Rainbow Electronics MAX723 User Manual

19-0113; Rev. 2; 1/95

EVALUATION KIT MANUAL

FOLLOWS DATA SHEET

3.3V/5V/Adjustable-Output,
Step-Up DC-DC Converters

_______________General Description

The MAX756/MAX757 are CMOS step-up DC-DC switch­ing regulators for small, low input voltage or battery-pow­ered systems. The MAX756 accepts a positive input
voltage down to 0.7V and converts it to a higher pin­selectable output voltage of 3.3V or 5V. The MAX757 is
an adjustable version that accepts an input voltage down
to 0.7V and generates a higher adjustable output voltage
in the range from 2.7V to 5.5V. Typical full-load efficiencies
for the MAX756/MAX757 are greater than 87%.

The MAX756/MAX757 provide three improvements over
previous devices. Physical size is reduced—the high
switching frequencies (up to 0.5MHz) made possible by
MOSFET power transistors allow for tiny (<5mm diameter)
surface-mount magnetics. Efficiency is improved to 87%
(10% better than with low-voltage regulators fabricated in
bipolar technology). Supply current is reduced to 60µA
by CMOS construction and a unique constant-off-time
pulse-frequency modulation control scheme.

________________________Applications

3.3V to 5V Step-Up Conversion
Palmtop Computers
Portable Data-Collection Equipment
Personal Data Communicators/Computers
Medical Instrumentation
2-Cell & 3-Cell Battery-Operated Equipment
Glucose Meters

____________________________Features

♦ Operates Down to 0.7V Input Supply Voltage
♦ 87% Efficiency at 200mA
♦ 60µA Quiescent Current
♦ 20µA Shutdown Mode with Active Reference and

LBI Detector

♦ 500kHz Maximum Switching Frequency
♦ ±1.5% Reference Tolerance Over Temperature
♦ Low-Battery Detector (LBI/LBO)
♦ 8-Pin DIP and SO Packages

______________Ordering Information

PART TEMP. RANGE PIN-PACKAGE

MAX756CPA

MAX756CSA 0°C to +70°C 8 SO
MAX756C/D 0°C to +70°C Dice*
MAX756EPA -40°C to +85°C 8 Plastic DIP
MAX756ESA -40°C to +85°C 8 SO
MAX757CPA
MAX757CSA 0°C to +70°C 8 SO
MAX757C/D 0°C to +70°C Dice*
MAX757EPA -40°C to +85°C 8 Plastic DIP
MAX757ESA -40°C to +85°C 8 SO

* Dice are tested at TA= +25°C only.

0°C to +70°C 8 Plastic DIP

0°C to +70°C 8 Plastic DIP

MAX756/MAX757

__________Typical Operating Circuit

INPUT

2V to V

OUT

0.1µF

150µF

8

6

4

22µH

LOW-BATTERY
DETECTOR OUTPUT

5

1

2

3

LBI

SHDN

MAX756

3/5

REF

________________________________________________________________

LX

OUT

LBO

GND

7

1N5817

OUTPUT

5V at 200mA

or

3.3V at 300mA

100µF

_________________Pin Configurations

TOP VIEW

1

SHDN

2

3/5

REF

LBO

SHDN

REF

LBO

MAX756

3
4

DIP/SO

1
2

FB

MAX757

3
4

DIP/SO

Maxim Integrated Products

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

8

LX

7

GND

6

OUT

5

LBI

8

LX

7

GND

6

OUT

5

LBI

1

3.3V/5V/Adjustable-Output,
Step-Up DC-DC Converters

ABSOLUTE MAXIMUM RATINGS

Supply Voltage (OUT to GND) ....................................-0.3V, +7V

Switch Voltage (LX to GND)........................................-0.3V, +7V

Auxiliary Pin Voltages (SHDN

3/5

, FB to GND)........................................-0.3V, (V

Reference Current (I
Continuous Power Dissipation (T

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

, LBI, LBO, REF,

) ....................................................2.5mA

REF

= +70°C)

A

OUT

+ 0.3V)

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

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.

