TK65025 データシートの表示（PDF） - Toko America Inc
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TK65025 Datasheet PDF : 12 Pages
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TK65025
series (surface mount); Matsuo 267 series (surface mount);
Sanyo OS-CON series (miniature through hold).
current capability in accordance with the maximum peak
current that could be calculated using Eq. (4). For a two
( ) ( ) ( ) [ ( ( ) () ) ] IO
=
VO
V
2
BB
D
2
D
fL
1
-
D
2f L
RS + R L + RSW
2
D
+ ROF IO(TGT) +
2f L
VBBR U
+
VF
−
VBB 1 -
D
2f L
RS
+ RL
−
f CS
V
2
BB
+
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 val-
ues 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 special considerations involved in designing
a converter with the TK65025 for use with two battery cells.
With two battery cells the TK65025 can provide substan-
tially more output current than a single cell input for the
same efficiency.
The concern is the possibility of saturating the inductor.
For a single cell input it was only necessary to choose the
Page 8
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
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
discontinuous 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 particularly suited for the higher output
current 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
February, 1997 Toko, Inc.
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