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UC2846: Current-Limit Shutdown Feature

Ryan Blanchard
Prodigy 20 points
Part Number: UC2846

Tool/software:

Looking at the above excerpt - wanting to confirm the shutdown function always requires input on Pin 16 to active the shutdown function, or if pulling more than 3mA on its own from Vref can cause this state to be entered.

Asking as I'm seeing a unit (Multi-Winding Flyback) that implements a short circuit function via:

  • Pin 16 is has a pull-down of 51.1ohms
  • Pin one is attached to both:
    • A resistor divider tied to Vref, per the app notes
    • Via a diode (anode tied to Pin 1 Node) it is tied to a resistor divider pulled up to the power output of a tertiary/aux winding. This essentially utilizing the droop of the aux winding during current limit or short events to pull down the current adj pin 1 value to limit the PWM DC. When this value is being pull-down (greater than 3mA draw), it is still latching vs. the desired outcome of recovering post-short removal (as the Shutdown is tied low). This is source of the initial question.

Any help or insight on this would be greatly appreciated.

  • 0
    TI__Guru 72955 points

    Hi,

    Pulling out 3mA from VREF does not shut down the IC since VREF loading can be 10mA.

    On your described approach, please indicate which application note and where in the application note. It is better to draw a circuit to show your idea to do the shutdown.

  • 0 in reply to Hong Huang
    Prodigy 20 points

    I've attached a simplified version of the circuit below (SHTDN Pin Squared-In Red):

    But just to simplify what I want to know - assuming based on the note below this, it does require both greater than 3mA current draw and shutdown to go high in order to latch the device off?

    The device will not latch off solely from the 3mA draw? I'm seeing anomalous behavior and just want to confirm the baseline notes from the datasheet. 

  • 0 in reply to Ryan Blanchard
    TI__Guru 72955 points

    Hi,

    Yes, the block diagram on page 8 shows pin 16 is to trigger shutdown and the thyristor needs > 3mA holding/latching current to keep the thyristor on so the latch off is achieved.