SLFS047C February   1984  – December 2024 TLC556 , TLC556M

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Pin Configuration and Functions
  6. 5 Specifications
    1. 5.1 Absolute Maximum Ratings
    2. 5.2 ESD Ratings
    3. 5.3 Recommended Operating Conditions
    4. 5.4 Thermal Information
    5. 5.5 Electrical Characteristics: VDD = 2V for TLC556C, VDD = 3V for TLC556I
    6. 5.6 Electrical Characteristics: VDD = 5V
    7. 5.7 Electrical Characteristics: VDD = 15V
    8. 5.8 Switching Characteristics
    9. 5.9 Typical Characteristics
  7. 6 Detailed Description
    1. 6.1 Overview
    2. 6.2 Functional Block Diagram (Each Timer)
    3. 6.3 Feature Description
      1. 6.3.1 Monostable Operation
      2. 6.3.2 Astable Operation
    4. 6.4 Device Functional Modes
  8. 7 Application and Implementation
    1. 7.1 Application Information
      1. 7.1.1 Designing for Improved ESD Performance
    2. 7.2 Typical Applications
      1. 7.2.1 Pulse-Width Modulation
        1. 7.2.1.1 Design Requirements
        2. 7.2.1.2 Detailed Design Procedure
        3. 7.2.1.3 Application Curve
      2. 7.2.2 Pulse-Position Modulation
        1. 7.2.2.1 Design Requirements
        2. 7.2.2.2 Detailed Design Procedure
        3. 7.2.2.3 Application Curves
  9. 8 Device and Documentation Support
    1. 8.1 Receiving Notification of Documentation Updates
    2. 8.2 Support Resources
    3. 8.3 Trademarks
    4. 8.4 Electrostatic Discharge Caution
    5. 8.5 Glossary
  10. 9 Revision History
  11. 10Mechanical, Packaging, and Orderable Information

6.3.2 Astable Operation

As shown in Figure 6-2, adding a second resistor, RB, to the circuit of Figure 6-1 and connecting the trigger input to the threshold input causes the timer to self-trigger and run as a multivibrator. The capacitor CT 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 CT charging and discharging between the threshold-voltage level (≅ 0.67 × VDD) and the trigger-voltage level (≅ 0.33 × VDD). As in the monostable circuit, charge and discharge times (and, therefore, the frequency and duty cycle) are independent of the supply voltage.

TLC556 TLC556M Circuit for Astable Operation
A. Decouple CONT voltage to ground with a capacitor to improve operation. Reevaluate for individual applications.
Figure 6-2 Circuit for Astable Operation
Equation 1. t H ≅ 0.693 × R A + R B × C T
Equation 2. t L ≅ 0.693 × R B × C T

Other useful relationships for period, frequency, and driver-referred and waveform-referred duty cycle are shown as follows:

Equation 3. T = t H + t L ≅ 0.693 × R A + 2 R B × C T
Equation 4. f = 1 T ≅ 1.44 R A + 2 R B × C T
Equation 5. O u t p u t   d r i v e r   d u t y   c y c l e = t L T ≅ R B R A + 2 R B
Equation 6. O u t p u t   w a v e f o r m   d u t y   c y c l e = t H T ≅ 1 - R B R A + 2 R B = R A + R B R A + 2 R B

These equations do not account for any propagation delay times from the TRIG and THRES inputs to DISCH output. These delay times add directly to the period and overcharge the capacitor, which creates differences between calculated and actual values that increase with frequency. In addition, the internal on-state resistance, ron, during discharge adds to RB to provide another source of timing error in the calculation when RB is very low. The following equations provide better agreement with measured values. Equation 7 and Equation 8 represent the actual low and high times when used at higher frequencies (beyond 100kHz) because propagation delay and discharge on resistance is added to the formulas. The value of CT includes both the nominal or deliberate timing capacitance, as well as parasitic capacitance on the PCB. Decoupling capacitance on CONT also affects the duty cycle, with an error contribution that depends on the capacitor leakage resistance. For additional discussion, see the Design low-duty-cycle timer circuits article.

Equation 7. t c ( H ) = C T × R A + R B × ln ⁡ 3 − e − t P D   r i s i n g C T × R B + r o n + t P D   f a l l i n g
Equation 8. t c ( L ) = C T × R B + r o n × ln ⁡ 3 − e − t P D   f a l l i n g C T × R A + R B + t P D   r i s i n g

These equations and those given earlier are similar in that a time constant is multiplied by the logarithm of a number or function. The limit values of the logarithmic terms must be between ln(2) at low frequencies, and ln(3) at extremely high frequencies. For a duty cycle close to 50%, an appropriate constant for the logarithmic terms can be substituted with good results. Output waveform duty cycles less than 50% require that tc(H) / tc(L) < 1 and possibly that RA ≤ ron. These conditions can be difficult to obtain.

TLC556 TLC556M Trigger and Threshold Voltage WaveformFigure 6-3 Trigger and Threshold Voltage Waveform