AN4501 datasheet(42/48 Pages) STMICROELECTRONICS

42 / 48 page

Calculation details

AN4501

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DocID026394 Rev 1

In steady state, by definition, the current starts at each switching cycle at the same value,

which means that the current increase during tON is exactly compensated the current

decrease during tOFF.

This property is known as the volt-second balance. As a consequence, on the inductor

voltage waveform (see Figure 21), the area under VL in during tON is equal to the area

above VL during tOFF.

Figure 21. Typical inductor waveforms in continuous conduction mode

Neglecting VSW and VF in the case of an ideal converter (without power loss and 100%

efficiency) we have:

Extracting the duty cycle in the last equation:

ΔI

LO FF

--------------------

OFF

OU T

–

FDIO DE

–

---------------------------------------------------------------

OFF

OU T

–

FDIODE

–

---------------------------------------------------------------

–

-------------

ΔI

LO N

ΔI

LO FF

LO N

LO FF

OFF

–

() D

OUT

–

()

–

()

OUT

–

()

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