ADP3155 データシートの表示（PDF） - Analog Devices
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ADP3155 Datasheet PDF : 14 Pages
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ADP3155
Tips for Selecting the Inductor Core
Ferrite designs have very low core loss, so the design should
focus on copper loss and on preventing saturation. Molypermalloy,
or MPP, is a low loss core material for toroids, and it yields the
smallest size inductor, but MPP cores are more expensive than
ferrite cores or the Kool Mµ® cores from Magnetics, Inc. The
lowest cost core is made of powdered iron, for example the #52
material from Micrometals, Inc., but yields the largest size
inductor.
COUT Selection—Determining the Capacitance
The minimum capacitance of the output capacitor is determined
from the requirement that the output be held up while the in-
ductor current ramps up (or down) to the new value. The mini-
mum capacitance should produce an initial dv/dt that is equal
(but opposite in sign) to the dv/dt obtained by multiplying the
di/dt in the inductor and the ESR of the capacitor:
( ) CMIN
=
IOMAX – IOMIN
RE(di / dt)
=
14.2
5.9 mΩ ×
A − 0.8 A
2.2 A/4.4 µH
= 4.5 mF
In the above equation the value of di/dt is calculated as the
smaller voltage across the inductor (i.e., VIN–VOUT rather than
VOUT) divided by the maximum inductance (4.4 µH) of the
Coiltronics CTX12-13855 inductor. The six parallel-connected
2700 µF capacitors have a total capacitance of 16,200 µF, so the
minimum capacitance requirement is met with ample margin.
RSENSE
The value of RSENSE is based on the required output current.
The current comparator of the ADP3155 has a threshold range
that extends from 0 mV to 125 mV (minimum). Note that the
full 125 mV range cannot be used for the maximum specified
nominal current, as headroom is needed for current ripple and
transients.
The current comparator threshold sets the peak of the inductor
current yielding a maximum output current, IOMAX, which equals
the peak value less half of the peak-to-peak ripple current. Solv-
ing for RSENSE allowing a 20% margin for overhead, and using
the minimum current sense threshold of 125 mV yields:
RSENSE = (125 mV)/[1.2(IOMAX + IRPP /2)] = 6.8 mΩ
Once RSENSE has been chosen, the peak short-circuit current
ISC(PK) can be predicted from the following equation:
ISC(PK) = (145 mV)/RSENSE = (145 mV)/(6.7 mΩ) = 21.5 A
The actual short-circuit current is less than the above calculated
ISC(PK) value because the off-time rapidly increases when the
output voltage drops below 1 V. The relationship between the
off-time and the output voltage is:
tOFF
≈
CT × 1 V
VO
360 kΩ
+
2
µA
With a short circuit across the output, the off-time will be about
70 µs. During that time the inductor current gradually decays.
The amount of decay depends on the L/R time constant in the
output circuit. With an inductance of 2.5 µH and total resis-
tance of 23 mΩ, the time constant will be 108 µs. This yields a
valley current of 11.3 A and an average short-circuit current of
about 16.3 A. To safely carry the short-circuit current, the sense
resistor must have a power rating of at least 16.3 A2 × 6.8 mΩ =
1.8 W.
Current Transformer Option
An alternative to using a low value and high power current sense
resistor is to reduce the sensed current by using a low cost cur-
rent transformer and a diode. The current can then be sensed
with a small-size, low cost SMT resistor. Using a transformer
with one primary and 50 secondary turns reduces the worst-case
resistor dissipation to a few mW. Another advantage of using
this option is the separation of the current and voltage sensing,
which makes the voltage sensing more accurate.
Power MOSFETs
Two external N-channel power MOSFETs must be selected for
use with the ADP3155, one for the main switch and an identical
one for the synchronous switch. The main selection parameters
for the power MOSFETs are the threshold voltage VGS(TH) and
the on resistance RDS(ON).
The minimum input voltage dictates whether standard threshold
or logic-level threshold MOSFETs must be used. For VIN > 8 V,
standard threshold MOSFETs (VGS(TH) < 4 V) may be used. If
VIN is expected to drop below 8 V, logic-level threshold MOSFETs
(VGS(TH) < 2.5 V) are strongly recommended. Only logic-level
MOSFETs with VGS ratings higher than the absolute maximum
of VCC should be used.
The maximum output current IOMAX determines the RDS(ON)
requirement for the two power MOSFETs. When the ADP3155
is operating in continuous mode, the simplifying assumption can
be made that one of the two MOSFETs is always conducting
the average load current. For VIN = 5 V and VOUT = 2.8 V, the
maximum duty ratio of the high side FET is:
DMAXHF = (1 – fMIN × tOFF) = (1 kHz–160 kHz × 2.2 µs) = 65%
The maximum duty ratio of the low side (synchronous rectifier)
FET is:
DMAXLF = 1 – DMAXHF = 35%
The maximum rms current of the high side FET is:
IRMSHS = [DMAXHF (ILVALLEY2 + ILPEAK2 + ILVALLEYILPEAK)/3]0.5
= 13.1 A rms
The maximum rms current of the low side FET is:
IRMSLS = [DMAXLF (ILVALLEY2 + ILPEAK2 + ILVALLEYILPEAK)/3]0.5
= 8.41 A rms
The RDS(ON) for each FET can be derived from the allowable
dissipation. If 5% of the maximum output power is allowed for
FET dissipation, the total dissipation will be:
PFETALL = 0.05 VOIOMAX = 2 W
Allocating half of the total dissipation for the high side FET and
half for the low side FET, the required minimum FET resis-
tances will be:
RDS(ON)HSF(MIN) = 1.33 W/(11.5 A)2 = 10 mΩ
RDS(ON)LSF(MIN) = 0.67 W/(8.41 A)2 = 9.5 mΩ
Note that there is a trade-off between converter efficiency and
cost. Larger FETs reduce the conduction losses and allow
higher efficiency, but increase the system cost. If efficiency is
not a major concern, the International Rectifier IRL3103 is an
economical choice for both the high side and low side positions.
Those devices have an RDS(ON) of 14 mΩ at VGS = 10 V and at
+25°C. The low side FET is turned on with at least 10 V. The
REV. A
–9–
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