MAX771ESA+T - Maxim Integrated Products, Inc.

_______________General Description

The MAX770–MAX773 step-up switching controllers pro-

vide 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 surface-

mount 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 out-

put 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 on-

board 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

Ordering Information continued at end of data sheet.

*Contact factory for dice specifications.

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

MAX770–MAX773

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

Low I

, Step-Up DC-DC Controllers

________________________________________________________________

Maxim Integrated Products

GND

AGND

REF

SHDN

V+

EXT

MAX770

MAX771

MAX772

DIP/SO

_________________Pin Configurations

FB AGND GNDV+

EXT N

REF

SHDN

ON/OFF

OUTPUT

12V

INPUT

2V TO VOUT

MAX771

__________Typical Operating Circuit

PART TEMP. RANGE PIN-PACKAGE

MAX770CPA 0°C to +70°C Plastic DIP

MAX770CSA 0°C to +70°C 8 SO

MAX770C/D 0°C to +70°C Dice*

MAX770EPA -40°C to +85°C 8 Plastic DIP

MAX770ESA -40°C to +85°C 8 SO

MAX770MJA -55°C to +125°C 8 CERDIP**

TOP VIEW

Pin Configurations continued at end of data sheet.

19-0202; Rev 2; 11/96

EVALUATION KIT MANUAL

FOLLOWS DATA SHEET

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.

MAX770–MAX773

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

Low I

, Step-Up DC-DC Controllers

2_______________________________________________________________________________________

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

ISGND ..................................................................................50mA

Continuous Power Dissipation (TA= +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, ILOAD = 0mA, TA= TMIN to TMAX, unless otherwise noted. Typical values are at TA= +25°C.)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Supply Current 85 110 µA

Standby Current 2 5 µA

Output Voltage (Note 1) V+ = 2.0V to 5.0V, over full load range 4.80 5.0 5.20 V

V+ = 2.0V to 12.0V, over full load range 11.52 12.0 12.48

V+ = 2.0V to 15.0V, over full load range 14.40 15.0 15.60

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

ILOAD = 700mA, VOUT = 5V 5 mV/V

Figure 2a, V+ = 3V, ILOAD = 30mA to 1A,

VOUT = 5V 20 mV/A

Maximum Switch On-Time tON(max) 12 16 20 µs

Minimum Switch Off-Time tOFF(min) 1.8 2.3 2.8 µs

Efficiency 87 %

Reference Voltage VREF IREF = 0µAMAX77_C 1.4700 1.5 1.5300 V

MAX77_E 1.4625 1.5 1.5375

MAX77_M 1.4550 1.5 1.5450

Output Voltage Line Regulation

(Note 2)

Output Voltage Load Regulation

(Note 2)

V+ = 4V, ILOAD = 500mA, VOUT = 5V

V+ = 10.0V, SHDN ≥1.6V (shutdown)

V+ = 16.5V, SHDN ≥1.6V (shutdown)

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

Minimum Start-Up Voltage MAX770/MAX771/MAX772 1.8 2.0 V

MAX770–772 (internal feedback resistors) 2.0 16.5

MAX770–772C/E (external resistors) 3.0 16.5

MAX770–772MJA (external resistors) 3.1 16.5

MAX773C/E 3.0 16.5

MAX773MJD 3.1 16.5

Input Voltage Range V

MAX770–MAX773

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

Low I

, Step-Up DC-DC Controllers

_______________________________________________________________________________________ 3

ELECTRICAL CHARACTERISTICS (continued)

(V+ = 5V, ILOAD = 0mA, TA= TMIN to TMAX, unless otherwise noted. Typical values are at TA= +25°C.)

