FAN6520A_06 Datasheet, PDF (12/15 Pages) Fairchild Semiconductor – Single Synchronous Buck PWM Controller

Low-Side Losses
Q2, however, switches on or off with its parallel Shottky
diode conducting, therefore V_DS Ý 0.5V. Since P_SW is
proportional to V_DS, Q2’s switching losses are negligible
and Q2 can be selected based on R_DS(ON) only.
Conduction losses for Q2 are given by:
P_COND = (1-D) × I_OUT2 × R_DS(ON)
(29)
where R_DS(ON) is the R_DS(ON) of the MOSFET at the
highest operating junction temperature and
D = ^{V-----O-----U-----T--} is the minimum duty cycle for the converter.
Since
VIN
D_MIN
<
20%
for
portable
computers,
(1-D)
≈
1
pro-
duces a conservative result, simplifying the calculation.
The maximum power dissipation (P_D(MAX) ) is a function
of the maximum allowable die temperature of the low-
side MOSFET, the θ_J-A, and the maximum allowable
ambient temperature rise:
PD_(MAX) = ^T----J----(--M------A----X--_θ--⁾-J---^–-_–---T--A--A-----⁽--M-----A-----X----⁾-
(30)
depends primarily on the amount of PCB area that can
be devoted to heat sinking. Refer to Fairchild Application
Note AN-1029 Maximum Power Enhancement Tech-
niques for SO-8 Power MOSFETs at http://www.fairchild-
semi.com/an/AN/AN-1029.pdf .
©2005 Fairchild Semiconductor Corporation
FAN6520A Rev. 1.0.5
12
www.fairchildsemi.com