MAX770–MAX773 5V/12V/15V or Adjustable, High-Efficiency, Low I , Step-Up DC-DC Controllers

MAX770–MAX773 5V/12V/15V or Adjustable, High-Efficiency, Low I , Step-Up DC-DC Controllers

19-0202; Rev 2; 11/96

EVALUATION KIT MANUAL

FOLLOWS DATA SHEET

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

Low I

Q

, Step-Up DC-DC Controllers

_______________General Description

The MAX770–MAX773 step-up switching controllers provide 90% efficiency over a 10mA to 1A load. A unique current-limited pulse-frequency-modulation (PFM) control scheme gives these devices the benefits of pulse-width-modulation (PWM) converters (high efficiency at heavy loads), while using less than 110µA of supply current (vs. 2mA to 10mA for PWM converters).

These ICs use tiny external components. Their high switching frequencies (up to 300kHz) allow surfacemount magnetics of 5mm height and 9mm diameter.

The MAX770/MAX771/MAX772 accept input voltages from 2V to 16.5V. Output voltages are preset at 5V,

(MAX770), 12V (MAX771), and 15V (MAX772); they can also be adjusted using two resistors.

The MAX773 accepts inputs from 3V to 16.5V. For a wider input range, it features an internal shunt regulator that allows unlimited higher input voltages. The MAX773’s output can be set to 5V, 12V, or 15V, or it can be adjusted with two resistors.

The MAX770–MAX773 drive external N-channel MOSFET switches, allowing them to power loads up to 15W. If less power is required, use the MAX756/MAX757 or

MAX761/MAX762 step-up switching regulators with onboard MOSFETs.

________________________Applications

Palmtops/Handy-Terminals

High-Efficiency DC-DC Converters

Battery-Powered Applications

Positive LCD-Bias Generators

Portable Communicators

Flash Memory Programmers

____________________________Features

90% Efficiency for 10mA to 1A Load Currents

Up to 15W Output Power

110µA Max Supply Current

5µA Max Shutdown Current

2V to 16.5V Input Range

(MAX770/MAX771/MAX772)

Internal Shunt Regulator for High Input Voltages

(MAX773)

Preset or Adjustable Output Voltages

MAX770: 5V or Adjustable

MAX771: 12V or Adjustable

MAX772: 15V or Adjustable

MAX773: 5V, 12V, 15V, or Adjustable

Current-Limited PFM Control Scheme

300kHz Switching Frequency

______________Ordering Information

PART

MAX770

CPA

MAX770CSA

MAX770C/D

MAX770EPA

MAX770ESA

MAX770MJA

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

Plastic DIP

8 SO

Dice*

8 Plastic DIP

8 SO

8 CERDIP**

Ordering Information continued at end of data sheet.

*Contact factory for dice specifications.

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

_________________Pin Configurations

__________Typical Operating Circuit

INPUT

2V TO VOUT

OUTPUT

12V

N

ON/OFF SHDN

MAX771

EXT

CS

REF

FB AGND GND

V+

TOP VIEW

EXT

1

V+

2

FB

3

SHDN

4

MAX770

MAX771

MAX772

DIP/SO

7

6

5

8

CS

GND

AGND

REF

Pin Configurations continued at end of data sheet.

________________________________________________________________

Maxim Integrated Products

1

For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800.

For small orders, phone 408-737-7600 ext. 3468.

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

Low I

Q

, Step-Up DC-DC Controllers

ABSOLUTE MAXIMUM RATINGS

Supply Voltages

V+ to GND.............................................................-0.3V to 17V

V+ to SGND.............................................................-0.3V to 7V

SGND........................................................-0.3V to (V+ + 0.3V)

EXT, CS, REF, LBO, LBI, SHDN, FB.............-0.3V to (V+ + 0.3V)

EXTH, EXTL ..................................................-0.3V to (V+ + 0.3V)

V5, V12, V15 .............................................................-0.3V to 17V

GND to AGND .........................................................0.1V to -0.1V

I

SGND

..................................................................................50mA

Continuous Power Dissipation (T

A

= +70°C)

8-Pin Plastic DIP (derate 9.09mW/°C above +70°C)....727mW

8-Pin SO (derate 5.88mW/°C above +70°C) ................471mW

8-Pin CERDIP (derate 8.00mW/°C above +70°C) ........640mW

14-Pin Plastic DIP

(derate 10.00mW/°C above +70°C) .............................800mW

14-Pin SO (derate 8.33mW/°C above +70°C) ..............667mW

14-Pin CERDIP (derate 9.09mW/°C above +70°C) ......727mW

Operating Temperature Ranges

MAX77_C_ _ ........................................................0°C to +70°C

MAX77_E_ _......................................................-40°C to +85°C

MAX77_MJ_ ...................................................-55°C to +125°C

Junction Temperatures

MAX77_C_ _/E_ _ ..........................................................+150°C

MAX77_MJ_..................................................................+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, T

A

= T

MIN to T

MAX

, unless otherwise noted. Typical values are at T

A

= +25°C.)

PARAMETER SYMBOL

Input Voltage Range

Minimum Start-Up Voltage

Supply Current

Standby Current

Output Voltage (Note 1)

Output Voltage Line Regulation

(Note 2)

CONDITIONS

MAX770–772 (internal feedback resistors)

MAX770–772C/E (external resistors)

MAX770–772MJA (external resistors)

MAX773C/E

MAX773MJD

MAX770/MAX771/MAX772

V+ = 16.5V, SHDN = 0V (normal operation)

V+ = 10.0V, SHDN

1.6V (shutdown)

V+ = 16.5V, SHDN

1.6V (shutdown)

V+ = 2.0V to 5.0V, over full load range

V+ = 2.0V to 12.0V, over full load range

V+ = 2.0V to 15.0V, over full load range

Figure 2a, V+ = 2.7V to 4.5V,

I

LOAD

= 700mA, V

OUT

= 5V

MIN

2.0

3.0

3.1

3.0

3.1

4.80

11.52

14.40

Figure 2a, V+ = 3V, I

LOAD

= 30mA to 1A,

V

OUT

= 5V

Output Voltage Load Regulation

(Note 2)

Maximum Switch On-Time

Minimum Switch Off-Time

Efficiency

Reference Voltage t

ON

(max) t

OFF

(min)

V

REF

V+ = 4V, I

LOAD

= 500mA, V

OUT

= 5V

MAX77_C

I

REF =

0

µ

A

MAX77_E

MAX77_M

12

1.8

1.4700

1.4625

1.4550

TYP MAX

16.5

16.5

16.5

16.5

16.5

2.0

1.8

85

2

110

5

4

5.0

5.20

12.0

12.48

15.0

15.60

UNITS

µ

µ

V

V

A

A

V

5 mV/V

20

16

2.3

87

1.5

1.5

1.5

20

2.8

1.5300

1.5375

1.5450

mV/A

µ s

µ s

%

V

2

_______________________________________________________________________________________

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

Low I

Q

, Step-Up DC-DC Controllers

ELECTRICAL CHARACTERISTICS (continued)

(V+ = 5V, I

LOAD

= 0mA, T

A

= T

MIN to T

MAX

, unless otherwise noted. Typical values are at T

A

= +25°C.)

