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LM5575-Q1: Low dropout operation

Srinivasu V
Intellectual 360 points
Part Number: LM5575-Q1

Hello Team,

we have designed Buck convertor with Vin 18 to 36V and  Vout=15V/1.5A.

During cold crank, the Vin can drop to 6V for 50ms. We need to configure,  in such way that the device just needs to transmit 6V directly to the output during the cold crank.



Can you please advise how we can make  to enter into low dropout operation and achieve 100% duty cycle?

Regards,

Srinivasu

  • 0
    TI__Mastermind 22301 points

    Srinivasu,

    The VCC UVLO is 5.35V typical. The datasheet describes that VCC is approximately equal to VIN at low input voltages (section 7.4.1).

    However if the VCC voltage falls below UVLO, then the part will just disable.

    You must be sure the VIN is above that UVLO voltage to enable the part.

    The duty cycle will automatically adjust to maximum during dropout, where VIN is lower than the programmed VOUT.

    The maximum duty cycle depends on the set frequency and the minimum off time.

    The maximum duty cycle equation is given in section 7.4.6 of the datasheet. DMAX = 1 – Fs × 500 ns

    I advise you use smaller switching frequencies to get a better duty cycle in dropout.

    However, during dropout you must also consider resistive voltage drops from conduction of the MOSFET RDS(ON) and inductor DCR.

    The MOSFET RDSON is 330mΩ and with 1.5A it will drop 0.5V from VIN to SW, the total drop will be more when adding in duty cycle and DCR.

    Please see http://www.ti.com/lit/an/slyt293/slyt293.pdf for more information on output voltage limitations.

    Hope this helped,

    -Orlando