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SE555: In Astable operation calculated frequency and measured frequency are not matching

Chaitanya Dasari
Intellectual 340 points

Part Number: SE555

Hi E2E 

I am using SE555 in Astable operation for generating a PWM signal of ~38kHz.I have chosen the resistors RA  RBand capacitor C as per the formula given in the datasheet under 8.3.2 A-stable Operation. 

Attaching that datasheet formula snippet for your reference.

After calculation, I have chosen the 

RA = 1.1kΩ,RA = 1.3kΩ,C = 10nF

As per the calculations  Frequency F= 38.9kHz and Duty Cycle D= 0.64%

I have tested the circuit with the above Resistors of ±1% Tolerance and C0G Capacitor ±1% Tolerance and measured  Frequency F= 35.1kHz and Duty Cycle D= 0.64 %

Attaching the capture for your reference 

Later we have updated the passives to RA = 1kΩ,RA = 1.2kΩ,C = 10nF

As per the calculations  Frequency F= 42.3kHz and Duty Cycle D= 0.65%

I have tested the circuit with the above Resistors of ±1% Tolerance and C0G Capacitor ±1% Tolerance and measured  Frequency F= 37.9kHz and Duty Cycle D= 0.64 %

Attaching the capture for your reference 

In both of the cases, I have seen a difference of ~4kHz, and this is more than ~10% difference on measured and calculated frequency. Can you please help me out why I am seeing this difference?

BR

Chaitanya

  • 0
    TI__Mastermind 31635 points

    Chaitanya,

    I have a few questions to see if I can help narrow down the issue. 

    1) Have you measured the resistors as well as the capacitor?

    2) Do you have part numbers you can share?

    The capacitor should be measured with an LCR meter with very short cables at the frequency of interest. COG/NPO usually have good tolerances but it would be good to double check the value if possible. 

    3) Have you accounted for the tolerance in threshold and trigger voltage levels?

    There can be up to 6% variation in the threshold voltage level, and 4% variation in the trigger voltage level.