PARAMETERS SYMBOL MIN

REF Load Regulation

REF Line Regulation

FB Trip-Point Voltage

FB Input Current

SHDN Input High Voltage

SHDN Input Low Voltage

SHDN Input Current

LBI Input Current

LBI Hysteresis

LBI Delay

LBI Threshold Voltage

LBO Leakage Current

LBO Output Voltage Low

Current-Limit Trip Level

CS Input Current

EXT Rise Time

EXT Fall Time

Supply Voltage in

Shunt Mode

V

FB

I

FB

V

IH

V

IL

V

OL

V

CS

V

SHUNT

CONDITIONS

0

µ

A

I

REF

100

µ

A

MAX77_C/E

MAX77_M

3V

V+

16.5V

MAX77_C

MAX77_E

MAX77_M

MAX77_C

MAX77_E

MAX77_M

V+ = 2.0V to 16.5V

MAX77_C/E, V+ = 2.0V to 16.5V

MAX77_M, V+ = 2.0V to 16.5V

V+ = 16.5V, SHDN = 0V or V+

MAX773, V+ = 16.5V, LBI = 1.5V

MAX773

5mV overdrive

MAX773, LBI falling

MAX77_C

MAX77_E

MAX77_M

MAX773, V+ = 16.5V, V

LBO

= 16.5V

MAX773, V+ = 5V, LBO sinking 1mA

V+ = 5V to 16.5V

V+ = 5V, 1nF from EXT to ground (Note 3)

V+ = 5V, 1nF from EXT to ground (Note 3)

MAX773, I

SHUNT

= 1mA to 20mA,

SGND = 0V, C

SHUNT

= 0.1

µ

F

1.4700

1.4625

1.4550

1.6

1.4700

1.4625

1.4550

170

5.5

TYP

4

MAX

10

4

40

15

100

1.50

1.5300

1.50

1.5375

1.50

1.5450

±20

±40

±60

0.01

0.1

200

0.01

55

55

0.4

0.2

±1

±20

20

2.5

1.50

1.5300

1.50

1.5375

1.50

1.5450

1.00

0.4

230

±1

6.3

UNITS

mV

µ

V/V

V nA

V

V

V

Note 1:

Output voltage guaranteed using preset voltages. See Figures 7a–7d for output current capability versus input voltage.

Note 2:

Output voltage line and load regulation depend on external circuit components.

Note 3:

For the MAX773, EXT is EXTH and EXTL shorted together.

µ

A

V mV

µ

A ns ns

V

µ

A nA mV

µs

_______________________________________________________________________________________

3

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

Low I

Q

, Step-Up DC-DC Controllers

__________________________________________Typical Operating Characteristics

(T

A

= +25°C, unless otherwise noted.)

MAX770

EFFICIENCY vs. OUTPUT CURRENT

(BOOTSTRAPPED)

MAX771

EFFICIENCY vs. OUTPUT CURRENT

(BOOTSTRAPPED)

MAX772

EFFICIENCY vs. OUTPUT CURRENT

(BOOTSTRAPPED)

100

100

100

V

IN

= 9V

V

IN

= 6V

V

OUT

= 15V, CIRCUIT OF FIGURE 2b

MAX772 SUBSTITUTED FOR MAX771

90

V

IN

= 4V

90

90

80 80

80

70

60

50

V

IN

= 3V

V

IN

= 3.5V

V

OUT

= 5V

CIRCUIT OF

FIGURE 2a

0.001

0.01

0.1

OUTPUT CURRENT (A)

1

MAX771

EFFICIENCY vs. OUTPUT CURRENT

(NON-BOOTSTRAPPED)

100

V

OUT

= 12V

CIRCUIT OF

FIGURE 2c

V

IN

= 9V

90

V

IN

= 6V

V

IN

= 5V

80

70

0.001

0.01

0.1

OUTPUT CURRENT (A)

1 10

70

60

50

0.001

V

IN

= 5V

V

IN

= 3V

V

OUT

= 12V

CIRCUIT OF

FIGURE 2b

0.01

0.1

OUTPUT CURRENT (A)

1

700

MAX770

LOAD CURRENT vs.

MINIMUM START-UP INPUT VOLTAGE

600

500

V

OUT

= 5V

CIRCUIT OF

FIGURE 2a

400

300

200

100

ABOVE 3.4V,

THE CIRCUIT

STARTS UP

UNDER

MAXIMUM

LOAD

CONDITIONS

0

1.0

1.5

2.0

2.5

3.0

MINIMUM START-UP INPUT VOLTAGE (V)

3.5

70

60

50

V

IN

= 12V

V

IN

= 9V

V

IN

= 6V

V

IN

= 5V

V

IN

= 3V

0.001

0.01

0.1

OUTPUT CURRENT (A)

1

500

MAX771

LOAD CURRENT vs.

MINIMUM START-UP INPUT VOLTAGE

400

V

OUT

= 12V

CIRCUIT OF

FIGURE 2b

300

200

100

ABOVE 3.5V

THE CIRCUIT

STARTS UP

UNDER

MAXIMUM

LOAD

CONDITIONS

0

2.0

2.5

3.0

3.5

MINIMUM START-UP INPUT VOLTAGE (V)

4.0

4

3

SUPPLY CURRENT vs. TEMPERATURE

V

OUT

= 12V, V

IN

= 5V

CIRCUIT OF FIGURE 2b

BOOTSTRAPPED MODE

ENTIRE

CIRCUIT

2

0.8

SUPPLY CURRENT vs. SUPPLY VOLTAGE

V

OUT

= 12V

0.6

BOOTSTRAPPED

CIRCUIT OF

FIGURE 2b

0.4

250

EXT RISE/FALL TIME vs. SUPPLY VOLTAGE

200

150

C

EXT

= 2200pF

C

EXT

= 1000pF

C

EXT

= 446pF

C

EXT

= 100pF

100

1

SCHOTTKY DIODE

LEAKAGE EXCLUDED

0

-75 -50 -25 0 25 50 75 100 125

TEMPERATURE (°C)

0.2

0

2

NON-BOOTSTRAPPED

CIRCUIT OF FIGURE 2c

4 6 8

SUPPLY VOLTAGE (V)

10 12

50

0

2

4

6

V+ (V)

8

4

_______________________________________________________________________________________

10 12

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

Low I

Q

, Step-Up DC-DC Controllers

____________________________Typical Operating Characteristics (continued)

(T

A

= +25°C, unless otherwise noted.)

250

REFERENCE OUTPUT RESISTANCE vs.

TEMPERATURE

200

150

100

50

50µA

10µA

100µA

0

-60 -40 -20 0 20 40 60 80

TEMPERATURE (

°

C)

100 120 140

REFERENCE vs. TEMPERATURE

1.506

1.504

1.502

1.500

1.498

1.496

1.494

1.492

-60 -40 -20 0 20 40 60 80

TEMPERATURE (

°

C)

100 120 140

16.5

MAXIMUM SWITCH ON-TIME vs.

TEMPERATURE

t ON(MAX) (µs)

16.0

15.5

-60 -30 0 30 60 90 120 150

TEMPERATURE (°C)

4.0

SHUTDOWN CURRENT vs. TEMPERATURE

3.5

3.0

2.5

2.0

1.5

V+ = 15V

V+ = 8V

1.0

0.5

V+ = 4V

0

-60 -40 -20 0 20 40 60 80

TEMPERATURE (

°

C)

100 120 140

2.30

MINIMUM SWITCH OFF-TIME vs.

TEMPERATURE

8.0

MAXIMUM SWITCH ON-TIME/

MINIMUM SWITCH OFF-TIME RATIO vs. TEMPERATURE

7.5

t OFF(MIN) (µs)

2.25

t OFF(MIN) RATIO

7.0

t ON(MAX)/

6.5

2.20

-60 -30

0 30 60

90

120 150

TEMPERATURE (°C)

6.0

-60 -30 0 30 60 90 120 150

TEMPERATURE (°C)

_______________________________________________________________________________________

5

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

Low I

Q

, Step-Up DC-DC Controllers

____________________________Typical Operating Characteristics (continued)

(Circuit of Figure 2a, T

A

= +25°C, unless otherwise noted.)

MAX770

HEAVY-LOAD SWITCHNG WAVEFORMS

MAX770

LIGHT-LOAD SWITCHING WAVEFORMS

V

OUT

0

I

LIM

I

LIM

2

0

A

B

ILIM

2

0

A

B

C

C

20

µ s/div

V

IN

= 2.9V, I

OUT

= 0.9A

A: EXT VOLTAGE, 5V/div

B: INDUCTOR CURRENT 1A/div

C: V

OUT

RIPPLE 100mV/div, AC-COUPLED

20

µ s/div

V+ = 3V, I

OUT

= 165mA

A: EXT VOLTAGE, 5V/div

B: INDUCTOR CURRENT, 1A/div

C: V

OUT

RIPPLE 100mV/div, AC-COUPLED

MAX770

LINE-TRANSIENT RESPONSE

MAX770

LOAD-TRANSIENT RESPONSE

A

4.5V

2.7V

0

0

A

B

B

2ms/div

I

OUT

= 0.7A

A: V

IN

, 2.7V TO 4.5V, 2V/div

B: V

OUT

RIPPLE, 100mV/div, AC-COUPLED

2ms/div

V

IN

= 3V

A: LOAD CURRENT 0.5A/div (0A to 1A)

B: V

OUT

RIPPLE, 100mV/div, AC-COUPLED

6

_______________________________________________________________________________________

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

Low I

Q

, Step-Up DC-DC Controllers

____________________________Typical Operating Characteristics (continued)

(Circuit of Figure 2a, T

A

= +25°C, unless otherwise noted.)

MAX770

EXITING SHUTDOWN

A

0

B

0

200

µ s/div

V

IN

= 3V, I

OUT

= 0.5A

A: SHDN, 2V/div

B: V

OUT

, 2V/div

______________________________________________________________Pin Description

MAX770

MAX771

MAX772

PIN

MAX773

1 —

NAME

EXT

FUNCTION

2

3

4

5

6

7

8

3

6

7

2

4

5

10

8

9

11

1

V+

FB

SHDN

REF

AGND

GND

CS

V12

V5

LBO

LBI

SGND

Gate drive for external N-channel power transistor

Power-supply input. Also acts as a voltage-sense point when in bootstrapped mode for the

MAX770/MAX771/MAX772, or as a shunt regulator when SGND is connected to ground for the

MAX773. Bypass to SGND with 0.1

µ

F when using the shunt regulator.

Feedback input for adjustable-output operation. Connect to ground for fixed-output operation. Use a resistor divider network to adjust the output voltage. See

Setting the Output Voltage section.

Active-high TTL/CMOS logic-level shutdown input. In shutdown mode, V

OUT is a diode drop below V+ (due to the DC path from V+ to the output) and the supply current drops to 5

µ

A maximum. Connect to ground for normal operation.

1.5V reference output that can source 100

µ

A for external loads. Bypass to GND with 0.1

µ

F.

The reference is disabled in shutdown.

Analog ground

High-current ground return for the output driver

Positive input to the current-sense amplifier. Connect the current-sense resistor between CS and GND.

Input sense point for 12V-output operation. Connect V

OUT to V12 for 12V-output operation.

Leave unconnected for adjustable-output operation.

Input sense point for 5V-output operation. Connect V

OUT to V5 for 5V-output operation. Leave unconnected for adjustable-output operation.

Low-battery output is an open-drain output that goes low when LBI is less than 1.5V. Connect to V+ through a pull-up resistor. Leave floating if not used. LBO is high impedance in shutdown mode.

Input to the internal low-battery comparator. Tie to GND or V+ if not used.

Shunt regulator ground. Leave unconnected if the shunt regulator is

not

used.

_______________________________________________________________________________________

7

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

Low I

Q

, Step-Up DC-DC Controllers

_________________________________________________Pin Description (continued)

MAX770

MAX771

MAX772

PIN

MAX773

— 12

NAME

EXTL

FUNCTION

13

14

EXTH

V15

Low-level gate/base drive for external power transistor. Connect to the gate of an external

N-channel MOSFET or to the base of an external NPN transistor.

High-level gate/base drive for external power transistor. Connect to EXTL when using an external

N-channel MOSFET. When using an external NPN transistor, connect a resistor R

BASE from

EXTH to the base of the NPN to set the maximum base-drive current.

Input sense point for 15V-output operation. Connect V

OUT to V15 for 15V-output operation.

Leave unconnected for adjustable-output operation

_______________Detailed Description

The MAX770–MAX773 are BiCMOS, step-up, switchmode power-supply controllers that provide preset 5V,

12V, and 15V output voltages, in addition to adjustableoutput operation. Their unique control scheme combines the advantages of pulse-frequency modulation

(low supply current) and pulse-width modulation (high efficiency with heavy loads), providing high efficiency over a wide output current range, as well as increased output current capability over previous PFM devices.

In addition, the external sense resistor and power transistor allow the user to tailor the output current capability for each application. Figure 1 shows the

MAX770–MAX773 block diagram.

The MAX770–MAX773 offer three main improvements over prior pulse-skipping control solutions: 1) the converters operate with tiny (5mm height and less than

9mm diameter) surface-mount inductors due to their

300kHz switching frequency; 2) the current-limited PFM control scheme allows 87% efficiencies over a wide range of load currents; and 3) the maximum supply current is only 110µA.

The MAX773 can be configured to operate from an internal 6V shunt regulator, allowing very high input/output voltages. Its output can be configured for an adjustable voltage or for one of three fixed voltages

(5V, 12V, or 15V), and it has a power-fail comparator for low-battery detection.

All devices have shutdown capability, reducing the supply current to 5µA max.

Bootstrapped/Non-Bootstrapped Modes

Figures 2 and 3 show standard application circuits for bootstrapped and non-bootstrapped modes. In bootstrapped mode, the IC is powered from the output

(V

OUT

, which is connected to V+) and the input voltage range is 2V to V

OUT

. The voltage applied to the gate of the external power transistor is switched from V

OUT to ground, providing more switch gate drive and thus reducing the transistor’s on resistance.

In non-bootstrapped mode, the IC is powered from the input voltage (V+) and operates with minimum supply current. In this mode, FB is the output voltage sense point. Since the voltage swing applied to the gate of the external power transistor is reduced (the gate swings from V+ to ground), the power transistor’s on resistance increases at low input voltages. However, the supply current is also reduced because V+ is at a lower voltage, and because less energy is consumed while charging and discharging the external MOSFET’s gate capacitance. The minimum input voltage for the

MAX770–MAX773 is 3V when using external feedback resistors. With supply voltages below 5V, bootstrapped mode is recommended.

Note: When using the MAX770/MAX771/MAX772 in non-bootstrapped mode, there is no preset output operation because V+ is also the output voltage sense point for fixed-output operation. External resistors must be used to set the output voltage.

Use 1% external feedback resistors when operating in adjustable-output mode (Figures 2c, 2d, 3b, 3d, 3e) to achieve an overall output voltage accuracy of ±5%.

The MAX773 can be operated in non-bootstrapped mode without using external feedback resistors because V+ does not act as the output voltage sense point with preset-output operation. To achieve highest efficiency, operate in bootstrapped mode whenever possible.

MAX773 Shunt-Regulator Operation

The MAX773 has an internal 6V shunt regulator that allows the device to step up from very high input voltages (Figure 4).

8

_______________________________________________________________________________________

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

Low I

Q

, Step-Up DC-DC Controllers

LBO V15 V12 V5 FB

V+

LBI

MAX773 ONLY

DUAL-MODE

COMPARATOR

MAX770–MAX773

N

N

SHDN

200mV

REF

1.5V

REFERENCE

ERROR

COMPARATOR

ONE-SHOT

Q TRIG

N

MAX770

MAX771

MAX772

BIAS

CIRCUITRY

V+

6V

ONE-SHOT

TRIG Q

S

F/F

R

Q

LOW-VOLTAGE

OSCILLATOR

CURRENT-SENSE

AMPLIFIER

EXT

CONTROL

2.5V

SGND

EXTH

MAX773

ONLY

EXTL

0.2V

0.1V

EXT

MAX770

MAX771

MAX772

CS

Figure 1. Block Diagram

Floating the shunt-regulator ground (SGND) disables the shunt regulator. To enable it, connect SGND to

GND. The shunt regulator requires 1mA minimum current for proper operation; the maximum current must not exceed 20mA. The MAX773 operates in non-bootstrapped mode when the shunt regulator is used, and

EXT swings between the 6V shunt-regulator voltage and GND.

When using the shunt regulator, use an N-channel power FET instead of an NPN power transistor as the power switch. Otherwise, excessive base drive will collapse the shunt regulator.

External Power-Transistor

Control Circuitry

PFM Control Scheme

The MAX770–MAX773 use a proprietary current-limited

PFM control scheme to provide high efficiency over a wide range of load currents. This control scheme combines the ultra-low supply current of PFM converters (or pulse skippers) with the high full-load efficiency of

PWM converters.

Unlike traditional PFM converters, the MAX770–

MAX773 use a sense resistor to control the peak inductor current. They also operate with high switching

_______________________________________________________________________________________

9

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

Low I

Q

, Step-Up DC-DC Controllers

V

IN

= 3V

C3

0.1

µ

F

C2

0.1

µ

F

V+

2

5

REF

4

SHDN

MAX770

3

FB

6

AGND

GND

7

EXT

1

L1

22

µ

H

C1

100

µ

F

D1

1N5817

N

MTP3055EL

CS

8

R

SENSE

75m

V

OUT

= 5V

@ 1A

C4

300

µ

F

V

IN

= 5V

C3

0.1

µ

F

C2

0.1

µ

F

V+

2

5

REF

4

SHDN

MAX771

3

FB

6

AGND

GND

7

EXT

1

L1

22

µ

H

C1

68

µ

F

D1

1N5817

N

Si9410DY

CS

8

R

SENSE

100m

V

OUT

= 12V

@ 0.5A

C4

200

µ

F

Figure 2a. 5V Preset Output, Bootstrapped Figure 2b. 12V Preset Output, Bootstrapped

V

IN

= 5V

C1

68

µ

F

5

REF

C2

0.1

µ

F

V+

2

C3

0.1

µ

F

4

SHDN

MAX770

MAX771

MAX772

6

AGND

EXT

1

CS

8

L1

22

µ

H

D1

1N5817

N

R

SENSE

100m

FB

3

GND

7

R2 = (R1)

(

V

OUT

-1

V

REF

)

V

REF

= 1.5V

Figure 2c. 12V Output, Non-Bootstrapped

R1

18k

R2

127k

V

OUT

= 12V

@ 0.5A

C4

200

µ

F

V

IN

= 4V

C3

0.1

µ

F

C2

0.1

µ

F

5

REF

V+

2

4

SHDN

MAX770

MAX771

MAX772

6

AGND

EXT

1

CS

8

L1

20

µ

H

C1

47

µ

F

D1

1N5817

N

Si9410DY

R

SENSE

FB

3

GND

7

R1

28k

R2 = (R1)

(

V

OUT

-1

)

V

REF

V

REF

= 1.5V

R2

140k

V

OUT

= 9V

C4

100

µ

F

Figure 2d. 9V Output, Bootstrapped frequencies (up to 300kHz), allowing the use of tiny external components.

As with traditional PFM converters, the power transistor is not turned on until the voltage comparator senses that the output is out of regulation. However, unlike traditional PFM converters, the MAX770–MAX773 switch using 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); there is no oscillator. 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 control circuitry allows the ICs to operate in continuous-conduction mode (CCM) while maintaining high efficiency with heavy loads. When the power switch is

10

______________________________________________________________________________________

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

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, Step-Up DC-DC Controllers

R4

63.4k

(1%)

R3

10k

(1%)

V+

3

1

14

2

V12

V15

V5 SGND

10

8

C3

0.1

µ

F

5

REF

LBI

MAX773

LBO

4

13

7

SHDN

EXTH

EXTL

CS

12

11

6

FB

GND

9

C2

0.1

µ

F

VIN

100k

C1

L1

22

µ

H

D1

1N5817

Si9410DY

N

V

OUT

= 12V

R

SENSE

C4

V

TRIP

(V)

MIN NOMINAL MAX

10.6 11.0 11.4

R4 = R3

(

V

V

REF

= 1.5V

V

TRIP

-1

)

REF

Figure 3a. 12V Preset Output, Bootstrapped, N-Channel

Power MOSFET

V

IN

= 5V

C1

47

µ

F

10

4

SGND

C2

0.1

µ

F

V+

3

EXTH

LBO EXTL

13

12

L1

150

µ

H

910

8

C3

0.1

µ

F

5

REF

LBI

MAX773

7

SHDN

CS

11

V15

V12

V5

FB

14

1

2

6

R

SENSE

0.4

D1

1N5818

V

OUT

= 24V

@ 30mA

ZTX694B

C4

150

µ

F

GND

9

R1

34k

R2

510k

R2 = (R1)

