8.3.1 User-Configurable RESET Delay Time

The rising RESET delay time (t_PD(r)) can be configured by installing a capacitor connected to the CT pin. The TPS3890-Q1 uses a CT pin charging current (I_CT) of 1.15 µA to help counter the effect of capacitor and board-level leakage currents that can be substantial in certain applications. The rising RESET delay time can be set to any value between 25 µs (no C_CT installed) and 30 s (C_CT = 26 µF).

The capacitor value needed for a given delay time can be calculated using Equation 1:

The slope of Equation 1 is determined by the time that the CT charging current (I_CT) takes to charge the external capacitor up to the CT comparator threshold voltage (V_CT). When RESET is asserted, the capacitor is discharged through the internal CT pulldown resistor (R_CT). When the RESET conditions are cleared, the internal precision current source is enabled and begins to charge the external capacitor and when the voltage on this capacitor reaches 1.22 V, RESET is deasserted. Note that in order to minimize the difference between the calculated RESET delay time and the actual RESET delay time, use a low-leakage type capacitor (such as a ceramic capacitor) and minimize parasitic board capacitance around this pin.