Rainbow Electronics MAX773 User Manual

_______________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

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 IQ, Step-Up DC-DC Controllers

________________________________________________________________

Maxim Integrated Products

1

1
2
3
4

8
7
6
5

CS
GND
AGND

REF

SHDN

FB

V+

EXT

MAX770
MAX771
MAX772

DIP/SO

_________________Pin Configurations

FB AGND GND

V+

CS

EXT

N

REF

SHDN

ON/OFF

OUTPUT

12V

INPUT

2V TO V

OUT

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 [phone].
For small orders, phone [phone] ext. 3468.

MAX770–MAX773

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

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, TA= T

MIN

to T

MAX

, 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

4

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,
I

LOAD

= 700mA, V

OUT

= 5V

5 mV/V

Figure 2a, V+ = 3V, I

LOAD

= 30mA to 1A,

V

OUT

= 5V

20 mV/A

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

µs

Minimum Switch Off-Time t

OFF

(min) 1.8 2.3 2.8

µs

Efficiency 87 %

Reference Voltage V

REF

I

REF =

0µA

MAX77_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, I

LOAD

= 500mA, V

OUT

= 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 IQ, Step-Up DC-DC Controllers

_______________________________________________________________________________________

3

ELECTRICAL CHARACTERISTICS (continued)

(V+ = 5V, I

LOAD

= 0mA, TA= T

MIN

to T

MAX

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

PARAMETERS

SYMBOL CONDITIONS MIN TYP MAX UNITS

REF Load Regulation

0µA ≤ I

REF

≤ 100µA

MAX77_C/E 4 10

mV

MAX77_M 4 15

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

µV/V

FB Trip-Point Voltage V

FB

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 I

FB

MAX77_C ±20

nAMAX77_E ±40

MAX77_M ±60

SHDN Input High Voltage V

IH

V+ = 2.0V to 16.5V 1.6 V

SHDN Input Low Voltage V

IL

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, V

LBO

= 16.5V 0.01 1.00

µA
LBO Output Voltage Low V

OL

MAX773, V+ = 5V, LBO sinking 1mA 0.1 0.4 V

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

mV

Current-Limit Trip Level V

CS

V+ = 5V to 16.5V 170 200 230 mV

V

SHUNT

MAX773, I

SHUNT

= 1mA to 20mA,

SGND = 0V, C

SHUNT

= 0.1µF

5.5 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

0

1

2

3

4

-75

-50 -25 0

25 50 75 100 125

SUPPLY CURRENT vs. TEMPERATURE

TEMPERATURE (°C)

SUPPLY CURRENT (mA)

V

OUT

= 12V, VIN = 5V
CIRCUIT OF FIGURE 2b
BOOTSTRAPPED MODE

ENTIRE
CIRCUIT

SCHOTTKY DIODE
LEAKAGE EXCLUDED

MAX770–3-07

0

0.2

0.4

0.6

0.8

2

4

6 8

10

12

SUPPLY CURRENT vs. SUPPLY VOLTAGE

SUPPLY VOLTAGE (V)

SUPPLY CURRENT (mA)

V

OUT

= 12V

NON-BOOTSTRAPPED
CIRCUIT OF FIGURE 2c

BOOTSTRAPPED
CIRCUIT OF
FIGURE 2b

MAX770–3-08

0

100

150

200

250

50

2

4

6 8

10

12

EXT RISE/FALL TIME vs. SUPPLY VOLTAGE

V+ (V)

EXT RISE/FALL TIME (ns)

C

EXT

= 2200pF

C

EXT

= 1000pF

C

EXT

= 446pF

C

EXT

= 100pF

MAX770–3-09

100

50

0.001 0.01 0.1 1

MAX772

EFFICIENCY vs. OUTPUT CURRENT

(BOOTSTRAPPED)

60

OUTPUT CURRENT (A)

EFFICIENCY (%)

