

V

OUT

12.48V

MAX765M, -11.52V

MAX766, -14.40V

V

V

OUT

OUT

12.48V

-15.60V

MAX76_C

MAX76_E

MAX76_M

0µA

I

REF

100µA

MAX76_C/E

MAX76_M

3V

V+

16V

0mA

I

LOAD

100mA

4V

V+

6V

10mA

I

LOAD

V

IN

= 5V

100mA,

V

OUT

= -5V

V

OUT

= -15V

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

3V

V+

16V

3V

V+

16V

MIN

3.0

3.5

-10

150

68

50

35

1.4700

1.4625

1.4550

1.6

90

2

1

1.5

1.5

4

4

40

0.008

260

120

120

105

1.5

0.12

80

82

MAX

16.0

120

5

10

±50

±70

±90

1.5300

1.5375

1.5450

10

15

100

V mV

µV/V

%/mA

%/V

±1

0.4

UNITS

V

µA mV nA mA

%

µA

V

V

2 _______________________________________________________________________________________

-5 V /-1 2 V /-1 5 V or Adjust a ble ,

H igh-Effic ie nc y, Low I

Q

DC-DC I nve rt e rs

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.)

PARAMETER

LX Leakage Current

LX On-Resistance

Peak Current at LX

Maximum Switch On-Time

Minimum Switch Off-Time

SYMBOL

I

PEAK t

ON t

OFF

I

I

I LX I + (V+)

20V

CONDITIONS

MAX76_C

MAX76_E

MAX76_M

V

V

OUT

OUT

I

I

+ (V+)

+ (V+)

10V

10V

MIN

0.5

12

1.8

TYP

1.4

0.75

16

2.3

MAX

±5

±10

±30

2.5

20

2.8

UNITS

µA

A

µs

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.

__________________________________________Typic a l Ope ra t ing Cha ra c t e rist ic s

(V+ = 5V, V

OUT

= -5V, T

A

= +25°C, unless otherwise noted.)

M AX764

EFFICIENCY vs. LOAD CURRENT

100

90

80

70

60

50

40

30

20

10

0

0.1

V+ = 5V

V+ = 10V

V+ = 15V

CIRCUIT OF FIGURE 2

V

OUT

= -5V ±4%

1 10 100

LOAD CURRENT (m A)

1000

M AX765

EFFICIENCY vs. LOAD CURRENT

100

90

80

70

60

50

40

30

20

10

0

0.1

V+ = 8V

V+ = 5V

CIRCUIT OF FIGURE 2

V

OUT

= -12V ±4%

1 10 100

LOAD CURRENT (m A)

1000

100

90

80

70

60

50

40

30

20

10

0

0.1

M AX766

EFFICIENCY vs. LOAD CURRENT

V+ = 5V

1

CIRCUIT OF FIGURE 2

V

OUT

= -15V ±4%

10 100

LOAD CURRENT (m A)

1000

_______________________________________________________________________________________ 3

-5 V /-1 2 V /-1 5 V or Adjust a ble ,

H igh-Effic ie nc y, Low I

Q

DC-DC I nve rt e rs

____________________________Typic a l Ope ra t ing Cha ra c t e rist ic s (c ont inue d)

(V+ = 5V, V

OUT

= -5V, T

A

= +25°C, unless otherwise noted.)

M AXIM UM OUTPUT CURRENT vs. SUPPLY VOLTAGE

600

500

CIRCUIT OF FIGURE 2

V

OUT

= -5V

400

300

200

V

OUT

= -12V

100

V

OUT

= -15V

0

3 4 5 6 7 8 9 10 11 12 13 14 15 16

SUPPLY VOLTAGE (V)

SHUTDOWN CURRENT vs. TEM PERATURE

4.0

3.5

3.0

2.5

2.0

1.5

V+ = 15V

V+ = 8V

1.0

0.5

0

-60

V+ = 4V

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

TEM PERATURE (°C)

SWITCH ON/OFF-TIM E RATIO vs. TEM PERATURE

7.2

7.1

7.0

6.9

6.8

6.7

6.6

6.5

V+ = 5V

6.4

6.3

6.2

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

TEM PERATURE (°C)

