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MC3486: Rise/Fall time

Cedrick
Expert 8270 points
Part Number: MC3486

Hi,

Good day.

Our customer is using the MC3486 on their existing design and they want to change this chip. In order to do that, they need the rise time and fall time for this device however, it is not provided on the datasheet. Can you please help us provide this for the customer?

Looking forward to your help on this. Thank you.


Regards,

Cedrick

  • 0
    TI__Guru 70312 points

    Cedrick,

    This is a very old device, so finding the characterization data will be difficult. You said that they want to change this chip, are they wanting the rise and fall time so the new device can match this parameter?

    Regards,

    Eric Hackett

  • 0 in reply to Eric Hackett
    TI__Mastermind 21543 points

    Hi Eric,

    I hope you are doing well. Looks like Cedrick and I are getting this question from two different directions (OEM and Tier 1). One thing I wanted to ask is if it made sense to calculate the worst-case output impedance of the output driver’s high-side and low-side FET using the datasheet parameters. Do you see any issue with this idea?  If our customer can then estimate the load resistance and load capacitance they could calculate their own worst-case rise and fall times. 

    Since we have guaranteed VOH and VOL numbers with load currents that are guaranteed across supply voltage range of 4.75 to 5.25 volts, can't we do the following:

    • Worst case RDSON for the high side FET of the output driver is at a VCC of 4.75V. 4.75V VCC and a 0.4mA load, the high-side FET will at worst pull high to 2.7V meaning its on state resistance is 5.125kΩ ( [4.75-2.7] / 0.4mA ).
    • Worst case RDSON of the low-side FET of the output driver is 0.5V VOH at 8mA load, which results in 100Ω ( [05 V-0.0 V] / 8 mA ).

    Let me know what you think,

    John

  • 0 in reply to John Griffith
    TI__Guru 70312 points

    John,

    This makes sense to me, the RDSon of the high-side seems high, but your math checks out. I'm looping in Hao to look at this as well.

    Regards,

    Eric Hackett

  • 0 in reply to Eric Hackett
    TI__Mastermind 21543 points

    Hi Eric,

    Thanks! I thought the same. I always heard the rule of thumb that the area of a PMOS is 4x that of a NMOS with similar on-state resistance, but it seems crazy to make the high-side 50x weaker than the low-side with a standard push-pull output. 

    Hi Hao! Hope you are doing well. Let us know what you think. 

    John 

  • 0 in reply to John Griffith
    TI__Mastermind 25705 points

    Hi John,

    I think your calculation makes sense (to be more precise, the N side impedance is about 62.5Ohm). The fact that the P side has much weaker driving strength may be due to the circuitry working like a follower rather than an amplifier. 

    There is some discussion at this website. 

    Emitter Follower Applications (gsu.edu)

    Regards,

    Hao

  • 0 in reply to Hao L
    TI__Mastermind 21543 points

    Thanks Hao! Not sure how I messed up my calculation, but thanks for correcting me! Let me follow up with my customer before closing the thread. 

    John