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PDF MAX651 Data sheet ( Hoja de datos )
| Número de pieza | MAX651 | |
| Descripción | Step-Down DC-DC Controllers | |
| Fabricantes | Maxim Integrated | |
| Logotipo |  |
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19-0225; Rev 3; 9/97
EVAALVUAAILTAIOBNLEKIT
5V/3.3V/3V or Adjustable, High-Efficiency,
Low IQ, Step-Down DC-DC Controllers
_______________General Description
The MAX649/MAX651/MAX652 BiCMOS, step-down DC-
DC switching controllers provide high efficiency over
three decades of load current. A unique, current-limited
pulse-frequency-modulated (PFM) control scheme gives
these devices the benefits of pulse-width-modulation
(PWM) converters (high efficiency at heavy loads), while
using only 100µA of supply current (vs. 2mA to 10mA for
PWM converters). The result is high efficiency over loads
ranging from 10mA to more than 2.5A.
These devices use miniature external components.
Their high switching frequency (up to 300kHz) allows
for less than 9mm diameter surface-mount inductors.
The MAX649/MAX651/MAX652 have dropout voltages
less than 1V and accept input voltages up to 16.5V.
Output voltages are preset at 5V (MAX649), 3.3V
(MAX651), and 3V (MAX652). These controllers can
also be adjusted to any voltage from 1.5V to the input
voltage by using two resistors.
These step-down controllers drive external P-channel
MOSFETs at loads greater than 10W. If less power is
required, use the MAX639/MAX640/MAX653 step-down
converters with on-chip FETs, which allow up to a
225mA load current.
________________________Applications
5V-to-3.3V Green PC Applications
High-Efficiency Step-Down Regulation
Minimum-Component DC-DC Converters
Battery-Powered Applications
____________________________Features
o More than 90% Efficiency (10mA to 1.5A Loads)
o More than 12.5W Output Power
o 100µA Max Quiescent Supply Current
o 5µA Max Shutdown Supply Current
o Less than 1.0V Dropout Voltage
o 16.5V Max Input Voltage
o 5V (MAX649), 3.3V (MAX651), 3V (MAX652),
or Adjustable Output Voltage
o Current-Limited Control Scheme
o Up to 300kHz Switching Frequency
______________Ordering Information
PART
TEMP. RANGE
PIN-PACKAGE
MAX649CPA
0°C to +70°C
8 Plastic DIP
MAX649CSA
0°C to +70°C
8 SO
MAX649C/D
0°C to +70°C
Dice*
MAX649EPA
-40°C to +85°C
8 Plastic DIP
MAX649ESA
-40°C to +85°C
8 SO
MAX649MJA
-55°C to +125°C 8 CERDIP**
Ordering Information continued at end of data sheet.
* Dice are tested at TA = +25°C.
**Contact factory for availability and processing to MIL-STD-883.
__________Typical Operating Circuit __________________Pin Configuration
INPUT
4V TO 16.5V
TOP VIEW
ON/OFF
V+
MAX651
SHDN
CS
EXT
REF OUT
FB GND
P
OUTPUT
3.3V
OUT 1
FB 2
SHDN 3
REF 4
MAX649
MAX651
MAX652
8 GND
7 EXT
6 CS
5 V+
DIP/SO
________________________________________________________________ Maxim Integrated Products 1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800.
For small orders, phone 1-800-835-8769.
http://www.Datasheet4U.com
1 page  
5V/3.3V/3V or Adjustable, High-Efficiency,
Low IQ, Step-Down DC-DC Controllers
____________________________Typical Operating Characteristics (continued)
(TA = +25°C, unless otherwise noted.)
EXT RISE AND FALL TIMES
vs. TEMPERATURE (1nF)
130
120 CEXT = 1nF
V+ = 5V, tRISE
110
100
90
80 V+ = 5V, tFALL
70
60
50 V+ = 12V, tRISE
40
30
20
-60 -40 -20
V+ = 12V, tFALL
0 20 40 60 80 100 120 140
TEMPERATURE (°C)
EXT RISE AND FALL TIMES
vs. TEMPERATURE (5nF)
500
CEXT = 5nF
450
400 V+ = 5V, tRISE
350
300 V+ = 12V, tRISE
250
V+ = 5V, tFALL
200
150
100 V+ = 12V, tFALL
50
-60 -40 -20 0 20 40 60 80 100 120 140
TEMPERATURE (°C)
DROPOUT VOLTAGE
vs. LOAD CURRENT
1000
MAX649, VOUT = 5V
900 MAX652, VOUT = 3V
800
700
600 MAX651, VOUT = 3.3V
500
400
300
200
100
0
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
LOAD CURRENT (A)
DROPOUT VOLTAGE
vs. TEMPERATURE
1100
MAX649
1000
900 MAX652
800 MAX651
700
600
-60 -40 -20
ILOAD = 1A
CIRCUIT OF FIGURE 1
0 20 40 60 80 100 120 140
TEMPERATURE (°C)
CS TRIP LEVEL
vs. TEMPERATURE
235
230
225
220
215
210
205
200
195
190
185
-60 -40 -20 0 20 40 60 80 100 120 140
TEMPERATURE (°C)
REFERENCE OUTPUT RESISTANCE
vs. TEMPERATURE
250
REFERENCE OUTPUT VOLTAGE
vs. TEMPERATURE
1.506
1.504
1.502
1.500
1.498
1.496
1.494
1.492
-60 -40 -20 0 20 40 60 80 100 120 140
TEMPERATURE (°C)
200 IREF = 10µA
150
IREF = 50µA
100
50 IREF = 100µA
0
-60 -40 -20 0 20 40 60 80 100 120 140
TEMPERATURE (°C)
_______________________________________________________________________________________ 5
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5 Page 
5V/3.3V/3V or Adjustable, High-Efficiency,
Low IQ, Step-Down DC-DC Controllers
Standard wire-wound and metal-film resistors have an
inductance high enough to degrade performance.
