FAN5236MTCX View Datasheet(PDF) - Fairchild Semiconductor
Part Name
Description
Manufacturer
FAN5236MTCX Datasheet PDF : 20 Pages
| |||
PRODUCT SPECIFICATION
FAN5236
These losses are given by:
PUPPER = PSW + PCOND
PSW
=
-V----D----S----×-----I--L-
2
×
2
×
tS
FSW
(19a)
PCOND
=
V-----O----U----T-
VIN
×
IO
U
2
T
×
RDS(ON)
(19b)
PUPPER is the upper MOSFET’s total losses, and PSW and
PCOND are the switching and conduction losses for a given
MOSFET. RDS(ON) is at the maximum junction temperature
(TJ). tS is the switching period (rise or fall time) and is t2+t3
Figure 15.
The driver’s impedance and CISS determine t2 while t3’s
period is controlled by the driver’s impedance and QGD.
Since most of tS occurs when VGS = VSP we can use a
constant current assumption for the driver to simplify the
calculation of tS:
VDS
C ISS
C RSS
C ISS
ID
VGS
QGS
QGD
VSP
VTH
QG(SW)
t1
t2
t3
CISS = CGS || CGD
4.5V
t4
t5
Figure 16. Switching losses and QG
5V
RD
VIN
HDRV
SW
C GD
RGATE
G
CGS
tS
=
-Q-----G----(--S---W----)- ≈ -----------------Q-----G----(--S---W----)-----------------
IDRIVER
-R----D---R--V--I--VC----E-C--R---–-+----V-R---S--G-P--A----T---E--
(20)
Most MOSFET vendors specify QGD and QGS. QG(SW) can
be determined as: QG(SW) = QGD + QGS – QTH where QTH is
the the gate charge required to get the MOSFET to it’s
threshold (VTH). For the high-side MOSFET, VDS = VIN,
which can be as high as 20V in a typical portable applica-
tion. Care should also be taken to include the delivery of the
MOSFET’s gate power (PGATE ) in calculating the power
dissipation required for the FAN5236:
PGATE = QG × VCC × FSW
(21)
where QG is the total gate charge to reach VCC.
Low-Side Losses
Q2, however, switches on or off with its parallel shottky
diode conducting, therefore VDS ≈ 0.5V. Since PSW is
proportional to VDS , Q2’s switching losses are negligible
and we can select Q2 based on RDS(ON) only.
Conduction losses for Q2 are given by:
PCOND
=
(1
–
D)
×
IO
U
2
T
×
RDS(ON)
(22)
where RDS(ON) is the RDS(ON) of the MOSFET at the highest
operating junction temperature and
D
=
V-----O----U----T-
VIN
is the minimum duty cycle for the converter.
Since DMIN < 20% for portable computers, (1-D) ≈ 1
produces a conservative result, further simplifying the
calculation.
The maximum power dissipation (PD(MAX) ) is a function of
the maximum allowable die temperature of the low-side
MOSFET, the θJ-A, and the maximum allowable ambient
temperature rise:
PD(MAX) = T----J---(-M-----A----X--θ-)---J–--–---T-A---A----(--M----A----X---)
(23)
θJ-A, depends primarily on the amount of PCB area that can
be devoted to heat sinking (see FSC app note AN-1029 for
SO-8 MOSFET thermal information).
Figure 17. Drive Equivalent Circuit
16
REV. 1.1.9 7/12/04
Share Link: 