ADP3154JRU-REEL Analog Devices Inc, ADP3154JRU-REEL Datasheet - Page 8

• Manufacturer Part Number: ADP3154JRU-REEL
• Description: IC PS DUAL PENTIUM III 20-TSSOP
• Manufacturer: Analog Devices Inc

Specifications of ADP3154JRU-REEL

• Rohs Status: RoHS non-compliant
• Applications: *
• Mounting Type: Surface Mount
• Package / Case: 20-TSSOP

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:
The minimum inductance gives a peak-to-peak ripple current of
2.55 A, or 15% of the maximum dc output current I
The inductor peak current in normal operation is:
The inductor valley current is:
The inductor for this application should have an inductance
of 3.3 H 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 M
C
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:
In the above equation the value of di/dt is calculated as the
smaller voltage across the inductor (i.e., V
V
connected 2200 F capacitors have a total capacitance of
4400 F, so the minimum capacitance requirement is met with
ample margin.
R
The value of R
The current comparator of the ADP3154 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, I
the peak value less half of the peak-to-peak ripple current. Solv-
ing for R
the minimum current sense threshold of 125 mV yields:
Once R
I
ADP3154
C
SC(PK)
OUT
SENSE
OUT
MIN
L
I
MIN
) divided by the maximum inductance. The two parallel-
SC(PK)
Selection–Determining the Capacitance
R
can be predicted from the following equation:
1
SENSE
SENSE
SENSE
I
OMAX
= (145 mV)/R
V t
O OFF
V
R
has been chosen, the peak short-circuit current
= (125 mV)/[1.2(I
, allowing a 20% margin for overhead, and using
E
RIPPLE p p
SENSE
I
I
–
LPEAK
LVALLEY
di dt
I
R
OMIN
E MAX
/
,
(
is based on the required output current.
= I
= I
0 8
SENSE
)
OMAX
.
®
LPEAK
cores from Magnetics, Inc.
2 0
.
5
= (145 mV)/(5.0 m ) = 29 A
+ I
OMAX
m
V
– I
RPP
17
RPP
10
3
/2 = 19.5 A
A
+ I
2 0
= 14.5 A
mV
s
.
RPP
1
V
IN
A
5 3
/ .
–V
/2)] = 5.0 m
3 0
.
OMAX
0 8
OUT
.
m
H
, which equals
rather than
OMAX
3 2
.
3840
.
H
F
–8–
The actual short-circuit current is less than the above calculated
I
output voltage drops below 1 V. The relationship between the
off-time and the output voltage is:
With a short circuit across the output, the off-time will be about
70 s. 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 H and total resis-
tance of 22 m , the time constant will be 108 s. 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 A
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 ADP3154, 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 V
and the on resistance R
The minimum input voltage dictates whether standard threshold
or logic-level threshold MOSFETs must be used. For V
standard threshold MOSFETs (V
V
(V
MOSFETs with V
of V
The maximum output current I
requirement for the two power MOSFETs. When the ADP3154
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 V
maximum duty ratio of the high side FET is:
The maximum duty ratio of the low side (synchronous rectifier)
FET is:
The maximum rms current of the high side FET is:
The maximum rms current of the low side FET is:
SC(PK)
D
IN
I
= 11.6 A rms
I
= 12.5 A rms
GS(TH)
MAXHF
RMSHS
RMSLS
is expected to drop below 8 V, logic-level threshold MOSFETs
CC
value because the off-time rapidly increases when the
should be used.
< 2.5 V) are strongly recommended. Only logic-level
= (1 – f
= [D
= [D
MAXLF
MAXHF
2
MIN
D
5.0 m = 20 W.
GS
MAXLF
(I
t
(I
OFF
ratings higher than the absolute maximum
LVALLEY
t
LVALLEY
DS(ON)
OFF
= 1 – D
) = (1 kHz–180 kHz 3.0 s) = 46%
360
.
2 + I
2 + I
V
IN
C
OMAX
O
k
T
MAXHF
GS(TH)
= 5 V and V
LPEAK
LPEAK
1
determines the R
V
2
2 + I
< 4 V) may be used. If
= 54%
2 + I
A
LVALLEY
LVALLEY
OUT
= 2.0 V, the
I
I
LPEAK
LPEAK
IN
DS(ON)
> 8 V,
REV. A
GS(TH)
)/3]
)/3]
0.5
0.5