EVALUATION KIT AVAILABLE
AVAILAB
LE
MAX756/MAX757
3.3V/5V/Adjustable-Output,
Step-Up DC-DC Converters
_______________General Description
The MAX756/MAX757 are CMOS step-up DC-DC switch-
ing regulators for small, low input voltage or battery-pow-
ered systems. The MAX756 accepts a positive input
voltage down to 0.7V and converts it to a higher pin-
selectable 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.
____________________________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
MAX756CPA
MAX756CSA
MAX756C/D
MAX756EPA
MAX756ESA
MAX757CPA
MAX757CSA
MAX757C/D
MAX757EPA
MAX757ESA
TEMP. RANGE
0°C to +70°C
0°C to +70°C
0°C to +70°C
-40°C to +85°C
-40°C to +85°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
-40°C to +85°C
-40°C to +85°C
PIN-PACKAGE
8 Plastic DIP
8 SO
Dice*
8 Plastic DIP
8 SO
8 Plastic DIP
8 SO
Dice*
8 Plastic DIP
8 SO
________________________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
Functional Diagrams
__________Typical Operating Circuit
INPUT
2V to V
OUT
* Dice are tested at T
A
= +25°C only.
_________________Pin Configurations
TOP VIEW
SHDN
1
2
8
7
LX
GND
OUT
LBI
150μF
5
1
LBI
SHDN
LX
8
OUTPUT
22μH
5V at 200mA
or
1N5817
3.3V at 300mA
100μF
3/5
REF
3
LBO
4
MAX756
6
5
DIP/SO
SHDN
FB
1
2
8
7
2
MAX756
3/5
OUT
6
LX
GND
OUT
LBI
3
REF
LBO
4
LOW-BATTERY
DETECTOR OUTPUT
REF
3
LBO
4
MAX757
6
5
GND
0.1μF
Pin Configurations appear at end of data sheet.
7
Functional Diagrams continued at end of data sheet.
UCSP is a trademark of Maxim Integrated Products, Inc.
DIP/SO
For pricing, delivery, and ordering information, please contact Maxim Direct
at 1-888-629-4642, or visit Maxim’s website at www.maximintegrated.com.
19-0113; Rev. 2; 1/95
MAX756/MAX757
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, LBI, LBO, REF,
3/5, FB to GND) ........................................-0.3V, (V
OUT
+ 0.3V)
Reference Current (I
REF
) ....................................................2.5mA
Continuous Power Dissipation (T
A
= +70°C)
Plastic DIP (derate 9.09mW/°C above +70°C) .............727mW
SO (derate 5.88mW/°C above +70°C) ..........................471mW
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
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
(Circuits of Figure 1 and Typical Operating Circuit, V
IN
= 2.5V, I
LOAD
= 0mA, T
A
= T
MIN
to T
MAX
, unless otherwise noted.)
PARAMETER
Output Voltage
CONDITIONS
MAX756, 3/5 = 0V, 0mA < I
LOAD
< 200mA
2V < V
IN
< 3V MAX756, 3/5 = 3V, 0mA < I
LOAD
< 300mA
MAX757, V
OUT
= 5V, 0mA < I
LOAD
< 200mA
Minimum Start-Up Supply Voltage I
LOAD
= 10mA
Minimum Operating Supply
Voltage (once started)
Quiescent Supply Current in
3.3V Mode (Note 1)
Battery Quiescent Current
Measured at V
IN
in Figure 1
Shutdown Quiescent Current
(Note 1)
Reference Voltage
Reference-Voltage Regulation
LBI Input Threshold
LBI Input Hysteresis
LBO Output Voltage Low
LBO Output Leakage Current
SHDN, 3/5 Input Voltage Low
SHDN, 3/5 Input Voltage High
SHDN, 3/5, FB, LBI Input Current
FB Voltage
Output Voltage Range
LBI = 1.25V, FB = 1.25V, SHDN = 0V or 3V,
3/5 = 0V or 3V
MAX757
MAX757, I
LOAD
= 0mA (Note 2)
1.22
2.7
1.25
1.6
±100
1.28
5.5
I
SINK
= 2mA
LBO = 5V
I
LOAD
= 20mA
I
LOAD
= 0mA, 3/5 = 3V, LBI = 1.25V, V
OUT
= 3.47V,
FB = 1.3V (MAX757 only)
Output set for 3.3V
SHDN = 0V, LBI = 1.25V, 3/5 = 3V, V
OUT
= 3.47V,
FB = 1.3V (MAX757 only)
No REF load, C
REF
= 0.1µF
3/5 = 3V, -20µA < REF load < 250µA, C
REF
= 0.22µF
With falling edge
1.23
1.22
60
MIN
4.8
3.17
4.8
TYP
5.0
3.30
5.0
1.1
0.7
60
MAX
5.2
3.43
5.2
1.8
V
V
µA
UNITS
V
µA
20
1.25
0.8
1.25
25
40
1.27
2.0
1.28
0.4
1
0.4
µA
V
%
V
mV
V
µA
V
V
nA
V
V
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
Maxim Integrated
MAX756/MAX757
3.3V/5V/Adjustable-Output,
Step-Up DC-DC Converters
__________________________________________Typical Operating Characteristics
(Circuit of Figure 1, T
A
= +25°C, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT
3.3V OUTPUT MODE
MAX756-1
EFFICIENCY vs. LOAD CURRENT
5V OUTPUT MODE
V
IN
= 3.3V
