LM555

LM555 Timer IC: Technical Summary

Overview

The LM555 is a highly stable integrated circuit designed for precision timing and pulse generation applications. As one of the most widely used timer ICs, it offers exceptional accuracy and flexibility in both monostable (one-shot) and astable (oscillator) modes. The device operates across a wide voltage range, making it suitable for diverse electronic designs. Its robust design ensures reliable performance in industrial, automotive, and consumer applications where precise timing control is required.

Key Features

• High Current Drive Capability: 200mA sink/source current enables direct driving of relays, LEDs, and small motors
• Adjustable Duty Cycle: Precise control over output pulse characteristics in astable mode
• Exceptional Temperature Stability: 0.005%/°C variation ensures consistent performance across operating conditions
• Wide Timing Range: Capable of generating timing pulses from microseconds to several hours
• Fast Response: Turn-off time less than 2μsec for rapid switching applications
• Wide Operating Voltage: Typically 4.5V to 16V (18V absolute maximum)
• Low Power Consumption: Typical supply current of 10mA at 15V

Applications

The LM555's versatility makes it suitable for numerous timing-related applications across multiple industries:

• Precision Timing Circuits: For industrial control systems and instrumentation
• Pulse Generation: Creating clock signals and trigger pulses
• Time Delay Generation: Sequential timing in automation systems
• Pulse Width Modulation (PWM): Motor speed control and power regulation
• Oscillator Circuits: Tone generation, LED flashers, and clock sources
• Voltage-Controlled Oscillators: When combined with external control voltage
• Missing Pulse Detectors: For safety monitoring applications
• Bounce-Free Switches: Contact debouncing in mechanical switches

Electrical Characteristics

The following table summarizes key electrical parameters (TA = 25°C, VCC = 5V to 15V unless otherwise specified):

• Supply Voltage Range: 4.5V to 16V (18V absolute max)
• Supply Current:
  • 5V: 3-6mA
  • 15V: 10-15mA
• Timing Error:
  • Monostable: 1% typical
  • Astable: 1.5% typical
• Output Current: 200mA maximum (sink or source)
• Trigger Voltage: 1.67V minimum (at VCC = 5V)
• Reset Current: 0.5mA typical
• Operating Temperature Range: 0°C to 70°C (commercial)

Functional Description

The LM555 contains 25 transistors, 2 diodes, and 15 resistors in a single silicon chip packaged in an 8-pin DIP or SOIC package. The internal architecture consists of:

• Voltage Divider Network: Creates reference voltages of 1/3 VCC and 2/3 VCC
• Two Comparators: Compare input signals to reference voltages
• SR Flip-Flop: Controls the output state
• Discharge Transistor: Provides capacitor discharge path
• Output Buffer: Provides high current drive capability

Package Information

The LM555 is available in multiple package options to suit different application requirements:

• 8-Pin PDIP (Plastic Dual In-line Package):
  • Package Code: N
  • Dimensions: 9.81mm x 6.35mm
  • Lead Pitch: 2.54mm
  • Operating Temperature: 0°C to 70°C
• 8-Pin SOIC (Small Outline IC):
  • Package Code: D
  • Dimensions: 4.9mm x 3.91mm
  • Lead Pitch: 1.27mm
  • Operating Temperature: -40°C to 85°C
• 8-Pin TSSOP (Thin Shrink Small Outline Package):
  • Package Code: PW
  • Dimensions: 3mm x 4.4mm
  • Lead Pitch: 0.65mm
  • Operating Temperature: -40°C to 125°C

Design Considerations

When implementing the LM555 in circuit designs, consider the following:

• Timing Accuracy: Use stable, low-tolerance components (1% resistors and low-leakage capacitors)
• Power Supply Decoupling: Place 0.1μF ceramic capacitor close to VCC pin
• Grounding: Use single-point grounding for timing components
• Noise Immunity: Keep timing components close to IC, use shielded cables for long runs
• Current Limiting: Add series resistors when driving inductive loads
• Thermal Considerations: Provide adequate heat sinking when operating near maximum ratings

Alternative Part Numbers

Several variants of the 555 timer are available for specific applications:

• LM555C: Commercial temperature range (0°C to 70°C)
• LM555I: Industrial temperature range (-40°C to 85°C)
• LM555M: Military temperature range (-55°C to 125°C)
• NE555: Original manufacturer's version
• SA555: Low-power version with reduced supply current
• TS555: CMOS version with rail-to-rail outputs