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参数资料
| 型号： | TK65130MTL |
| 厂商： | ASAHI KASEI POWER DEVICES CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING REGULATOR, 102 kHz SWITCHING FREQ-MAX, PDSO6 |
| 封装： | SOT-23L, SMT-6 |
| 文件页数： | 8/20页 |
| 文件大小： | 158K |
| 代理商： | TK65130MTL |
Page 16
January 2001 TOKO, Inc.
TK651xxM
DUAL-CELL APPLICATION
There are special considerations involved in designing a
converter with the TK651xx for use with two battery cells.
With two battery cells, the TK651xx can provide substantially
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
current capability in accordance with the maximum peak
current that could be calculated using Equation 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 Equation 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 increases in the inductor
during the on-time is faster than the rate of decay than
during the off-time. This is because there is more voltage
applied across the switching during the on-time (two battery
cells) than during the off-time (3 volts plus a diode minus
two 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 is necessary to choose a
lower inductor value to achieve a higher output current
rating (Equation 6 specifically shows “I
OUT” as a function of
“L”). This also implies 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 higher inductance
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 be for a single-cell input using the same
inductor value. The Toko D73 and D75 series inductors are
partially 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 rating
equal to the converter output current rating plus the single
cycle ripple current peak given Equation 4. With that
guideline followed, the risk of saturation becomes only a
dynamic problem. Under the situation of placing a dynamic
load on the output of the converter, saturation may occur.
Fortunately, unlike off-line powered converters, battery
powered converters tend to be quite forgiving of dynamic
saturation, due to the limitation of available power.
Start-up of the converter is an example of a practically
unavoidable dynamic load change (complicated by an
output operating point change) that can cause saturation of
the inductor. However, this particular phenomenon applies
to single-cell powered converters, too. Hence, saturation is
not entirely avoidable, yet does not cause system problems.
It is beyond the scope of this application note to quantify the
practical limitations of allowed dynamic saturation and how
stressful it may be to the various components involved. It
is left to the user to examine empirically the dynamic
saturation phenomenon and determine what performance
is acceptable. In most cases, no problem will be exhibited.
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