ADP3157 데이터 시트보기 (PDF) - Analog Devices
부품명
상세내역
일치하는 목록
ADP3157 Datasheet PDF : 12 Pages
| |||
ADP3157
10 mΩ ESR. The two capacitors have a total ESR of 5.0 mΩ when
connected in parallel, which gives adequate margin.
Inductor Selection
The minimum inductor value can be calculated from ESR, off-
time, dc output voltage and allowed peak-to-peak ripple voltage
using the following equation:
LMIN1
=
VOtOFF RE(MAX )
VRIPPLE, p − p
=
2.0 V
× 3 µs ×
10 mV
5.3
mΩ
=
3.2
µH
The minimum inductance gives a peak-to-peak ripple current of
2.55 A, or 15% of the maximum dc output current IOMAX.
The inductor peak current in normal operation is:
ILPEAK = IOMAX + IRPP/2 = 19.5 A
The inductor valley current is:
ILVALLEY = ILPEAK – IRPP = 14.5 A
The inductor for this application should have an inductance
of 3.3 µH at full load current and should not saturate at the
worst-case overload or short circuit current at the maximum
specified ambient temperature.
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.
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 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
RE(di / dt)
× 0.8
17 A – 1 A × 0.8
= 5 mΩ × (2.0 V / 3.0µH )
= 3840 µF
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 inductor. The two
parallel-connected 2200 µF capacitors have a total capacitance
of 4400 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 ADP3157 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)] = 5.0 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)/(5.0 mΩ) = 29 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
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 3.3 µH and total resis-
tance of 22 mΩ, the time constant will be 73 µs. This yields an
average short-circuit current of about 20 A. To safely carry the
short-circuit current, the sense resistor must have a power rating
of at least 20 A2 × 5.0 mΩ = 2.0 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 ADP3157, 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).
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 ADP3157
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–180 kHz × 3.0 µs) = 46%
The maximum duty ratio of the low side (synchronous rectifier)
FET is:
DMAXLF = 1 – DMAXHF = 54%
The maximum rms current of the high side FET is:
IRMSHS = [DMAXHF (ILVALLEY2 + ILPEAK2 + ILVALLEYILPEAK)/3]0.5
= 11.6 A rms
–8–
REV. A
Share Link: 