70

80

90

V

OUT

= 15V, CIRCUIT OF FIGURE 2b

MAX772 SUBSTITUTED FOR MAX771

VIN = 12V

VIN = 9V
VIN = 6V
VIN = 5V

VIN = 3V

MAX770–3-03

50

60

70

80

90

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

V

OUT

= 12V
CIRCUIT OF
FIGURE 2b

MAX770–3-02

100

50

0.001 0.01 0.1 1

MAX770

EFFICIENCY vs. OUTPUT CURRENT

(BOOTSTRAPPED)

60

OUTPUT CURRENT (A)

EFFICIENCY (%)

70

80

90

VIN = 3V

VIN = 3.5V

V

OUT

= 5V
CIRCUIT OF
FIGURE 2a

VIN = 4V

MAX770–3-01

MAX770–MAX773

5V/12V/15V or Adjustable, High-Efficiency,
Low IQ, Step-Up DC-DC Controllers

4

_______________________________________________________________________________________

__________________________________________Typical Operating Characteristics

(TA = +25°C, unless otherwise noted.)

70

80

90

100

0.001

0.01

0.1

10

1

MAX771

EFFICIENCY vs. OUTPUT CURRENT

(NON-BOOTSTRAPPED)

OUTPUT CURRENT (A)

EFFICIENCY (%)

VIN = 9V

VIN = 6V

VIN = 5V

V

OUT

= 12V
CIRCUIT OF
FIGURE 2c

MAX770–3-04

0

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

V

OUT

= 5V
CIRCUIT OF
FIGURE 2a

MAX770–3-05

0

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

V

OUT

= 12V
CIRCUIT OF
FIGURE 2b

MAX770–3-06

MAX770–MAX773

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

Low IQ, Step-Up DC-DC Controllers

_______________________________________________________________________________________

5

250

0

-60 -20 60 140

REFERENCE OUTPUT RESISTANCE vs.

TEMPERATURE

50

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)

I

CC

(µA)

20 100-40 0 8040 120

3.5

3.0

2.5

2.0

1.5

1.0

0.5
0

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)

t

ON(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)

t

OFF(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)

t

ON(MAX)/

t

OFF(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 IQ, Step-Up DC-DC Controllers

6

_______________________________________________________________________________________

VIN = 2.9V, I

OUT

= 0.9A
A: EXT VOLTAGE, 5V/div
B: INDUCTOR CURRENT 1A/div
C: V

OUT

RIPPLE 100mV/div, AC-COUPLED

MAX770

HEAVY-LOAD SWITCHNG WAVEFORMS

20µs/div

V

OUT

0 A

I

LIM

I

LIM

2

B

C

0

V+ = 3V, I

OUT

= 165mA
A: EXT VOLTAGE, 5V/div
B: INDUCTOR CURRENT, 1A/div
C: V

OUT

RIPPLE 100mV/div, AC-COUPLED

MAX770

LIGHT-LOAD SWITCHING WAVEFORMS

20µs/div

0

B

A

C

I

LIM

2

I

OUT

= 0.7A

A: V

IN

, 2.7V TO 4.5V, 2V/div

B: V

OUT

RIPPLE, 100mV/div, AC-COUPLED

MAX770

LINE-TRANSIENT RESPONSE

A

B

4.5V

2.7V

0

2ms/div

VIN = 3V
A: LOAD CURRENT 0.5A/div (0A to 1A)
B: V

OUT

RIPPLE, 100mV/div, AC-COUPLED

MAX770

LOAD-TRANSIENT RESPONSE

2ms/div

A

B

0

____________________________Typical Operating Characteristics (continued)

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

MAX770–MAX773

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

Low IQ, Step-Up DC-DC Controllers

_______________________________________________________________________________________

7

VIN = 3V, I

OUT

= 0.5A
A: SHDN, 2V/div
B: V

OUT

, 2V/div

MAX770

EXITING SHUTDOWN

A

B

0

0

200µs/div

______________________________________________________________Pin Description

____________________________Typical Operating Characteristics (continued)

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

PIN

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
8 11 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, V

OUT

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 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.

