TK65015MTL Ver la hoja de datos (PDF) - Toko America Inc
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TK65015MTL Datasheet PDF : 8 Pages
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TK65015
The implication from Eq. (2) is that the inductor will
operate in discontinuous mode. From a practical stand-
point for the TK65015, this is essentially guaranteed when
using a single battery cell to power the converter.
Now, plugging in worst case conditions, the inductor
value can be determined by simply transforming the above
equation in terms of “L”:
requirements. The following series of Toko inductors work
especially well with the TK65015: 10RF, 12RF, 3DF, D73,
and D75. The 5CA series can be used for isolated-output
applications, although such design objectives are not
considered here.
Other Converter Components
[ ] LMIN
=
V2
I(MIN)
D(MIN)
2 f I (MAX) O(MAX) VO(MIN) + VF(MAX)
(3)
− VI(MIN)
2
In choosing a diode, parameters worthy of consideration
are: forward voltage, reverse leakage, and capacitance.
The biggest efficiency loss in the converter is due to the
diode forward voltage. A schottky diode is typically chosen
where “VF(MAX)” is best approximated by the diode forward
voltage at about two-thirds of the peak diode current value.
to minimize this loss.
Reverse leakage current is generally higher in schottkys
The peak diode current is the same as the peak input
than in pn-junction diodes. If the converter spends a good
current, the peak switch current, and the peak inductor
current. The formula is:
2 deal of the battery lifetime operating at very light load (i.e.,
the system under power is frequently in a stand-by mode),
then the reverse leakage current could become a substan-
I PK = VID
fL
(4)
tial fraction of the entire average load current, thus degrad-
ing battery life. So don’t dramatically oversize the schottky
diode if this is the case.
Diode capacitance isn’t likely to make much of an
Some reiteration is implied because “L” is a function of
undesirable contribution to switching loss at this relatively
“VF” which is a function of “IPK” which, in turn, is a function
of “L”. The best way into this loop is to first approximate
low switching frequency. It can, however, increase the
snubber dissipation requirement.
“VF”, determine “L”, determine “IPK”, and then determine a
new “VF”. Then, if necessary, reiterate.
When selecting the actual inductor, it is necessary to
The snubber is composed of a series RC network from
the switch pin to ground (or to the output or input if
preferred). Its function is to dampen the resonant LC circuit
make sure that the peak current rating of the inductor (i.e.,
which rings during the inductor current deadtime. When
the current which causes the core to saturate) is greater
the current flowing in the inductor through the output diode
than the maximum peak current that the inductor will
decays to zero, the parasitic capacitance at the switch pin
encounter. To determine the maximum peak current, use
from the switch, the diode, and the inductor winding has
Eq. (4) again, but this time plugging in maximum values for
energy which rings back into the inductor, flowing back into
“VI” and “D”, and minimum values for “f “ and “L”.
It may also be necessary when selecting the inductor to
the battery. If there is no snubbing, it is feasible that the
switch pin voltage could ring below ground. Although the
check the rms current rating of the inductor. Whereas
IC is well protected against latchup, this ringing may be
peak current rating is determined by core saturation, rms
undesirable due to radiated noise. In order to do an
current rating is determined by wire size and power dissi-
effective job, the snubber capacitor should be large (e.g.,
pation in the wire resistance. The inductor rms current is
5~20 times) in comparison to the parasitic capacitance. If
given by:
it is unnecessarily large, then it dissipates extra energy
every time the converter switches. The resistor of the
I L(RMS) = I PK
D+
VO
I PK f L
+ VF − VI
(5) snubber should be chosen such that it drops a substantial
voltage as the ringing parasitic capacitance attempts to
pull the snubber capacitor along for the ride. If the resistor
3
is too small (e.g., zero), then the snubber capacitance just
adds to the ringing energy. If the resistor is too large (e.g.,
where “IPK” is the same maximized value that was just used
to check against inductor peak current rating, and the term
in the numerator within the radical that is added to the
[on-time] duty ratio, “D”, is the off-time duty ratio.
Toko America, Inc. offers a wide range of inductor
values and sizes to accomodate varying power level
infinite) then it effectively disengages the snubber capaci-
tor from fighting the ringing.
The output capacitor, the capacitor connected from the
diode cathode to ground, has the function of averaging the
current pulses delivered through the inductor while holding
a relatively smooth voltage for the converter load. Typi-
February, 1996 TOKO, Inc.
2-2-96
Page 5
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