Design Goals

Design Description

The oscillator circuit generates a square wave at a selected frequency. This is done by charging and discharging the capacitor, C₁ through the resistor, R₁. The oscillation frequency is determined by the RC time constant of R₁ and C₁, and the threshold levels set by the resistor network of R₂, R₃, and R₄. The maximum frequency of the oscillator is limited by the toggle rate of the comparator and the capacitance load at the output. This oscillator circuit is commonly used as a time reference or a supervisor clock source.

Design Notes

1. Comparator toggle rate and output capacitance are critical considerations when designing a high-speed oscillator.
2. Select C₁ to be large enough to minimize the errors caused by stray capacitance.
3. If using a ceramic capacitor, select a COG or NPO type for best stability over temperature.
4. Select lower value resistors for the R₂, R₃, R₄ resistor network to minimize the effects of stray capacitance.
5. Adjust R₂, R₃, and R₄ to create a duty cycle other than 50%.

Design Steps

1. When R₂ = R₃ = R₄, the resistor network sets the oscillator trip points of the non-inverting input at one-third and two-thirds of the supply.
2. When the output is high, the upper trip point is set at two-thirds of the supply to bring the output back low.
   ${\text{V}}_{\text{o}}{\text{ = V}}_{\text{s}}\left(\frac{{\text{R}}_{\text{3}}}{{\text{(R}}_{\text{2}}{\text{∥R}}_{\mathrm{4}}{\text{)+R}}_{\text{3}}}\right)\text{ = }\frac{\text{2}}{\text{3}}{\text{V}}_{\text{s}}\text{ = 3.33V}$
3. When the output is low, the lower trip point is set at one-third of the supply to bring the output back high.
   ${\text{V}}_{\text{o}}{\text{ = V}}_{\text{s}}\left(\frac{{\text{R}}_{\mathrm{3}}{\text{∥R}}_{\text{4}}}{{\text{(R}}_{\text{3}}{\text{∥R}}_{\text{4}}{\text{)+R}}_{\text{2}}}\right)\text{ = }\frac{\text{1}}{\text{3}}{\text{V}}_{\text{s}}\text{ = 1.67V}$
4. The timing of the oscillation is controlled by the charging and discharging rate of the capacitor C₁ through the resistor R₁. This capacitor sets the voltage of the inverting input of the comparator. Calculate the time to discharge the capacitor.
   ${\text{V}}_{\text{c}}{\text{ = V}}_{\text{i}}{\text{e}}^{\text{-}\frac{\text{t}}{{\text{R}}_1{\text{C}}_1}}$
   $\frac{\text{1.67}}{\text{3.33}}{\text{ = e}}^{\text{-}\frac{\text{t}}{{\text{R}}_1{\text{C}}_1}}$
   ${\text{t = 0.69R}}_1{\text{C}}_1$
5. Calculate the time to charge the capacitor.
   ${\text{V}}_{\mathrm{i}}{\text{ = V}}_{\text{c}}\left({\text{1-e}}^{\text{-}\frac{\text{t}}{\text{RC}}}\right)$
   $\text{1.67 = 3.33}\left({\text{1-e}}^{\text{-}\frac{\text{t}}{\text{RC}}}\right)$
   $\frac{\text{1.67}}{\text{3.33}}{\text{ = e}}^{\text{-}\frac{\text{t}}{\text{RC}}}$
   ${\text{t = 0.69R}}_1{\text{C}}_1$
6. The time for the capacitor to charge or discharge is given by 0.69R₁C₁. With a target oscillator frequency of 1 MHz, the time to charge or discharge should be 500ns.
   ${\text{0.69R}}_{\text{1}}{\text{C}}_{\mathrm{1}}\text{ = 500ns}$
   ${\text{R}}_{\text{1}}{\text{C}}_{\text{1}}\text{ = 724ns}$
7. Select C₁ as 100pF and R₁ as 6.8kΩ (the closest real world value).

Design Simulations

Transient Simulation Results

Design References

See circuit spice simulation file, SBOMAO3.

Design Featured Comparator

Design Alternate Comparator