NO-LOAD SUPPLY CURRENT vs. SUPPLY VOLTAGE

100

95

90

85

80

75

70

65

60

3 4 5 6 7 8 9 10 11 12 13 14 15 16

SUPPLY VOLTAGE (V)

M AXIM UM SWITCH ON-TIM E vs. TEM PERATURE

17.0

16.8

16.6

16.4

16.2

16.0

15.8

15.6

V+ = 15V

V+ = 5V

15.4

15.2

15.0

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

TEM PERATURE (°C)

START-UP SUPPLY VOLTAGE vs. OUTPUT CURRENT

4

3

2

8

7

6

5

1

0

0

CIRCUIT OF FIGURE 2

50 100 150 200

OUTPUT CURRENT (m A)

250 300

NO-LOAD SUPPLY CURRENT vs. TEM PERATURE

85

80

75

70

65

60

55

110

105

100

95

90

V+ = 15V

V+ = 5V

50

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

TEM PERATURE (°C)

M INIM UM SWITCH OFF-TIM E vs. TEM PERATURE

2.60

2.55

2.50

2.45

2.40

2.35

V+ = 5V

V+ = 15V

2.30

2.25

2.20

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

TEM PERATURE (°C)

LX LEAKAGE CURRENT vs. TEM PERATURE

10,000

I

V

OUT

I

+ (V+) = 20V

1000

100

10

1

20 30 40 50 60 70 80 90 100 110 120 130

TEM PERATURE (°C)

4 _______________________________________________________________________________________

-5 V /-1 2 V /-1 5 V or Adjust a ble ,

H igh-Effic ie nc y, Low I

Q

DC-DC I nve rt e rs

____________________________Typic a l Ope ra t ing Cha ra c t e rist ic s (c ont inue d)

(V+ = 5V, V

OUT

= -5V, T

A

= +25°C, unless otherwise noted.)

LX ON-RESISTANCE vs. TEM PERATURE

PEAK CURRENT AT LX vs. TEM PERATURE

REFERENCE OUTPUT RESISTANCE vs. TEM PERATURE

2.2

0.95

250

2.0

1.8

I

V

OUT

I

+ (V+) = 10V

0.90

I

V

OUT

I

+ (V+) = 20V

200

I

REF

= 10

µ

A

0.85

1.6

I

V

OUT

I

+ (V+) = 15V

1.4

0.80

I

V

OUT

I

+ (V+) = 15V

150

I

REF

= 50

µ

A

100

0.75

1.2

1.0

I

V

OUT

I

+ (V+) = 20V

0.8

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

TEM PERATURE (°C)

REFERENCE OUTPUT vs. TEM PERATURE

0.70

I

V

OUT

I

+ (V+) = 10V

0.65

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

TEM PERATURE (°C)

1.506

1000

50

I

REF

= 100

µ

A

0

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

TEM PERATURE (°C)

SUPPLY CURRENT vs. SUPPLY VOLTAGE

1.504

100

I

LOAD

= 100m A

1.502

1.500

10

1.498

1.496

1.494

1.492

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

TEM PERATURE (°C)

1

I

LOAD

= 0m A

0.1

0.01

0 2

CIRCUIT OF FIGURE 2

4 6 8 10 12 14

SUPPLY VOLTAGE (V)

16

_______________________________________________________________________________________ 5

-5 V /-1 2 V /-1 5 V or Adjust a ble ,

H igh-Effic ie nc y, Low I

Q

DC-DC I nve rt e rs

____________________________Typic a l Ope ra t ing Cha ra c t e rist ic s (c ont inue d)

(V+ = 5V, V

OUT

= -5V, T

A

= +25°C, unless otherwise noted.)

