Comparator has independent trip voltages

Gregory Billiard, Fike Corp, Blue Springs, MO -- EDN, 7/22/1999

For an inverting amplifier with hysteresis, resistors R_A, R_B, and R_F determine the crossover voltages (Figure 1). Unfortunately, using three resistors to set the upper and lower trip voltages creates a dependence between the two trip voltages: It's impossible to set one voltage without affecting the other. However, you can achieve the same output-voltage response with two independent trip voltages by using a comparator with an open-collector output, such as the LM393 or LM339. In Figure 2, the resistor divider of R₁, R₂, and R₃ determines the upper trip voltage, V₁. R₄ and R₅ determine the lower trip voltage, V₂.

When V_IN is between 0 and V₁, V_OUT is high and prohibits any current flowing through R₄ and R₅, which sets V₂ to V_CC and thus keeps the output of Comparator B high. When the output of Comparator B is high, V₁=V_CCXR₃/(R₁+R₂+R₃). When V_IN exceeds V₁, V_OUT goes low, to 0.7V, allowing current to flow through the resistor divider. V₂ then changes from V_CC to 0.7V+V_CCR₅/(R₄+R₅). On this change, the output of Comparator B also goes low, bringing V₁ to 0.7XR₃/(R₂+R₃). V₁ is now lower than V_IN, so V_OUT goes low. V_OUT stays low until V_IN drops below V₂. When V_IN drops below V₂, V_OUT goes high again; V₂=V_CC; and R₁, R₂, and R₃ set V₁.

You can easily set either trip voltage without affecting the other. However, V₁ must be greater than V₂ or both trip voltages will be equal to V₁. A disadvantage of this circuit is that it requires two comparators, but comparators usually come in dual packages. (DI #2383)