mc33468sn-20atr ON Semiconductor, mc33468sn-20atr Datasheet - Page 4

• Manufacturer Part Number: mc33468sn-20atr
• Description: Variable Frequency Micropower Dc-to-dc Converter
• Manufacturer: ON Semiconductor

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
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
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
4
The MC33468 is a monolithic power switching regulator
The MC33468 converter operates as a fixed on–time,
V IN
The following converter design characteristics must be chosen:
the ripple voltage should be kept to a low value since it directly affects regulation.
Capacitor C O should have a low equivalent series resistance (ESR).
NOTE: 1. V sat – Saturation voltage of the switching transistor.
V in – Nominal Operating dc input voltage
V O – Desired dc output voltage
I O – Desired dc output current
V ripple(pp) – Desired peak–to–peak output ripple voltage. For best performance
n – Estimated circuit efficiency.
Calculation
Figure 4. Typical 2.0 V Application with BJT
V ripple(pp)
Figure 5. Design Equations for Step–Up
I L(avg)
I L(pk)
t on
L
MC33468
V DD
EXT
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.
startup circuit is needed.
Oscillator
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:
Feedback Comparator
comparator noninverting input through an internal resistive
divider. The comparator inverting input is connected to an
internally trimmed reference.
normal conditions, output switch conduction is initiated and
terminated by the oscillator, off–time is controlled by the high
speed voltage feedback comparator.
Gnd
V OUT
Since the V DD pin is connected to the input no external
The oscillator frequency, is internally programmed to
The output voltage is sensed and fed to a high speed
With a voltage mode ripple converter operating under
(V
t
in
[
(n)(V
*
Equation
(t on )(I
f
osc
D
V sat )(t on )
(C
L
t on
in
P
I in
O
) 2 (t on )
O
)
O
MOTOROLA ANALOG IC DEVICE DATA
+
)
f osc
D
+
(180 kHz)
0.75
V OUT
+
4.16 s