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LM3409HV initial ON delay and external Vcc

Tim Chao Li
Expert 4415 points
Other Parts Discussed in Thread: LM3409HV

Hi,

With the LM3409HV, we noticed that there's ~80us of turn ON delay the first time we assert the EN pin compared to a negligible amount when EN hasn't been low for too long.

Turns out, this is because (Vin-Vcc) needs to charge to more than 3.75V for the part to operate, and with the eval kit's 1uF cap on Vcc, this takes roughly those 80us. Once EN is back to L, it discharges to ~4V in ~6.5ms. The data sheet never specifies how long after EN=L the VCC regulator shuts down.

This E2E post suggests that the solution is to power Vcc externally: http://e2e.ti.com/support/power_management/led_driver/f/192/p/177725/1130535.aspx#1130535

Could you please confirm how to do this safely? I was thinking of a zener from Vin (K) to Vcc (A), and a resistor to GND, but don't know whether this could damage the internal regulator.

  • 0
    TI__Expert 4415 points

    I was able to find a solution to this issue:

    You cannot power VCC externally. It is VIN referenced and only an 8V internal driver. At 7V regulation, there is not enough headroom to externally drive it. 

    However, you can use a diode “or” connection to UVLO pin instead. Keep EN high. This has worked in the past for camera flash applications.

  • 0 in reply to Tim Chao Li
    Prodigy 20 points

    Because I have the same Delay-problem in my application: do you mean to use the  UVLO-Pin instead of the Enable Pin? So to wire-or the inputvoltage-resistor-divider and the PWM-signal? Or maybe could you upload a rough schematic how it was solved in the camera flash application?

    Thanks a lot,

    br

    Andreas Egger