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UCC21710: input capacitance spec for buffer drive current requirement

Kelvin Le THS
Expert 7855 points
Part Number: UCC21710

Hi,

I'm trying to figure out what's the min buffer drive requirement (current source/sink capability.  e.g. +/-16mA) to drive the UCC21710 input.  MCU->buffer->UCC21710.

I didn't see the input capacitance spec in the UCC datasheet.

Idrive > Cin*dV/dt.

If I assume 100pF for Cin, dV=5V, dt=30ns, the Idrive > 16mA.  If I include PCB & filter capacitance, then the Idrive will be a bit higher.

please let me know what i can assume about Cin of UCC21710.

-Kelvin

  • 0
    TI__Expert 8265 points

    Hi Kelvin,

    When you said Input pin capacitance, I believe that you interested in INP and INN pins. Is that right?

    INP and INN pins have internal pull down [55K ohms] - so these pull down resistance will limit the Idrive current to ~90uA at 5V. 

    UCC21710 features a 40ns internal deglitch filter to IN+, IN- and RST/EN pin. Any signal less than 40ns can be filtered out from the input pins.

    For noisy systems, external low pass filter can be added externally to the input pins with 100 ohms and 100pF - so that high frequency noise will be filtered out.

    Let me know if it answers your question by pressing the Green button.

  • 0 in reply to Sasikala Thangam
    TI__Expert 7855 points

    Yes the UCC input capacitance at INP and INN. any idea what this is?

  • +1 in reply to Kelvin Le THS
    TI__Expert 8265 points

    Hi Kelvin,

    We recommend an RC filter at the input pins with 100pF and 100 ohms to bypass the high frequency noise. Input current is limited with internal resistance, so even if there is higher capacitance on the board, it will not cause higher drive current [if you limit the voltage to 5V]. But increasing capacitance will limit the input switching frequency, Hope it answers your question.

  • 0 in reply to Sasikala Thangam
    TI__Expert 7855 points

    the peak current into the cap at the transition is limited by the 100ohm series resistor.