AN2928 datasheet(12/21 Pages) STMICROELECTRONICS

12 / 21 page

Design equations for the modified buck converter

AN2928

12/21

Figure 4.

Sawtooth signal

Equation 24

IRMS = root-mean squared drain current (A)

IPP = peak-to-peak current (A) (difference between I_MAX and I_MIN)

The real drain current waveform is given in Figure 5. As can be observed, the signal is quite

similar to the signal in Figure 4, except that there is no current during the off-time stage

(IO = 0) and the on and off-times are reversed. Therefore, for real current Equation 24 is

modified to become Equation 25.

Square RMS drain current is calculated using the equation:

Equation 25

IAVR_ON = average current during tON time (A) (see Figure 5).

And finally it is possible to calculate the continuous conduction losses:

Equation 26

RDS(ON) = static drain-source on resistance for working MOSFET temperature (Ω).

AM00369

lAVG

IPP

i(t)

(1- D)T

lMAX

tON

tOFF

lMIN

RM S

---

() t

---

() ti

() t··

∫

–

() ⋅

⋅

MAX

MI N

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

⎝⎠

⎛⎞

--------

∫

RM S

MAX

MI N

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

⎝⎠

⎛⎞

--------

AVR ON

--------

⋅⋅

CON

RM S

DS ON

()

⋅

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