FAN6520B Fairchild Semiconductor, FAN6520B Datasheet - Page 11

• Manufacturer Part Number: FAN6520B
• Description: Single Synchronous Buck PWM Controller
• Manufacturer: Fairchild Semiconductor

FAN6520B Rev. 1.0.3
Most MOSFET vendors specify Q
can be determined as: Q
where Q
FET to its threshold (V
V
table application. Care should also be taken to include
the delivery of the MOSFET’s gate power (P
culating
FAN6520B:
where Q
Low-Side Losses
Q2, however, switches on or off with its parallel shottky
diode conducting, therefore V
proportional to V
and we can select Q2 based on R
DS
P
t
s
GATE
= V
-------------------- -
I
Q
IN
DRIVER
G
TH
G SW
= Q
, which can be as high as 20V in a typical por-
the
is the total gate charge to reach VCC.
is the gate charge required to get the MOS-
G
power
DS
VCC
----------------------------------------------------- -
----------------------------------------------- -
R
, Q2’s switching losses are negligible
DRIVER
VCC V
TH
dissipation
F
Q
SW
). For the high-side MOSFET,
G SW
G(SW)
–
+
R
DS
SP
GATE
= Q
DS(ON)
GD
0.5V. Since P
required
GD
and Q
only.
+ Q
GATE
GS
GS
. Q
for
) in cal-
– Q
SW
G(SW)
(16)
(17)
the
TH
is
11
Conduction losses for Q2 are given by:
where R
highest operating junction temperature and
Since D
produces a conservative result, further simplifying the
calculation.
The maximum power dissipation (P
of the maximum allowable die temperature of the low-
side MOSFET, the
ambient temperature rise:
can be devoted to heat sinking (see Fairchild app note
AN-1029 for SO-8 MOSFET thermal information).
D
J-A
P
=
, depends primarily on the amount of PCB area that
P
COND
D MAX
V
--------------
V
OUT
MIN
IN
DS(ON)
= (1-D)
is the minimum duty cycle for the converter.
< 20% for portable computers, (1-D)
=
T
------------------------------------------------ -
is the R
J MAX
I
OUT
J-A
J A
2
, and the maximum allowable
–
DS(ON)
–
T
R
A MAX
DS(ON)
of the MOSFET at the
D(MAX
) ) is a function
www.fairchildsemi.com
(19)
(18)
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