SLFS022I September   1973  – September 2014 NA555 , NE555 , SA555 , SE555

PRODUCTION DATA.  

  1. Features
  2. Applications
  3. Description
  4. Simplified Schematic
  5. Revision History
  6. Pin Configuration and Functions
  7. Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 Handling Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Electrical Characteristics
    5. 7.5 Operating Characteristics
    6. 7.6 Typical Characteristics
  8. Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 Mono-stable Operation
      2. 8.3.2 A-stable Operation
      3. 8.3.3 Frequency Divider
    4. 8.4 Device Functional Modes
  9. Applications and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Applications
      1. 9.2.1 Missing-Pulse Detector
        1. 9.2.1.1 Design Requirements
        2. 9.2.1.2 Detailed Design Procedure
        3. 9.2.1.3 Application Curves
      2. 9.2.2 Pulse-Width Modulation
        1. 9.2.2.1 Design Requirements
        2. 9.2.2.2 Detailed Design Procedure
        3. 9.2.2.3 Application Curves
      3. 9.2.3 Pulse-Position Modulation
        1. 9.2.3.1 Design Requirements
        2. 9.2.3.2 Detailed Design Procedure
        3. 9.2.3.3 Application Curves
      4. 9.2.4 Sequential Timer
        1. 9.2.4.1 Design Requirements
        2. 9.2.4.2 Detailed Design Procedure
        3. 9.2.4.3 Application Curves
  10. 10Power Supply Recommendations
  11. 11Device and Documentation Support
    1. 11.1 Related Links
      1. 11.1.1 Trademarks
      2. 11.1.2 Electrostatic Discharge Caution
    2. 11.2 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

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8 Detailed Description

8.1 Overview

The xx555 timer is a popular and easy to use for general purpose timing applications from 10 µs to hours or from < 1mHz to 100 kHz. In the time-delay or mono-stable mode of operation, the timed interval is controlled by a single external resistor and capacitor network. In the a-stable mode of operation, the frequency and duty cycle can be controlled independently with two external resistors and a single external capacitor. Maximum output sink and discharge sink current is greater for higher VCC and less for lower VCC.

8.2 Functional Block Diagram

fbd_lfs022.gif
A. Pin numbers shown are for the D, JG, P, PS, and PW packages.
B. RESET can override TRIG, which can override THRES.

8.3 Feature Description

8.3.1 Mono-stable Operation

For mono-stable operation, any of these timers can be connected as shown in Figure 9. If the output is low, application of a negative-going pulse to the trigger (TRIG) sets the flip-flop (Q goes low), drives the output high, and turns off Q1. Capacitor C then is charged through RA 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 flip-flop (Q goes high), drives the output low, and discharges C through Q1.

app_fig1_lfs022.gifFigure 9. Circuit for Monostable Operation

Monostable operation is initiated when TRIG voltage falls below the trigger threshold. Once initiated, the sequence ends only if TRIG is high for at least 10 µs before the end of the timing interval. When the trigger is grounded, the comparator storage time can be as long as 10 µs, which limits the minimum monostable pulse width to 10 µs. Because of the threshold level and saturation voltage of Q1, the output pulse duration is approximately tw = 1.1RAC. Figure 11 is a plot of the time constant for various values of RA and C. The threshold levels and charge rates both are directly proportional to the supply voltage, VCC. The timing interval is, therefore, independent of the supply voltage, so long as the supply voltage is constant during the time interval.

Applying a negative-going trigger pulse simultaneously to RESET and TRIG during the timing interval discharges C 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 used, it should be connected to VCC.

app_fig2_lfs022.gifFigure 10. Typical Monostable Waveforms
app_fig3_lfs022.gifFigure 11. Output Pulse Duration vs Capacitance

8.3.2 A-stable Operation

As shown in Figure 12, adding a second resistor, RB, to the circuit of Figure 9 and connecting the trigger input to the threshold input causes the timer to self-trigger and run as a multi-vibrator. The capacitor C charges through RA and RB and then discharges through RB only. Therefore, the duty cycle is controlled by the values of RA and RB.

This astable connection results in capacitor C charging and discharging between the threshold-voltage level (≈ 0.67 × VCC) and the trigger-voltage level (≈ 0.33 × VCC). As in the mono-stable circuit, charge and discharge times (and, therefore, the frequency and duty cycle) are independent of the supply voltage.

app_fig4_lfs022.gifFigure 12. Circuit for Astable Operation
app_fig5_lfs022.gifFigure 13. Typical Astable Waveforms

Figure 12 shows typical waveforms generated during astable operation. The output high-level duration tH and low-level duration tL can be calculated as follows:

Equation 1. eq1_slfs022.gif
Equation 2. eq2_slfs022.gif

Other useful relationships are shown below:

Equation 3. eq3_slfs022.gif
Equation 4. eq4_slfs022.gif
Equation 5. eq5_slfs022.gif
Equation 6. eq6_slfs022.gif
Equation 7. eq7_slfs022.gif
app_fig7_lfs022.gifFigure 14. Free-Running Frequency

8.3.3 Frequency Divider

By adjusting the length of the timing cycle, the basic circuit of Figure 9 can be made to operate as a frequency divider. Figure 15 shows a divide-by-three circuit that makes use of the fact that re-triggering cannot occur during the timing cycle.

app_fig10_lfs022.gifFigure 15. Divide-by-Three Circuit Waveforms

8.4 Device Functional Modes

Table 1. Function Table

RESET TRIGGER VOLTAGE(1) THRESHOLD VOLTAGE(1) OUTPUT DISCHARGE SWITCH
Low Irrelevant Irrelevant Low On
High <1/3 VCC Irrelevant High Off
High >1/3 VCC >2/3 VCC Low On
High >1/3 VCC <2/3 VCC As previously established
(1) Voltage levels shown are nominal.