PARAMETERS SYMBOL CONDITIONS MIN TYP MAX UNITS

REF Load Regulation 0µA ≤IREF ≤100µAMAX77_C/E 4 10 mV

MAX77_M 4 15

REF Line Regulation 3V ≤V+ ≤16.5V 40 100 µV/V

FB Trip-Point Voltage VFB

MAX77_C 1.4700 1.50 1.5300

VMAX77_E 1.4625 1.50 1.5375

MAX77_M 1.4550 1.50 1.5450

FB Input Current IFB

MAX77_C ±20

nAMAX77_E ±40

MAX77_M ±60

SHDN Input High Voltage VIH V+ = 2.0V to 16.5V 1.6 V

SHDN Input Low Voltage VIL MAX77_C/E, V+ = 2.0V to 16.5V 0.4 V

SHDN Input Current ±1 µA

LBI Input Current MAX773, V+ = 16.5V, LBI = 1.5V ±20 nA

LBI Hysteresis MAX773 20

LBI Threshold Voltage MAX773, LBI falling

MAX77_C 1.4700 1.50 1.5300

VMAX77_E 1.4625 1.50 1.5375

MAX77_M 1.4550 1.50 1.5450

LBO Leakage Current MAX773, V+ = 16.5V, VLBO = 16.5V 0.01 1.00 µA

LBO Output Voltage Low VOL MAX773, V+ = 5V, LBO sinking 1mA 0.1 0.4 V

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

Current-Limit Trip Level VCS V+ = 5V to 16.5V 170 200 230 mV

VSHUNT MAX773, ISHUNT = 1mA to 20mA,

SGND = 0V, CSHUNT = 0.1µF5.5 6.3 V

CS Input Current 0.01 ±1 µA

EXT Rise Time V+ = 5V, 1nF from EXT to ground (Note 3) 55 ns

EXT Fall Time V+ = 5V, 1nF from EXT to ground (Note 3) 55 ns

Supply Voltage in

Shunt Mode

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.

LBI Delay 5mV overdrive 2.5 µs

MAX77_M, V+ = 2.0V to 16.5V 0.2

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

SUPPLY CURRENT vs. TEMPERATURE

TEMPERATURE (°C)

SUPPLY CURRENT (mA)

VOUT = 12V, VIN = 5V

CIRCUIT OF FIGURE 2b

BOOTSTRAPPED MODE

ENTIRE

CIRCUIT

SCHOTTKY DIODE

LEAKAGE EXCLUDED

MAX770–3-07

0.2

0.4

0.6

0.8

246 8 10 12

SUPPLY CURRENT vs. SUPPLY VOLTAGE

SUPPLY VOLTAGE (V)

SUPPLY CURRENT (mA)

VOUT = 12V

NON-BOOTSTRAPPED

CIRCUIT OF FIGURE 2c

BOOTSTRAPPED

CIRCUIT OF

FIGURE 2b

MAX770–3-08

100

150

200

250

246 8 10 12

EXT RISE/FALL TIME vs. SUPPLY VOLTAGE

V+ (V)

EXT RISE/FALL TIME (ns)

CEXT = 2200pF

CEXT = 1000pF

CEXT = 446pF

CEXT = 100pF

MAX770–3-09

100

50 0.001 0.01 0.1 1

MAX772

EFFICIENCY vs. OUTPUT CURRENT

(BOOTSTRAPPED)

OUTPUT CURRENT (A)

EFFICIENCY (%)

VOUT = 15V, CIRCUIT OF FIGURE 2b

MAX772 SUBSTITUTED FOR MAX771

VIN = 12V

VIN = 9V

VIN = 6V

VIN = 5V

VIN = 3V

MAX770–3-03

100

0.001 0.01 1

MAX771

EFFICIENCY vs. OUTPUT CURRENT

(BOOTSTRAPPED)

OUTPUT CURRENT (A)

EFFICIENCY (%)

0.1

VIN = 6V

VIN = 5V

VIN = 3V

VIN = 9V

VOUT = 12V

CIRCUIT OF

FIGURE 2b

MAX770–3-02

100

50 0.001 0.01 0.1 1

MAX770

EFFICIENCY vs. OUTPUT CURRENT

(BOOTSTRAPPED)

OUTPUT CURRENT (A)

EFFICIENCY (%)

VIN = 3V

VIN = 3.5V

VOUT = 5V

CIRCUIT OF

FIGURE 2a

VIN = 4V

MAX770–3-01

MAX770–MAX773

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

Low I

, Step-Up DC-DC Controllers

4_______________________________________________________________________________________

__________________________________________Typical Operating Characteristics

(TA = +25°C, unless otherwise noted.)

100

0.001 0.01 0.1 10

MAX771

EFFICIENCY vs. OUTPUT CURRENT

(NON-BOOTSTRAPPED)

OUTPUT CURRENT (A)

EFFICIENCY (%)

VIN = 9V

VIN = 6V

VIN = 5V

VOUT = 12V

CIRCUIT OF

FIGURE 2c

MAX770–3-04

100

200

300

400

500

600

700

1.0

MAX770

LOAD CURRENT vs.

MINIMUM START-UP INPUT VOLTAGE

MINIMUM START-UP INPUT VOLTAGE (V)

LOAD CURRENT (mA)

3.01.5 2.5 3.52.0

ABOVE 3.4V,

THE CIRCUIT

STARTS UP

UNDER

MAXIMUM

LOAD

CONDITIONS

VOUT = 5V

CIRCUIT OF

FIGURE 2a

MAX770–3-05

100

200

300

400

500

2.0

MAX771

LOAD CURRENT vs.

MINIMUM START-UP INPUT VOLTAGE

MINIMUM START-UP INPUT VOLTAGE (V)

LOAD CURRENT (mA)

4.0

2.5 3.5

3.0

ABOVE 3.5V

THE CIRCUIT

STARTS UP

UNDER

MAXIMUM

LOAD

CONDITIONS

VOUT = 12V

CIRCUIT OF

FIGURE 2b

MAX770–3-06

MAX770–MAX773

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

Low I

, Step-Up DC-DC Controllers

_______________________________________________________________________________________

250

0-60 -20 60 140

REFERENCE OUTPUT RESISTANCE vs.

TEMPERATURE

MAX770–3-10

TEMPERATURE (°C)

REFERENCE OUTPUT RESISTANCE (Ω)

20 100

150

-40 0 8040 120

100

200

100µA

50µA

10µA

1.502

-60 -20 60 140

REFERENCE vs. TEMPERATURE

MAX770–3-11

TEMPERATURE (°C)

REFERENCE (V)

20 100-40 0 8040 120

1.500

1.498

1.496

1.494

1.492

1.504

1.506

4.0

-60 -20 60 140

SHUTDOWN CURRENT vs. TEMPERATURE

MAX770–3-12

TEMPERATURE (°C)

ICC (µA)

20 100-40 0 8040 120

3.5

3.0

2.5

2.0

1.5

1.0

0.5

V+ = 15V

V+ = 4V

V+ = 8V

15.5

16.0

16.5

-60 -30 0 30 60 90 120 150

MAXIMUM SWITCH ON-TIME vs.

TEMPERATURE

TEMPERATURE (°C)

tON(MAX) (µs)

MAX770–3-13

2.20

2.25

2.30

-60 -30 0 30 60 90 120 150

MINIMUM SWITCH OFF-TIME vs.

TEMPERATURE

TEMPERATURE (°C)

tOFF(MIN) (µs)

MAX770–3-14

6.0

7.0

8.0

-60 -30 0 30 60 90 120 150

MAXIMUM SWITCH ON-TIME/

MINIMUM SWITCH OFF-TIME RATIO

vs. TEMPERATURE

TEMPERATURE (°C)

tON(MAX)/tOFF(MIN) RATIO

MAX770–3-15

7.5

6.5

____________________________Typical Operating Characteristics (continued)

(TA = +25°C, unless otherwise noted.)

MAX770–MAX773

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

Low I

, Step-Up DC-DC Controllers

6_______________________________________________________________________________________

VIN = 2.9V, IOUT = 0.9A

A: EXT VOLTAGE, 5V/div

B: INDUCTOR CURRENT 1A/div

C: VOUT RIPPLE 100mV/div, AC-COUPLED

MAX770

HEAVY-LOAD SWITCHNG WAVEFORMS

20µs/div

VOUT

0 A

ILIM

2 B

V+ = 3V, IOUT = 165mA

A: EXT VOLTAGE, 5V/div

B: INDUCTOR CURRENT, 1A/div

C: VOUT RIPPLE 100mV/div, AC-COUPLED

MAX770

LIGHT-LOAD SWITCHING WAVEFORMS

20µs/div

ILIM

IOUT = 0.7A

A: VIN, 2.7V TO 4.5V, 2V/div

B: VOUT RIPPLE, 100mV/div, AC-COUPLED

MAX770

LINE-TRANSIENT RESPONSE

4.5V

2.7V

2ms/div

VIN = 3V

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

B: VOUT RIPPLE, 100mV/div, AC-COUPLED

MAX770

LOAD-TRANSIENT RESPONSE

2ms/div

____________________________Typical Operating Characteristics (continued)

(Circuit of Figure 2a, TA = +25°C, unless otherwise noted.)

MAX770–MAX773

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

Low I

, Step-Up DC-DC Controllers

_______________________________________________________________________________________ 7

VIN = 3V, IOUT = 0.5A

A: SHDN, 2V/div

B: VOUT, 2V/div

MAX770

EXITING SHUTDOWN

200µs/div

______________________________________________________________Pin Description

____________________________Typical Operating Characteristics (continued)

(Circuit of Figure 2a, TA = +25°C, unless otherwise noted.)

