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ADP1822 Datasheet(PDF) 14 Page - Analog Devices

Part No. ADP1822
Description  PWM, Step-Down DC-to-DC Controller with Margining and Tracking
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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ADP1822 Datasheet(HTML) 14 Page - Analog Devices

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ADP1822
Rev. B | Page 14 of 24
APPLICATION INFORMATION
SELECTING THE INPUT CAPACITOR
The output ripple voltage is a function of the inductor ripple
current and the capacitor impedance at the switching frequency.
For high ESR capacitors, the impedance is dominated by the
ESR, while for low ESR capacitors the impedance is dominated
by the capacitance. Determine if the capacitor is high ESR or
low ESR by comparing the zero frequency formed by the
capacitance and the ESR to the switching frequency:
The input capacitor absorbs the switched input current of the
dc-to-dc converter, allowing the input source to deliver smooth
dc current. Choose an input capacitor whose impedance at the
switching frequency is lower than the input source impedance.
Use low equivalent series resistance (ESR) capacitors, such as
low ESR tantalum, ceramic, or organic electrolyte (such as
Sanyo OS-CON) types. For all types of capacitors, make sure
that the current rating of the capacitor is greater than the input
rms ripple current, which is approximately
1
×
×
OUT
IN
IN
OUT
LOAD
IN_RMS
V
V
V
V
I
I
(1)
OUTPUT LC FILTER
The output LC filter smoothes the switched voltage at SW,
making the output an almost dc voltage. Choose the output LC
filter to achieve the desired output ripple voltage. Since the
output LC filter is part of the regulator negative-feedback
control loop, the choice of the output LC filter components
affects the regulation control-loop stability.
Choose an inductor value such that the inductor ripple current
is approximately 1/3 of the maximum dc output load current.
Using a larger value inductor results in a physical size larger
than required, and using a smaller value results in increased
losses in the inductor and/or MOSFET switches.
Choose the inductor value by
=
IN
OUT
OUT
L
SW
V
V
V
I
f
L
1
)
Δ
)(
(
1
(2)
where:
L
is the inductor value.
fSW
is the switching frequency.
VOUT
is the output voltage.
VIN
is the input voltage.
ΔI
L
is the inductor ripple current, typically 1/3 of the maximum
dc load current.
Choose the output capacitor to set the desired output voltage
ripple. The ADP1822 functions with output capacitors that have
both high and low ESR. For high ESR capacitors, such as tantalum
or electrolytic types, many parallel connected capacitors may be
required to achieve the desired output ripple voltage. When
choosing an output capacitor, consider ripple current rating,
capacitance, and ESR. Make sure that the ripple current rating is
higher than the maximum inductor ripple current (ΔIL).
()(
)
ESR
C
f
OUT
ESRZ
π
2
1
=
(3)
where:
fESRZ
is the frequency of the output capacitor ESR zero.
COUT
is the output capacitance.
ESR
is the equivalent series resistance of the capacitor.
If fESRZ is much less than the switching frequency, then the capacitor
is high ESR, and the ESR dominates the impedance at the switching
frequency. If fESRZ is much greater than the switching frequency, the
capacitor is low ESR, and the impedance is dominated by the
capacitance at the switching frequency.
When using capacitors whose impedance is dominated by ESR
at the switching frequency (such as tantalum or aluminum
electrolytic capacitors), approximate the output voltage ripple
current by
)
(
Δ
Δ
ESR
I
V
L
OUT
(4)
where:
ΔV
OUT
is the output ripple voltage.
ΔI
L
is the inductor ripple current.
ESR is the total equivalent series resistance of the output
capacitor (or the parallel combination of ESR of all parallel-
connected output capacitors).
Make sure that the ripple current rating of the output capacitor(s) is
greater than the maximum inductor ripple current.
For output capacitors whose ESR is much lower than the
capacitive impedance at the switching frequency, the capacitive
impedance dominates the output ripple current. In this case,
determine the ripple voltage by
()( )
SW
OUT
L
OUT
f
C
I
V
8
Δ
Δ
(5)
where:
fSW is the switching frequency.
C
is the output capacitance.
OUT


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