(

V

OUT

-1

)

V

REF

V

REF

= 1.5V

Figure 3b. 24V Output, Non-Bootstrapped, NPN Power

Transistor

V

IN

= 5V

C1

C2

0.1

µ

F

V+

3

10

SGND

4

LBO

C3

0.1

µ

F

8

REF

MAX773

EXTH

13

EXTL

12

CS

11

L1

22

µ

H

D1

1N5817

V

OUT

= 15V

N

Si9410DY

R

SENSE

C4

7

6

SHDN

FB

5

LBI

GND

9

V15

V12

V5

2

14

1

Figure 3d. 16V Output, Bootstrapped, N-Channel

Power MOSFET

VIN

C1

L1

20

µ

H

C2

0.1

µ

F

D1

1N5817

V

OUT

= 16V

V+

3

4

LBO

8

C3

0.1

µ

F

5

REF

LBI

MAX773

7

SHDN

10

SGND

EXTH

13

EXTL

12

CS

11

V15

V12

14

V5

FB 6

1

2

GND

9

R

SENSE

R1

13.7k

N

Si9410DY

C4

R2

133k

R2 = (R1)

(

V

-1

)

V

REF

V

REF

= 1.5V

Figure 3c. 15V Preset Output, Non-Bootstrapped N-Channel

Power MOSFET

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11

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

Low I

Q

, Step-Up DC-DC Controllers

V

IN

= 24V TO 28V

C1

47

µ

F

R

SHUNT

3k

L1

250

µ

H

C2

0.1

µ

F

10

SGND

4

LBO v+

3

EXTH

13

EXTL

12

8

C3

0.1

µ

F

5

REF

LBI

MAX773

7

SHDN

GND

9

CS

11

V15

V12

V5

FB

2

6

14

1

R1

11.3k (1%)

D1

MUR115

N

Si9420DY

R

SENSE

1.0

R2

732k (1%)

V

OUT

= 100V

@ 10mA

C4

100

µ

F

V

R2 = (R1)

REF

= 1.5V

(

-1

V

REF

)

V

IN

R

SHUNT

MAX773

3 V+

C2

0.1

µ

F

6V (typ)

R

SHUNT =

V

IN (MIN)

- V

SHUNT

(

MAX

)

I

SHUNT

*

*

SEE TEXT FOR I

SHUNT

CALCULATION

10 SGND

Figure 3e. 100V Output, Shunt Regulator, N-Channel Power

MOSFET

Figure 4. MAX773 Shunt Regulator turned on, it stays on until either 1) the maximum ontime one-shot turns it off (typically 16µs later), or 2) the switch current reaches the peak current limit set by the current-sense resistor.

To increase light-load efficiency, the current limit for the first two pulses is set to one-half the peak current limit.

If those pulses bring the output voltage into regulation, the error comparator holds the MOSFET off and the current limit remains at one-half the peak current limit. If the output voltage is still out of regulation after two pulses, the current limit for the next pulse is raised to the peak current limit set by the external sense resistor

(see inductor current waveforms in the Typical

Operating Characteristics).

The MAX770–MAX773 switching frequency is variable

(depending on load current and input voltage), causing variable switching noise. However, the subharmonic noise generated does not exceed the peak current limit times the filter capacitor equivalent series resistance

(ESR). For example, when generating a 12V output at

500mA from a 5V input, only 180mV of output ripple occurs using the circuit of Figure 2b.

Low-Voltage Start-Up Oscillator

The MAX770/MAX771/MAX772 feature a low input voltage start-up oscillator that guarantees start-up with no load down to 2V when operating in bootstrapped mode and using internal feedback resistors. At these low voltages, the supply voltage is not large enough for proper error-comparator operation and internal biasing. The start-up oscillator has a fixed 50% duty cycle and the

MAX770/MAX771/MAX772 disregard the error-comparator output when the supply voltage is less than

2.5V. Above 2.5V, the error-comparator and normal oneshot timing circuitry are used. The low voltage start-up circuitry is disabled if non-bootstrapped mode is selected (FB is not tied to ground).

The MAX773 does not provide the low-voltage 50% duty-cycle oscillator. Its minimum start-up voltage is 3V for all modes.

External Transistor

An N-FET power switch is recommended for the

MAX770/MAX771/MAX772.

The MAX773 can drive either an N-channel MOSFET

(N-FET) or an NPN because it provides two separate

12

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

Low I

Q

, Step-Up DC-DC Controllers

drive outputs (EXTH and EXTL) that operate 180° out of phase (Figures 3a and 3b). In Figure 3b, the resistor in series with EXTH limits the base current, and EXTL (which is connected directly to the base) turns the transistor off.

Shutdown Mode

When SHDN is high, the MAX770–MAX773 enter shutdown mode. In this mode, the internal biasing circuitry is turned off (including the reference) and V

OUT falls to a diode drop below V

IN

(due to the DC path from the input to the output). In shutdown mode, the supply current drops to less than 5µA. SHDN is a

TTL/CMOS logic-level input. Connect SHDN to GND for normal operation.

The MAX773’s shunt regulator is

not

disabled in shutdown mode.

Low-Battery Detector

The MAX773 provides a low-battery comparator that compares the voltage on LBI to the reference voltage.

When the LBI voltage is below V

REF,

LBO (an opendrain output) goes low. The low-battery comparator’s

20mV of hysteresis adds noise immunity, preventing repeated triggering of LBO. Use a resistor-divider network between V+, LBI, and GND to set the desired trip voltage

V

TRIP

. LBO is high impedance in shutdown mode.

__________________Design Procedure

Setting the Output Voltage

To set the output voltage, first determine the mode of operation, either bootstrapped or non-bootstrapped.

Bootstrapped mode provides more output current capability, while non-bootstrapped mode reduces the supply current (see Typical Operating Characteristics).

If a decaying voltage source (such as a battery) is used, see the additional notes in the Low Input Voltage

Operation section.

Use the MAX770/MAX771/MAX772 unless one or more of the following conditions applies. If one or more of the following is true, use the MAX773:

1) An NPN power transistor will be used as the power switch

2) The LBI/LBO function is required

3) The shunt regulator must accommodate a high input voltage

4) Preset-output non-bootstrapped operation is desired—for example, to reduce the no-load supply current in a 5V to 12V application.

MAX770

MAX771

MAX772

MAX773

GND

FB

Figure 5. Adjustable Output Circuit

R1

R2

V

OUT

R1 = 10k TO 500k

R2 = R1

(

V

-1

V

REF

)

V

REF

= 1.5V

See Table 1 for a summary of operating characteristics and requirements for the ICs in bootstrapped and nonbootstrapped modes.

The MAX770–MAX773’s output voltage can be adjusted from very high voltages down to 3V, using external resistors R1 and R2 configured as shown in Figure 5.

For adjustable-output operation, select feedback resistor R1 in the range of 10k

Ω to 500k

. R2 is given by:

R2 = (R1)

(

––––– -1

V

REF

) where V

REF equals 1.5V.

For preset-output operation, tie FB to GND (this forces bootstrapped-mode operation for the

MAX770/MAX771/MAX772).

