A word of caution for KiCad users attempting to use this LTspice model: the subcircuit syntax between LTspice (a derivative of Berkeley SPICE) and Ngspice (which KiCad uses) is not always directly compatible. The subcircuits often reference other models, and their internal syntax may not translate perfectly, leading to errors. It is generally best to keep TL494 simulation work within LTspice itself.
fosc=1.2RT×CTf sub o s c end-sub equals the fraction with numerator 1.2 and denominator cap R sub cap T cross cap C sub cap T end-fraction 2. Output Control (Pin 13)
To accurately simulate the device, one must understand its four distinct internal blocks.
If you’re trying to use the LTspice TL494 model in KiCad (which uses ngspice), you’ll face fundamental syntax differences. LTspice uses “A-devices” (AND, OR, DFLOP, SCHMITT) that ngspice does not directly support. To port the TL494 to ngspice, you’d need to replace these A-devices with equivalent behavioral models or hand-crafted subcircuits. This is a significant undertaking and beyond the scope of this article. tl494 ltspice
or f = 1.2 / (RT × CT) , depending on the model.
The PWM duty cycle is determined by comparing the control voltage on the FB pin (pin 3) with the sawtooth waveform on the dead-time comparator. As the voltage on pin 3 increases from 0.5V to 3.5V, the duty cycle can vary from 0% to nearly 100%.
: Create a resistive voltage divider from your main power supply output rail and feed it into the non-inverting input (Pin 1). Connect a stable reference voltage divider (sourced from Pin 14 VREFcap V sub cap R cap E cap F end-sub ) to the inverting input (Pin 2). A word of caution for KiCad users attempting
Despite your best efforts, you will likely encounter simulation errors. Here is a guide to the most common ones and their solutions.
You can create the following circuit in LTspice:
Since you requested a "good paper," I have structured this response as a technical application note. It covers the operating theory of the TL494, a guide to modeling it in LTspice, and a practical design example. fosc=1
TI provides PSPICE models for the TL494 on its product page. These models are generally compatible with LTspice.
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This is perhaps the most frequent complaint. Users report seeing a constant 12V at the emitter pins with no switching activity.