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TL494BD
Switchmode Pulse Width Modulation Control Circuit
AN983/D
A Simplified Power Supply
Design Using the TL494
Control Circuit
Prepared by: Jade H. AlberkrackON Semiconductor
Bipolar IC Division
This bulletin describes the operation and characteristics of
the TL494 SWITCHMODE� V oltage Regulator and shows
its application in a 400–watt off–line power supply.
The TL494 is a fixed–frequency pulse width modulation
control circuit, incorporating the primary building blocks
required for the control of a switching power supply. (See
Figure 1). An internal linear sawtooth oscillator is
frequency–programmable by two external components, RT
and CT. The oscillator frequency is determined by:
fosc� 1.1�CT
Output pulse width modulation is accomplished by
comparison of the positive sawtooth waveform across
capacitor CT to either of two control signals. The NOR gates,
which drive output transistors Q1 and Q2, are enabled only
when the flip–flop clock–input line is in low state. This
happens only during that portion of time when the sawtooth
voltage is greater than the control signals. Therefore, an
increase in control–signal amplitude causes a corresponding
linear decrease of output pulse width. (Refer to the timing
diagram shown in Figure 2).
The control signals are external inputs that can be fed into
the dead–time control (Figure 1, Pin 4), the error amplifier
inputs (Pins 1, 2, 15, 16), or the feedback input (Pin 3). The
dead–time control comparator has an effective 120 mV
input offset which limits the minimum output dead time to
approximately the first 4% of the sawtooth–cycle time. This
would result in a maximum duty cycle of 96% with the
output mode control (Pin 13) grounded, and 48% with it
connected to the reference line. Additional dead time may be
imposed on the output by setting the dead–time control input
to a fixed voltage, ranging between 0 to 3.3 V
Figure 1. TL494 Block DiagramDead–Time
Control
Feedback/P.W.M.
Comparator Input
Output Mode Control
This document may contain references to devices which are no
