Functional Description

The basic oscillator (switching) frequency is controlled by an

Figure 10

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 50V 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 30% or 50% 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 50V

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

shown in

Figure 8 Charging current is provided by the mas-

ter circuit Discharging is through all the circuits slaved to

Test Circuits

TLH10056 – 3

FIGURE 1 Error Amplifier Test Circuit

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FIGURE 2 Current Limit Sense

Amplifier Test Circuit

TLH10056 – 5

TLH10056 – 6

FIGURE 3 Common Emitter Configuration

Test Circuit and Waveform

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TLH10056 – 8

FIGURE 4 Emitter Follower Configuration

Test Circuit and Waveform

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FIGURE 5 Dead Time and Feedback

Control Test Circuit

4