ADP3159 Ver la hoja de datos (PDF) - Analog Devices
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ADP3159 Datasheet PDF : 16 Pages
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ADP3159/ADP3179
There are many useful references for quickly designing a power
inductor. Table II gives some examples.
Table II. Magnetics Design References
Magnetic Designer Software
Intusoft (http://www.intusoft.com)
Designing Magnetic Components for High-Frequency DC-DC
Converters
McLyman, Kg Magnetics
ISBN 1-883107-00-08
Selecting a Standard Inductor
The companies listed in Table III can provide design consul-
tation and deliver power inductors optimized for high power
applications upon request.
Table III. Power Inductor Manufacturers
Coilcraft
(847) 639-6400
http://www.coilcraft.com
Coiltronics
(561) 752-5000
http://www.coiltronics.com
Sumida Electric Company
(408) 982-9660
http://www.sumida.com
COUT Selection—Determining the ESR
The required equivalent series resistance (ESR) and capacitance
drive the selection of the type and quantity of the output capaci-
tors. The ESR must be small enough to contain the voltage
deviation caused by a maximum allowable CPU transient cur-
rent within the specified voltage limits, giving consideration also
to the output ripple and the regulation tolerance. The capaci-
tance must be large enough that the voltage across the capacitor,
which is the sum of the resistive and capacitive voltage deviations,
does not deviate beyond the initial resistive deviation while the
inductor current ramps up or down to the value corresponding
to the new load current. The maximum allowed ESR also repre-
sents the maximum allowed output resistance, ROUT.
The cumulative errors in the output voltage regulation cuts into
the available regulation window, VWIN. When considering dynamic
load regulation this relates directly to the ESR. When consider-
ing dc load regulation, this relates directly to the programmed
output resistance of the power converter.
Some error sources, such as initial voltage accuracy and ripple
voltage, can be directly deducted from the available regulation
window, while other error sources scale proportionally to the
amount of voltage positioning used, which, for an optimal design,
should utilize the maximum that the regulation window will allow.
The error determination is a closed-loop calculation, but it can
be closely approximated. To maintain a conservative design while
avoiding an impractical design, various error sources should
be considered and summed statistically.
The output ripple voltage can be factored into the calculation by
summing the output ripple current with the maximum output
current to determine an effective maximum dynamic current
change. The remaining errors are summed separately according
to the formula:
VWIN = (V∆ –VVID × 2 kVID ) ×
(5)
1 – IO
IO + IO∆
kRCS 2
+
kCSF
2
2
+
kRT 2
+
kEA 2
=
95
mV
where kVID = 0.5% is the initial programmed voltage tolerance
from the graph of TPC 6, kRCS = 2% is the tolerance of the
current sense resistor, kCSF = 10% is the summed tolerance of
the current sense filter components, kRT = 2% is the tolerance of
the two termination resistors added at the COMP pin, and
kEA = 8% accounts for the IC current loop gain tolerance
including the gm tolerance.
The remaining window is then divided by the maximum output
current plus the ripple to determine the maximum allowed ESR
and output resistance:
RE ( MAX )
= ROUT ( MAX )
=
VWIN
IO + IO∆
= 95 mV
15 A + 3.8 A
= 5 mΩ
(6)
The output filter capacitor bank must have an ESR of less than
5 mΩ. One can, for example, use five ZA series capacitors from
Rubycon which would give an ESR of 4.8 mΩ. Without ADOPT
voltage positioning, the ESR would need to be less than 3 mΩ,
yielding a 50% increase to eight Rubycon output capacitors.
COUT—Checking the Capacitance
As long as the capacitance of the output capacitor is above a
critical value and the regulating loop is compensated with ADOPT,
the actual value has no influence on the peak-to-peak deviation
of the output voltage to a full step change in the load current.
The critical capacitance can be calculated as follows:
COUT (CRIT )
=
RE
IO
×VOUT
×L
= 15 A × 1.5 µH = 2.6 mF
(7)
5 mΩ × 1.7
The critical capacitance for the five ZA series Rubycon capaci-
tors is 2.6 mF while the equivalent capacitance is 5 mF. The
capacitance is safely above the critical value.
RSENSE
The value of RSENSE is based on the maximum required output
current. The current comparator of the ADP3159 has a mini-
mum current limit threshold of 69 mV. Note that the 69 mV
value cannot be used for the maximum specified nominal cur-
rent, as headroom is needed for ripple current and tolerances.
–8–
REV. A
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