Operating Temperature Ranges:

MAX75_C_ _ ........................................................0°C to +70°C

MAX75_E_ _......................................................-40°C to +85°C

Junction Temperature......................................................+150°C

Storage Temperature Range............................... -65°to +160°C

Lead Temperature (soldering, 10sec)........................... +300°C

ELECTRICAL CHARACTERISTICS

(Circuits of Figure 1 and Typical Operating Circuit, VIN= 2.5V, I

= 0mA, TA= T

LOAD

MIN

to T

, unless otherwise noted.)

MAX

MAX756/MAX757

Output Voltage

Minimum Start-Up Supply Voltage
Minimum Operating Supply

Voltage (once started)

Quiescent Supply Current in

3.3V Mode (Note 1)

Battery Quiescent Current
Measured at V

in Figure 1

IN

Shutdown Quiescent Current

(Note 1)
Reference Voltage
Reference-Voltage Regulation

LBO Output Voltage Low

LBO Output Leakage Current

SHDN, 3/5, FB, LBI Input Current

Output Voltage Range

MAX756, 3/5 = 0V, 0mA < I

2V < VIN< 3V

I

LOAD

I

LOAD

I

= 0mA, 3/5 = 3V, LBI = 1.25V, V

LOAD

FB = 1.3V (MAX757 only)

SHDN = 0V, LBI = 1.25V, 3/5 = 3V, V
FB = 1.3V (MAX757 only)

MAX756, 3/5 = 3V, 0mA < I
MAX757, V

= 10mA
= 20mA

= 0.1µF

REF

= 5V, 0mA < I

OUT

OUT

OUT

3/5 = 3V, -20µA < REF load < 250µA, C

I

= 2mA

SINK

LBI = 1.25V, FB = 1.25V, SHDN = 0V or 3V,
3/5 = 0V or 3V

MAX757, I

= 0mA (Note 2)

LOAD

LOAD
LOAD

LOAD

= 3.47V,

= 3.47V,

= 0.22µF

REF

< 200mA
< 300mA

< 200mA

4.8 5.0 5.2

3.17 3.30 3.43 V

4.8 5.0 5.2

1.1 1.8

60

60Output set for 3.3V

UNITSMIN TYP MAXCONDITIONSPARAMETER

V
V0.7

µA

µA

µA20 40

V1.23 1.25 1.27No REF load, C

%0.8 2.0

V1.22 1.25 1.28With falling edgeLBI Input Threshold

mV25LBI Input Hysteresis

V0.4

µA1LBO = 5V

V0.4SHDN, 3/5 Input Voltage Low
V1.6SHDN, 3/5 Input Voltage High

nA±100

V1.22 1.25 1.28MAX757FB Voltage
V2.7 5.5

Note 1: Supply current from the 3.3V output is measured with an ammeter between the 3.3V output and OUT pin. This current

correlates directly with actual battery supply current, but is reduced in value according to the step-up ratio and efficiency.

Note 2: Minimum value is production tested. Maximum value is guaranteed by design and is not production tested.

2 _______________________________________________________________________________________

3.3V/5V/Adjustable-Output,
Step-Up DC-DC Converters

__________________________________________Typical Operating Characteristics

(Circuit of Figure 1, TA= +25°C, unless otherwise noted.)

