ADP1621ARMZ-R7(RevB) 데이터 시트보기 (PDF) - Analog Devices
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ADP1621ARMZ-R7 Datasheet PDF : 32 Pages
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Data Sheet
ADP1621
Assuming the ripple current is 30% of 1/(1 − D) times the max-
imum load current, a reasonable choice for the inductor value is
L = VIN × D × (1 − D)
(9)
0.3 × f SW × I LOAD,MAX
From this starting point, modify the inductance to obtain the
right balance of size, cost, and output voltage ripple while
maintaining the inductor ripple current between 20% and 40%
of 1/(1 − D) times the maximum load current. Keep in mind
that the inductor saturation current must be greater than the
peak inductor current. Magnetically shielded inductors are
generally recommended, although they cost slightly more than
unshielded inductors.
Also, losses due to the inductor winding resistance reduce the
efficiency of the boost converter. This power loss is given by
PL,W
=
I LOAD
1−D
2
× RW
(10)
OUTPUT CAPACITOR SELECTION
The output capacitor maintains the output voltage and supplies
current to the load while the external MOSFET is on.
The value and characteristics of the output capacitor greatly
affect the output voltage ripple and stability of the converter.
The amount of peak-to-peak output voltage ripple, ΔVOUT, can
be approximated by
∆V OUT
≈ I LOAD
1−D
+
∆I L
2
×
( ) 2π×
1
f SW × COUT
2
+ ESR2
+
2π×
f SW
× ESL
2
(12)
where ΔIL is the peak-to-peak inductor ripple current, fSW is the
switching frequency, COUT is the output capacitance, ESR is the
effective ESR of COUT, and ESL is the effective equivalent series
inductance of COUT.
where PL,W is the power dissipation in the winding of the
inductor, and RW is the winding resistance.
INPUT CAPACITOR SELECTION
The bulk input capacitor provides a low impedance path for the
inductor ripple current. Capacitor C1 in Figure 1 represents a
bulk input capacitor. Choose a bulk input capacitor whose
impedance at the switching frequency is lower than the
impedance of the voltage source VIN.
The preferred bulk input capacitor is a 10 µF to 100 µF ceramic
capacitor because it has low equivalent series resistance (ESR) and
low impedance. Aluminum electrolytic and aluminum polymer
capacitors can also be used as the bulk input capacitors. The bulk
input capacitor does not need to be placed very close to the IN
and PIN pins. Aluminum electrolytic capacitors are the cheapest
and generally have high ESR values, which increase dramatically at
temperatures less than 0°C. Some aluminum electrolytic capacitors
have ESR less than 20 mΩ, but their capacitances are generally
greater than 800 µF. Aluminum polymer capacitors are more
expensive than the aluminum electrolytic ones, but are generally
cheaper than the ceramic capacitors for the same amount of
capacitance. Polymer capacitors have relatively low ESR, with
some models having less than 10 mΩ.
Regardless of the type of capacitor used, make sure the ripple
current rating of the bulk input capacitor, ICIN,RMS, is greater than
ICIN ,RMS =
1 × ∆IL
32
(11)
where ΔIL is the peak-to-peak inductor ripple current.
In addition to the bulk input capacitor, a bypass input capacitor is
required. The function of the bypass capacitor is to locally filter the
input voltage to the ADP1621 and maintain the input voltage at a
steady value during switching transitions. The bypass capacitor is
typically a 0.1 µF or greater ceramic capacitor and should be placed
as close as possible to the IN and PIN pins of the ADP1621.
Capacitors C3 and C4 in Figure 1 represent the bypass capacitors.
Because the output capacitor is typically greater than 40 µF, the
ESR dominates the output capacitance impedance and thus the
output voltage ripple. The use of low ESR, ceramic dielectric
capacitors is preferred, although aluminum electrolytic,
tantalum, OS-CON™ (from Sanyo), and aluminum polymer
capacitors can be used. At higher switching frequencies, the ESL
of the output capacitor may also be a factor in determining the
output voltage ripple. Multiple capacitors can be connected in
parallel to reduce the effective ESR and ESL. Keep in mind that
the capacitance of a given capacitor typically degrades with
increased temperature and bias voltage. Consult the capacitor
manufacturer’s data sheet when determining the actual
capacitance of a capacitor under certain conditions.
Ensure that the output capacitor ripple current rating, I , COUT,RMS
is greater than
ICOUT ,RMS = I LOAD ×
D
1− D
(13)
DIODE SELECTION
The diode conducts the inductor current to the output capacitor
and load while the MOSFET is off. The average diode current is
the load current:
I DIODE,AVE = I LOAD
(14)
The rms diode current in continuous conduction mode is given by
I DIODE,RMS
=
I LOAD
1− D
×
1− D
(15)
where D is the duty cycle.
The power dissipated in the diode is
PDIODE = VD × I LOAD
(16)
where VD is the forward-voltage drop of the diode.
Rev. B | Page 15 of 32
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