TL494_15 Datasheet, PDF (10/33 Pages) Unisonic Technologies – VOLTAGE MODE PWM CONTROL CIRCUIT

TL494
SLVS074G – JANUARY 1983 – REVISED JANUARY 2015
9 Detailed Description
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9.1 Overview
The design of the TL494 not only incorporates the primary building blocks required to control a switching power
supply, but also addresses many basic problems and reduces the amount of additional circuitry required in the
total design. The TL494 is a fixed-frequency pulse-width-modulation (PWM) control circuit. Modulation of output
pulses is accomplished by comparing the sawtooth waveform created by the internal oscillator on the timing
capacitor (CT) to either of two control signals. The output stage is enabled during the time when the sawtooth
voltage is greater than the voltage control signals. As the control signal increases, the time during which the
sawtooth input is greater decreases; therefore, the output pulse duration decreases. A pulse-steering flip-flop
alternately directs the modulated pulse to each of the two output transistors. For more information on the
operation of the TL494, see the application notes located on ti.com.
9.2 Functional Block Diagram
RT ⁶
CT ⁵
DTC ^{4 ≈0.1 V}
Oscillator
Dead-Time Control
Comparator
1IN+ ¹
1IN− ²
≈0.7 V
Error Amplifier 1
+
−
PWM
Comparator
2IN+ ¹⁶
2IN− ¹⁵
Error Amplifier 2
+
−
OUTPUT CTRL
(see Function Table)
13
1D
C1
Pulse-Steering
Flip-Flop
Reference
Regulator
Q1 _{8 C1}
9 _E1
Q2 _{11 C2}
10
E2
12 _VCC
14
REF
FEEDBACK ³
0.7 mA
7 _GND
9.3 Feature Description
9.3.1 5-V Reference Regulator
The TL494 internal 5-V reference regulator output is the REF pin. In addition to providing a stable reference, it
acts as a preregulator and establishes a stable supply from which the output-control logic, pulse-steering flip-flop,
oscillator, dead-time control comparator, and PWM comparator are powered. The regulator employs a band-gap
circuit as its primary reference to maintain thermal stability of less than 100-mV variation over the operating free-
air temperature range of 0°C to 70°C. Short-circuit protection is provided to protect the internal reference and
preregulator; 10 mA of load current is available for additional bias circuits. The reference is internally
programmed to an initial accuracy of ±5% and maintains a stability of less than 25-mV variation over an input
voltage range of 7 V to 40 V. For input voltages less than 7 V, the regulator saturates within 1 V of the input and
tracks it.
10
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