Configure the MAX773 for a preset voltage of 5V, 12V, or

15V by connecting the output to the corresponding sense input pin (i.e., V5, V12, or V15). FB must be tied to ground for preset-output operation. Leave all unused sense input pins unconnected. Failure to do so will cause an incorrect output voltage. The MAX773 can provide a preset output voltage in both bootstrapped and nonbootstrapped modes.

Figures 2 and 3 show various circuit configurations for bootstrapped/non-bootstrapped, preset/adjustable operation.

Shunt-Regulator Operation

When using the shunt regulator, connect SGND to ground and place a 0.1µF capacitor between V+ and SGND, as close to the IC as possible. Increase C2 to 1.0µF to improve shunt regulators performance with heavy loads.

Select R

SHUNT such that 1mA

I

SHUNT

20mA.

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13

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

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Q

, Step-Up DC-DC Controllers

Table 1. Bootstrapped vs. Non-Bootstrapped Operation

PARAMETER

Gate Drive

FET On Resistance

Gate-Drive Capacitive Losses

No-Load Supply Current

Possible Input Voltage Range

Normally Recommended Input

Voltage Range

BOOTSTRAPPED*

GND to V

OUT

Lower

Higher

Higher

2V to 16.5V (MAX770/MAX771/MAX772),

(internal feedback resistors)

3V to 16.5V (MAX770/MAX771/MAX772),

(external feedback resistors)

3V to 16.5V (MAX773)

2V to 5V (MAX770/MAX771/MAX772),

3V to 5V (MAX773)

Fixed Output Available MAX770–MAX773(N)

Adjustable Output Available MAX770–MAX773(N)

*MAX773(S) indicates shunt mode; MAX773(N) indicates NOT in shunt mode.

NON-BOOTSTRAPPED

GND to V+

Higher

Lower

Lower

3V to 16.5V

(MAX770/MAX771/MAX772),

3V and up (MAX773)

5V to 16.5V

(MAX770/MAX771/MAX772),

5V and up (MAX773)

MAX773(N)/MAX773(S)

MAX770/MAX771/MAX772/

MAX773(N)/MAX773(S)

Use an N-channel FET as the power switch when using the shunt regulator (see MAX773 Shunt-Regulator

Operation in the Detailed Description). The shunt-regulator current powers the MAX773 and also provides the

FET gate-drive current, which depends largely on the

FET’s total gate charge at V

GS

= 5V. To determine the shunt-resistor value, first determine the maximum shunt current required.

I

SHUNT

= I

SUPP

+ I

GATE

See N-Channel MOSFETs in the Power-Transistor

Selection section to determine I

GATE

.

Determine the shunt-resistor value using the following equation:

V

IN

(min) - V

SHUNT

(max)

R

SHUNT

(max) = ————————————

I

SHUNT where V

SHUNT

(max) is 6.3V.

The shunt regulator is not disabled in shutdown mode, and continues to draw the calculated shunt current.

If the calculated shunt regulator current exceeds 20mA, or if the shunt current exceeds 5mA and less shunt regulator current is desired, use the circuit of Figure 6 to provide increased drive and reduced shunt current when driving N-FETs with large gate capacitances.

Select I

SHUNT

= 3mA. This provides adequate biasing current for this circuit, although higher shunt currents can be used.

R

SHUNT

C2

0.1

µ

F

V+

3

SGND

10

C1

EXTH

13

EXTL

12

100

NPN

2N2222A

MAX773

PNP

2N2907A

N

L1

20

µ

H

D1

R2

CS

11

R

SENSE

FB

6

R1

To prevent the shunt regulator from drawing current in shutdown mode, place a switch in series with the shunt resistor.

V

IN

V

OUT

C4

Figure 6. Increased N-FET Gate Drive when Using the Shunt

Regulator

14

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

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Q

, Step-Up DC-DC Controllers

3.5

3.0

2.5

V

OUT

= 5V

L = 22

µ

H

R

SENSE

= 40m

R

SENSE

= 50m

2.0

R

SENSE

= 75m

1.5

1.0

R

SENSE

= 100m

0.5

R

SENSE

= 200m

0

2 3 4

INPUT VOLTAGE (V)

5

Figure 7a. Maximum Output Current vs. Input Voltage

(V

OUT

= 5V)

3.5

3.0

2.5

2.0

V

OUT

= 15V

L = 22

µ

H

R

SENSE

= 40m

R

SENSE

= 50m

R

SENSE

= 75m

1.5

1.0

R

SENSE

= 100m

0.5

R

SENSE

= 200m

0

2 4 6 8 10

INPUT VOLTAGE (V)

12 14 16

Figure 7c. Maximum Output Current vs. Input Voltage

(V

OUT

= 15V)

Determining R

SENSE

The

Typical Operating Characteristics graphs show the output current capability for various modes, sense resistors, and input/output voltages. Use these graphs, along with the theoretical output current curves shown in Figures 7a-7d, to select R

SENSE

. These theoretical curves assume that an external N-FET power switch is used. They were derived using the minimum (worstcase) current-limit comparator threshold value, and the inductance value. No tolerance was included for

R

SENSE

. The voltage drop across the diode was assumed to be 0.5V, and the drop across the power switch r

DS(ON) and coil resistance was assumed to be

0.3V. To use the graphs, locate the graph with the appropriate output voltage or the graph having the nearest output voltage higher than the desired output voltage. On this graph, find the curve for the largest

3.5

3.0

2.5

2.0

V

OUT

= 12V

L = 22

µ

H

R

SENSE

= 40m

R

SENSE

= 50m

R

SENSE

= 75m

1.5

1.0

R

SENSE

= 100m

0.5

R

SENSE

= 200m

0

2 4 6 8

INPUT VOLTAGE (V)

10 12

Figure 7b. Maximum Output Current vs. Input Voltage

(V

OUT

= 12V)

0.8

V

OUT

= 24V

L =150

µ

H

0.6

R

SENSE

= 100m

0.4

R

SENSE

= 200m

0.2

R

SENSE

= 400m

0

2 6 10

INPUT VOLTAGE (V)

14

Figure 7d. Maximum Output Current vs. Input Voltage

(V

OUT

= 24V) sense-resistor value with an output current that is adequate at the lowest input voltage.

Determining the Inductor (L)

Practical inductor values range from 10µH to 300µH.

20µH is a good choice for most applications. In applications with large input/output differentials, the IC’s output current capability will be much less when the inductance value is too low, because the IC will always operate in discontinuous mode. If the inductor value is too low, the current will ramp up to a high level before the current-limit comparator can turn off the switch. The minimum on-time for the switch (t

ON

(min)) is approximately 2µs; select an inductor that allows the current to ramp up to I

LIM

/2 in no less than 2µs.

Choosing a value of I

LIM

/2 allows the half-size current pulses to occur, increasing light-load efficiency and minimizing output ripple.