EFFICIENCY vs. LOAD CURRENT

90

80

70

60

EFFICIENCY (%)

50

40

1M

100k

10k

1k

SWITCHING FREQUENCY (Hz)

100

10

3.3V OUTPUT MODE

VIN = 2.0V

VIN = 1.2V

0.1 10 1000

1 100

LOAD CURRENT (mA)

SWITCHING FREQUENCY

vs. LOAD CURRENT

5V MODE

10µ 10m 1

100µ 1m 100m

LOAD CURRENT (A)

MINIMUM START-UP INPUT VOLTAGE

1.8

1.6

3.3V MODE

V

= 2.5V

IN

vs. LOAD CURRENT

MAX756-1

EFFICIENCY (%)

MAX756-4

EFFICIENCY vs. LOAD CURRENT

90

80

70

60

50

40

500

400

300

200

QUIESCENT CURRENT (µA)

100

0

5V OUTPUT MODE

VIN = 3.3V

VIN = 1.25V

0.1 10 1000

1 100

LOAD CURRENT (mA)

QUIESCENT CURRENT

vs. INPUT VOLTAGE

CURRENT MEASURED AT V

V

= 5V

OUT

V

1

2

3

INPUT VOLTAGE (V)

MAX756-7

OUT

VIN = 2.5V

IN

= 3.3V

4

10

8

MAX756-2

MAXIMUM OUTPUT CURRENT (mA)

MAX756-5

SHUTDOWN QUIESCENT CURRENT (µA)

5

REFERENCE VOLTAGE

LOAD REGULATION

MAXIMUM OUTPUT CURRENT

800
700

600
500
400
300
200
100

0

50

40

30

20

10

0

vs. INPUT VOLTAGE

3.3V MODE

02

13

INPUT VOLTAGE (V)

SHUTDOWN QUIESCENT CURRENT

vs. INPUT VOLTAGE

CURRENT MEASURED AT V

12 5

3

INPUT VOLTAGE (V)

5V MODE

4

IN

4

MAX756-8

MAX756/MAX757

MAX756-3

5

MAX756-6

1.4

1.2

START-UP INPUT VOLTAGE (V)

1.0

0.8
1

10 100

LOAD CURRENT (mA)

3.3V MODE

1000

6

4

VREF LOAD REGULATION (mV)

2

0

0

50 100 150 200

LOAD CURRENT (µA)

V

= 3.3V

OUT

250

_________________________________________________________________________________________________

3

3.3V/5V/Adjustable-Output,
Step-Up DC-DC Converters

_____________________________Typical Operating Characteristics (continued)

(Circuit of Figure 1, TA= +25°C, unless otherwise noted.)

LOAD-TRANSIENT RESPONSE

OUTPUT
VOLTAGE
50mV/div

OUTPUT

CURRENT

0mA to 200mA

MAX756/MAX757

= 2.5V

V

IN

HORIZONTAL = 50µs/div
5V Mode

V



SHDN

2V/div

V



OUT

2V/div

VIN = 2.5V
HORIZONTAL = 5ms/div
5V Mode

START-UP DELAY

3V

0V

5V

0V

______________________________________________________________Pin Description

PIN

MAX756 MAX757

1

2 3/5 Selects the main output voltage setting; 5V when low, 3.3V when high.

– FB

1

–

2

NAME FUNCTION

SHDN

Shutdown Input disables SMPS when low, but the voltage reference and low-battery com­parator remain active.

Feedback Input for adjustable output operation. Connect to an external voltage divider
between OUT and GND.

3 REF

4 LBO

5 LBI

6 OUT

7 GND Power Ground. Must be low impedance; solder directly to ground plane.
8 LX 1A, 0.5Ω N-Channel Power MOSFET Drain

4 _______________________________________________________________________________________

3

4

5

6

7
8

1.25V Reference Voltage Output. Bypass with 0.22µF to GND (0.1µF if there is no external
reference load). Maximum load capability is 250µA source, 20µA sink.

Low-Battery Output. An open-drain N-channel MOSFET sinks current when the voltage at
LBI drops below +1.25V.

Low-Battery Input. When the voltage on LBI drops below +1.25V, LBO sinks current.
Connect to VINif not used.

Connect OUT to the regulator output. It provides bootstrapped power to both devices,
and also senses the output voltage for the MAX756.

