Adjusting Hysteresis in Microprocessor Reset ICs

Microprocessor-based systems often include a 3-pin microprocessor reset IC. These devices monitor a single-supply supply voltage and provide a system reset signal in the event of an undervoltage condition. Typically, this IC has a fixed hysteresis voltage (the difference in threshold voltage when VCC rises and falls), which can be adjusted using a simple circuit (Figure 1) to increase design flexibility.

In the circuit of Figure 1, RH and RP are used to adjust the hysteresis and determine the reset period.

As VIN rises above 1.0V, the /RESET output is low, indicating that the input voltage is below the monitoring threshold. Current flows from VIN through RP to the internal MOSFET driver, and then through RH to GND, generating an offset voltage on RH. Since the internal voltage reference is referenced to GND, the offset voltage is added to the threshold value when VCC rises. The new rising threshold value can be calculated as follows:

When VIN rises and crosses this threshold and remains there for longer than the reset period, /RESET is released from the reset state and the current through RH decreases, restoring the VCC threshold to the nominal value.

Assume that a microprocessor reset IC (MAX6383XR31D1) has a reset threshold of 3.08V and a pull-up resistor (RP) of 10kΩ. If a rising threshold of 3.18V (100mV hysteresis) is desired, the above equation yields RH (ignoring the effects of supply current and MOSFET output driver on-resistance) of 324.68Ω. The closest 1% standard resistor is 324Ω. The

oscilloscope image (Figure 2) shows the circuit in action. The rising threshold is measured to be 3.1984V and the falling threshold to be 3.0891V, yielding 109.3mV hysteresis. The 9.3mV difference (relative to the calculated 100mV) is primarily due to the MOSFET on-resistance, the device supply current, and resistor tolerances.

Figure 2 Test waveform of the circuit in Figure 1, 100mV hysteresis, that is, the voltage difference between VCC (CH1) at the intersection of the rising and falling edges of /RESET (CH2) is 100mV

Note that RH adds hysteresis voltage to the VOL (logic low voltage) of the /RESET output. In this example, the maximum value of VOL measured was 127mV. Therefore, you should ensure that the circuit connected to /RESET can tolerate a higher VOL.