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LM555 Pulse width modulation

K.Hamamoto
Guru 12155 points
Other Parts Discussed in Thread: LM555

Hi

Our customer are considering LM555 as the PWM pulse generator.

I have a few question for this use.

 - How much voltage can output at Vcc=5V.

      (Can voltage output 5V at Vcc=5V?)

 - How much minimum duty and maximum duty cycle can control by Pin5?

       Is possible from 0% to 100%?

 - Is the frequency calculation below

       F= 1/T = (1.44*C) / Ra

 - Can I have an example of a circuit that satisfies the following requirements?

--- (customer requirement ) ------

  Vout p-p = 0 ~ 5V, Output frequency = 900Hz

 Duty = 0 ~ 100%

Regards,

K

  • 0
    TI__Genius 16890 points

    Hello,

    Using Figure 19 in the datasheet set RA = 40.2k and C = 0.01uF.  These are starting values and will need to be adjusted by the customer.  Use 5V for Vcc, 0V to 5V for the modulation input and an 900Hz, 0v to 5V pulse on the trigger input.  The pulse on the trigger input should have a very high duty cycle, 90% or more.  This value will need to be experimented with by the customer. 

    The output will be 0V to about 4.5V.  It will not go to 5V.  When the modulation input is a low voltage the output will be a small duty cycle.  The smallest duty cycle is the inverse of the trigger input.  So, if the trigger input is a 90% duty cycle the smallest output duty cycle is 10%.  As the modulation voltage is increased the output duty cycle will increase up to around 90% maximum. 

    They will need to adjust components and inputs to get the desired values. 

    Mike