FN6288 Rev 5.00

Page 21 of 22

Oct 7, 2008

ISL6540A

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the largest component of power dissipation for both the

upper and the lower MOSFETs. These losses are distributed

between the two MOSFETs according to duty factor (see the

equations below). The upper MOSFET exhibits turn-on and

turn-off switching losses as well as the reverse recover loss,

while the synchronous rectifier exhibits body-diode

conduction losses during the leading and trailing edge dead

times.

These equations do not include the gate-charge losses that

are proportional to the total gate charge and the switching

frequency and partially dissipated by the internal gate

resistance of the MOSFETs. Ensure that both MOSFETs are

within their maximum junction temperature at high ambient

temperature by calculating the temperature rise according to

package thermal-resistance specifications. A separate

heatsink may be necessary depending upon MOSFET

power, package type, ambient temperature and air flow.

2

I

12

--------

+



,L

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



1D

–

 P

DEAD

+



=

I



12

------

+



 V

DT





I



12

------

–



 V

DL





+



=

I



12

------

+



 t

OFF



I



12

------

–



 t

ON



+

VIN F



S



=

2

I

12

--------

+



,U

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



++



=



S



=

(EQ. 22)