NAME FUNCTION

MAX773

1 — EXT Gate drive for external N-channel power transistor

2 3 V+

3 6 FB

4 7 SHDN

5 8 REF

6 — AGND Analog ground

7 9 GND High-current ground return for the output driver

811 CS

— 1 V12

— 2 V5

MAX770

MAX771

MAX772

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 0.1µF 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, VOUT is a diode drop

below V+ (due to the DC path from V+ to the output) and the supply current drops to 5µA

maximum. Connect to ground for normal operation.

1.5V reference output that can source 100µA for external loads. Bypass to GND with 0.1µF.

The reference is disabled in shutdown.

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

Input sense point for 12V-output operation. Connect VOUT to V12 for 12V-output operation.

Leave unconnected for adjustable-output operation.

Input sense point for 5V-output operation. Connect VOUT to V5 for 5V-output operation. Leave

unconnected for adjustable-output operation.

— 4 LBO

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

—10 SGND Shunt regulator ground. Leave unconnected if the shunt regulator is not used.

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.

MAX770–MAX773

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

Low I

, Step-Up DC-DC Controllers

8_______________________________________________________________________________________

_______________Detailed Description

The MAX770–MAX773 are BiCMOS, step-up, switch-

mode power-supply controllers that provide preset 5V,

12V, and 15V output voltages, in addition to adjustable-

output operation. Their unique control scheme com-

bines 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 con-

verters 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/out-

put 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 boot-

strapped mode, the IC is powered from the output

(VOUT, which is connected to V+) and the input voltage

range is 2V to VOUT. The voltage applied to the gate of

the external power transistor is switched from VOUT 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 volt-

age, 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 high-

est efficiency, operate in bootstrapped mode when-

ever 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).

NAME FUNCTION

MAX770

MAX771

MAX772 MAX773

—13 EXTH

—12 EXTL 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.

—14 V15 Input sense point for 15V-output operation. Connect VOUT to V15 for 15V-output operation.

Leave unconnected for adjustable-output operation

_________________________________________________Pin Description (continued)

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 RBASE from

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

Floating the shunt-regulator ground (SGND) disables

the shunt regulator. To enable it, connect SGND to

GND. The shunt regulator requires 1mA minimum cur-

rent for proper operation; the maximum current must

not exceed 20mA. The MAX773 operates in non-boot-

strapped 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 pow-

er 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 com-

bines 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 induc-

tor current. They also operate with high switching

MAX770–MAX773

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

Low I

, Step-Up DC-DC Controllers

_______________________________________________________________________________________ 9

V+

EXT

CONTROL

1.5V

REFERENCE

Q TRIG

QS F/F

QTRIG

LOW-VOLTAGE

OSCILLATOR 2.5V

0.1V0.2V

ONE-SHOT

CURRENT-SENSE

AMPLIFIER

DUAL-MODE

COMPARATOR

LBO V15 V12 V5 FB

LBI

REF

200mV

ERROR

COMPARATOR

SHDN

V+

SGND

EXTH

EXTL

EXT

MAX773 ONLY

MAX770

MAX771

MAX772

MAX770

MAX771

MAX772

BIAS

CIRCUITRY

MAX770–MAX773

MAX773

ONLY

Figure 1. Block Diagram

MAX770–MAX773

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 tra-

ditional 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 contin-

uous-conduction mode (CCM) while maintaining high

efficiency with heavy loads. When the power switch is

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

Low I

, Step-Up DC-DC Controllers

10 ______________________________________________________________________________________

MAX770

VIN = 3V

REF

SHDN

AGND

GND

EXT

0.1µF

100µF

22µHD1

1N5817

MTP3055EL

RSENSE

75mΩC4

300µF

0.1µF

V+

VOUT = 5V

@ 1A

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

Figure 2c. 12V Output, Non-Bootstrapped Figure 2d. 9V Output, Bootstrapped

MAX770

MAX771

MAX772

VIN = 5V

REF

SHDN

AGND

GND

EXT

22µHD1

1N5817

18k

200µF

0.1µF

V+

68µF

VOUT = 12V

@ 0.5A

127k

RSENSE

100mΩ

0.1µF

VOUT

VREF

R2 = (R1) ( -1)

VREF = 1.5V

MAX770

MAX771

MAX772

REF

SHDN

AGND

GND

EXT

47µF

20µHD1

1N5817

28k

100µF

0.1µF

V+ VOUT = 9V

140k

RSENSE

0.1µF

VOUT

VREF

R2 = (R1) ( -1)

VREF = 1.5V

Si9410DY

VIN = 4V

MAX771

VIN = 5V

REF

SHDN

AGND

GND

EXT

0.1µF

68µF

22µHD1

1N5817

Si9410DY

RSENSE

100mΩC4

200µF

0.1µF

V+

VOUT = 12V

@ 0.5A

MAX770–MAX773

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

Low I

, Step-Up DC-DC Controllers

______________________________________________________________________________________ 11

GND

MAX773

0.1µF

VOUT

= 12V

V+

63.4k

(1%)

