For monostable operation, Figure 6-2 shows how either of the timers can be connected. If the output is low, application of a negative-going pulse to the trigger (TRIG) sets the internal latch; the output goes high, and discharge pin (DISCH) becomes open drain. Capacitor C_T then is charged through R_A until the voltage across the capacitor reaches the threshold voltage of the threshold (THRES) input. If TRIG has returned to a high level, the output of the threshold comparator resets the internal latch, the output goes low, the discharge pin goes low, which quickly discharges capacitor C_T.

Figure 6-2 Circuit for Monostable Operation

Monostable operation is initiated when TRIG voltage is less than the trigger threshold. If initiated, the sequence ends only if TRIG is high for at least 1µs before the end of the timing interval. When the trigger is grounded, the comparator storage time can be as long as 1µs, which limits the minimum monostable pulse duration to 1µs. The output pulse duration is approximately t_w = 1.1 × R_AC_T. The threshold levels and charge rates both are directly proportional to the supply voltage, V_CC. The timing interval is, therefore, independent of the supply voltage, so long as the supply voltage is constant during the time interval. In monostable applications, set the trip point of the trigger input by a voltage applied to CONT.

Applying a negative-going trigger pulse simultaneously to RESET and TRIG during the timing interval discharges capacitor C_T and reinitiates the cycle, commencing on the positive edge of the reset pulse. The output is held low as long as the reset pulse is low. To prevent false triggering, when RESET is not asserted low, connect RESET to V_CC. If the RESET function is required and the pin is driven by external logic or a microcontroller, use a pullup resistor to V_CC (such as 10kΩ) to prevent the RESET pin from floating. If the RESET function is not required, short the RESET pin directly to the V_CC pin.