AN2928 datasheet(6/21 Pages) STMICROELECTRONICS

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Design equations for the modified buck converter

AN2928

6/21

Design equations for the modified buck converter

This section provides all the calculations required for a designer to develop an application

with the modified buck converter working in FOT and CCM. The equations are described

step-by-step, following an application design procedure. First, the basic equations for this

type of converter are shown, then the components for the tOFF time are calculated, the

proper power diode and power MOSFET is selected, and finally the power inductor

calculation is demonstrated.

2.1

Basic equations for the modified buck converter

Figure 3 shows basic circuit stage during tON and tOFF time, with indicated voltage and

component references used in the equations.

The voltage across the inductor L is calculated using the following equation:

Equation 1

VL= inductor voltage (V)

L = inductance (H)

IL = inductor current (A)

Figure 3.

Modified buck converter - theory of operation

--------

⋅

VIN

LEDs

tON

VLED

VIN

LEDs

load

VLED

VL = VLED

LEDs

load

VL = VIN - VLED

tOFF

tON time

tOFF time

AM00368

AN2928 Datasheet(PDF) 6 Page - STMicroelectronics