MAX756-2
MAXIMUM OUTPUT CURRENT
vs. INPUT VOLTAGE
700
600
500
400
300
200
100
3.3V MODE
5V MODE
MAX756-3
90
V
IN
= 2.0V
80
V
IN
= 1.2V
EFFICIENCY (%)
90
800
MAXIMUM OUTPUT CURRENT (mA)
80
V
IN
= 2.5V
EFFICIENCY (%)
70
V
IN
= 1.25V
70
60
60
50
50
40
0.1
1
10
100
1000
LOAD CURRENT (mA)
40
0.1
1
10
100
1000
LOAD CURRENT (mA)
0
0
1
2
3
4
5
INPUT VOLTAGE (V)
SWITCHING FREQUENCY
vs. LOAD CURRENT
MAX756-4
QUIESCENT CURRENT
vs. INPUT VOLTAGE
MAX756-5
SHUTDOWN QUIESCENT CURRENT
vs. INPUT VOLTAGE
SHUTDOWN QUIESCENT CURRENT (μA)
CURRENT MEASURED AT V
IN
40
MAX756-6
1M
500
CURRENT MEASURED AT V
IN
QUIESCENT CURRENT (μA)
400
50
SWITCHING FREQUENCY (Hz)
100k
5V MODE
10k
3.3V MODE
300
V
OUT
= 5V
30
1k
200
20
100
V
IN
= 2.5V
10
10μ
100μ
1m
10m
100m
1
LOAD CURRENT (A)
100
V
OUT
= 3.3V
0
1
2
3
INPUT VOLTAGE (V)
4
5
10
0
1
2
3
INPUT VOLTAGE (V)
4
5
MINIMUM START-UP INPUT VOLTAGE
vs. LOAD CURRENT
MAX756-7
REFERENCE VOLTAGE
LOAD REGULATION
MAX756-8
1.8
10
START-UP INPUT VOLTAGE (V)
1.6
VREF LOAD REGULATION (mV)
8
1.4
6
1.2
4
V
OUT
= 3.3V
1.0
3.3V MODE
0.8
1
10
100
1000
LOAD CURRENT (mA)
2
0
0
50
100
150
200
250
LOAD CURRENT (μA)
Maxim Integrated
3
MAX756/MAX757
3.3V/5V/Adjustable-Output,
Step-Up DC-DC Converters
_____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 1, T
A
= +25°C, unless otherwise noted.)
LOAD-TRANSIENT RESPONSE
START-UP DELAY
3V
OUTPUT
VOLTAGE
50mV/div
V
SHDN
2V/div
0V
5V
OUTPUT
CURRENT
0mA to 200mA
V
OUT
2V/div
0V
V
IN
= 2.5V
HORIZONTAL = 50μs/div
5V Mode
V
IN
= 2.5V
HORIZONTAL = 5ms/div
5V Mode
______________________________________________________________Pin Description
PIN
MAX756 MAX757
NAME
FUNCTION
Shutdown Input disables SMPS when low, but the voltage reference and low-battery com-
parator remain active.
Selects the main output voltage setting; 5V when low, 3.3V when high.
Feedback Input for adjustable output operation. Connect to an external voltage divider
between OUT and GND.
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 V
IN
if not used.
Connect OUT to the regulator output. It provides bootstrapped power to both devices,
and also senses the output voltage for the MAX756.
Power Ground. Must be low impedance; solder directly to ground plane.
1A, 0.5Ω N-Channel Power MOSFET Drain
1
2
–
1
–
2
SHDN
3/5
FB
3
3
REF
4
4
LBO
5
5
LBI
6
7
8
6
7
8
OUT
GND
LX
4
Maxim Integrated
MAX756/MAX757
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).
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 typi-
cally 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 bat-
tery voltage range is from V
OUT
+ V
D
to less than 1V
(where V
D
is 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 resis-
tors, 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
OUT
= (V
REF
) [(R2 + R1) / R2]
where V
REF
= 1.25V.
To simplify resistor selection:
R1 = (R2) [(V
OUT
/ V
REF
) - 1]
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.
Pulse-Frequency
Modulation Control Scheme
A unique minimum off time, current-limited, pulse-frequen-
cy 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
(V
SW
) of about 500mV under high output loads. The
value of V
SW
decreases with light current loads.
Conventional PWM converters generate constant-fre-
quency switching noise, whereas this architecture pro-
duces 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 ref-
erence is unloaded, bypass it with at least 0.1µF.
Low-Battery Detection
The MAX756/MAX757 contain on-chip circuitry for low-
battery detection. If the voltage at LBI falls below the reg-
ulator’s internal reference voltage (1.25V), LBO (an open-
drain output) sinks current to GND. The low-battery mon-
itor's threshold is set by two resistors, R3 and R4 (Figure
1), which forms a voltage divider between the input volt-
age and the LBI pin. The threshold voltage is set by R3
and R4 using the following equation:
R3 = [(V
IN
/ V
REF
) - 1] (R4)
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 dri-
ven from signal sources that exceed the main supply
Maxim Integrated
5
