FAN5234QSCX_NL Fairchild Semiconductor, FAN5234QSCX_NL Datasheet - Page 11

• Manufacturer Part Number: FAN5234QSCX_NL
• Manufacturer: Fairchild Semiconductor

Specifications of FAN5234QSCX_NL

• Dc To Dc Converter Type: Synchronous Buck Controller/PWM DC to DC Controller
• Number Of Outputs: 1
• Pin Count: 16
• Input Voltage: 5 to 24V
• Output Voltage: 0.9 to 5.5V
• Package Type: QSOP
• Mounting: Surface Mount
• Operating Temperature Classification: Commercial
• Operating Temperature (min): -10C
• Operating Temperature (max): 85C
• Lead Free Status / Rohs Status: Compliant

FAN5234
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
P
P
MOSFET. R
(T
(Figure 8).
The driver’s impedance and C
period is controlled by the driver's impedance and Q
Since most of t
constant current assumption for the driver to simplify the
calculation of t
REV. 1.0.10 5/3/04
UPPER
UPPER
COND
J
). t
C
P
P
COND
SW
ISS
V
S
V
is the upper MOSFET's total losses, and P
V
DS
GS
= P
V
is the switching period (rise or fall time) and is t2+t3
I
are the switching and conduction losses for a given
TH
D
SP
= C
=
SW
=


5V
GS
Figure 8. Switching losses and Q
DS(ON)
Figure 9. Drive Equivalent Circuitt
V
--------------------- -
R
+ P


DS
S
D
S
V
--------------
|| C
V
:
2
occurs when V
COND
OUT
t1
×
IN
C
GD
Q
I
is at the maximum junction temperature
L
ISS
GS


×
×
2
where:
t2
I
OUT
×
SW
HDRV
t
Q
S
2
 F

G(SW)
×
ISS
SW
R
GS
C
Q
DS ON
RSS
determine t2 while t3’s
GD
t3
= V
G
(
SP
R
)
GATE
C
we can use a
GS
C
t4
GD
C
ISS
G
SW
VIN
t5
4.5V
and
GD
(15a)
(15b)
.
Most MOSFET vendors specify Q
be determined as: Q
the gate charge required to get the MOSFET to it's threshold
(V
be as high as 20V in a typical portable application. Care
should also be taken to include the delivery of the
MOSFET's gate power (P
dissipation required for the FAN5234:
where Q
Low-Side Losses
Q2, however, switches on or off with its parallel shottky
diode conducting, therefore V
portional to V
we can select Q2 based on R
Conduction losses for Q2 are given by::
where R
operating junction temperature and
Since D
produces a conservative result, further simplifying the
calculation.
The maximum power dissipation (P
the maximum allowable die temperature of the low-side
MOSFET, the θ
temperature rise:
θ
be devoted to heat sinking (see FSC app note AN-1029 for
SO-8 MOSFET thermal information).
D
J-A
TH
P
=
, depends primarily on the amount of PCB area that can
P
GATE
t
). For the high-side MOSFET, V
S
D MAX
V
--------------
P
(
V
=
COND
MIN
OUT
DS(ON)
G
IN
= Q
-------------------- -
I
Q
is the total gate charge to reach VCC.
DRIVER
G SW
)
< 20% for portable computers, (1-D) ≈ 1
is the minimum duty cycle for the converter.
G
(
DS
=
=
× VCC × F
is the R
J-A
T
------------------------------------------------- -
(
, Q2's switching losses are negligible and
)
1 D
J MAX
≈
, and the maximum allowable ambient
(
–
G(SW)
---------------------------------------------------- -


---------------------------------------------- -
R
) I
DS(ON)
DRIVER
θ
)
×
J A
VCC V
SW
–
GATE
–
= Q
Q
OUT
T
A MAX
G SW
DS(ON)
(
GD
(
DS
2
of the MOSFET at the highest
–
+
) in calculating the power
×
R
≈ 0.5V. Since P
+ Q
R
)
SP
GATE
GD
PRODUCT SPECIFICATION
)
DS ON
D(MAX)
only.
GS
(
DS
and Q


– Q
= VIN, which can
)
) is a function of
TH
GS
. Q
where Q
SW
G(SW)
is pro-
(16)
(18)
TH
(19)
(17)
can
11
is