ZHCSIK0A July 2018 – October 2021 TPS3430
PRODUCTION DATA
The TPS3430 uses a CRST pin charging current (ICRST) of 375 nA. When using an external capacitor, the rising WDO delay time can be set to any value between 700 μs (CCRST = 100 pF) and 3.2 seconds (CCRST = 1 μF). The typical ideal capacitor value needed for a given delay time can be calculated using Equation 1, where CCRST is in microfarads and tRST is in seconds:
To calculate the minimum and maximum watchdog reset delay time use Equation 2 and Equation 3, respectively.
The slope of Equation 1 is determined by the time the CRST charging current (ICRST) takes to charge the external capacitor up to the CRST comparator threshold voltage (VCRST). When WDO is asserted, the capacitor is discharged through the internal CRST pulldown resistor. When the WDO conditions are cleared, the internal precision current source is enabled and begins to charge the external capacitor; when VCRST = 1.21 V, WDO is unasserted. Note to minimize the difference between the calculated WDO delay time and the actual WDO delay time, use a use a high-quality ceramic dielectric COG, X5R, or X7R capacitor and minimize parasitic board capacitance around this pin. Table 8-2 lists the watchdog reset delay time ideal capacitor values for CCRST.
| CCRST | WDO DELAY TIME (tRST) | UNIT | ||
|---|---|---|---|---|
| MIN(1) | TYP | MAX(1) | ||
| 100 pF | 0.61 | 0.70 | 0.80 | ms |
| 1 nF | 3.21 | 3.61 | 4.08 | ms |
| 10 nF | 29.2 | 32.6 | 36.8 | ms |
| 100 nF | 289 | 323 | 364 | ms |
| 1 μF | 2886 | 3227 | 3644 | ms |