TIM E TO ENTER/EXIT SHUTDOWN LOAD-TRANSIENT RESPONSE

0V

A

A

0V

2ms/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

B

LINE-TRANSIENT RESPONSE

0V

5ms/div

CIRCUIT OF FIGURE 2, V

OUT

= -5V, I

LOAD

= 100mA

A: V

OUT

, 50mV/div, AC-COUPLED

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

A

B

0mA

5ms/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

DISCONTINUOUS CONDUCTION AT

HALF AND FULL CURRENT LIM IT

0A

0V

5

µ 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

C

A

B

B

6 _______________________________________________________________________________________

-5 V /-1 2 V /-1 5 V or Adjust a ble ,

H igh-Effic ie nc y, Low I

Q

DC-DC I nve rt e rs

____________________________Typic a l Ope ra t ing Cha ra c t e rist ic s (c ont inue d)

(V+ = 5V, V

OUT

= -5V, T

A

= +25°C, unless otherwise noted.)

DISCONTINUOUS CONDUCTION AT

HALF CURRENT LIM IT

CONTINUOUS CONDUCTION AT

FULL CURRENT LIM IT

A

B

A

B

0A

0V

C

0A

0V

C

5

µ 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

5

µ 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

3

4

5

6, 7

8

______________________________________________________________Pin De sc ript ion

PIN

1

2

NAME

OUT

FB

SHDN

REF

GND

V+

LX

FUNCTION

Sense Input for Fixed-Output Operation (V

FB

= V

REF

). OUT must be connected to V

OUT

.

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 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.

Ground

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.

_______________________________________________________________________________________ 7

-5 V /-1 2 V /-1 5 V or Adjust a ble ,

H igh-Effic ie nc y, Low I

Q

DC-DC I nve rt e rs

FB

COMPARATOR

REF

SHDN ERROR

COMPARATOR

MAX764

MAX765

MAX766

OUT

V+

V+

TRIG

ONE-SHOT

Q

Q TRIG

ONE-SHOT

N

S Q

R

CURRENT

COMPARATOR

1.5V

REFERENCE

FROM V+

FROM OUT

P

LX

CURRENT

CONTROL CIRCUITS

0.2V

(FULL

CURRENT)

0.1V

(HALF

CURRENT)

FROM V+

GND

Figure 1. Block Diagram

_______________De t a ile d De sc ript ion

Ope ra t ing Princ iple

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:

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

OUT

). The principal benefit of this arrangement is that it applies the largest available signal to the gate of the internal P-channel power MOS-

FET. 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 I V

OUT

I plus a diode drop (when the

8 _______________________________________________________________________________________

-5 V /-1 2 V /-1 5 V or Adjust a ble ,

H igh-Effic ie nc y, Low I

Q

DC-DC I nve rt e rs

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

C1

120

µ

F

20V

V

IN

C1

120

µ

F

20V

C3

0.1

µ

F

1

C2

0.1

µ

F

3

OUT

SHDN

MAX764

MAX765

MAX766

2

FB

V+

7

V+

LX

6

8

4

REF

5

GND

L1

47

µ

H

D1

1N5817

C4

68

µ

F

20V

V

OUT

PRODUCT

MAX764

MAX765

MAX766

1

OUT

3

2

SHDN

MAX764

MAX765

MAX766

FB

V+

6

LX

8

D1

1N5817

4

REF

OUTPUT

VOLTAGE (V)

-5

-12

-15

Figure 2. Fixed Output Voltage Operation

C2

0.1

µ

F

R2

R1

C3

0.1

µ

F

5

GND

INPUT

VOLTAGE (V)

3 to 15

3 to 8

3 to 5

V+

7

L1

47

µ

H

Figure 3. Adjustable Output Voltage Operation

C4

68

µ

F

20V

V

OUT

-1V to

-16V

PFM Cont rol Sc he m e

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.)

Shut dow n M ode

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

-5 V /-1 2 V /-1 5 V or Adjust a ble ,

H igh-Effic ie nc y, Low I

Q

DC-DC I nve rt e rs

M ode s of Ope ra t ion

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 ind uc tor starts at zero, rises to a p eak value, then ramps down to zero on each cycle. Although efficiency is still excellent, the output ripple may increase slightly.

