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SNVSB03D December 2018 – January 2020

PRODUCTION DATA.

8.3.2 User-Programmable Reset Time Delay

The reset time delay can be set to a minimum value of 50 µs by leaving the CT pin floating, or a maximum value of approximately 6.2 seconds by connecting 10 µF delay capacitor. The reset time delay (t_D) can be programmed by connecting a capacitor no larger than 10 µF between CT pin and GND.

The relationship between external capacitor (C_{CT_EXT}) in Farads at CT pin and the time delay (t_D) in seconds is given by Equation 2.

Equation 2. t_D = -ln (0.29) x R_CT x C_{CT_EXT} + t_D (no cap)

Equation 2 is simplified to Equation 3 by plugging R_CT and t_{D(no cap)} given in Electrical Characteristics section:

Equation 3. t_D = 618937 x C_{CT_EXT} + 50 µs

Equation 4 solves for external capacitor value (C_{CT_EXT}) in units of Farads where t_D is in units of seconds

Equation 4. C_{CT_EXT} = (t_D- 50 µs) ÷ 618937

The reset delay varies according to three variables: the external capacitor variance (C_CT), CT pin internal resistance (R_CT) provided in the Electrical Characteristics table, and a constant. The minimum and maximum variance due to the constant is shown in Equation 5 and Equation 6.

Equation 5. t_{D (minimum)} = -ln (0.36) x R_{CT (min)} x C_{CT (min)} + t_{D (no cap, min)}

Equation 6. t_{D (maximum)} = -ln (0.26) x R_{CT (max)} x C_{CT (max)} + t_{D (no cap, max)}

The recommended maximum delay capacitor for the TPS3840 is limited to 10 µF as this ensures there is enough time for the capacitor to fully discharge when the reset condition occurs. When a voltage fault occurs, the previously charged up capacitor discharges, and if the monitored voltage returns from the fault condition before the delay capacitor discharges completely, the delay capacitor will begin charging from a voltage above zero volts and the reset delay will be shorter than expected. Larger delay capacitors can be used so long as the capacitor has enough time to fully discharge during the duration of the voltage fault.