3.3V/5V/Adjustable-Output,
Step-Up DC-DC Converters

_______________Detailed Description

Operating Principle

The MAX756/MAX757 combine a switch-mode regulator
with an N-channel MOSFET, precision voltage reference,
and power-fail detector in a single monolithic device.
The MOSFET is a “sense-FET” type for best efficiency,
and has a very low gate threshold voltage to ensure
start-up under low-battery voltage conditions (1.1V typ).

Pulse-Frequency

Modulation Control Scheme

A unique minimum off time, current-limited, pulse-frequen­cy modulation (PFM) control scheme is a key feature of
the MAX756/MAX757. This PFM scheme combines the
advantages of pulse-width modulation (PWM) (high output
power and efficiency) with those of a traditional PFM
pulse-skipper (ultra-low quiescent currents). There is no
oscillator; at heavy loads, switching is accomplished
through a constant peak-current limit in the switch, which
allows the inductor current to self-oscillate between this
peak limit and some lesser value. At light loads, switching
frequency is governed by a pair of one-shots, which set a
minimum off-time (1µs) and a maximum on-time (4µs).
The switching frequency depends on the load and the
input voltage, and can range as high as 500kHz.

The peak switch current of the internal MOSFET power
switch is fixed at 1A ±0.2A. The switch's on resistance
is typically 0.5Ω, resulting in a switch voltage drop
(VSW) of about 500mV under high output loads. The
value of VSWdecreases with light current loads.

Conventional PWM converters generate constant-fre­quency switching noise, whereas this architecture pro­duces variable-frequency switching noise. However,
the noise does not exceed the switch current limit times
the filter-capacitor equivalent series resistance (ESR),
unlike conventional pulse-skippers.

Voltage Reference

The precision voltage reference is suitable for driving
external loads such as an analog-to-digital converter.
It has guaranteed 250µA source-current and 20µA
sink-current capability. The reference is kept alive
even in shutdown mode. If the reference drives an
external load, bypass it with 0.22µF to GND. If the ref­erence is unloaded, bypass it with at least 0.1µF.

Control-Logic Inputs

The control inputs (3/5, SHDN) are high-impedance
MOS gates protected against ESD damage by normally
reverse-biased clamp diodes. If these inputs are dri­ven from signal sources that exceed the main supply

voltage, the diode current should be limited by a series
resistor (1MΩ suggested). The logic input threshold
level is the same (approximately 1V) in both 3.3V and
5V modes. Do not leave the control inputs floating.

__________________Design Procedure

Output Voltage Selection

The MAX756 output voltage can be selected to 3.3V or
5V under logic control, or it can be left in one mode or
the other by tying 3/5 to GND or OUT. Efficiency varies
depending upon the battery and the load, and is typi­cally better than 80% over a 2mA to 200mA load range.
The device is internally bootstrapped, with power
derived from the output voltage (via OUT). When the
output is set at 5V instead of 3.3V, the higher internal
supply voltage results in lower switch-transistor on
resistance and slightly greater output power.
Bootstrapping allows the battery voltage to sag to less
than 1V once the system is started. Therefore, the bat­tery voltage range is from V
(where VDis the forward drop of the Schottky rectifier).
If the battery voltage exceeds the programmed output
voltage, the output will follow the battery voltage. In
many systems this is acceptable; however, the output
voltage must not be forced above 7V.

The output voltage of the MAX757 is set by two resis­tors, R1 and R2 (Figure 1), which form a voltage divider
between the output and the FB pin. The output voltage
is set by the equation:

V

= (V

OUT

where V
To simplify resistor selection:

Since the input bias current at FB has a maximum
value of 100nA, large values (10kΩ to 200kΩ) can be
used for R1 and R2 with no significant loss of accuracy.
For 1% error, the current through R1 should be at least
100 times FB’s bias current.