10k

(1%)

SGND

LBO

EXTH

EXTL

V12

V15

REF

LBI

SHDN

RSENSE

Si9410DY

0.1µFVIN

22µH

1N5817

100k

R4 = R3 ( VTRIP -1)

VREF

MIN NOMINAL MAX

10.6 11.0 11.4

VTRIP (V)

VREF = 1.5V

GND

MAX773

0.1µF

V+

EXTL

V12

7V5

SGND

LBO

REF

LBI

SHDN

RSENSE

0.4Ω

150µF

0.1µFD1

1N5818

V15 14

EXTH 13

47µF3

150µH

910Ω

34k

510k

ZTX694B

VOUT = 24V

@ 30mA

VIN = 5V

VOUT

VREF

R2 = (R1) ( -1)

VREF = 1.5V

GND

MAX773

0.1µF

V+

EXTL

6V15

V12

SGND

LBO

REF

SHDN

VIN = 5V

RSENSE C4

0.1µFD1

1N5817

EXTH 13

22µH

VOUT = 15V

LBI

Si9410DY

GND

MAX773

0.1µF

V+

EXTL

V15

V12

LBO

REF

LBI

SHDN

VIN

RSENSE

1N5817

EXTH 13

20µHVOUT = 16V

SGND

0.1µF

FB 6

133k

13.7k VOUT

VREF

R2 = (R1) ( -1)

VREF = 1.5V

Si9410DY

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

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

Transistor

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

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

Power MOSFET

MAX770–MAX773

turned on, it stays on until either 1) the maximum on-

time 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 volt-

age 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 volt-

ages, 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-com-

parator output when the supply voltage is less than

2.5V. Above 2.5V, the error-comparator and normal one-

shot timing circuitry are used. The low voltage start-up

circuitry is disabled if non-bootstrapped mode is select-

ed (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

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

Low I

, Step-Up DC-DC Controllers

12 ______________________________________________________________________________________

GND

MAX773

0.1µF

EXTL

V15

V12

LBO

REF

LBI

SHDN

412

RSENSE

1.0Ω

100µF

MUR115

EXTH 13

250µH

VOUT = 100V

@ 10mA

SGND N

47µF

FB 6R2

732k (1%)

11.3k (1%)

Si9420DY

RSHUNT

0.1µF

VIN = 24V TO 28V

VOUT

VREF

R2 = (R1) ( -1)

VREF = 1.5V

MAX773

RSHUNT

0.1µF

SGND

6V (typ)

V+

VIN

RSHUNT = VIN (MIN) - VSHUNT (MAX)

I SHUNT *

* SEE TEXT FOR ISHUNT CALCULATION

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

MOSFET Figure 4. MAX773 Shunt Regulator

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 shut-

down mode. In this mode, the internal biasing circuit-

ry is turned off (including the reference) and VOUT

falls to a diode drop below VIN (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 shut-

down 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 VREF,LBO (an open-

drain 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

VTRIP. 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.

See Table 1 for a summary of operating characteristics

and requirements for the ICs in bootstrapped and non-

bootstrapped modes.

The MAX770–MAX773’s output voltage can be adjust-

ed 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 resis-

tor R1 in the range of 10kΩto 500kΩ. R2 is given by:

VOUT

R2 = (R1) (––––– -1)

VREF

where VREF 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 non-

bootstrapped 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 RSHUNT such that 1mA ≤ISHUNT ≤20mA.

MAX770–MAX773

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

Low I

, Step-Up DC-DC Controllers

______________________________________________________________________________________ 13

MAX770

MAX771

MAX772

MAX773

GND

FB VOUT

R1 = 10k TO 500k

VOUT

VREF

R2 = R1 ( -1)

VREF = 1.5V

Figure 5. Adjustable Output Circuit

MAX770–MAX773

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-regu-

lator current powers the MAX773 and also provides the

FET gate-drive current, which depends largely on the

FET’s total gate charge at VGS = 5V. To determine the

shunt-resistor value, first determine the maximum shunt

current required.

ISHUNT = ISUPP + IGATE

See

N-Channel MOSFETs

in the

Power-Transistor

Selection

section to determine IGATE.

Determine the shunt-resistor value using the following

equation:

VIN(min) - VSHUNT(max)

RSHUNT(max) = ————————————

ISHUNT

where VSHUNT(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 reg-

ulator 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 ISHUNT = 3mA. This provides adequate biasing

current for this circuit, although higher shunt currents

can be used.

