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PDF MAX1771 Data sheet ( Hoja de datos )
| Número de pieza | MAX1771 | |
| Descripción | 12V or Adjustable / High-Efficiency / Low IQ / Step-Up DC-DC Controller | |
| Fabricantes | Maxim Integrated | |
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19-0263; Rev 1; 7/95
EVFAOLLULAOTWIOSNDKAITTAMSAHNEUEATL
12V or Adjustable, High-Efficiency,
Low IQ, Step-Up DC-DC Controller
_______________General Description ____________________________Features
The MAX1771 step-up switching controller provides
90% efficiency over a 30mA to 2A load. A unique cur-
rent-limited pulse-frequency-modulation (PFM) control
scheme gives this device 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).
This controller uses miniature external components. Its
high switching frequency (up to 300kHz) allows sur-
face-mount magnetics of 5mm height and 9mm diame-
ter. It accepts input voltages from 2V to 16.5V. The
output voltage is preset at 12V, or can be adjusted
using two resistors.
The MAX1771 optimizes efficiency at low input voltages
and reduces noise by using a single 100mV current-limit
threshold under all load conditions. A family of similar
devices, the MAX770–MAX773, trades some full-load
efficiency for greater current-limit accuracy; they provide
a 200mV current limit at full load, and switch to 100mV
for light loads.
o 90% Efficiency for 30mA to 2A Load Currents
o Up to 24W Output Power
o 110µA Max Supply Current
o 5µA Max Shutdown Current
o 2V to 16.5V Input Range
o Preset 12V or Adjustable Output Voltage
o Current-Limited PFM Control Scheme
o Up to 300kHz Switching Frequency
o Evaluation Kit Available
______________Ordering Information
PART
TEMP. RANGE
PIN-PACKAGE
The MAX1771 drives an external N-channel MOSFET
MAX1771CPA
switch, allowing
is required, use
tiht etoMpAoXw7e5r6lo/MadAsX7u5p7toor2M4WAX. 7If6le1Ds/MsaAptXoa7wS6e2hr eet4UMM.AAcXXo1177m7711CCS/DA
step-up switching regulators with on-board MOSFETs.
MAX1771EPA
An evaluation kit is available. Order the MAX1771EVKIT-SO.
MAX1771ESA
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
8 Plastic DIP
8 SO
Dice*
8 Plastic DIP
8 SO
________________________Applications
MAX1771MJA -55°C to +125°C
8 CERDIP**
Positive LCD-Bias Generators
Flash Memory Programmers
* Contact factory for dice specifications.
** Contact factory for availability and processing to MIL-STD-883B.
High-Power RF Power-Amplifier Supply
Palmtops/Hand-Held Terminals
Battery-Powered Applications
Portable Communicators
__________________Pin Configuration
__________Typical Operating Circuit
INPUT
2V TO VOUT
ON/OFF
MAX1771 EXT
SHDN
CS
REF
V+
FB AGND GND
N
OUTPUT
12V
TOP VIEW
EXT 1
V+ 2
FB 3
SHDN 4
MAX1771
DIP/SO
8 CS
7 GND
6 AGND
5 REF
________________________________________________________________ Maxim Integrated Products 1
Call toll free 1-800-998-8800 for free samples or literature.
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12V or Adjustable, High-Efficiency,
Low IQ, Step-Up DC-DC Controller
____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 2a, TA = +25°C, unless otherwise noted.)
HEAVY-LOAD SWITCHING WAVEFORMS
A
B
VOUT
0V
ILIM
0A
C
MEDIUM-LOAD SWITCHING WAVEFORMS
A VOUT
0V
ILIM
B
0A
C
2µs/div
VIN = 5V, IOUT = 900mA, VOUT = 12V
A: EXT VOLTAGE, 10V/div
B: INDUCTOR CURRENT, 1A/div
C: VOUT RIPPLE, 50mV/div, AC-COUPLED
LINE-TRANSIENT RESPONSE
A
10µs/div
VIN = 5V, IOUT = 500mA, VOUT = 12V
A: EXT VOLTAGE, 10V/div
B: INDUCTOR CURRENT, 1A/div
DataSheet4U.comC: VOUT RIPPLE, 50mV/div, AC-COUPLED
LOAD-TRANSIENT RESPONSE
7V
5V A
500mA
0A
0V
BB
5ms/div
IOUT = 700mA, VOUT = 12V
A: VIN, 5V to 7V, 2V/div
B: VOUT RIPPLE, 100mV/div, AC-COUPLED
5ms/div
VIN = 6V, VOUT = 12V
A: LOAD CURRENT, 0mA to 500mA, 500mA/div
B: VOUT RIPPLE, 100mV/div, AC-COUPLED
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12V or Adjustable, High-Efficiency,
Low IQ, Step-Up DC-DC Controller
Power Transistor Selection
Diode Selection
Use an N-channel MOSFET power transistor with the The MAX1771’s high switching frequency demands a
MAX1771.