The MAX756/MAX757 contain on-chip circuitry for low­battery detection. If the voltage at LBI falls below the reg­ulator’s internal reference voltage (1.25V), LBO (an open­drain output) sinks current to GND. The low-battery mon­itor's threshold is set by two resistors, R3 and R4 (Figure

1), which forms a voltage divider between the input volt­age and the LBI pin. The threshold voltage is set by R3
and R4 using the following equation:

= 1.25V.

REF

R1 = (R2) [(V

R3 = [(V

) [(R2 + R1) / R2]

REF

OUT

/ V

IN

REF

+ VDto less than 1V

OUT

/ V

) - 1]

REF

Low-Battery Detection

) - 1] (R4)

MAX756/MAX757

_______________________________________________________________________________________ 5

3.3V/5V/Adjustable-Output,
Step-Up DC-DC Converters

V

IN

C1

R3

R4

C3

0.1µF

MAX756/MAX757

Figure 1. Standard Application Circuit

150µF

5

LBI

MAX757

1

SHDN

3

REF

LX

OUT

FB

LBO

GND

7

L1
22µH

D1

1N5817

8

6

2

4

R1

R2

where VINis the desired threshold of the low-battery
detector, R3 and R4 are the input divider resistors at
LBI, and V

is the internal 1.25V reference.

REF

Since the LBI current is less than 100nA, large resistor
values (typically 10kΩ to 200kΩ) can be used for R3
and R4 to minimize loading of the input supply.

When the voltage at LBI is below the internal threshold,
LBO sinks current to GND. A pull-up resistor of 10kΩ
or more connected from LBO to V
when driving CMOS circuits. Any pull-up resistor con-

can be used

OUT

nected to LBO should not be returned to a voltage
source greater than V
threshold, the LBO output is off. The low-battery com-

. When LBI is above the

OUT

parator and reference voltage remain active when the
MAX756/MAX757 is in shutdown mode.

If the low-battery comparator is not used, connect LBI
to VINand leave LBO open.

Inductor Selection

The inductors should have a saturation (incremental)
current rating equal to or greater than the peak switch­current limit, which is 1.2A worst-case. However, it’s
generally acceptable to bias the inductor into satura­tion by 20%, although this will reduce the efficiency.

The 22µH inductor shown in the typical applications cir­cuit is sufficient for most MAX756/MAX757 application
circuits. Higher input voltages increase the energy
transferred with each cycle, due to the reduced
input/output differential. Minimize excess ripple due to
increased energy transfer by reducing the inductor
value (10µH suggested).

V

OUT

C2
100µF

The inductor’s DC resistance significantly affects effi­ciency. For highest efficiency, limit L1’s DC resistance
to 0.03Ω or less. See Table 1 for a list of suggested
inductor suppliers.

Table 1. Component Suppliers

PRODUCTION

METHOD

Surface-Mount AVX

Miniature
Through-Hole

Low-Cost
Through-Hole

AVX USA: [phone], FAX [phone]

CoilCraft USA: [phone], FAX [phone]
Coiltronics USA: [phone], FAX [phone]
Collmer

Semiconductor USA: [phone]
Motorola USA: [phone], FAX [phone]
Nichicon USA: [phone], FAX [phone]

Nihon USA: [phone], FAX [phone]

Sanyo OS-CON USA: [phone]

Sprague USA: [phone], FAX [phone]
Sumida USA: [phone]

United
Chemi-Con USA: [phone], FAX [phone]

A 100µF, 10V surface-mount (SMT) tantalum capacitor
typically provides 50mV output ripple when stepping
up from 2V to 5V at 200mA. Smaller capacitors, down
to 10µF, are acceptable for light loads or in applica­tions that can tolerate higher output ripple.

