Applications Information

Many comparators oscillate in the linear region of operation because of noise or undesired parasitic feedback. This tends to occur when the voltage on one input is equal or very close to the voltage on the other input. The MAX49017 has internal 2.5mV hysteresis to counter parasitic effects and noise.

The hysteresis in a comparator creates two trip points: one for upper threshold (V_TRIP+) and one for lower threshold (V_TRIP-) for voltage transitions on the input signal (Figure 1). The difference between the trip points is the hysteresis band (V_HYS). When the comparator’s input voltages are equal, the hysteresis effectively causes one comparator input to move quickly past the other, thus taking the input out of the region where oscillation occurs. Figure 1 illustrates the case in which IN_- has a fixed voltage applied, and IN_+ is varied. If the inputs were reversed, the figure would be the same, except with an inverted output.

In applications requiring more than the internal 2.5mV hysteresis of the devices, additional hysteresis can be added with two external resistors. Since these comparators are intended to use in very low-power systems, care must be taken to minimize power dissipation in the additional circuitry.

Regardless of which approach is employed to add external hysteresis, the external hysteresis will be V_DD dependent. Figure 2 shows the simplest circuit for adding external hysteresis. In this example, the hysteresis is defined by:

$\mathrm{Hysteresis}=\frac{R_G}{R_F}x{V}_{\mathrm{DD}}$

Where R_G is the source resistance and RF is the feedback resistance. The comparison threshold is V_REF.

Because the MAX49017 is intended for very low power-supply systems, the highest impedance circuits should be used wherever possible. The offset error due to input-bias current is proportional to the total impedance seen at the input. For example, selecting components for Figure 2, with a target of 50mV hysteresis, a 5V supply, and choosing an R_F of 10MΩ gives R_G as 100kΩ. The total impedance seen at IN+ is therefore 10MΩ || 100kΩ, or 99kΩ. The typical input bias current of MAX49017 is 2nA; therefore, the error due to source impedance is less than 200μV.

Power-supply bypass capacitors are not typically needed, but use 100nF bypass capacitors close to the device’s supply pins when supply impedance is high, supply leads are long, or excessive noise is expected on the supply lines. Minimize signal trace lengths to reduce stray capacitance. A ground plane and surface-mount components are recommended. If the REF pin is decoupled, use a new low-leakage capacitor.

The MAX49017 is ideal for window detectors (undervoltage/overvoltage detectors). The Typical Application Circuit shows how a window detector circuit can detect the related car battery's voltage level with an undervoltage of 2.9V and an overvoltage of 4.2V. Choose different thresholds by changing the values of R1, R2, and R3. OUTA provides an active-low undervoltage indication, and OUTB provides an active-low overvoltage indication.

1. Select R1. The input bias current into INB- is normally less than 5nA, so the current through R1 should exceed 250nA for the thresholds to be accurate. In this example, choose R1 = 0.25MΩ (1.252V/5µA).
2. Calculate R2 + R3. The overvoltage threshold should be 4.2V when V_IN is rising. The design equation is as follows:

3. Calculate R2. The undervoltage threshold should be 2.9V when V_IN is falling. The design equation is as follows:

4. ​Calculate R3:

5. Choose standard 1% resistors for R1 = 249kΩ, R2 = 110kΩ, and R3 = 475kΩ.
6. Verify the resistor values. The equations are as follows, evaluated for the above example.