high-speed rectifier. Schottky diodes such as the
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.
When selecting an N-FET, three important parameters
are the total gate charge (Qg), on-resistance (rDS(ON)),
and reverse transfer capacitance (CRSS).
1N5817–1N5822 are recommended. Make sure the
Schottky diode’s average current rating exceeds the
peak current limit set by RSENSE, and that its break-
down voltage exceeds VOUT. For high-temperature
applications, Schottky diodes may be inadequate due
to their high leakage currents; high-speed silicon
diodes such as the MUR105 or EC11FS1 can 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.
Qg takes into account all capacitances associated with
charging the gate. Use the typical Qg value for best
Capacitor Selection
results; the maximum value is usually grossly over-
Output Filter Capacitor
specified since it is a guaranteed limit and not the mea- The primary criterion for selecting the output filter capac-
sured value. The typical total gate charge should be itor (C4) is low effective series resistance (ESR). The
50nC or less. With larger numbers, the EXT pins may product of the peak inductor current and the output filter
not be able to adequately drive the gate. The EXT capacitor’s ESR determines the amplitude of the ripple
rise/fall time varies with different capacitive loads as seen on the output voltage. Two OS-CON 150µF, 16V
shown in the Typical Operating Characteristics.
output filter capacitors in parallel with 35mΩ of ESR each
The two most significant losses contributing to the typically provide 75mV ripple when stepping up from 5V
N-FET’s
losses.
power
Select
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high-
or in
CRSS to minimize these losses.
applications that can tolerate higher output ripple.
Determine the maximum required gate-drive current
from the Qg specification in the N-FET data sheet.
The MAX1771’s maximum allowed switching frequency
Since the output filter capacitor’s ESR affects efficien-
cy, use low-ESR capacitors for best performance. See
Table 1 for component selection.
during normal operation is 300kHz; but at start-up, the
maximum frequency can be 500kHz, so the maximum
current required to charge the N-FET’s gate is
f(max) x Qg(typ). Use the typical Qg number 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:
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 MAX1771. The input voltage source impedance
determines the size of the capacitor required at the V+
input. As with the output filter capacitor, a low-ESR
IGATE (max) = (500kHz) (17nC) = 8.5mA.
The bypass capacitor on V+ (C2) must instantaneously
furnish the gate charge without excessive droop (e.g.,
capacitor is recommended. For output currents up to
1A, 68µF (C1) is adequate, although smaller bypass
capacitors may also be acceptable.
less than 200mV):
Bypass the IC with a 0.1µF ceramic capacitor (C2)
Qg
∆V+ = ——
C2
placed as close to the V+ and GND pins as possible.
Reference Capacitor
Continuing with the example, ∆V+ = 17nC/0.1µF = 170mV.
Figure 2a’s application circuit uses an 8-pin Si9410DY
Bypass REF with a 0.1µF capacitor (C3). REF can
source up to 100µA of current for external loads.
surface-mount N-FET that has 50mΩ on-resistance with
4.5V VGS, and a guaranteed VTH of less than 3V. Figure
2b’s application circuit uses an MTD20N03HDL logic-
Feed-Forward Capacitor
In adjustable output voltage and non-bootstrapped
modes, parallel a 47pF to 220pF capacitor across R2,
level N-FET with a guaranteed threshold voltage (VTH)
of 2V.
as shown in Figures 2 and 3. Choose the lowest capac-
itor value that insures stability; high capacitance values
may degrade line regulation.
______________________________________________________________________________________ 11
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| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet MAX1771.PDF ] | |
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