7.3.1 Voltage Reference

The reference output voltage is a band gap derived 2.048 V that is trimmed to achieve typically 0.2% accuracy over the full operating temperature range of −40°C to 125°C. The trim procedure employs a curvature correction algorithm to compensate for the base emitter thermal nonlinearity inherent in band gap design topologies. The reference accuracy and the set resistor tolerance determine the magnitude and precision of the programmable hysteresis. In situations where reference noise filtering is required, TI recommends a 5-µF capacitor in series with a 190-Ω resistor to ground.

7.3.2 Comparator

7.3.3 Hysteresis

False triggering on noise coupled into the signal path is a common problem for comparator based threshold detectors. One of the most effective solutions is to add hysteresis. Hysteresis is a circuit signal path characteristic where an amplitude delay is introduced to the normal input. Positive hysteresis forces the signal to pass the normal switch point before the output makes a low to high transition while negative hysteresis does the opposite. This is a memory effect. The comparator behaves differently based on which direction the signal is going.

The LM7300 has been designed with a unique way of introducing hysteresis. The set points are completely independent of each other, the power supply, and the input or output conditions. The HYSTP pin sets positive hysteresis and the HYSTN pin sets the negative hysteresis in a simple way using two resistors. The pins can be tied together for the same hysteresis or tied to separate voltage taps for asymmetric hysteresis, or tied to the reference for no hysteresis. When the precision reference is used to drive the voltage tap resistor divider precise, stable threshold levels can be obtained. The maximum recommended hysteresis is about 130 mV. This places the HYSTP and HYSTN pin voltages at V_REF – 130 mV, which is approximately the center of their input common mode range at 2.7 V. For the typical example, a differential input signal voltage, V_IN, is applied between INP and INN, the noninverting and inverting inputs of the comparator. A DC switch or threshold voltage, V_TH, is set on the negative input to keep the output off when the signal is above and on when it goes below this level. For a precision threshold tie the INN pin to V_REF. With the output, off the circuit is in the minimum power state. Figure 13 through Figure 21 demonstrate the different configurations for setting the upper threshold V_IH and the lower threshold V_IL and their relationship to the input trip point V_REF, by the following formulas.

Equation 1.

Figure 12. Typical Micropower Application to Set Asymmetric Positive and
Negative Hysteresis of −10 mV, 3 mV

Figure 13. Typical Micropower Application to Set Asymmetric Positive and
Negative Hysteresis of −10 mV, 3 mV

When V_ID = 0, INN = INP = V_TH

Figure 15 shows the configuration with no hysteresis when the HYSTP and HYSTN pins are connected together to V_REF. TI does not recommend this configuration because it has the highest level of false triggers due to the system noise.

Figure 14. Typical Configuration for No Hysteresis

Figure 15. Typical Configuration for No Hysteresis

Figure 17 shows the configuration with symmetric hysteresis when the HYSTP and HYSTN pins are connected to the same voltage that is less than V_REF. The two trip points set a hysteresis band around the input threshold voltage V_REF, such that the positive band is equal to the negative band.

This configuration controls the false triggering mentioned in Figure 15. For precise level detection applications, TI recommends symmetric hysteresis values less than 5 mV to 10 mV.

Figure 16. Symmetric Hysteresis ±5 mV

Figure 17. Symmetric Hysteresis ±5 mV

Figure 19 shows the case for negative hysteresis by biasing only the HYSN pin to a voltage less than V_REF.

Figure 18. Typical Configuration for Negative Hysteresis = −10 mV

Figure 19. Typical Configuration for Negative Hysteresis = −10 mV

The case for setting only a positive hysteresis is demonstrated in Figure 21.

Figure 20. Connections for Positive Hysteresis = 10 mV

Figure 21. Connections for Positive Hysteresis = 10 mV

In the general case, as demonstrated with both positive and negative hysteresis bands in Figure 22, noise within these bands has no effect on the state of the comparator output. In Example 1 the noise is well behaved and in band. The output is clean and well behaved. In Example 2, a significant amount of out of band noise is present; however, due to hysteresis no false triggers occur on the rising positive or falling negative edges. The hysteresis forces the signal level to move higher or lower before the output is set to the opposite state.

Figure 22. Output Response With Input Noise Less than Hysteresis Band