• State TI Thinks Resolved
  • Locked Locked
  • Replies 1 reply
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 331 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

LMZ14201: Is DCM Boundary Equation Correct? (Equation 18)

ErnestHanks_55
Intellectual 460 points
Part Number: LMZ14201

I'm designing in 2x LMZ14201H for +24<->+15 (and +24<->-15) rail conversions.

Equation 18 calculates the DCM & CCM boundary conditions... However the resulting values seem exceptionally high. My experiences with DCM in the past have it switch modes at low currents...

Here are my parameters:

Vin = 24V

Vo = 15V

Fsw = 300kHz (Ron=383kOhm)

Equation 18: IDCB = Vo*(Vin-Vo) / (2*15uH * fsw * Vin).

This yields IDCB = 625mA, on only a 1A Max supply!

This switcher operates in DCM up until ~62% of load current.

This seems quite high... Is there an error in the datasheet?

  • 0
    TI__Mastermind 18940 points

    Hello Ernest, 

    Modules have a fixed inductance value which has to handle a wide range of input and output conditions and switching frequencies. The choice of inductance is usually a tradeoff between size, conversion efficiency, vin and vout range, and switching frequency range. 

    With 300kHz of switching frequency, 15uH fixed internal inductance and 24Vin to 15Vout conversion, the DCM boundary will be at around 625mA as indicated by the equation. 

    Is the DCM boundary an issue for your application?

    The knob you can turn is the switching frequency. You can increase the switching frequency (through the Ron resistor) and shift the DCM boundary to a lower current point. The tradeoff would be higher power dissipation with the higher switching frequency. 

    Cheers, 
    Denislav