6 Comparator with Hysteresis

The designer can use the 555 timer without an external timing element to create a comparator with input hysteresis. In the configuration shown in Figure 6-1, the internal resistor divider presents thresholds of ? × VDD for the high comparator and ? × VDD for the low comparator. When an input voltage is applied to the connected TRIG and THRESH pins, the circuit functions as a comparator with input hysteresis. The high threshold for a positive-transitioning input signal is ? × VDD and the low threshold for a negative-transitioning input signal is ? × VDD. Upon transitioning beyond either threshold, the internal flip-flop latches the output state and will not clear unless the input signal transitions beyond the other threshold or if RESET is toggled. Due to the fixed configuration of the 555 timer, the comparator circuit functions with inverse polarity as seen in the simulation in Figure 6-2.

The main limitation of this circuit is that the designer cannot independently adjust the high and low threshold levels, as they are fixed by the internal resistor divider. Note that there is some flexibility to be had with the use of the CONT pin, as any impedance on this node will adjust the high and low thresholds and can be calculated using simple resistor divider math. Connecting a resistor from the CONT pin to ground will lower both the high and low thresholds, whereas connecting a resistor from the CONT pin to VDD will increase both the high and low thresholds. The designer can also apply a voltage directly to the CONT pin to adjust the threshold voltages. In all cases, it is not possible to independently adjust one threshold without impacting the other.

The TINA-TI simulation results presented in Figure 6-2 displays the output for a 1-kHz input sine wave with an amplitude of 4.5 Vpp and a DC offset of 2.5 V.

Smart DACs can be configured to act as a comparator with hysteresis with two independent threshold voltages. The input voltage applied to the DAC53701 feedback pin is compared against the threshold voltage set in the DAC. The GPI pin on the DAC53701 can be programmed to switch between two threshold voltages. If the output of the DAC is connected to the GPI pin, the DAC will switch threshold voltages when the output changes due to the voltage on VFB, creating the hysteresis. These two threshold voltages are programmed into the margin-high and margin-low registers, and the GPI is enabled for margin-high, margin-low function. An RC filter can be added to the GPI feedback as seen in Figure 6-3 to provide a small delay to avoid any instability on the GPI pin due to noise.

Figure 6-4 shows a simulation of the DAC53701 output responding to the sine wave applied to the feedback pin and the 3-V and 1-V threshold values.

The smart DAC presents a superior solution when used as a comparator with hysteresis by offering independently configurable thresholds, whereas the 555 timer is limited in threshold configurability by its fixed internal resistor dividers.