______________________________________________________________________________________

15

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

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Q

, Step-Up DC-DC Controllers

MAX770

MAX771

MAX772

EXT

CS

N

R

SENSE

MAX773

EXTH

EXTL

CS

I

B

R

BASE

I

C(PEAK)

L

NPN

R

SENSE

Figure 8a. Use an N-Channel MOSFET with the

MAX770/MAX771/MAX772

Figure 8c. Using an NPN Transistor with the MAX773

MAX773

EXTH

EXTL

CS

N

L

R

SENSE

Figure 8b. Using an N-Channel MOSFET with the MAX773

The standard operating circuits use a 22µH inductor.

If a different inductance value is desired, select L such that:

L

V

IN

(max) x t

ON

(min)

——————————

I

LIM

/ 2

Larger inductance values tend to increase the start-up time slightly, while smaller inductance values allow the coil current to ramp up to higher levels before the switch turns off, increasing the ripple at light loads.

Inductors with a ferrite core or equivalent are recommended; powder iron cores are not recommended for use with high switching frequencies. Make sure the inductor’s saturation current rating (the current at which the core begins to saturate and the inductance starts to fall) exceeds the peak current rating set by R

SENSE

.

However, it is generally acceptable to bias the inductor into saturation by approximately 20% (the point where the inductance is 20% below the nominal value). For highest efficiency, use a coil with low DC resistance, preferably under 20m

. To minimize radiated noise, use a toroid, a pot core, or a shielded coil.

Table 2 lists inductor suppliers and specific recommended inductors.

Power Transistor Selection

Use an N-channel MOSFET power transistor with the

MAX770/MAX771/MAX772 (Figure 8a).

Use an N-FET whenever possible with the MAX773. An

NPN transistor can be used, but be extremely careful when determining the base current (see NPN

Transistors section). An NPN transistor is not recommended when using the shunt regulator.

N-Channel MOSFETs

To ensure the external N-channel MOSFET (N-FET) is turned on hard, use logic-level or low-threshold

N-FETs when the input drive voltage is less than 8V. This applies even in bootstrapped mode, to ensure start-up.

N-FETs provide the highest efficiency because they do not draw any DC gate-drive current, but they are typically more expensive than NPN transistors. When using an N-FET with the MAX773, connect EXTH and EXTL to the N-FET’s gate (Figure 8b).

When selecting an N-FET, three important parameters are the total gate charge (Q g

), on resistance (r

DS(ON)

), and reverse transfer capacitance (C

RSS

).

Q g takes into account all capacitances associated with charging the gate. Use the typical Q g value for best results; the maximum value is usually grossly overspecified since it is a guaranteed limit and not the measured value. The typical total gate charge should be

50nC or less. With larger numbers, the EXT pins may not be able to adequately drive the gate. The EXT rise/fall time with various capacitive loads as shown in the

Typical Operating Characteristics.

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

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The two most significant losses contributing to the

N-FET’s power dissipation are I

2

R losses and switching losses. Select a transistor with low r

DS(ON) and low

C

RSS to minimize these losses.

Determine the maximum required gate-drive current from the Q g specification in the N-FET data sheet.

The MAX773’s maximum allowed switching frequency during normal operation is 300kHz; but at start-up the maximum frequency can be 500kHz, so the maximum current required to charge the N-FET’s gate is f(max) x Q g

(typ). Use the typical Q g number from the transistor data sheet. For example, the Si9410DY has a

Q g

(typ) of 17nC (at V

GS

= 5V), therefore the current required to charge the gate is:

I

GATE

(max) = (500kHz) (17nC) = 8.5mA.

The bypass capacitor on V+ (C2) must instantaneously furnish the gate charge without excessive droop (e.g., less than 200mV):

Q g

V+ = ——

C2

Continuing with the example,

V+ = 17nC/0.1µF = 170mV.

Use I

GATE when calculating the appropriate shunt resistor. See the Shunt Regulator Operation section.

Figure 2a’s application circuit uses an MTD3055EL logic-level N-FET with a guaranteed threshold voltage

(V

TH

) of 2V. Figure 2b’s application circuit uses an

8-pin Si9410DY surface-mount N-FET that has 50m

Ω on resistance with 4.5V V

GS

, and a guaranteed V

TH of less than 3V.

NPN Transistors

The MAX773 can drive NPN transistors, but be extremely careful when determining the base-current requirements. Too little base current can cause excessive power dissipation in the transistor; too much base current can cause the base to oversaturate, so the transistor remains on continually. Both conditions can damage the transistor.

When using the MAX773 with an NPN transistor, connect EXTL to the transistor’s base, and connect R

BASE between EXTH and the base (Figure 8c).

To determine the required peak inductor current,

I

C(PEAK

), observe the Typical Operating Characteristics efficiency graphs and the theoretical output current capability vs. input voltage graphs to determine a sense resistor that will allow the desired output current.

Divide the 170mV worst-case (smallest) voltage across the current-sense amplifier V

CS

(max) by the senseresistor value. To determine I

B

, set the peak inductor current (I

LIM) equal to the peak transistor collector current I

C(PEAK)

. Calculate I

B as follows:

I

B

= I

LIM

Use the worst-case (lowest) value for ß given in the transistor’s electrical specification, where the collector current used for the test is approximately equal to I

LIM

.

It may be necessary to use even higher base currents

(e.g., I

B

= I

LIM

/10), although excessive I

B may impair operation by extending the transistor’s turn-off time.

R

BASE is determined by:

(

V

EXTH

- V

BE

- V

CS

(min )

)

R

BASE

= ————————————–

I

B

Where V

EXTH is the voltage at V+ (in bootstrapped mode V

EXTH is the output voltage), V

BE is the 0.7V transistor base-emitter voltage, V

CS

(min) is the voltage drop across the current-sense resistor, and I

B is the minimum base current that forces the transistor into saturation. This equation reduces to (V+ - 700mV -

170mV) / I

B

.

For maximum efficiency, make R

BASE as large as possible, but small enough to ensure the transistor is always driven near saturation. Highest efficiency is obtained with a fast-switching NPN transistor

(f

T

150MHz) with a low collector-emitter saturation voltage and a high current gain. A good transistor to use is the Zetex ZTX694B.

Diode Selection

The MAX770–MAX773’s high switching frequency demands a high-speed rectifier. Schottky diodes such as the 1N5817–1N5822 are recommended. Make sure that the Schottky diode’s average current rating exceeds the peak current limit set by R

SENSE

, and that its breakdown voltage exceeds V

OUT

. For high-temperature applications, Schottky diodes may be inadequate due to their high leakage currents; high-speed silicon diodes may be used instead. 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 (C2) is low effective series resistance (ESR).

The product of the peak inductor current and the output filter capacitor’s ESR determines the amplitude of the ripple seen on the output voltage. An OS-CON 300µF,

6.3V output filter capacitor has approximately 50m

Ω of

ESR and typically provides 180mV ripple when stepping up from 3V to 5V at 1A (Figure 2a).

______________________________________________________________________________________

17

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

Low I

Q

, Step-Up DC-DC Controllers

Smaller capacitors are acceptable for light loads or in applications that can tolerate higher output ripple.

Since the output filter capacitor’s ESR affects efficiency, use low-ESR capacitors for best performance. The smallest low-ESR surface-mount tantalum capacitors currently available are the Sprague 595D series. Sanyo

OS-CON organic semiconductor through-hole capacitors and the Nichicon PL series also exhibit low ESR.

