SNAS558M February   2000  – July 2016 LMC555


  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Pin Configuration and Functions
  6. Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
  7. Parameter Measurement Information
  8. Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 Low-Power Dissipation
      2. 8.3.2 Various Packages and Compatibility
      3. 8.3.3 Operates in Both Astable and Monostable Mode
    4. 8.4 Device Functional Modes
      1. 8.4.1 Monostable Operation
      2. 8.4.2 Astable Operation
  9. Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Application
      1. 9.2.1 Design Requirements
      2. 9.2.2 Detailed Design Procedure
      3. 9.2.3 Application Curve
    3. 9.3 Frequency Divider
      1. 9.3.1 Design Requirements
      2. 9.3.2 Application Curve
    4. 9.4 Pulse Width Modulator
      1. 9.4.1 Design Requirements
      2. 9.4.2 Application Curve
    5. 9.5 Pulse Position Modulator
      1. 9.5.1 Design Requirements
      2. 9.5.2 Application Curve
    6. 9.6 50% Duty Cycle Oscillator
      1. 9.6.1 Design Requirements
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
  12. 12Device and Documentation SupportChanged layout of National Semiconductor Data Sheet to TI format
    1. 12.1 Receiving Notification of Documentation Updates
    2. 12.2 Community Resources
    3. 12.3 Trademarks
    4. 12.4 Electrostatic Discharge Caution
    5. 12.5 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

9 Application and Implementation


Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.

9.1 Application Information

The LMC555 timer can be used a various configurations, but the most commonly used configuration is in monostable mode. A typical application for the LMC555 timer in monostable mode is to turn on an LED for a specific time duration. A pushbutton is used as the trigger to output a high pulse when trigger pin is pulsed low. This simple application can be modified to fit any application requirement.

9.2 Typical Application

Figure 9 shows the schematic of the LM555 that flashes an LED in monostable mode.

LMC555 LMC555_example.gif Figure 9. Schematic of Monostable Mode to Flash an LED

9.2.1 Design Requirements

The main design requirement for this application requires calculating the duration of time for which the output stays high. The duration of time is dependent on the R and C values (as shown in monostable figure) and can be calculated by: t= 1.1*R*C seconds.

Equation 6. t = 1.1 × R × C

9.2.2 Detailed Design Procedure

To allow the LED to flash on for a noticeable amount of time, a 5-second time delay was chosen for this application. By using the equation:

Equation 7. t = 1.1 × R × C seconds


  • RC equals 4.545

If R is chosen as 100 kΩ, C = 45.4 µF. The values of R = 100 kΩ and C = 47 µF was chosen based on standard values of resistors and capacitors.

A momentary push button switch connected to ground is connected to the trigger input with a 10-kΩ current limiting resistor pull up to the supply voltage. When the push button is pressed, the trigger pin goes to GND. An LED is connected to the output pin with a current limiting resistor in series from the output of the LMC555 to GND. The reset pin is not used and was connected to the supply voltage.

9.2.3 Application Curve

The data shown in Figure 10 was collected with the circuit used in the typical applications section. The LM555 was configured in the monostable mode with a time delay of 5.17 s. The waveforms correspond to:

  • Top Waveform (Blue) – Capacitor voltage
  • Middle Waveform (Purple) – Trigger
  • Bottom Waveform (Green) – Output

As the trigger pin pulses low, the capacitor voltage starts charging and the output goes high. The output goes low as soon as the capacitor voltage reaches 2/3 of the supply voltage, which is the time delay set by the R and C value. For this example, the time delay is 5.17 seconds.

LMC555 app-curve.gif Figure 10. Trigger, Capacitor Voltage, and Output Waveforms in Monostable Mode

9.3 Frequency Divider

The monostable circuit of Figure 11 can be used as a frequency divider by adjusting the length of the timing cycle. Figure 12 shows the waveforms generated in a divide by three circuit.

LMC555 866904.png Figure 11. Monostable (One-Shot)

9.3.1 Design Requirements

Design a frequency divider by adjusting the length of the timing cycle.

9.3.2 Application Curve

LMC555 866914.png Figure 12. Frequency Divider Waveforms

9.4 Pulse Width Modulator

When the timer is connected in the monostable mode and triggered with a continuous pulse train, the output pulse width can be modulated by a signal applied to the control voltage terminal. Figure 13 shows the circuit, and in Figure 14 are some waveform examples.

LMC555 866920.png Figure 13. Pulse Width Modulator

9.4.1 Design Requirements

Modulator the output pulse width by the signal applied to the control voltage terminal.

9.4.2 Application Curve

LMC555 866915.png Figure 14. Pulse Width Modulator Waveforms

9.5 Pulse Position Modulator

This application uses the timer connected for astable operation, as in Figure 15, with a modulating signal again applied to the control voltage terminal. The pulse position varies with the modulating signal, since the threshold voltage and hence the time delay is varied. Figure 16 shows the waveforms generated for a triangle wave modulation signal.

LMC555 866921.png Figure 15. Pulse Position Modulator

9.5.1 Design Requirements

Using astable operation vary the pulse position with a modulating signal applied to the control voltage terminal.

9.5.2 Application Curve

LMC555 866916.png Figure 16. Pulse Position Modulator Waveforms

9.6 50% Duty Cycle Oscillator

The frequency of oscillation is:

Equation 8. f = 1/(1.4 RCC)
LMC555 866906.png Figure 17. 50% Duty Cycle Oscillator

9.6.1 Design Requirements

An oscillator with a 50% duty cycle output.