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CD4563B: Determining "Rs" for support component resistor

John Klingelhoeffer
Prodigy 90 points

The data sheet for the TI CD4536B includes a support component labeled "Rs".  However, the data sheet does not say what Rs does,  how it enters into the frequency calculation for the oscillator, nor how to optimize the value for Rs.    

Page 3-313 of the data sheet shows in the note: Rs ~= 5  to 10 x Rt, yet graph Figure 12 indicates Rs equal to 2x Rt, and graph Figures 13 and 14 compare temperature deviation of the oscillator with Rs equal to zero ohms and to 120 k ohms (some, but not much difference, except at lower Vdd values).  

So, the questions are:  What does Rs 'do' for the CD4536B?  How does the value of Rs work into the timing equation for the oscillator?  How does one go about optimizing the value of Rs, and indeed what IS optimized by changing the value of Rs?  

  • 0
    TI__Mastermind 46480 points

    I am not sure. usually RS controls duty cycle but I have found no verbage to confirm this.

    Here is one statement I found int the competitor datashet that was not much help. They do not explain it either. The best I can tell it does not affect the frequency since it is not included in the formulas.

    IN 1 (Pin 3) −The device’s internal counters advance on the negative−going edge of this input. IN 1 may be used as an external clock input or used in conjunction with OUT 1 and OUT 2 to form an RC oscillator. When an external clock is used, both OUT 1 and OUT2 may be left unconnected or used to drive 1 LSTTL or several CMOS loads.