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CD4047B: CD4047BE

Ray Linehan
Prodigy 10 points
Part Number: CD4047B

Dear Sir / Madam,

 

Recently on a Motor Gate Drive Circuit we replaced an obsolete NXP device with TEXAS INSTRUMENT device CD4047BE (CMOS Low-Power Astable Multivibrator).

 

However the frequency reduced significantly (from 890KHz to 670KHz) with the TI component.

The device is being run in Astable Mulitivibrator mode (free running / Outputs Pins 10 & 11)

 and the datasheet specified the formula;

 

tA = 4.40 * R * C.

 

In our application;

R = R1 = 10K (10,000 ohms)

C = C1 = 22pF (22 * 10e-12).

This gives tA  = 0.968*e-6

and F = 1.033MHz.

 

Why is the frequency then = 670KHz in practice?

 

VDD = +15V

VSS = Agnd

Pins 4,5,6 &14 are tied to +15V & Pins 7,8,9 &12 are tied to Agnd

 

OSC O/P(Pin 13) is unused.

There is a 10K Resistor between pins 13 & 14 and it is decoupled to Agnd with a 100nF Capacitor.

 

Best Regards,

Ray L.

 

  • 0
    TI__Guru* 96947 points
    Hi Ray and welcome to the forums!
    Many times the equations you find in datasheets are heavily simplified. I recently derived the full form of the oscillator equation for this type of chip, and it's absolutely massive. It includes variables beyond the external components such as the internal delays and input capacitances... it's just not very easy to work with. I can see why the original designers of parts like this would want to provide simplified versions of the equations that only apply over certain component values to help customers more quickly narrow down their design requirement.

    With any type of logic/RC based oscillator, I always recommend prototyping to 'hone in' on the frequency you want. In this case, the system was designed for a different IC, so it's not surprising to hear that there's a significant frequency shift when just swapping the IC and not tuning anything else.

    It looks like this device was characterized with 1nF, 10nF, 100nF, and 1uF capacitors, and you're using a 22pF capacitor.

    The frequency accuracy seems dependent on capacitance, and is at its worst at 15V (Fig 11-13):
    1uF: 1.4%
    10nF: 0.6%
    1nF: 1.9%

    Since there's no data at 22pF, I can only guess that the accuracy degrades significantly with reduced capacitance.

    I would recommend tweaking the RC values to get your desired frequency.