To prevent the shunt regulator from drawing current in

shutdown mode, place a switch in series with the shunt

resistor.

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

Low I

, Step-Up DC-DC Controllers

14 ______________________________________________________________________________________

MAX773(N)/MAX773(S)MAX770–MAX773(N)Fixed Output Available

Higher

Lower

GND to VOUT

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

3V to 5V (MAX773)

MAX770–MAX773(N)

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)

BOOTSTRAPPED*

MAX770/MAX771/MAX772/

MAX773(N)/MAX773(S)

Adjustable Output Available

LowerGate-Drive Capacitive Losses HigherFET On Resistance GND to V+Gate Drive

5V to 16.5V

(MAX770/MAX771/MAX772),

5V and up (MAX773)

Normally Recommended Input

Voltage Range

LowerNo-Load Supply Current

3V to 16.5V

(MAX770/MAX771/MAX772),

3V and up (MAX773)

Possible Input Voltage Range

NON-BOOTSTRAPPEDPARAMETER

Table 1. Bootstrapped vs. Non-Bootstrapped Operation

MAX773

SGND

RSHUNT

EXTL 100Ω

V+

0.1µF

VIN

20µH

NPN

2N2222A

D1 VOUT

RSENSE

PNP

2N2907A

310

EXTH

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

Regulator

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

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 RSENSE. These theoretical

curves assume that an external N-FET power switch is

used. They were derived using the minimum (worst-

case) current-limit comparator threshold value, and the

inductance value. No tolerance was included for

RSENSE. The voltage drop across the diode was

assumed to be 0.5V, and the drop across the power

switch rDS(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

sense-resistor value with an output current that is ade-

quate at the lowest input voltage.

Determining the Inductor (L)

Practical inductor values range from 10 µH to 300µH.

20µH is a good choice for most applications. In appli-

cations 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 (tON(min))

is approximately 2µs; select an inductor that allows

the current to ramp up to ILIM/2 in no less than 2µs.

Choosing a value of ILIM/2 allows the half-size current

pulses to occur, increasing light-load efficiency and

minimizing output ripple.

MAX770–MAX773

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

Low I

, Step-Up DC-DC Controllers

______________________________________________________________________________________ 15

MAXIMUM OUTPUT CURRENT (A)

INPUT VOLTAGE (V)

0.5

1.0

1.5

2.0

2.5

3.0

3.5

2 3 4 5

RSENSE = 40mΩ

RSENSE = 50mΩ

RSENSE = 75mΩ

RSENSE = 200mΩ

RSENSE = 100mΩ

VOUT = 5V

L = 22µH

MAXIMUM OUTPUT CURRENT (A)

INPUT VOLTAGE (V)

0.5

1.0

1.5

2.0

2.5

3.0

3.5

2 4 6 8 10 12

RSENSE = 200mΩ

RSENSE = 100mΩ

RSENSE = 40mΩ

RSENSE = 50mΩ

RSENSE = 75mΩ

VOUT = 12V

L = 22µH

Figure 7a. Maximum Output Current vs. Input Voltage

(VOUT = 5V) Figure 7b. Maximum Output Current vs. Input Voltage

(VOUT = 12V)

Figure 7c. Maximum Output Current vs. Input Voltage

(VOUT = 15V) Figure 7d. Maximum Output Current vs. Input Voltage

(VOUT = 24V)

MAXIMUM OUTPUT CURRENT (A)

INPUT VOLTAGE (V)

0.5

1.0

1.5

2.0

2.5

3.0

3.5

2 4 6 8 10 12 14 16

RSENSE = 200mΩ

RSENSE = 100mΩ

VOUT = 15V

L = 22µH

RSENSE = 40mΩ

RSENSE = 50mΩ

RSENSE = 75mΩ

MAXIMUM OUTPUT CURRENT (A)

02INPUT VOLTAGE (V)

0.8

6 10 14

0.2

0.4

0.6

VOUT = 24V

L =150µH

RSENSE = 100mΩRSENSE = 200mΩ

RSENSE = 400mΩ

MAX770–MAX773

The standard operating circuits use a 22µH inductor.

If a different inductance value is desired, select L

such that: VIN(max) x tON(min)

L ≥——————————

ILIM/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 recom-

mended; 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 RSENSE.

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 20mΩ. To minimize radiated noise,

use a toroid, a pot core, or a shielded coil.

