ADP3152 查看數據表（PDF） - Analog Devices
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ADP3152 Datasheet PDF : 12 Pages
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ADP3152
The output filter capacitor must have an ESR of less than
5.9 mΩ. One can use, for example, six FA type capacitors from
Panasonic, with 2700 µF capacitance, 10 V voltage rating, and
34 mΩ ESR. The six capacitors have a total typical ESR of
~ 5 mΩ when connected in parallel.
Inductor Selection
The minimum inductor value can be calculated from ESR, off
time, dc output voltage and allowed peak-to-peak ripple voltage.
LMIN1 = VOtOFF RE(MAX ) = 2.8 × 2.2 µ × 5.9 m = 2.6 µH
V RIPPLE , p −p
14 m
The minimum inductance gives a peak-to-peak ripple current of
2.15 A, or 15% of the maximum dc output current IOMAX.
The inductor peak current in normal operation is:
ILPEAK = IOMAX + IRPP/2 = 15.3 A
The inductor valley current is:
ILVALLEY = ILPEAK – IRPP = 13 A
The inductor for this application should have an inductance
of 2.6 µH at full load current and should not saturate at the
worst-case overload or short circuit current at the maximum
specified ambient temperature. A suitable inductor is the
CTX12-13855 from Coiltronics, which is 4.4 µH at 1 A and
about 2.5 µH at 14.2 A.
Tips for Selecting 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.
CO 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 which 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 =
14.2 – 0.8
= 4.5 mF
RE (di/dt ) 5.9 m (2.2 / 4. 4 µH )
In the above equation the value of di/dt is calculated as the
smaller voltage across the inductor (i.e., VIN–VO rather than VO)
divided by the maximum inductance (4.4 µH) of the CTX12-
13855 inductor from Coiltronics. The parallel-connected six
2700 µF/10 V FA series capacitors from Panasonic have a total
capacitance of 16,200 µF, so the minimum capacitance require-
ment is met with ample margin.
RSENSE
The value of RSENSE is based on the required output current.
The current comparator of the ADP3152 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, tran-
sients and inductor core saturation.
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 and allowing a margin for tolerances inside the
ADP3152 and in the external component values yields:
RSENSE = (125 mV)/[1.2(IOMAX + IRPP/2)] = 6.8 mΩ
A practical solution is to use three 20 mΩ resistors in parallel,
with an effective resistance of about 6.7 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 × 1V
VO + 2 µA
360 kΩ
With a short across the output, the off time will be about
70 µs. During that off time the inductor current gradually de-
cays. 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, which 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 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. If we use a transformer
with one primary and 50 secondary turns, the worst-case resistor
dissipation is reduced to a fraction of a mW. Another advantage
of using this option is the separation of the current and voltage
sensing, which makes the voltage sensing more accurate.
Power MOSFET
Two external N-channel power MOSFETs must be selected for
use with the ADP3152, one for the main switch, and an identi-
cal one for the synchronous switch. The main selection param-
eters for the power MOSFETs are the threshold voltage VGS(TH)
and the on resistance RDS(ON).
–8–
REV. 0
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