8.3.4 SENSE Input

The SENSE input can vary from ground to 5.5 V (7.0 V, absolute maximum), regardless of the device supply voltage used. The SENSE pin is used to monitor the critical voltage rail. If the voltage on this pin drops below V_ITN, then RESET is asserted. When the voltage on the SENSE pin exceeds the positive threshold voltage, RESET deasserts after the user-defined RESET delay time.

The internal comparator has built-in hysteresis to ensure well-defined RESET assertions and deassertions even when there are small changes on the voltage rail being monitored.

The TPS3890-Q1 device is relatively immune to short transients on the SENSE pin. Glitch immunity is dependent on threshold overdrive, as illustrated in Figure 19 for V_ITN and Figure 18 for V_ITP. Although not required in most cases, for noisy applications good analog design practice is to place a 1-nF to 10-nF bypass capacitor at the SENSE input to reduce sensitivity to transient voltages on the monitored signal.

The adjustable version (TPS389001-Q1) can be used to monitor any voltage rail down to 1.15 V using the circuit shown in Figure 24.

Figure 24. Using the TPS389001-Q1 to Monitor a User-Defined Threshold Voltage

The target threshold voltage for the monitored supply (V_ITx(MON)) and the resistor divider values can be calculated by using Equation 2 and Equation 3, respectively:

Equation 3 can be used to calculate either the negative threshold or the positive threshold by replacing V_ITx with either V_ITN or V_ITP, respectively.

Resistors with high values minimize current consumption; however, the input bias current of the device degrades accuracy if the current through the resistors is too low. Therefore, choosing an R_TOTAL value so that the current through the resistor divider is at least 100 times larger than the SENSE input current is simplest. See the Optimizing Resistor Dividers at a Comparator Input application report for more details on sizing input resistors.