FREQ datasheet(36/56 Pages) FUJITSU

36 / 56 page

MB39A214A

Limitation : development tool vendor use only

FUJITSU SEMICONDUCTOR CONFIDENTIAL

DS405-00007-1v0-E

Selection of fly-back diode

This device is improved by adding the fly-back diode when the conversion efficiency improvement or the

suppression of the low-side FET fever is desired, although those are unnecessary to execute normally. The

effect is achieved in the condition where the switching frequency is high or output voltage is lower. Select

schottky barrier diode (SBD) that the forward current is as small as possible. In this DC/DC control IC, the

period for the electric current flow into fly-back diode is limited to dead time period because the

synchronous rectification system is adopted. (as for the dead time, see “Output Block” in “ELECTRICAL

CHARACTERISTICS”). Each rating for the fly-back diode can be calculated by the following formula.

ID ≥ IOUT_MAX fOSC

(tD1 + tD2)

: Forward current rating of SBD [A]

IOUT_MAX : Maximum load current [A]

fOSC

: Switching frequency [Hz]

tD1, tD2

: Dead time [s]

ΔIL

IFSM ≥ IOUT_MAX +

IFSM

: Peak forward surge current ratings of SBD [A]

IOUT_MAX : Maximum load current [A]

ΔIL

: Ripple current peak-to-peak value of inductor [A]

VR_Fly > VIN

VR_Fly

: Reverse voltage of fly-back diode direct current [V]

VIN

: Power supply voltage [V]

FREQ Datasheet(PDF) 36 Page - fujitsu semiconductor