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In the time delay mode of operation, the time is precisely controlled by one external resistor and capacitor.

For a stable operation as an oscillator, the free running frequency and the duty cycle are both accurately controlled with two external resistors and one capacitor.

The circuit may be triggered and ne555 on falling waveforms, and the output structure can source or sink up to mA.

555 Timer IC

Short-circuits can cause excessive heating. These values are typical. This is done for all couples of connected pin combinations while the other pins are floating. This is done for all pins. Specified with trigger input high.


No protection against excessive pin 7 current is necessary, providing the package dissipation rating is not exceeded. Time measured from a positive pulse from 0 V to 0. Trigger is tied to threshold.

NE555 Timer IC

Minimum pulse width required for triggering Figure 4. Supply current versus supply voltage Figure 5. Delay time versus temperature Figure 6. Low output voltage versus output sink current Figure 7. Low output voltage versus output sink current Figure 8.

High output voltage drop versus output Figure Delay time versus supply voltage Figure As shown in Figure 12, the external capacitor is initially held discharged by a transistor inside the timer. Once triggered, the circuit remains in this state until the set time has elapsed, even if it is triggered again during this interval.

Note that because the charge rate and the threshold level of the comparator are both directly proportional to supply voltage, the timing interval is independent of supply. Applying a negative ne555n simultaneously to the reset terminal pin 4 and the trigger terminal pin 2 during the timing cycle discharges the external capacitor and causes the cycle to start over.

NEN STMicroelectronics | Ciiva

The timing cycle now starts on the positive edge of the reset pulse. During datasheeet time the reset pulse is applied, the output is driven to its LOW state. When a negative trigger pulse is applied to pin 2, the flip-flop is set, releasing the shortcircuit across the external capacitor and driving the output HIGH.


Figure 13 shows the actual waveforms generated in this mode of operation. When Reset is not used, it should be tied high to avoid any possibility of unwanted triggering. The external capacitor charges through R1 and R2 and discharges through R2 only.

Thus the duty cycle can be set accurately by adjusting the ratio of these two resistors. As in the triggered mode, the charge and discharge times and, therefore, frequency are independent of the supply voltage. The charge time output HIGH is given by: The duty cycle is given by: