TK65015 Ver la hoja de datos (PDF) - Toko America Inc
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TK65015 Datasheet PDF : 8 Pages
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TK65015
( ) ( ) ( ) [ ( ( ) () ) ] IO
=
VO
+
V BB 2 D
2
D
fL
1
-
D
2f L
D
R OF IO(TGT) +
VBBR U +
2f L
2
RS + RL + RSW
VF
−
VBB 1 -
D
2f L
RS
+
RL
−
f CS
VBB2 +
2
VO + VF + VO + VF − VBB
2 VO + VF
2
(6)
Higher-Order Design Equation
The equation above was developed as a closed form
approximation for the design variable that required the
least approximation to allow a closed form. In this case,
that variable was “IO” (e.g., as opposed to “L”).
The approximations made in the equation development
have the primary consequence that error is introduced as
resistive losses become relatively large. As it is normally
a practical design goal to ensure that resistive losses will
be relatively small, this seems acceptable. The variables
used are:
IO Output current capability
IO(TGT) Targeted output current capability
VO Output voltage
VF Diode forward voltage
VBB Battery voltage, unloaded
D Oscillating duty ratio of main switch
f
Oscillator frequency
L Inductance value
RS Source resistance (battery + filter)
RL Inductor winding resistance
RSW Switch on-state resistance
ROF Output filter resistance
RU ESR of upstream output capacitor
CS Snubber capacitance
Deriving a design solution with this equation is neces-
sarily an iterative process. Use worst case tolerances as
described for inductor selection, plugging in different
values for “L” to approximately achieve an “IO” equal to the
targeted value. Then, fine tune the parasitic values as
needed and, if necessary, readjust “L” again and reiterate
the process.
DUAL-CELL APPLICATION
There are some risks involved in designing a converter
with the TK65015 for use with two battery cells. But with
some precautions taken it can be done and can provide
substantially more output current than a single cell input
for the same efficiency.
The risk lies in the possibility of saturating the inductor.
For a single cell input it was only necessary to choose the
current capability in accordance with the maximum peak
current that could be calculated using Eq. (4). For a two
cell input the peak current is not so readily determined
because the inductor can go into continuous mode.
When this happens, the increase of current during the on-
time remains more-or-less the same (i.e., approximately
equal to the peak current as calculated using Eq. (4)), but
the inductor current doesn’t start from zero. It starts from
where it had decayed to during the previous off-time.
There is no deadtime associated with a single switching
period when in continuous mode because the inductor
2
current never decays to zero within one cycle.
The cause for continuous mode operation is readily
seen by noting that the rate of current increase in the
inductor during the on-time is faster than the rate of decay
during the off-time. The reason for that is because there
is more voltage applied across the switch during the
on-time (two battery cells) than during the off-time (3 volts
plus a diode drop minus two battery cells). That situation,
in conjunction with a switch duty ratio of about 50%,
implies that the current can’t fall as much as it can rise
during a cycle. So when a switching cycle begins with
zero current in the inductor, it ends with current still
flowing.
Continuous mode operation implies that the inductor
value no longer restricts the output current capability.
With discontuous mode operation, it was necessary to
choose a lower inductor value to achieve a higher output
current rating. (Eq. (6) specifically shows “IO” as a function
of “L”.) This also implied higher ripple current from the
battery. In continuous mode operation, one can choose
a larger inductor value intentionally if it is desirable to
minimize ripple current. The catch is that high inductance
and high current rating together generally imply larger
inductor size. But generally this unrestricted inductor
value allows more freedom in the converter design.
The dual cell input and the continuous current rating
imply that the peak current in the inductor will be at least
twice as high as it would for a single cell input using the
same inductor value. The Toko D73 and D75 series
inductors are particular suited for the higher output cur-
rent capability of the dual cell configuration.
For operation at a fixed maximum load, the inductor
can be kept free of saturation by choosing its peak current
rating equal to the converter output current rating plus the
single cycle ripple current peak given by Eq. (4). With that
guideline followed, the risk of saturation becomes only a
February, 1996 TOKO, Inc.
2-2-96
Page 7
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