• State
  • Locked Locked
  • Replies 2 replies
  • Subscribers 62 subscribers
  • Views 350 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM3430 in buck boost SEPIC configuration

Ed Walker
Prodigy 5 points

It is standard practice to use a BOOST controller in a SEPIC (buck boost) topology.
SEPIC is "boost derived" in that the single inductor is replaced with a coupled inductor.

I am looking for confirmation, and any examples, of LM3430 being used in a SEPIC topology.
This will be used in conjunction with LM3432 for a DHC dynamic headroom control led driver.

  • 0
    Prodigy 5 points

    I have 6 strings of 6 series LEDS in each string.
    27.5V @ 8mA to 30.8V @ 33mA string voltage.

    INPUT is 18-32VDC.

    If operated in boost mode only, LM3430, would there be too much dissipation in the LM3432 at 27.5Vstring and 32Vin?
    It seems that boost mode may be the region of operation the majority of the time, with the boost not switching at high Vin and low Vstring.

    How does the LM3430 react if the Vin does not need to be boosted to maintain headroom for the LM3432?
    The LM3430 datasheet indicates it will go to 0% duty (no boost).

  • 0 in reply to Ed Walker
    Prodigy 5 points

    Regarding to Heat dissipation of LM3432:

    While LM3430 + LM3432 operated together, LM3430 is working in boost mode and VDHC pins were connected properly through a gain setting resistor, the headroom will be optimized according to the LED voltages and therefore, only minimal heat dissipation occur on LM3432.

    In customer’s case of VIN = 32V while string voltage at 27.5V, the LM3430 boost will not operate (in theory as it can achieve 0% duty cycle) and LM3432 will be seeing 4.5V potential difference.

    With 8mA per string, 6 strings in total drawing 48mA and the total power dissipated by LM3432 would be 0.216W.

    If string voltage becomes 30.8V and current becomes 33mA, LM3432 will be seeing 1.2V potential difference and the total power dissipated by LM3432 would be 0.238W.

    Regarding to LM3430 with high VIN:

    In theory, LM3430 can go to 0% duty cycle without switching. So in theory, the boost will not have any action while VIN is higher than the LED voltages (plus necessary head-room voltages for LM3432).

    But the actual response and the transient will require detail testing to confirm.