ADP3041 查看數據表（PDF） - Analog Devices
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ADP3041 Datasheet PDF : 16 Pages
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ADP3041
As a rule, powdered iron cores saturate softly, whereas ferrite cores
saturate abruptly. Open drum core inductors tend to saturate
gradually and are low cost and small in size, making these
types of inductors attractive in many applications. However,
care must be exercised in their placement because they have
high magnetic fields. In applications that are sensitive to mag-
netic fields, shielded geometrics are recommended.
In addition, inductor losses must be considered. Both core and
copper losses contribute to loss in converter efficiency. To mini-
mize core losses, look for inductors rated for operation at high
switching frequencies. To minimize copper losses, it is best to
use low dc resistance inductors. Typically, it is best to use an
inductor with a dc resistance lower than 20 mΩ per µH.
The inductor value can be estimated using
( ) L = VOUT –VIN × MSLOPE
(2)
where MSLOPE is the scaling factor for the proper slope compensation.
MSLOPE
= 1.456
fSW
(3)
Choose the closest standard inductor value as a starting point.
The corresponding peak inductor current can then be calculated.
( ) ( ) IL PEAK
=
IOUT
× VOUT
VIN
+
1 VIN × VOUT –VIN
2 L ×VOUT × fS
(4)
It is recommended to try several different inductor values, sizes,
and types to find the best inductor for the application. In gen-
eral, large inductor values lead to lower ripple current, less
output noise, and either larger size or higher dc resistance.
Conversely, low inductor values lead to higher ripple current,
more noise, and either smaller size or lower dc resistance. The
final inductor selection should be based on the best trade-off of
size, cost, and performance.
Capacitor Selection
The ADP3041 requires an input capacitor to reduce the switching
ripple and noise on the IN pin. The value of the input capaci-
tor depends on the application. For most applications, a minimum
of 10 µF is required. For applications that are running close to
current limit or that have large transient loads, input capacitors
in the range of 22 µF to 47 µF are required.
The selection of the output capacitor also depends on the
application. Given the allowable output ripple voltage, ⌬VOUT,
the criteria for selecting the output capacitor can be calcu-
lated using
( ) COUT
≥ 8 × IOUT
fS
VOUT – VIN
×VOUT × ∆VOUT
(5)
( ) ESRCOUT
≤
∆VOUT
IL PEAK
(6)
When selecting an output capacitor, make sure that the ripple
current rating is sufficient to cover the rms switching current
of the ADP3041. The ripple current in the output capacitor is
given by
( ) IRMS COUT
= IOUT
VOUT – VIN
VIN
(7)
Multilayer ceramic capacitors are a good choice since they have
low ESR, high ripple current rating, and a very small package
size. Tantalum or OS-CON capacitors can be used; however,
they have a larger package size and higher ESR. Table II lists
some capacitor manufacturers. Consult the manufacturer for
more information.
Table II. Capacitor Manufacturers
Vendor
AVX
Murata
Sanyo
Taiyo Yuden
Phone No.
843-448-9411
770-436-1300
408-749-9714
858-554-0755
Web Address
www.avxcorp.com
www.murata.com
www.sanyovideo.com
www.t-yuden.com
Diode Selection
In specifying a diode, consideration must be given to speed,
forward current, forward voltage drop, reverse leakage current,
and the breakdown voltage. The output diode should be rated to
handle the maximum output current. If the output can be sub-
jected to accidental short circuits, then the diode must be rated to
handle currents up to the current limit of the ADP3041. The
breakdown rating of the diode must exceed the output voltage. A
high speed diode with low forward drop and low leakage will help
improve the efficiency of the converter by lowering the losses of
the diode. Schottky diodes are recommended.
Loop Compensation
Like most current programmed PWM converters, the ADP3041
needs compensation to maintain stability over the operating
conditions of the particular application. For operation at duty
cycles above 50%, the choice of inductor is critical in maintain-
ing stability. If the slope of the inductor current is too small or
too large, the circuit will be unstable. See the Inductor Selection
section for more information on choosing the proper inductor.
The ADP3041 provides a pin (COMP) for compensating the
voltage feedback loop. This is done by connecting a series RC
network from the COMP pin to GND (see Figure 2). For most
applications, the compensation resistor, RC, should be in the
range of 5 kΩ < RC < 400 kΩ, and the compensation capacitor,
CC, in the range of 100 pF < CC < 10 nF. Further details for
selecting the compensation components follow.
–12–
REV. D
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