LM494 National Semiconductor, LM494 Datasheet - Page 4

• Manufacturer Part Number: LM494
• Description: Pulse Width Modulated Control Circuit
• Manufacturer: National Semiconductor

Figure 10
Functional Description
The basic oscillator (switching) frequency is controlled by an
external resistor (R
between the values of R
The level of the sawtooth wave form is compared with an
error voltage by the pulse width modulated comparator The
output of the PWM Comparator directs the pulse steering
flip-flop and the output control logic
The error voltage is generated by the error amplifier The
error amplifier boosts the voltage difference between the
output and the 5 0V internal reference See Figure 7 for
error amp sensing techniques The second error amp is typi-
cally used to implement current-limiting
The output control logic selects either push-pull or single-
ended operation of the output transistors (see Figure 6 )
The dead time control prevents on-state overlap of the out-
put transistors as can be seen in Figure 5 The dead time is
approximately 3 0% or 5 0% of the total period if the dead
time control is grounded This dead time can be increased
by connecting the dead time control to a voltage up to 5 0V
The frequency response of the error amps (Figure 11) can
be modified by using external resistors and capacitors
These components are typically connected between the
compensation terminal and the inverting input of the error
amps
The switching frequency of two or more LM494 circuits can
be synchronized The timing capacitor C
shown in Figure 8 Charging current is provided by the mas-
ter circuit Discharging is through all the circuits slaved to
the master R
Test Circuits
FIGURE 1 Error Amplifier Test Circuit
FIGURE 2 Current Limit Sense
T
is required only for the master circuit
Amplifier Test Circuit
T
) and capacitor (C
T
C
T
and frequency is shown in
T
) The relationship
T
is connected as
TL H 10056–3
TL H 10056–4
4
FIGURE 3 Common Emitter Configuration
FIGURE 4 Emitter Follower Configuration
FIGURE 5 Dead Time and Feedback
Test Circuit and Waveform
Test Circuit and Waveform
Control Test Circuit
TL H 10056 – 5
TL H 10056 – 6
TL H 10056 – 7
TL H 10056 – 8
TL H 10056 – 9