TL494_15 Datasheet, PDF (12/33 Pages) Unisonic Technologies – VOLTAGE MODE PWM CONTROL CIRCUIT

TL494
SLVS074G – JANUARY 1983 – REVISED JANUARY 2015
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Feature Description (continued)
9.3.5 Pulse-Width Modulation (PWM)
The comparator also provides modulation control of the output pulse width. For this, the ramp voltage across
timing capacitor C_T is compared to the control signal present at the output of the error amplifiers. The timing
capacitor input incorporates a series diode that is omitted from the control signal input. This requires the control
signal (error amplifier output) to be ∼0.7 V greater than the voltage across C_T to inhibit the output logic, and
ensures maximum duty cycle operation without requiring the control voltage to sink to a true ground potential.
The output pulse width varies from 97% of the period to 0 as the voltage present at the error amplifier output
varies from 0.5 V to 3.5 V, respectively.
9.3.6 Error Amplifiers
Both high-gain error amplifiers receive their bias from the V_I supply rail. This permits a common-mode input
voltage range from –0.3 V to 2 V less than V_I. Both amplifiers behave characteristically of a single-ended single-
supply amplifier, in that each output is active high only. This allows each amplifier to pull up independently for a
decreasing output pulse-width demand. With both outputs ORed together at the inverting input node of the PWM
comparator, the amplifier demanding the minimum pulse out dominates. The amplifier outputs are biased low by
a current sink to provide maximum pulse width out when both amplifiers are biased off.
9.3.7 Output-Control Input
The output-control input determines whether the output transistors operate in parallel or push-pull. This input is
the supply source for the pulse-steering flip-flop. The output-control input is asynchronous and has direct control
over the output, independent of the oscillator or pulse-steering flip-flop. The input condition is intended to be a
fixed condition that is defined by the application. For parallel operation, the output-control input must be
grounded. This disables the pulse-steering flip-flop and inhibits its outputs. In this mode, the pulses seen at the
output of the dead-time control/PWM comparator are transmitted by both output transistors in parallel. For push-
pull operation, the output-control input must be connected to the internal 5-V reference regulator. Under this
condition, each of the output transistors is enabled, alternately, by the pulse-steering flip-flop.
9.3.8 Output Transistors
Two output transistors are available on the TL494. Both transistors are configured as open collector/open
emitter, and each is capable of sinking or sourcing up to 200 mA. The transistors have a saturation voltage of
less than 1.3 V in the common-emitter configuration and less than 2.5 V in the emitter-follower configuration. The
outputs are protected against excessive power dissipation to prevent damage, but do not employ sufficient
current limiting to allow them to be operated as current-source outputs.
9.4 Device Functional Modes
When the OUTPUT CTRL pin is tied to ground, the TL494 is operating in single-ended or parallel mode. When
the OUTPUT CTRL pin is tied to V_REF, the TL494 is operating in normal push-pull operation.
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