Table 2 lists inductor suppliers and specific recom-

mended 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 recom-

mended 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 typi-

cally 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 (Qg), on resistance (rDS(ON)),

and reverse transfer capacitance (CRSS).

Qgtakes into account all capacitances associated with

charging the gate. Use the typical Qgvalue for best

results; the maximum value is usually grossly over-

specified since it is a guaranteed limit and not the mea-

sured 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

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

Low I

, Step-Up DC-DC Controllers

16 ______________________________________________________________________________________

MAX770

MAX771

MAX772

EXT

RSENSE

MAX773

NPN

EXTH

EXTL

RSENSE

RBASE

IC(PEAK)

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

MAX770/MAX771/MAX772

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

Figure 8c. Using an NPN Transistor with the MAX773

MAX773

EXTH

EXTL

RSENSE

The two most significant losses contributing to the

N-FET’s power dissipation are I2R losses and switching

losses. Select a transistor with low rDS(ON) and low

CRSS to minimize these losses.

Determine the maximum required gate-drive current

from the Qgspecification 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 Qg(typ). Use the typical Qgnumber from the

transistor data sheet. For example, the Si9410DY has a

Qg(typ) of 17nC (at VGS = 5V), therefore the current

required to charge the gate is:

IGATE (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): Qg

∆V+ = ——

Continuing with the example, ∆V+ = 17nC/0.1µF = 170mV.

Use IGATE 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

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

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

on resistance with 4.5V VGS, and a guaranteed VTH 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 exces-

sive power dissipation in the transistor; too much base

current can cause the base to oversaturate, so the tran-

sistor remains on continually. Both conditions can dam-

age the transistor.

When using the MAX773 with an NPN transistor, con-

nect EXTL to the transistor’s base, and connect RBASE

between EXTH and the base (Figure 8c).

To determine the required peak inductor current,

IC(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 VCS(max) by the sense-

resistor value. To determine IB, set the peak inductor

current (ILIM) equal to the peak transistor collector cur-

rent IC(PEAK). Calculate IBas follows:

IB= ILIM/ß

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 ILIM.

It may be necessary to use even higher base currents

(e.g., IB= ILIM/10), although excessive IBmay impair

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

RBASE is determined by:

(VEXTH - VBE - VCS(min))

RBASE = ————————————–

Where VEXTH is the voltage at V+ (in bootstrapped

mode VEXTH is the output voltage), VBE is the 0.7V

transistor base-emitter voltage, VCS(min) is the voltage

drop across the current-sense resistor, and IBis the

minimum base current that forces the transistor into

saturation. This equation reduces to (V+ - 700mV -

170mV) / IB.

For maximum efficiency, make RBASE as large as pos-

sible, but small enough to ensure the transistor is

always driven near saturation. Highest efficiency is

obtained with a fast-switching NPN transistor

(fT≥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 RSENSE, and that

its breakdown voltage exceeds VOUT. For high-temper-

ature 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 for-

ward 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 50mΩof

ESR and typically provides 180mV ripple when

stepping up from 3V to 5V at 1A (Figure 2a).

MAX770–MAX773

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

Low I

, Step-Up DC-DC Controllers

______________________________________________________________________________________ 17

MAX770–MAX773

Smaller capacitors are acceptable for light loads or in

applications that can tolerate higher output ripple.

Since the output filter capacitor’s ESR affects efficien-

cy, 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 capaci-

tors 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 imped-

ance 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

(VTRIP(falling)), select R3 between 10kΩand 500kΩ

(Figure 9), and calculate R4 as follows:

VTRIP - VREF

R4 = (R3) (———————)

VREF

where VREF = 1.5V.

The rising trip voltage is higher because of the com-

parator’s approximately 20mV of hysteresis, and is

determined by: R4

VTRIP (rising) = (VREF + 20mV) (1 + ——)

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., 100kΩ) between LBO

and V+. Tie LBI to GND and leave LBO floating if the

low-battery detector is not used.

__________Applications Information

MAX773 Operation with High

Input/Output Voltages

The MAX773’s shunt regulator input allows high volt-

ages 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 volt-

age. Use an external logic-level N-FET as the power

switch, since only 6V of VGS 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 non-

bootstrapped mode than in bootstrapped mode, since

bootstrapped mode typically provides much higher

VGS voltages. To avoid this condition, make sure VEXT

is above the VTH 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 predeter-

mined minimum value. Excessive loads with low input

voltages can also cause this condition.