— 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 IQ, Step-Up DC-DC Controllers

8

_______________________________________________________________________________________

_______________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).

PIN

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 V

OUT

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 R

BASE

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 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

MAX770–MAX773

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

Low IQ, Step-Up DC-DC Controllers

_______________________________________________________________________________________

9

V+

EXT

CONTROL

1.5V

REFERENCE

Q TRIG

QS

F/F

R

QTRIG

LOW-VOLTAGE

OSCILLATOR

2.5V

0.1V0.2V

ONE-SHOT

ONE-SHOT

CURRENT-SENSE

AMPLIFIER

DUAL-MODE
COMPARATOR

LBO V15 V12 V5 FB

LBI

REF

200mV

ERROR

COMPARATOR

SHDN

V+

SGND

6V

EXTH

EXTL

EXT

CS

MAX773 ONLY

MAX770
MAX771
MAX772

MAX770
MAX771
MAX772

BIAS

CIRCUITRY

N

N

N

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 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

5V/12V/15V or Adjustable, High-Efficiency,
Low IQ, Step-Up DC-DC Controllers

10

______________________________________________________________________________________

MAX770

VIN = 3V

REF

SHDN

FB

AGND

GND

N

7

EXT

CS

C2

0.1µF

C1

100µF

L1

22µH

D1

1N5817

MTP3055EL

R

SENSE

75mΩ

C4
300µF

C3

0.1µF

5

4

3

6

1

8

2

V+

V

OUT

= 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

N

7

EXT

CS

FB

L1

22µH

D1

1N5817

R1

18k

C4
200µF

C3

0.1µF

5

4

6

1

8

3

2

V+

C1

68µF

V

OUT

= 12V

@ 0.5A

R2

127k

R

SENSE

100mΩ

C2

0.1µF

V

OUT

V

REF

R2 = (R1) ( -1)

V

REF

= 1.5V

VIN = 5V

C2

0.1µF

0.1µF

5

REF

C3

4

SHDN

2

V+

MAX771

L1

22µH

1N5817

68µF
D1

C1

V

= 12V

OUT

@ 0.5A

3

FB

6

AGND

GND

7

EXT

1

8

CS

C2

0.1µF

5

REF

C3

0.1µF
4

SHDN

6

AGND

V

= 1.5V

OUT

V

REF

R2 = (R1) ( -1)

V

REF

2

V+

MAX770
MAX771
MAX772

GND

7

EXT

1

8

CS

3

FB

VIN = 4V

L1

20µH

N
Si9410DY

R

SENSE

100mΩ

D1

1N5817

N
Si9410DY

R

SENSE

R1

28k

47µF

C4
200µF

C1

V

= 9V

OUT

C4
100µF

R2

140k

MAX770–MAX773

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

Low IQ, Step-Up DC-DC Controllers

______________________________________________________________________________________

11

GND

MAX773

C3

0.1µF

V

OUT

= 12V

V+

R4

63.4k
(1%)

R3

10k

(1%)

SGND

LBO

EXTH

8

5

7

6

EXTL

CS

V12
V15

V5
REF

LBI

SHDN

FB

2

14

1

10

4

13
12

11

9

R

SENSE

N

Si9410DY

C4

C2

0.1µF

V

IN

L1
22µH

D1

1N5817

100k

C1

R

4 = R3

(

V

TRIP

-1

)

V

REF

MIN NOMINAL MAX

10.6 11.0 11.4

V

TRIP

(V)

3

VREF = 1.5V

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

VIN = 5V

C1

C3

0.1µF

C2

0.1µF

3

SGND

LBO

REF

V+

EXTH

EXTL

CS

10

4

8

L1

22µH

13
12

11

MAX773

7

SHDN

6

FB

5

LBI

GND

9

V15
V12

V5

14
1

2

D1

1N5817

N

Si9410DY

R

SENSE

VIN = 5V

C1

47µF

C3

0.1µF
V15

V12

L1

150µH

910Ω

13

12

11

CS

R

34k

SENSE

0.4Ω

R1

14
1

2

V5

6

FB

D1

V

OUT

1N5818

@ 30mA

ZTX694B

C4

150µF

R2

510k

V

= 1.5V

OUT

V

REF

R2 = (R1) ( -1)

