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PDF MAX756 Data sheet ( Hoja de datos )
| Número de pieza | MAX756 | |
| Descripción | 3.3V/5V/Adjustable-Output / Step-Up DC-DC Converters | |
| Fabricantes | Maxim Integrated | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de MAX756 (archivo pdf) en la parte inferior de esta página. Total 8 Páginas | ||
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19-0113; Rev. 2; 1/95
EVFAOLLULAOTWIOSNDKAITTAMSAHNEUEATL
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.
________________________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
__________Typical Operating Circuit
____________________________Features
o Operates Down to 0.7V Input Supply Voltage
o 87% Efficiency at 200mA
o 60µA Quiescent Current
o 20µA Shutdown Mode with Active Reference and
LBI Detector
o 500kHz Maximum Switching Frequency
o ±1.5% Reference Tolerance Over Temperature
o Low-Battery Detector (LBI/LBO)
o 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
* Dice are tested at TA = +25°C only.
_________________Pin Configurations
INPUT
2V to VOUT
1
SHDN
150µF
5
LBI
8
LX
2
3/5
MAX756
6
OUT
OUTPUT
22µH
5V at 200mA
or
1N5817 3.3V at 300mA
100µF
0.1µF
3
REF
4
LBO
GND
7
LOW-BATTERY
DETECTOR OUTPUT
TOP VIEW
SHDN 1
3/5 2
REF 3
LBO 4
MAX756
DIP/SO
8 LX
7 GND
6 OUT
5 LBI
SHDN 1
FB 2
REF 3
LBO 4
MAX757
DIP/SO
8 LX
7 GND
6 OUT
5 LBI
________________________________________________________________ Maxim Integrated Products 1
Call toll free 1-800-998-8800 for free samples or literature.
1 page  
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-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
(VSW) of about 500mV under high output loads. The
value of VSW 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.
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
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 VOUT + VD to less than 1V
(where VD 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:
VOUT = (VREF) [(R2 + R1) / R2]
where VREF = 1.25V.
To simplify resistor selection:
R1 = (R2) [(VOUT / VREF) - 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.
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 = [(VIN / VREF) - 1] (R4)
_______________________________________________________________________________________ 5
5 Page | ||
| Páginas | Total 8 Páginas | |
| PDF Descargar | [ Datasheet MAX756.PDF ] | |
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