• State
  • Locked Locked
  • Replies 1 reply
  • Subscribers 47 subscribers
  • Views 228 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM2917-N: Response time equation for C2

steven avritch
Prodigy 60 points
Part Number: LM2917-N

The datasheet says the response time of the LM2917 is set by C2, but I cannot find an equation that relates response time to C2

In the lab, with a 1uF value for C2, I measure a response time of apx 500ms.  

I see an equation that defines ripple based on C2 and other component values... but no way to calculate response time based on C2

Is there an equation that relates C2 to response time?

  • 0
    TI__Guru* 93105 points

    Hi Steven,

    The LM2917-N documentation is vague about response time. There is mention about it on datasheet Pg. 14, section 0.2.1.2.1 - Choosing R1 and C1, where it states:

    R1 can be chosen independent of ripple. However, response time, or the time it takes VOUT to stabilize at a new voltage, increases as the size of C2 increases, so a compromise between ripple, response time, and linearity must be chosen carefully.

    And on Page 10:

    A charge pump system is used to translate the frequency of this square wave to a voltage. At the start of every positive half cycle of the input signal a 180-µA constant current charges C1 until its voltage has increased by VCC/2. The capacitor is held at that voltage until the input signal begins a negative half cycle. Then the 180-µA constant current discharges capacitor C1 until its voltage has dropped by VCC/2. This voltage is held until the next positive half cycle and the process repeats. This generates pulses of current flowing into and out of capacitor C1 at the same frequency as the input signal. For every full cycle, the charge pump mirrors both current pulses as positive current pulses into the parallel combination of resistor R1 and capacitor C2.

    If we consider the F-to-V converter circuit shown in datasheet Fig. 19, it has R1 = 100 kilohms and C2 is 1 uF. I believe they are the same values you are using in your circuit. The parallel combination of R1, C2 is sourced current from the charge pump as stated above. Taking these R1,C2 values and charging them with a current source 1 time constant would be approximately 100 ms (t = R1C2) and 5 time constants would be 500 ms. That is line with what you are observing in the lab. You can reduce C2 to a lower capacitance and that will decrease the response time, but ripple will become larger as dictated by Eq. 6 provided on Pg. 14.

    Regards, Thomas

    Precision Amplifiers Applications Engineering