V

REF

= 24V

C2

0.1µF
3

V+

10

SGND

4

LBO

8

REF

5

LBI

7

SHDN

EXTH

EXTL

MAX773

GND

9

V

IN

C1

V

= 15V

OUT

C4

C3

0.1µF

4

LBO

8

REF

5

LBI

7

SHDN

10

SGND

V+

MAX773

GND

3

13

EXTH

12

EXTL

11

CS

14

V15

1

V12

2

V5

6

FB

9

C2

0.1µF

R

SENSE

20µH

13.7k

L1

R1

1N5817

N

Si9410DY

V

= 16V

OUT

D1

C4

R2

133k

V

= 1.5V

OUT

V

REF

R2 = (R1) ( -1)

V

REF

MAX770–MAX773

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

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

______________________________________________________________________________________

GND

MAX773

C3

0.1µF

v+

EXTL

CS

V5

8

5

7

V15
V12

LBO

REF

LBI

SHDN

4

12
11

2

14
1

9

R

SENSE

1.0Ω

C4
100µF

D1

MUR115

EXTH

13

3

L1

250µH

V

OUT

= 100V

@ 10mA

SGND

N

10

C1

47µF

FB

6

R2

732k (1%)

R1

11.3k (1%)

Si9420DY

R

SHUNT

3k

C2

0.1µF

VIN = 24V TO 28V

V

OUT

V

REF

R2 = (R1) ( -1

)

V

REF

= 1.5V

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

Figure 4. MAX773 Shunt Regulator

V

IN

R

SHUNT

MAX773

3

V+

C2

0.1µF

V

IN (MIN)

R

SHUNT =

I

*

SEE TEXT FOR I

- V

SHUNT (MAX)

SHUNT *

CALCULATION

SHUNT

6V (typ)

10

SGND

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 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 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.

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 10kΩ to 500kΩ. R2 is given by:

V

OUT

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.

MAX770–MAX773

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

Low IQ, Step-Up DC-DC Controllers

______________________________________________________________________________________

13

MAX770
MAX771
MAX772
MAX773

R1

R2

GND

FB

V

OUT

R1 = 10k TO 500k

V

OUT

V

REF

R2 = R1 ( -1

)

V

REF

= 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-regu­lator 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.

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.

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 IQ, Step-Up DC-DC Controllers

14

______________________________________________________________________________________

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

Higher

Lower

GND to V

OUT

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

CS

FB

SGND

R

SHUNT

N

EXTL

100Ω

V+

C1

C2

0.1µF

V

IN

L1
20µH

NPN
2N2222A

R2

R1

D1

V

OUT

C4

R

SENSE

PNP
2N2907A

3

10

13

12

11

6

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 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

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 (tON(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.

MAX770–MAX773

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

Low IQ, Step-Up DC-DC Controllers

______________________________________________________________________________________

15

MAXIMUM OUTPUT CURRENT (A)

0

INPUT VOLTAGE (V)

0.5

1.0

1.5

2.0

2.5

3.0

3.5

2 3 4 5

R

SENSE

= 40mΩ

R

SENSE

= 50mΩ

R

SENSE

= 75mΩ

R

SENSE

= 200mΩ

R

SENSE

= 100mΩ

VOUT = 5V
L = 22µH

Figure 7a. Maximum Output Current vs. Input Voltage
(V

OUT

= 5V)

Figure 7b. Maximum Output Current vs. Input Voltage
(V

OUT

= 12V)

Figure 7c. Maximum Output Current vs. Input Voltage
(V

OUT

= 15V)

Figure 7d. Maximum Output Current vs. Input Voltage
(V

OUT

= 24V)

MAXIMUM OUTPUT CURRENT (A)

0

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

R

SENSE

= 200mΩ

R

SENSE

= 100mΩ

VOUT = 15V
L = 22µH

R

SENSE

= 40mΩ

R

SENSE

= 50mΩ

R

SENSE

= 75mΩ

3.5
V

= 12V

OUT

L = 22µH

3.0

2.5

2.0

1.5

1.0

MAXIMUM OUTPUT CURRENT (A)

0.5

R

= 40mΩ

SENSE

R

= 50mΩ

SENSE

R

= 75mΩ

SENSE

R

= 100mΩ

SENSE

R

= 200mΩ

0

2 4 6 8 10 12

INPUT VOLTAGE (V)

SENSE

0.8

V

= 24V

OUT

L =150µH

0.6

R

= 100mΩ

SENSE

0.4

0.2

MAXIMUM OUTPUT CURRENT (A)

0

2

6 10 14

INPUT VOLTAGE (V)

R

R

SENSE

SENSE

= 200mΩ

= 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 ≥ ——————————

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 20mΩ. 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 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 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 (Qg), on resistance (r

DS(ON)

),

and reverse transfer capacitance (C

RSS

).

