Constant-current charging starts on the timing capacitor when the sensing-line voltage reaches the TL7700-SEP sense voltage. When the capacitor voltage exceeds the threshold level of the output drive comparator, RESET changes from a low to a high level. The output pulse duration is the time between the point when the SENSE-pin voltage exceeds the threshold level and the point when the RESET output changes from a low level to a high level. When the TL7700-SEP is used for system power-on reset, the output pulse duration, t_po, must be set longer than the power rise time. The value of t_po in seconds is:

where:

• C_T is the timing capacitor in farads

• ΔV_CT is the change in voltage at the CT pin (1.5-V reference voltage)

• I_CT is the timing capacitor charge current (typically 15 µA)

There is a limit on the device response speed. Even if C_t = 0, t_po is not 0, but approximately 5 µs to 10 µs. Therefore, when the TL7700-SEP is used as a comparator with hysteresis without connecting C_t, switching speeds (t_r/t_f, t_po/t_pd, and so forth) must be considered.