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

Low I

, Step-Up DC-DC Controllers

18 ______________________________________________________________________________________

MAX773

LBI LBO

GND

V+

VIN

100k

R3 LOW-BATTERY

OUTPUT

VTRIP

VREF

R4 = R3 ( -1)

VREF = 1.5V

Figure 9. Input Voltage Monitor Circuit

Starting Up under Load

The

Typical Operating Characteristics

show the Start-

Up Voltage vs. Load Current graph for bootstrapped-

mode operation. This graph depends on the type

of power switch used. The MAX770–MAX773 are

not designed to start up under full load in boot-

strapped mode with low input voltages.

Layout Considerations

Due to high current levels and fast switching wave-

forms, 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

lead, and the output filter capacitor ground lead to a

single point (star ground configuration). Also, minimize

lead lengths to reduce stray capacitance, trace resis-

tance, and radiated noise. Place input bypass capaci-

tor 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 efficien-

cy. If desired, place a 4.7µF directly across the V+ and

GND pins (in parallel with the 0.1µF C2 bypass capaci-

tor) to reduce the noise at V+.

MAX770–MAX773

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

Low I

, Step-Up DC-DC Controllers

______________________________________________________________________________________ 19

Table 2. Component Suppliers

PRODUCTION INDUCTORS CAPACITORS TRANSISTORS

Surface Mount

Matsuo

267 series

Sprague

595D series

Through Hole

Sumida

CD54 series

CDR125 series

Coiltronics

CTX20 series

Motorola

1N5817–1N5822

MUR115 (high voltage)

Nihon

EC10 series

DIODES

Sumida

RCH855 series

RCH110 series

Renco

RL1284-18

Sanyo

OS-CON series

Nichicon

PL series

United Chemi-Con

LXF series

NPN

Zetex

ZTX694B

Coiltronics

Matsuo USA: (714) 969-2491 (714) 960-6492

Japan: 81-6-337-6450 81-6-337-6456

Nichicon USA: (847) 843-7500 (847) 843-2798

Nihon USA: (805) 867-2555 (805) 867-2698

Renco USA: (516) 586-5566 (516) 586-5562

Sanyo USA: (619) 661-6835 (619) 661-1055

Japan: 81-7-2070-6306 81-7-2070-1174

Sumida USA: (847) 956-0666

Japan: 81-3-3607-5111 81-3-3607-5144

United Chemi-Con USA: (714) 255-9500 (714) 255-9400

N-FET

Siliconix

Si9410DY

Si9420DY (high voltage)

Motorola

MTP3055EL

MTD20N03HDL

USA: (561) 241-7876 (561) 241-9339

SUPPLIER PHONE FAX

Zetex USA: (516) 543-7100 (516) 864-7630

UK: 44-61-627-4963 44-61-627-5467

MAX770–MAX773

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

Low I

, Step-Up DC-DC Controllers

20 ______________________________________________________________________________________

V15

EXTH

EXTL

LBO

V+

V12

MAX773

SGND

GND

REF

SHDN

LBI

DIP/SO

___Ordering Information (continued)

____Pin Configurations (continued)

TOP VIEW

_________________Chip Topographies

TRANSISTOR COUNT: 501;

SUBSTRATE CONNECTED TO V+.

TRANSISTOR COUNT: 501;

SUBSTRATE CONNECTED TO V+.

EXTH

EXTL

SGND

GND

V5 V12 V15

V+

LBO

LBI

SHDN REF

0.126"

(3.200mm)

0.080"

(2.032mm)

MAX770/MAX771/MAX772

MAX773

V+

0.126"

(3.200mm)

0.080"

(2.032mm)

EXT

GND

AGND

SHDN REF

14 CERDIP-55°C to +125°CMAX773MJD 14 Narrow SO-40°C to +85°CMAX773ESD 14 Plastic DIP-40°C to +85°CMAX773EPD Dice*0°C to +70°CMAX773C/D 14 SO0°C to +70°CMAX773CSD 14 Plastic DIP0°C to +70°C

MAX773CPD 8 CERDIP-55°C to +125°CMAX772MJA 8 SO-40°C to +85°CMAX772ESA 8 Plastic DIP-40°C to +85°CMAX772EPA Dice*0°C to +70°CMAX772C/D 8 SO0°C to +70°CMAX772CSA 8 Plastic DIP0°C to +70°C

MAX772CPA 8 CERDIP-55°C to +125°CMAX771MJA 8 SO-40°C to +85°CMAX771ESA 8 Plastic DIP-40°C to +85°CMAX771EPA Dice*0°C to +70°CMAX771C/D 8 SO0°C to +70°CMAX771CSA 8 Plastic DIP0°C to +70°C

MAX771CPA PIN-PACKAGETEMP. RANGEPART

*Contact factory for dice specifications.