Design Goals

Design Description

This circuit utilizes a triangle wave generator and comparator to generate a 500kHz pulse-width-modulated (PWM) waveform with a duty cycle that is inversely proportional to the input voltage. An op amp and comparator (U₃ and U₄) generate a triangle waveform which is applied to the inverting input of a second comparator (U₂). The input voltage is applied to the non-inverting input of U₂. By comparing the input waveform to the triangle wave, a PWM waveform is produced. U₂ is placed in the feedback loop of an error amplifier (U₁) to improve the accuracy and linearity of the output waveform.

Design Notes

1. Use a comparator with push-pull output and minimal propagation delay.
2. Use an op amp with sufficient slew rate, GBW, and voltage output swing.
3. Place the pole created by C₁ below the switching frequency and well above the audio range.
4. V_ref must be low impedance (for example, output of an op amp).

Design Steps

1. Set the error amplifier inverting signal gain.
   $\mathrm{Gain}=-\frac{{\mathrm{R}}_4}{{\mathrm{R}}_3}=-1\frac{\mathrm{V}}{\mathrm{V}}$
   $\text{Select }\mathrm{}{\mathrm{R}}_3={\mathrm{R}}_4=10\mathrm{k\Omega }$
2. Determine R₁ and R₂ to divide V_ref to cancel the non-inverting gain.
   ${\mathrm{V}}_{\mathrm{o\_dc}}=\left(1+\frac{{\mathrm{R}}_4}{{\mathrm{R}}_3}\right)\left(\frac{{\mathrm{R}}_2}{{\mathrm{R}}_1+{\mathrm{R}}_2}\right)\times \mathrm{Vref}$
   $\begin{array}{l}{\mathrm{R}}_1={\mathrm{R}}_2={\mathrm{R}}_3={\mathrm{R}}_4=10\mathrm{k\Omega }\text{, }\end{array}{\mathrm{V}}_{\mathrm{o\_dc}}=2.5\mathrm{V}$
3. The amplitude of V_tri must be chosen such that it is greater than the maximum amplitude of V_i (2.0V) to avoid 0% or 100% duty cycle in the PWM output signal. Select V_tri to be 2.1V. The amplitude of V₁ = 2.5V.
   ${\mathrm{V}}_{\mathrm{tri}}\text{ (Amplitude)}=\frac{{\mathrm{R}}_5}{{\mathrm{R}}_6}\times {\mathrm{V}}_1\left(\mathrm{Amplitude}\right)$
   $\text{Select }\mathrm{}\mathrm{}{\mathrm{R}}_6\text{ to be }\mathrm{}10\mathrm{k\Omega }\mathrm{}, \text{then compute }\mathrm{}\mathrm{}{\mathrm{R}}_5$
   ${\mathrm{R}}_5=\frac{{\mathrm{V}}_{\mathrm{tri}}\left(\mathrm{Amplitude}\right)\times {\mathrm{R}}_6}{{\mathrm{V}}_1 \left(\mathrm{Amplitude}\right)}=8.4\mathrm{k\Omega }\approx 8.45\mathrm{k\Omega }\text{ (Standard Value)}$
4. Set the oscillation frequency to 500kHz.
   ${\mathrm{f}}_{\mathrm{t}}=\frac{{\mathrm{R}}_6}{4\times {\mathrm{R}}_7\times {\mathrm{R}}_5\times {\mathrm{C}}_3}$
   $\text{Set }\mathrm{}{\mathrm{C}}_3=100\mathrm{pF},\text{ then compute }\mathrm{}\mathrm{}{\mathrm{R}}_7$
   ${\mathrm{R}}_7=\frac{{\mathrm{R}}_6}{4\times {\mathrm{f}}_{\mathrm{t}}\times {\mathrm{R}}_5\times {\mathrm{C}}_3}=5.92\mathrm{k\Omega }\approx 5.90\mathrm{k\Omega }\text{ (Standard Value)}$
5. Choose C₁ to limit amplifier bandwidth to below switching frequency.
   ${\mathrm{f}}_{\mathrm{p}}=\frac{1}{2\times \mathrm{\pi }\times {\mathrm{R}}_4\times {\mathrm{C}}_1}$
   ${\mathrm{C}}_1=100\mathrm{pF}\to {\mathrm{f}}_{\mathrm{p}}=159\mathrm{kHz}$
6. Select C₂ to filter noise from V_ref.
   $\mathrm{}{\mathrm{C}}_2\mathrm{}=\mathrm{}100\mathrm{nF}\text{ (Standard Value)}$
   ${\mathrm{f}}_{\mathrm{div}}=\frac{1}{2\times \mathrm{\pi }\times {\mathrm{C}}_2\times \frac{{\mathrm{R}}_1\times {\mathrm{R}}_2}{{\mathrm{R}}_1+{\mathrm{R}}_2}}=320\mathrm{}\mathrm{Hz}$

Design Simulations

DC Simulation Results

Transient Simulation Results

Design References

Texas Instruments, Simulation for PWM Generator Circuit, circuit SPICE simulation file

Texas Instruments, Analog PWM Generator 5V, 500kHz PWM Output, reference design

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