Qgtakes into account all capacitances associated with
charging the gate. Use the typical Qgvalue 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

.

5V/12V/15V or Adjustable, High-Efficiency,
Low IQ, Step-Up DC-DC Controllers

16

______________________________________________________________________________________

MAX770
MAX771
MAX772

N

EXT

CS

R

SENSE

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

N

EXTH
EXTL

CS

L

R

SENSE

I

C(PEAK)

MAX773

I

R

B

EXTH

EXTL

CS

BASE

L

NPN

R

SENSE

The two most significant losses contributing to the
N-FET’s power dissipation are I2R 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 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:

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
(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 VTHof
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 VCS(max) by the sense­resistor value. To determine IB, set the peak inductor
current (I

LIM)

equal to the peak transistor collector cur-

rent I

C(PEAK)

. Calculate IBas follows:

IB= I

LIM

/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 I

LIM

.
It may be necessary to use even higher base currents
(e.g., IB= I

LIM

/10), although excessive IBmay impair

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

BASE

is determined by:

(

V

EXTH

- VBE- V

CS

(min))

R

BASE

= ————————————–

I

B

Where V

EXTH

is the voltage at V+ (in bootstrapped

mode V

EXTH

is the output voltage), VBEis 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 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
(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 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 50mΩ of
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 IQ, 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 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 low­ESR 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 10kΩ and 500kΩ

(Figure 9), and calculate R4 as follows:

V

TRIP - VREF

R4 = (R3)(———————

)

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., 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 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 VGSare 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
VGSvoltages. To avoid this condition, make sure V

EXT

is above the VTHof 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.

5V/12V/15V or Adjustable, High-Efficiency,
Low IQ, Step-Up DC-DC Controllers

18

______________________________________________________________________________________

MAX773

LBI LBO

GND

V+

R4

V

IN

R5
100k

R3

LOW-BATTERY

OUTPUT

V

TRIP

V

REF

R4 = R3 ( -1

)

V

REF

= 1.5V

Figure 9. Input Voltage Monitor Circuit

Starting Up under Load

The

Typical Operating Characteristics

show the Start­Up 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

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+.

MAX770–MAX773

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

Low IQ, 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: [phone] [phone]

Japan: 81-6-337-6450 81-6-337-6456
Nichicon USA: [phone] [phone]
Nihon USA: [phone] [phone]
Renco USA: [phone] [phone]

Sanyo

USA: [phone] [phone]

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

USA: [phone]

Japan: 81-3-3607-5111 81-3-3607-5144
United Chemi-Con USA: [phone] [phone]

N-FET

Siliconix

Si9410DY

Si9420DY (high voltage)

Motorola

MTP3055EL
MTD20N03HDL

USA: [phone] [phone]

SUPPLIER PHONE FAX

Zetex

USA: [phone] [phone]

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

MAX770–MAX773

5V/12V/15V or Adjustable, High-Efficiency,
Low IQ, Step-Up DC-DC Controllers

20

______________________________________________________________________________________

14
13
12
11
10

9
8

1
2
3
4
5
6
7

V15
EXTH
EXTL
CS

LBO

V+

V5

V12

MAX773

SGND
GND

REF

SHDN

FB

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

CS

SGND

GND
GND

V5 V12 V15

V+

LBO

LBI

FB

SHDN REF

0.126"

(3.200mm)

0.080"

(2.032mm)

MAX770/MAX771/MAX772

MAX773

V+

FB

0.126"

(3.200mm)

0.080"

(2.032mm)

EXT

CS

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.