Datasheet LMC555 (Texas Instruments) - 10
| Manufacturer | Texas Instruments |
| Description | CMOS Timer |
| Pages / Page | 30 / 10 — LMC555. www.ti.com. Device Functional Modes (continued). 8.4.2 Astable … |
| Revision | M |
| File Format / Size | PDF / 1.4 Mb |
| Document Language | English |
LMC555. www.ti.com. Device Functional Modes (continued). 8.4.2 Astable Operation
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LMC555
SNAS558M – FEBRUARY 2000 – REVISED JULY 2016
www.ti.com Device Functional Modes (continued) 8.4.2 Astable Operation
If the circuit is connected as shown in Figure 6 (Trigger and Threshold terminals connected together) it will trigger itself and free run as a multivibrator. The external capacitor charges through RA + RB and discharges through RB. Thus the duty cycle may be precisely set by the ratio of these two resistors.
Figure 6. Astable (Variable Duty Cycle Oscillator)
In this mode of operation, the capacitor charges and discharges between 1/3 VS and 2/3 VS. As in the triggered mode, the charge and discharge times, and therefore the frequency are independent of the supply voltage. Figure 7 shows the waveform generated in this mode of operation. VCC = 5 V Top Trace: Output 5 V/Div. TIME = 20 µs/Div. Bottom Trace: Capacitor Voltage 1 V/Div. RA = 3.9 kΩ RB = 9 kΩ C = 0.01 µF
Figure 7. Astable Waveforms
The charge time (output high) is given by t1 = 0.693 (RA + RB)C (1) And the discharge time (output low) by: t2 = 0.693 (RB)C (2) Thus the total period is: T = t1 + t2 = 0.693 (RA + 2RB)C (3) The frequency of oscillation is: (4) Figure 8 may be used for quick determination of these RC Values. The duty cycle, as a fraction of total period that the output is low, is: RB D = RA + 2RB (5) 10 Submit Documentation Feedback Copyright © 2000–2016, Texas Instruments Incorporated Product Folder Links: LMC555 Document Outline 1 Features 2 Applications 3 Description Table of Contents 4 Revision History 5 Pin Configuration and Functions 6 Specifications 6.1 Absolute Maximum Ratings 6.2 ESD Ratings 6.3 Recommended Operating Conditions 6.4 Thermal Information 6.5 Electrical Characteristics 7 Parameter Measurement Information 8 Detailed Description 8.1 Overview 8.2 Functional Block Diagram 8.3 Feature Description 8.3.1 Low-Power Dissipation 8.3.2 Various Packages and Compatibility 8.3.3 Operates in Both Astable and Monostable Mode 8.4 Device Functional Modes 8.4.1 Monostable Operation 8.4.2 Astable Operation 9 Application and Implementation 9.1 Application Information 9.2 Typical Application 9.2.1 Design Requirements 9.2.2 Detailed Design Procedure 9.2.3 Application Curve 9.3 Frequency Divider 9.3.1 Design Requirements 9.3.2 Application Curve 9.4 Pulse Width Modulator 9.4.1 Design Requirements 9.4.2 Application Curve 9.5 Pulse Position Modulator 9.5.1 Design Requirements 9.5.2 Application Curve 9.6 50% Duty Cycle Oscillator 9.6.1 Design Requirements 10 Power Supply Recommendations 11 Layout 11.1 Layout Guidelines 11.2 Layout Example 12 Device and Documentation Support 12.1 Receiving Notification of Documentation Updates 12.2 Community Resources 12.3 Trademarks 12.4 Electrostatic Discharge Caution 12.5 Glossary 13 Mechanical, Packaging, and Orderable Information
LMC555
SNAS558M – FEBRUARY 2000 – REVISED JULY 2016
www.ti.com Device Functional Modes (continued) 8.4.2 Astable Operation
If the circuit is connected as shown in Figure 6 (Trigger and Threshold terminals connected together) it will trigger itself and free run as a multivibrator. The external capacitor charges through RA + RB and discharges through RB. Thus the duty cycle may be precisely set by the ratio of these two resistors.
Figure 6. Astable (Variable Duty Cycle Oscillator)
In this mode of operation, the capacitor charges and discharges between 1/3 VS and 2/3 VS. As in the triggered mode, the charge and discharge times, and therefore the frequency are independent of the supply voltage. Figure 7 shows the waveform generated in this mode of operation. VCC = 5 V Top Trace: Output 5 V/Div. TIME = 20 µs/Div. Bottom Trace: Capacitor Voltage 1 V/Div. RA = 3.9 kΩ RB = 9 kΩ C = 0.01 µF
Figure 7. Astable Waveforms
The charge time (output high) is given by t1 = 0.693 (RA + RB)C (1) And the discharge time (output low) by: t2 = 0.693 (RB)C (2) Thus the total period is: T = t1 + t2 = 0.693 (RA + 2RB)C (3) The frequency of oscillation is: (4) Figure 8 may be used for quick determination of these RC Values. The duty cycle, as a fraction of total period that the output is low, is: RB D = RA + 2RB (5) 10 Submit Documentation Feedback Copyright © 2000–2016, Texas Instruments Incorporated Product Folder Links: LMC555 Document Outline 1 Features 2 Applications 3 Description Table of Contents 4 Revision History 5 Pin Configuration and Functions 6 Specifications 6.1 Absolute Maximum Ratings 6.2 ESD Ratings 6.3 Recommended Operating Conditions 6.4 Thermal Information 6.5 Electrical Characteristics 7 Parameter Measurement Information 8 Detailed Description 8.1 Overview 8.2 Functional Block Diagram 8.3 Feature Description 8.3.1 Low-Power Dissipation 8.3.2 Various Packages and Compatibility 8.3.3 Operates in Both Astable and Monostable Mode 8.4 Device Functional Modes 8.4.1 Monostable Operation 8.4.2 Astable Operation 9 Application and Implementation 9.1 Application Information 9.2 Typical Application 9.2.1 Design Requirements 9.2.2 Detailed Design Procedure 9.2.3 Application Curve 9.3 Frequency Divider 9.3.1 Design Requirements 9.3.2 Application Curve 9.4 Pulse Width Modulator 9.4.1 Design Requirements 9.4.2 Application Curve 9.5 Pulse Position Modulator 9.5.1 Design Requirements 9.5.2 Application Curve 9.6 50% Duty Cycle Oscillator 9.6.1 Design Requirements 10 Power Supply Recommendations 11 Layout 11.1 Layout Guidelines 11.2 Layout Example 12 Device and Documentation Support 12.1 Receiving Notification of Documentation Updates 12.2 Community Resources 12.3 Trademarks 12.4 Electrostatic Discharge Caution 12.5 Glossary 13 Mechanical, Packaging, and Orderable Information