8.2.1.2 Detailed Design Procedure

The maximum frequency is limited by the large signal propagation delay of the comparator and by the capacitive loading at the output, which limits the output slew rate.

Figure 26. Square Wave Oscillator Timing Thresholds

Consider the output of Figure 25 to be high to analyze the circuit. That implies that the inverted input (V_C) is lower than the noninverting input (V_A). This causes the C₁ to be charged through R₄, and the voltage V_C increases until it is equal to the noninverting input. The value of V_A at this point is:

Equation 8.

If R₁ = R₂ = R₃, then V _A1 = 2 V_cc/3

At this point the comparator switches pulling down the output to the negative rail. The value of V_A at this point is:

Equation 9.

If R₁ = R₂ = R₃, then V_A2 = V_CC/3.

The capacitor C₁ now discharges through R₄, and the voltage V_C decreases until it is equal to V_A2, at which point the comparator switches again, bringing it back to the initial stage. The time period is equal to twice the time it takes to discharge C₁ from 2V_CC/3 to V_CC/3, which is given by R₄C₁·ln2. Hence the formula for the frequency is:

The LMV7239 should be used for a symmetrical output. The LMV7235 will require a pullup resistor on the output to function, and will have a slightly asymmetrical output due to the reduced sourcing current.