ADP3157JR Analog Devices Inc, ADP3157JR Datasheet - Page 8

• Manufacturer Part Number: ADP3157JR
• Description: IC CNTRL SYNC PENTIUM III 16SOIC
• Manufacturer: Analog Devices Inc

Specifications of ADP3157JR

• Rohs Status: RoHS non-compliant
• Applications: Controller, Intel Pentium® III
• Voltage - Input: 12V
• Number Of Outputs: 1
• Voltage - Output: 1.3 ~ 3.5 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (3.9mm Width)

ADP3157
10 m ESR. The two capacitors have a total ESR of 5.0 m 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:
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 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:
In the above equation the value of di/dt is calculated as the
smaller voltage across the inductor (i.e., V
V
parallel-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 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, 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:
C
OUT
OUT
SENSE
MIN
L
MIN
) divided by the maximum inductance inductor. The two
Selection–Determining the Capacitance
R
1
SENSE
SENSE
I
OMAX
V t
O OFF
V
R di dt
, allowing a 20% margin for overhead, and using
= (125 mV)/[1.2(I
RIPPLE p p
SENSE
E
–
I
I
( / )
LPEAK
LVALLEY
I
R
OMIN
E MAX
,
(
is based on the required output current.
= I
= I
)
0 8
OMAX
.
®
LPEAK
cores from Magnetics, Inc.
2 0
.
5
+ I
OMAX
m
V
– I
RPP
17
RPP
10
/2 = 19.5 A
3
A
+ I
( .
2 0
–
= 14.5 A
mV
s
RPP
1
IN
V
A
/2)] = 5.0 m
5 3
–V
/ .
OMAX
.
3 0
OUT
0 8
m
.
H
, which equals
rather than
)
OMAX
3 2
.
3840
.
H
F
–8–
Once R
I
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 73 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 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 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 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 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:
SC(PK)
SC(PK)
IN
D
I
= 11.6 A rms
GS(TH)
RMSHS
MAXHF
is expected to drop below 8 V, logic-level threshold MOSFETs
CC
I
SC(PK)
can be predicted from the following equation:
value because the off-time rapidly increases when the
should be used.
SENSE
= [D
< 2.5 V) are strongly recommended. Only logic-level
= (1–f
= (145 mV)/R
has been chosen, the peak short-circuit current
MAXHF
2
MIN
D
5.0 m = 2.0 W.
GS
MAXLF
(I
LVALLEY
ratings higher than the absolute maximum
t
t
OFF
OFF
DS(ON)
SENSE
) = (1 kHz–180 kHz
= 1 – D
2 + I
360
.
V
= (145 mV)/(5.0 m ) = 29 A
IN
C
OMAX
O
LPEAK
k
T
MAXHF
GS(TH)
= 5 V and V
1
2 + I
V
determines the R
2
< 4 V) may be used. If
= 54%
A
LVALLEY
OUT
I
3.0 s) = 46%
LPEAK
= 2.8 V, the
IN
)/3]
DS(ON)
> 8 V,
REV. A
0.5
GS(TH)