TK65015 데이터 시트보기 (PDF) - Toko America Inc
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TK65015 Datasheet PDF : 8 Pages
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TK65015
SINGLE-CELL APPLICATION
Theory of Operation
The converter operates with one terminal of an inductor
connected to the DC input and the other terminal con-
nected to the switch pin of the IC. When the switch is
turned on, the inductor current ramps up. When the switch
is turned off (or “lets go” of the inductor), the voltage flies
up as the inductor seeks out a path for its current. A diode,
also connected to the switching node, provides a path of
conduction for the inductor current to the boost converter’s
output capacitor. The TK65015 monitors the voltage of the
output capacitor and has a 3 volt threshold at which the
converter switching becomes disactivated. So the output
capacitor charges up to 3 volts and regulates there,
provided that we don’t draw more current from the output
than the inductor can provide. The primary task, then, in
designing a boost converter with the TK65015 is to deter-
mine the inductor value which will provide the amount of
current needed to guarantee that the output voltage will be
able to maintain regulation up to a specified maximum load
current. Secondary tasks include choosing the diode,
output capacitor, snubber, and filtering if desired.
The TK65015 runs with a fixed oscillator frequency and
it regulates by applying or skipping pulses to the internal
power switch. This regulation method is called pulse burst
modulation (PBM).
Reset Feature
The TK65015 also features an output voltage monitor
which provides a reset signal to a microprocessor or other
external system controller. When the output voltage is
below the reset threshold (which is less than the regulation
threshold), the reset signal is asserted low, indicating that
the system controller (e.g., microprocessor) should be in a
reset mode. Such a condition might exist during startup of
the converter or under an overload fault condition. This
method of reset control can be used to prevent improper
system operation which might occur at low supply voltage
levels.
The TK65015 has a reset threshold between 2.48 and
2.70 volts.
Breakdown of a Switching Cycle
Although the derivation of equations is not discussed,
the user will more easily be able to understand (and if
desired, reproduce) the design equations if we begin by
more precisely describing how the converter operates
over a switching cycle.
From an oscillator standpoint, the switching cycle con-
sists of only an on-time and an off-time. But from an
inductor current standpoint, the switching cycle breaks
down into three important sections: on-time, off-time, and
deadtime. The on-time of the switch and the inductor
current are synonymous. During the on-time, the inductor
current increases. During the off-time of the switch, the
inductor current decreases as it flows into the output. When
(and if) the inductor current reaches zero, that marks the
end of the inductor current off-time. For the rest of the cycle,
the inductor current remains at zero. Since no energy is
being either stored or delivered, that remaining time is
called deadtime. This mode of the inductor current decay-
ing to zero every cycle is called discontinuous mode. In
summary, energy is stored in the inductor during the
on-time, delivered to the output during the off-time, and
remains at zero during the deadtime.
Unless otherwise specified, the term off-time refers to
the inductor current, not to the switch.
Inductor Selection
It is under the condition of lowest input voltage that the
boost converter output current capability is the lowest for a
given inductance value. Three other significant param-
eters with worst case values for calculating the inductor
value are: highest switching frequency, lowest duty ratio
(of the switch on-time to the total switching period), and
highest diode forward voltage. Other parameters which
can affect the required inductor value, but for simplicity will
not be considered in this first analysis are: the series
resistance of the DC input source (i.e., the battery), the
series resistance of the internal switch, the series resis-
tance of the inductor itself, ESR of the output capacitor,
input and output filter losses, and snubber power loss.
The converter reaches maximum output current capabil-
ity when the switch runs at the oscillator frequency, without
pulses being skipped. The output current of the boost
converter is then given by the equation:
( ) IO =
VI2D2
2
2 f L VO + VF − VI
(1)
where “VI” is the input voltage, “D” is the on-time duty ratio
of the switch, “f “ is the switching (oscillator) frequency, “L”
is the inductor value, “VO” is the output voltage, and “VF” is
the diode forward voltage. It is important to note that this
equation makes the assumption stated in equation form:
( ) VI ≤ VO + VF (1 - D)
(2)
Page 4
2-2-96
February, 1996 TOKO, Inc.
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