MC33468 Просмотр технического описания (PDF) - ON Semiconductor
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MC33468 Datasheet PDF : 8 Pages
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MC33468
DEFINITIONS
Quiescent Bias Current – Current which is used to operate
the switching regulator chip and is not delivered to the load.
Leakage Current – Current drawn through a transistor
junction, under a specified collector voltage, when the
transistor is off.
FUNCTIONAL DESCRIPTION
Introduction
The MC33468 is a monolithic power switching regulator
optimized for dc–to–dc converter applications where power
drain must be minimized. The MC33468 uses Variable
Frequency Modulation to step up the input DC voltage to a
higher accurate output voltage. Potential applications
include low power consumer products and battery powered
portable products. Typical application circuits are shown in
Figure 4.
Operating Description
The MC33468 converter operates as a fixed on–time,
variable off–time voltage mode ripple regulator. Operation
is intended to be in the discontinuous mode, where the
inductor current ramps up to a peak value which is greater
than or equal to twice the value of the dc input current during
the on–time of the transistor switch. During the off–time of
the transistor switch, the inductor current ramps down to
zero and remains at zero until another switching cycle
begins.
Since the VDD pin is connected to the input no external
startup circuit is needed.
Oscillator
The oscillator frequency, is internally programmed to
180 kHz. The duty ratio of the oscillator is designed for a
constant value of 0.75 nominal. Hence the nominal on–time
of the power switch is:
ton
+
D
fosc
+
0.75
(180 kHz)
+
4.16
µs
Feedback Comparator
The output voltage is sensed and fed to a high speed
comparator noninverting input through an internal resistive
divider. The comparator inverting input is connected to an
internally trimmed reference.
With a voltage mode ripple converter operating under
normal conditions, output switch conduction is initiated and
terminated by the oscillator, off–time is controlled by the
high speed voltage feedback comparator.
Figure 4. Typical 2.0 V Application with BJT
VOUT
VDD
VIN
VOUT
EXT
Gnd
Figure 5. Design Equations for Step–Up
Calculation
Equation
ton
D
fosc
L
t
(n)(Vin)2(ton)
PO
IL(avg)
IL(pk)
Iin
(Vin * Vsat)(ton)
L
Vripple(pp)
[ (ton)(IO)
(CO)
The following converter design characteristics must be chosen:
Vin – Nominal Operating dc input voltage
VO – Desired dc output voltage
IO – Desired dc output current
Vripple(pp) – Desired peak–to–peak output ripple voltage. For best performance
the ripple voltage should be kept to a low value since it directly affects regulation.
Capacitor CO should have a low equivalent series resistance (ESR).
NOTE: 1. Vsat – Saturation voltage of the switching transistor.
n – Estimated circuit efficiency.
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