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UCC2803-EP: Calculating Raux value

Asha C
Intellectual 385 points
Part Number: UCC2803-EP
Other Parts Discussed in Thread: UCC2803

Tool/software:

In my case,  i have Vaux as 9 to 9.5V. How to calculate Raux since internal zener diode will never be operational as our Vinput is 9V.

Through Webench it says 10ohm.

  • 0
    TI__Guru 74115 points

    Hi,

    Raux is for filtering instead of to help clamp VCC. 10-ohm is a rule of thumb for spikes filtering. Its value can be changed during the bench test.

  • 0 in reply to Hong Huang
    Intellectual 385 points

    Hello,

    Could you please help me then which Rmax is it referring here? Isnt it Raux?

  • 0 in reply to Asha C
    TI__Guru 74115 points

    Hi,

    That Rmax is referring to the circled resistors in your design for startup which is ok to reply on the internal 13.5V zener clamping. But right after the startup, VCC voltage abs max needs to be < 12V based on the abs max specs. 

  • 0 in reply to Hong Huang
    Intellectual 385 points

    Hello Huang,

    In continuation with the previous query,

    We have a start up current of about 0.2mA for UCC2803 and later should provide 4.1V for the startup duration( default 4ms), For providing this we need to have charge CVCC to 4.1V from the Higher voltage via higher resistors. In this case via 430k + 430k . If this is the case then it would take longer time to charge the capacitor to 4.1V and later it starts to generate the PWM. If this is the case , then for generating Vout it would take longer time is it?

    Or Am i missing anything here?

  • 0 in reply to Asha C
    TI__Guru 74115 points

    Hi,

    The soft start time 4ms is counted after VCC reaches its UVLO on. The time to make VCC reach its UVLO on depends on the Rmax and the Vcc capacitor but this time is not the 4ms soft start time since IC is not in operation yet.

  • 0 in reply to Hong Huang
    Intellectual 385 points

    Hello,

    Under electrical spec , i see Soft start has a max value of 10ms, then in that case should we calculate the capacitor value for 10ms start up time?

  • 0 in reply to Hong Huang
    Intellectual 385 points

    Hello,

    Thank you , got it.

    As per datasheet, Maximum current capability of Vcc pin is 30mA , but in simulation i am observing higher current than 20mA.

    This is mainly due to the gate current of MOSFET. 

    Is it only in simulation it is consuming more Ig current , but in actual use case it will be limited to less than 30mA?

  • 0 in reply to Asha C
    TI__Guru 74115 points

    Hi,

    Soft start time is its specs achieved thought internal circuit, no capacitor is needed.

    The IC VCC current is combined the IC bias and the MOSFET. In simulation, IC VCC is fixed at a pre-selected value. In real circuit, the VCC current is in a range for bias. Your external circuit needs to make sure not to push current over 30mA into the IC. The IC has no current limit circuit inside. The IC max junction temperature needs to check to make sure not to exceed its abs max on top of abs max current.