ADP3152AR Analog Devices, ADP3152AR Datasheet - Page 8

• Manufacturer Part Number: ADP3152AR
• Description: 5-Bit Programmable Synchronous Switching Regulator Controller for Pentium II Processor
• Manufacturer: Analog Devices

ADP3152
The output filter capacitor must have an ESR of less than
5.9 m . One can use, for example, six FA type capacitors from
Panasonic, with 2700 F capacitance, 10 V voltage rating, and
34 m ESR. The six capacitors have a total typical ESR of
~ 5 m 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.
The minimum inductance gives a peak-to-peak ripple current of
2.15 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 2.6 H 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 H at 1 A and
about 2.5 H 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 M
lowest cost core is made of powdered iron, for example the #52
material from Micrometals, Inc., but yields the largest size
inductor.
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
divided by the maximum inductance (4.4 H) of the CTX12-
13855 inductor from Coiltronics. The parallel-connected six
2700 F/10 V FA series capacitors from Panasonic have a total
capacitance of 16,200 F, so the minimum capacitance require-
ment is met with ample margin.
O
C
L
Selection—Determining the Capacitance
MIN1
MIN
I
V
OMAX
R
O
V
E
t
OFF
I
RIPPLE , p p
LPEAK
(di/dt )
I
LVALLEY
– I
R
E( MAX )
OMIN
= I
OMAX
= I
®
LPEAK
cores from Magnetics, Inc. The
5.9 m (2.2 / 4. 4 H )
2.8 2.2
+ I
RPP
– I
14.2 – 0.8
/2 = 15.3 A
RPP
14 m
= 13 A
IN
–V
5.9 m
O
rather than V
OMAX
2.6 H
4.5 mF
.
O
)
–8–
R
The value of R
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 I
the peak value less half of the peak-to-peak ripple current. Solv-
ing for R
ADP3152 and in the external component values yields:
A practical solution is to use three 20 m resistors in parallel,
with an effective resistance of about 6.7 m .
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 across the output, the off time will be about
70 s. 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 H and total resis-
tance of 23 m , the time constant will be 108 s, 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 A
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 V
and the on resistance R
SC(PK)
SC(PK)
SENSE
R
I
SC(PK)
SENSE
can be predicted from the following equation:
value because the off time rapidly increases when the
SENSE
SENSE
= (145 mV)/R
= (125 mV)/[1.2(I
has been chosen, the peak short-circuit current
and allowing a margin for tolerances inside the
SENSE
is based on the required output current.
t
OFF
SENSE
DS(ON)
OMAX
360 k
= (145 mV)/(6.7 m ) = 21.5 A
.
V
C
O
T
+ I
RPP
1V
2 A
/2)] = 6.8 m
OMAX,
2
which equals
.8 m =
REV. 0
GS(TH)