FAN5234QSC_Q Fairchild Semiconductor, FAN5234QSC_Q Datasheet - Page 12

• Manufacturer Part Number: FAN5234QSC_Q
• Description: Switching Converters, Regulators & Controllers PWM Controller
• Manufacturer: Fairchild Semiconductor

Specifications of FAN5234QSC_Q

• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

© 2004 Fairchild Semiconductor Corporation
FAN5234 • Rev. 2.0.0
High-Side Losses
Figure 9 shows a MOSFET's switching interval, with the
upper graph being the voltage and current on the drain-
to-source and the lower graph detailing V
with a constant current charging the gate. The x-axis
therefore is also representative of gate charge (Q
C
C
(as V
the lower graph are either specified or can be derived
from MOSFET datasheets.
Assuming switching losses are about the same for both
the rising edge and falling edge, Q1's switching losses,
occur during the shaded time when the MOSFET has
voltage across it and current through it.
These losses are given by:
P
where:
P
P
given MOSFET. R
temperature (T
time) and is t2+t3 in Figure 9.
P
P
UPPER
UPPER
COND
ISS
GD
COND
SW
receives the current from the gate driver during t3
V
= C
V
DS
=
V
=
DS
V
I
TH
GS
are the switching and conduction losses for a
D
= P
⎛
⎜
⎜
⎝
is the upper MOSFET's total losses and P
SP
⎛
⎜
⎜
⎝
is falling). The gate charge (Q
V
V
GD
Figure 9. Switching Losses and Q
V
5V
Figure 10. Drive Equivalent Circuit
OUT
DS
IN
SW
R
+ C
2
D
×
⎞
⎟
⎟
⎠
+ P
I
×
J
L
t1
GS
I
). t
C
OUT
COND
×
Q
, and it controls t1, t2, and t4 timing.
ISS
2
S
GS
2
DS(ON)
×
is the switching period (rise or fall
×
(C
t
t2
R
s
ISS
DS
⎞
⎟
⎟
⎠
SW
HDRV
Q
f
(
is at the maximum junction
ON
= C
SW
G(SW)
)
C
GS
Q
GD
GD
t3
|| C
G
GD
R
)
G
GATE
) parameters on
C
t4
GS
C
C
GD
ISS
GS
G
vs. time
t5
4.5V
SW
V
IN
(21)
(22)
(20)
and
G
).
12
The driver’s impedance and C
period is controlled by the driver's impedance and Q
Since most of t
current assumption for the driver to simplify the
calculation of t
Most MOSFET vendors specify Q
can be determined as:
Q
where Q
MOSFET to its threshold (V
For the high-side MOSFET, V
high as 20V in a typical portable application. Care
should be taken to include the delivery of the
MOSFET's gate power (P
dissipation required for the FAN5234:
where Q
Low-Side Losses
Q2 switches on or off with its parallel Schottky diode
conducting;
proportional to V
and Q2 is selected based on R
Conduction losses for Q2 are given by:
where R
highest operating junction temperature and
Since D
a conservative result, simplifying the calculation.
The maximum power dissipation (P
of the maximum allowable die temperature of the low-
side MOSFET, the Θ
ambient temperature rise:
Θ
can be devoted to heat sinking (see AN-1029 —
Maximum Power Enhancement Techniques for SO-8
Power MOSFET for MOSFET thermal information) .
P
P
D =
P
t
s
G(SW)
JA
G
COND
D
ATE
(
=
MAX
depends primarily on the amount of PCB area that
V
Q
I
V
DRIVER
OUT
=
)
=
= Q
IN
G
MIN
=
(
Q
(
G
SW
1
DS(ON)
TH
G
T
−
is the total gate charge to reach V
GD
<20% for portable computers, (1-D)≈1 produces
)
is the minimum duty cycle for the converter.
( J
×
D
MAX
=
is the gate charge required to get the
V
)
+ Q
×
therefore,
S
⎛
⎜
⎜
⎝
CC
S
:
I
is the R
)
Θ
R
OUT
occurs when V
DS
GS
−
×
JA
DRIVER
, Q2's switching losses are negligible
T
V
f
2
SW
– Q
A
Q
CC
×
(
MAX
R
G
JA
−
(
TH
DS
SW
+
, and the maximum allowable
DS(ON)
)
V
R
(
GATE
ON
)
V
SP
TH
GATE
DS
)
).
DS
) in calculating the power
≈ 0 .5V.
ISS
DS(ON)
of the MOSFET at the
GS
⎞
⎟
⎟
⎠
= V
determine t2 while t3’s
= V
GD
D(MAX)
IN
only.
SP
, which can be as
and Q
Since
, use a constant
) is a function
CC
www.fairchildsemi.com
GS
.
P
. Q
SW
G(SW)
(24)
(23)
(25)
(26)
(27)
GD
is
.