See Table 2.

Input Bypass Capacitors

The input bypass capacitor (C1) reduces peak currents drawn from the voltage source and also reduces noise at the voltage source caused by the switching action of the MAX770–MAX773. 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 recommended. For output currents up to 1A, 150µF (C1) is adequate, although smaller bypass capacitors may also be acceptable.

Bypass the IC with a 0.1µF ceramic capacitor (C2) placed close to the V+ and GND pins.

Reference Capacitor

Bypass REF with a 0.1µF capacitor (C3). REF can source up to 100µA of current.

Setting the Low-Battery-Detector Voltage

To set the low-battery detector’s falling trip voltage

(V

TRIP

(falling)), select R3 between 10k

Ω and 500k

(Figure 9), and calculate R4 as follows:

R4 = (R3)

(

V

TRIP

- V

REF

———————

)

V

REF where V

REF

= 1.5V.

The rising trip voltage is higher because of the comparator’s approximately 20mV of hysteresis, and is determined by:

R4

V

TRIP

(rising) = (V

REF

+ 20mV) (1 + ——)

R3

Connect a high value resistor (larger than R3 + R4) between LBI and LBO if additional hysteresis is required.

Connect a pull-up resistor (e.g., 100k

) between LBO and V+. Tie LBI to GND and leave LBO floating if the low-battery detector is not used.

V

IN

R4

R3

V+

LBI

MAX773

LBO

R5

100k

LOW-BATTERY

OUTPUT

GND

R4 = R3

V

TRIP

(

-1

)

V

REF

V

REF

= 1.5V

Figure 9. Input Voltage Monitor Circuit

__________Applications Information

MAX773 Operation with High

Input/Output Voltages

The MAX773’s shunt regulator input allows high voltages to be converted to very high voltages. Since the

MAX773 runs off the 6V shunt (bootstrapped operation is not allowed), the IC will not see the high input voltage. Use an external logic-level N-FET as the power switch, since only 6V of V

GS are available. Also, make sure all external components are rated for very high output voltage. Figure 3e shows a circuit that converts

28V to 100V.

Low Input Voltage Operation

When using a power supply that decays with time

(such as a battery), the N-FET transistor will operate in its linear region when the voltage at EXT approaches the threshold voltage of the FET, dissipating excessive power. Prolonged operation in this mode may damage the FET. This effect is much more significant in nonbootstrapped mode than in bootstrapped mode, since bootstrapped mode typically provides much higher

V

GS voltages. To avoid this condition, make sure V

EXT is above the V

TH of the FET, or use a voltage detector

(such as the MAX8211) to put the IC in shutdown mode once the input supply voltage falls below a predetermined minimum value. Excessive loads with low input voltages can also cause this condition.

18

______________________________________________________________________________________

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

Low I

Q

, Step-Up DC-DC Controllers

Starting Up under Load

The

Typical Operating Characteristics show the Start-

Up Voltage vs. Load Current graph for bootstrappedmode operation. This graph depends on the type of power switch used. The MAX770–MAX773 are not designed to start up under full load in bootstrapped mode with low input voltages.

Layout Considerations

Due to high current levels and fast switching waveforms, which radiate noise, proper PC board layout is essential. Protect sensitive analog grounds by using a star ground configuration. Minimize ground noise by connecting GND, the input bypass capacitor ground

Table 2. Component Suppliers

PRODUCTION INDUCTORS CAPACITORS

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. Place input bypass capacitor C2 as close as possible to V+ and GND.

Excessive noise at the V+ input may falsely trigger the timing circuitry, resulting in short pulses at EXT. If this occurs it will have a negligible effect on circuit efficiency. If desired, place a 4.7µF directly across the V+ and

GND pins (in parallel with the 0.1µF C2 bypass capacitor) to reduce the noise at V+.

Surface Mount

Sumida

CD54 series

CDR125 series

Coiltronics

CTX20 series

Matsuo

267 series

Sprague

595D series

TRANSISTORS

N-FET

Siliconix

Si9410DY

Si9420DY (high voltage)

Motorola

MTP3055EL

MTD20N03HDL

DIODES

Nihon

EC10 series

Through Hole

Sumida

RCH855 series

RCH110 series

Renco

RL1284-18

Sanyo

OS-CON series

Nichicon

PL series

United Chemi-Con

LXF series

NPN

Zetex

ZTX694B

Motorola

1N5817–1N5822

MUR115 (high voltage)

SUPPLIER

Coiltronics

Matsuo

Nichicon

Nihon

Renco

Sanyo

Sumida

United Chemi-Con

Zetex

PHONE

USA: (561) 241-7876

USA: (714) 969-2491

Japan: 81-6-337-6450

USA: (847) 843-7500

USA: (805) 867-2555

USA: (516) 586-5566

USA: (619) 661-6835

Japan: 81-7-2070-6306

USA: (847) 956-0666

Japan: 81-3-3607-5111

USA: (714) 255-9500

USA: (516) 543-7100

UK: 44-61-627-4963

FAX

(561) 241-9339

(714) 960-6492

81-6-337-6456

(847) 843-2798

(805) 867-2698

(516) 586-5562

(619) 661-1055

81-7-2070-1174

81-3-3607-5144

(714) 255-9400

(516) 864-7630

44-61-627-5467

______________________________________________________________________________________

19

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

Low I

Q

, Step-Up DC-DC Controllers

___Ordering Information (continued)

PART

MAX771

CPA

MAX771CSA

MAX771C/D

MAX771EPA

MAX771ESA

MAX771MJA

MAX772

CPA

MAX772CSA

MAX772C/D

MAX772EPA

MAX772ESA

MAX772MJA

MAX773

CPD

MAX773CSD

MAX773C/D

MAX773EPD

MAX773ESD

MAX773MJD

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

-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

-55°C to +125°C

*Contact factory for dice specifications.

PIN-PACKAGE

8 Plastic DIP

8 SO

Dice*

8 Plastic DIP

8 SO

8 CERDIP

8 Plastic DIP

8 SO

Dice*

8 Plastic DIP

8 SO

8 CERDIP

14 Plastic DIP

14 SO

Dice*

14 Plastic DIP

14 Narrow SO

14 CERDIP

____Pin Configurations (continued)

_________________Chip Topographies

MAX770/MAX771/MAX772

EXT

V+

FB

SHDN

0.080"

(2.032mm)

REF

TRANSISTOR COUNT: 501;

SUBSTRATE CONNECTED TO V+.

MAX773

CS

0.126"

(3.200mm)

GND

AGND

TOP VIEW

V5 V12 V15

V12

1

V5

2

V+

3

LBO

4

LBI

5

FB

6

SHDN

7

MAX773

9

8

14

V15

13

EXTH

12

EXTL

11

CS

10

SGND

GND

REF

V+

LBO

LBI

FB

EXTH

EXTL

CS

SGND

0.126"

(3.200mm)

GND

GND

DIP/SO

SHDN

0.080"

(2.032mm)

TRANSISTOR COUNT: 501;

SUBSTRATE CONNECTED TO V+.

REF

20

______________________________________________________________________________________

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