'SWITCHMODE' related articles 1

  1. 2006/12/16 TL494 - SWITCHMODE Pulse Width Modulation Control Circuit
SWITCHMODE PULSE WIDTH MODULATION CONTROL CIRCUIT
The TL494 is a fixed frequency, pulse width modulation control circuit designed primarily for SWITCHMODE power supply control.

• Complete Pulse Width Modulation Control Circuitry
• On–Chip Oscillator with Master or Slave Operation
• On–Chip Error Amplifiers
• On–Chip 5.0 V Reference
• Adjustable Deadtime Control
• Uncommitted Output Transistors Rated to 500 mA Source or Sink
• Output Control for Push–Pull or Single–Ended Operation
• Undervoltage Lockout

Description
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.  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 its 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 deadtime control, the error amplifier inputs, or the feedback input. The deadtime control comparator has an effective 120 mV input offset which limits the minimum output deadtime to approximately the first 4% of the sawtooth–cycle time. This would result in a maximum duty cycle on a given output of 96% with the output control grounded, and 48% with it connected to the reference line. Additional deadtime may be imposed on the output by setting the deadtime–control input to a fixed voltage, ranging between 0 V to 3.3 V
The pulse width modulator comparator provides a means for the error amplifiers to adjust the output pulse width from the maximum percent on–time, established by the deadtime control input, down to zero, as the voltage at the feedback pin varies from 0.5 V to 3.5 V. Both error amplifiers have a common mode input range from –0.3 V to (VCC – 2V), and may be used to sense power–supply output voltage and current. The error–amplifier outputs are active high and are ORed together at the noninverting input of the pulse–width modulator comparator. With this configuration, the amplifier that demands minimum output on time, dominates control of the loop.

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