INDUCTORS CAPACITORS

Sumida
CD54-220 (22µH)
CoilCraft
DT3316-223
Coiltronics
CTX20-1

Sumida
RCH654-220

CoilCraft
PCH-27-223

[phone]

Japan: [phone], FAX (+81-) 7-5256-4158

Japan: [phone], FAX (+81-) 3-3494-7414

Japan: [phone], FAX (+81-720-) 70-1174

Japan: [phone], FAX (+81-3-) 3607-5428

TPS series
Sprague

595D series

Sanyo OS-CON

OS-CON series
low-ESR organic
semiconductor

Nichicon

PL series
low-ESR
electrolyic

United Chemi-Con

LXF series

Capacitor Selection

6 _______________________________________________________________________________________

3.3V/5V/Adjustable-Output,
Step-Up DC-DC Converters

The ESR of both bypass and filter capacitors affects
efficiency. Best performance is obtained by using spe­cialized low-ESR capacitors, or connecting two or more
filter capacitors in parallel. The smallest low-ESR SMT
tantalum capacitors currently available are Sprague
595D series, which are about half the size of competing
products. Sanyo OS-CON organic semiconductor
through-hole capacitors also exhibit very low ESR, and
are especially useful for operation at cold tempera­tures. Table 1 lists suggested capacitor suppliers.

MINIMUM

SHDN

OFF-TIME

ONE-SHOT

3/5

MAXIMUM

ON-TIME

ONE-SHOT

TRIG Q

ONE-SHOT

ONE-SHOT

F/F

S

R

Rectifier Diode

For optimum performance, a switching Schottky diode,
such as the 1N5817, is recommended. 1N5817 equiv­alent diodes are also available in surface-mount pack­ages from Collmer Semiconductor in Dallas, TX, phone
[phone]. The part numbers are SE014 or
SE024. For low output power applications, a pn junc­tion switching diode, such as the 1N4148, will also
work well, although efficiency will suffer due to the
greater forward voltage drop of the pn junction diode.

V

IN

TRIGQ

LX

Q

N

GND

OUT

MAX756/MAX757

V

OUT

LBO

LBI

Figure 2. MAX756 Block Diagram

_______________________________________________________________________________________ 7

MAX756

N

REFERENCE

REF

3.3V/5V/Adjustable-Output

3.3V/5V/Adjustable-Output,
Step-Up DC-DC Converters

Step-Up DC-DC Converters

PC Layout and Grounding

The MAX756/MAX757 high peak currents and high-fre­quency operation make PC layout important for mini-

___________________Chip Topography

SHDN

LX

mizing ground bounce and noise. The distance
between the MAX756/MAX757’s GND pin and the
ground leads of C1 and C2 in Figure 1 must be kept to
less than 0.2" (5mm). All connections to the FB and LX
pins should also be kept as short as possible. To

3/5 (MAX756)

FB (MAX757)

obtain maximum output power and efficiency and mini­mum output ripple voltage, use a ground plane and

GND

solder the MAX756/MAX757 GND (pin 7) directly to the
ground plane.

GND

OUT

LBI

MAX756/MAX757

REF

LBO

0.080"

(2.03mm)

TRANSISTOR COUNT: 758
SUBSTRATE CONNECTED TO OUT

________________________________________________________Package Information

INCHES MILLIMETERS

DIM



A

A1

B
C

HE

D
E
e
H
h
L

α

MIN

0.053

0.004

0.014

0.007

0.189

0.150

0.228

0.010

0.016
0˚

MAX

MIN

0.069

1.35

0.010

0.10

0.019

0.35

0.010

0.19

0.197

4.80

0.157

3.80

1.27 BSC0.050 BSC





0.244

0.020

0.050



5.80

0.25

0.40

8˚

0˚

0.122"

(3.10mm)

MAX

1.75

0.25

0.49

0.25

5.00

4.00


6.20

0.50

1.27

8˚

21-325A

D

A

0.127mm

e

A1

B

0.004in.

h x 45˚

α

8-PIN PLASTIC

C

L

SMALL-OUTLINE

PACKAGE

8 _______________________________________________________________________________________