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LMZ14203H: LMZ14203 minimal input voltage

Dorian Chambon
Prodigy 10 points
Part Number: LMZ14203H

Hello Support,

We are currently using the LMZ14203H to convert a +32V DC input into an 18V DC output. Our +32V bus maintains a steady state within a range of +/- 3%, although in emergency situations, it has the potential to drop as low as 20V in 30ms.

Our aim is to determine the minimum input voltage required to ensure normal operation in emmergency cases, as outlined in the application note: "Understanding Output Voltage Limitations of DC/DC Buck Converters." However, some crucial parameters missing in the datasheet:

  • Specifications for resistors on both high-side and low-side switches, as well as for the inductor.
  • Duty cycle limits.

Although we've conducted a Webench Electrical Simulation, we've encountered limitations. The tool executes an input step, causing the voltage to drop from 32V to 20V with a high dv/dt, which does not accurately reflect our operating conditions.

We will conduct tests on one unit and inquire whether it is representative for all samples ?

In addition does the TINA TI Model is representative is these conditions ?

Your guidance on resolving these uncertainties would be greatly appreciated.

Best regards, 

CD.

  • 0
    TI__Mastermind 30430 points

    Hello,

    Bucks have duty cycle limitations and also a vout to vin differential due to the I-R drops associated with the HSFET and buck inductor.

    Taking a dropout curve is a good way of determining this differential, or minimum input voltage required,

    I would suggest evaluating this in your application. Across the output loading range, reduce the input voltage and record when the output voltage falls outside of your acceptable tolerance.

    You can then use your engineering best judgement to consider if the headroom you have is acceptable for your application.

    The variance from part to part should not be substantially large and I would only use TINA as a first pass before verifying on the final application board.

    I would expect a minimum of 20V input should cause no problem in ensuring you can achieve 18V output with no drop at the device's maximum load current specification.