__________________De sign Proc e dure

Se t t ing t he Out put V olt a ge

The MAX764/MAX765/MAX766’s output voltage can be adjusted from -1.0V to -16V using external resistors R1 and R2, c onfig ured as shown in Fig ure 3. For adjustable-output operation, select feedback resistor

R1 = 150k

. R2 is given by: where V

REF

= 1.5V.

R2 = (R1)

I

V

OUT

———

V

REF

I

For fixed-output operation, tie FB to REF.

I nduc t or Se le c t ion

In b oth c ontinuous- and d isc ontinuous-c ond uc tion 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 100m

. To minimize radiated noise, use a toroid , p ot c ore, or shield ed c oil.

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.

Diode Se le c t ion

The MAX764/MAX765/MAX766’s high switching freq uenc y d emand s a hig h-sp eed rec tifier. 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 c an use hig h-sp eed silic on d iod es, suc h 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.

Ca pa c it or Se le c t ion

Output Filter Capacitor

The p rimary c riterion for selec ting the outp ut 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 c ap ac itor 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 68µF or larger, low-ESR c apac itor 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 ac tion of the MAX764–MAX766. The inp ut voltag e source impedance determines the size of the capacitor req uired at the V+ inp ut. As with the outp ut 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 ______________________________________________________________________________________

-5 V /-1 2 V /-1 5 V or Adjust a ble ,

H igh-Effic ie nc y, Low I

Q

DC-DC I nve rt e rs

Reference Capacitor

Bypass REF with a 0.1µF capacitor (C3). The REF output can source up to 100µA for external loads.

La yout Conside ra t ions

Proper PC board layout is essential to reduce noise generated by high current levels and fast switching waveforms. Minimize g round noise b y c onnec ting

GND, the input bypass capacitor ground lead, and the

Table 1. Component Suppliers

PRODUCTION METHOD

Surface Mount

Miniature Through-Hole

Low-Cost Through-Hole

INDUCTORS

Sumida

CD75/105 series

Coiltronics

CTX series

Coilcraft

DT/D03316 series

Sumida

RCH895 series

Renco

RL1284 series

AVX

Coilcraft

Coiltronics

SUPPLIER

Matsuo

Motorola

Nichicon

Nihon

Renco

Sanyo OS-CON

Sprague Electric Co.

Sumida

USA:

USA:

USA:

PHONE

(803) 448-9411

(708) 639-6400

(407) 241-7876

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 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.

CAPACITORS

Matsuo

267 series

Sprague

595D/293D series

AVX

TPS series

Sanyo

OS-CON series (very low ESR)

Nichicon

PL series

DIODES

Nihon

EC10QS02L (Schottky)

EC11FS1 (high-speed silicon)

Motorola

1N5817, 1N5818, (Schottky)

MUR105 (high-speed silicon)

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

______________________________________________________________________________________ 11

-5 V /-1 2 V /-1 5 V or Adjust a ble ,

H igh-Effic ie nc y, Low I

Q

DC-DC I nve rt e rs

_Orde ring I nform a t ion (c ont inue d)

PART

MAX766CPA

MAX766CSA

MAX766C/D

MAX766EPA

MAX766ESA

MAX766MJA

TEMP. RANGE

0°C to +70°C

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

PIN-PACKAGE

8 Plastic DIP

8 SO

Dice*

8 Plastic DIP

8 SO

8 CERDIP**

* Dice are tested at T

A

= +25°C, DC parameters only.

**Contact factory for availability and processing to MIL-STD-883.

___________________Chip Topogra phy

LX

OUT

FB

0 . 1 4 5 "

( 3 6 8 3

µ

m )

V +

V +

SHDN

REF

0 . 0 8 0 "

( 2 0 3 2

µ

m )

TRANSISTOR COUNT: 443

SUBSTRATE CONNECTED TO V+

GN D

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 __________________M a x im I nt e gra t e d Produc t s, 1 2 0 Sa n Ga brie l Drive , Sunnyva le , CA 9 4 0 8 6 (4 0 8 ) 7 3 7 -7 6 0 0

© 1994 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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