Surface-mount (chip) resistors have very little induc-
tance and are well suited for use as current-sense
resistors. A wire resistor made by IRC works well in
through-hole applications. Because this resistor is a
band of metal shaped as a “U”, its inductance is less
than 10nH (an order of magnitude less than metal film
resistors). Resistance values between 5mΩ and 0.1Ω
are available (see Table 1).
Inductor Selection
Practical inductor values range from 10µH to 50µH or more.
The circuit operates in discontinuous-conduction mode if:
V+ ≤
VOUT x (R + 1)
————————
+
—V—D +
VSW
RR
R, the switch on-time/off-time ratio, equals 6.7. VD is the
diode’s drop, and VSW is the voltage drop across the
P-channel FET. To get the full output capability in
discontinuous-conduction mode, choose an inductor
value no larger than:
RSENSE x 12µs x (V+ - VSW - VOUT)
L(max) = —————————————————
VCS
where VCS is the current-sense voltage.
In both the continuous and discontinuous modes, the
lower limit of the inductor is more important. With a
small inductor value, the current rises faster and over-
shoots the desired peak current limit because the cur-
rent-limit comparator cannot respond fast enough. This
reduces efficiency slightly and, more importantly, could
cause the current rating of the external components
to be exceeded. Calculate the minimum inductor value
as follows:
(V+(max) - VSW - VOUT) x 0.3µs
L(min) = ————————————––——
∆I x ILIM(min)
where ∆I is the percentage of inductor-current over-
shoot, where ILIM = VCS/RSENSE and 0.3µs is the time
it takes the comparator to switch. An overshoot of 10%
is usually not a problem. Inductance values above the
minimum work well if the maximum value defined above
is not exceeded. Smaller inductance values cause
higher output ripple because of overshoot. Larger val-
ues tend to produce physically larger coils.
For highest efficiency, use a coil with low DC resis-
tance; a value smaller than 0.1V/ILIM works best. To
minimize radiated noise, use a toroid, pot core, or
shielded-bobbin inductor. Inductors with a ferrite core
or equivalent are recommended. Make sure the induc-
tor’s saturation-current rating is greater than ILIM(max).
However, it is generally acceptable to bias the inductor
into saturation by about 20% (the point where the
inductance is 20% below its nominal value).
The peak current of Figure 1 is 2.35A for a 1.5A output.
The inductor used in this circuit is specified to drop by
10% at 2.2A (worst case); a curve provided by the
manufacturer shows that the inductance typically drops
by 20% at 3.1A. Using a slightly underrated inductor
can sometimes reduce size and cost, with only a minor
impact on efficiency. The MAX649/MAX651/MAX652
current limit prevents any damage from an underrated
inductor’s low inductance at high currents.
Table 1 lists inductor types and suppliers for various
applications. The efficiencies of the listed surface-
mount inductors are nearly equivalent to those of the
larger size through-hole versions.
Diode Selection
The MAX649/MAX651/MAX652’s high switching fre-
quency demands a high-speed rectifier (commonly
called a catch diode when used in switching-regulator
circuits). Schottky diodes, such as the 1N5817 through
1N5822 families (and their surface-mount equivalents),
are recommended. Choose a diode with an average
current rating equal to or greater than ILIM(max) and a
voltage rating higher than V+(max). For high-tempera-
ture applications, where Schottky diodes can be
inadequate because of high leakage currents, use
high-speed silicon diodes instead. At heavy loads and
high temperatures, the disadvantages of a Schottky
diode’s high leakage current may outweigh the benefits
of its low forward voltage. Table 1 lists diode types and
suppliers for various applications.
External Switching Transistor
The MAX649/MAX651/MAX652 drive P-channel
enhancement-mode MOSFET transistors only. The
choice of power transistor is primarily dictated by the
input voltage and the peak current. The transistor's
on-resistance, gate-source threshold, and gate
capacitance must also be appropriately chosen. The
drain-to-source and gate-to-source breakdown voltage
ratings must be greater than V+. The total gate-charge
specification is normally not critical, but values should
be less than 100nC for best efficiency. The MOSFET
should be capable of handling the peak current and,
for maximum efficiency, have a very low on-resistance
at that current. Also, the on-resistance must be low for
the minimum available VGS, which equals V+(min).
Select a transistor with an on-resistance between 50%
and 100% of the current-sense resistor. The Si9430
transistor chosen for the Typical Operating Circuit has
______________________________________________________________________________________ 11
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11 Page | ||
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| PDF Descargar | [ Datasheet MAX651.PDF ] | |
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