FAN5250 Fairchild Semiconductor, FAN5250 Datasheet - Page 14

• Manufacturer Part Number: FAN5250
• Description: Mobile Processor Core-Voltage Regulator
• Manufacturer: Fairchild Semiconductor

FAN5250
Output Capacitor Selection
The output capacitor serves two major functions in a switch-
ing power supply. Along with the inductor it filters the
sequence of pulses produced by the switcher, and it supplies
the load transient currents. The filtering requirements are a
function of the switching frequency and the ripple current
allowed, and are usually easy to satisfy in high frequency
converters.
The load transient requirements are a function of the slew
rate (di/dt) and the magnitude of the transient load current.
Modern microprocessors produce transient load rates in
excess of 10A/µs. High frequency ceramic capacitors placed
beneath the processor socket initially supply the transient
and reduce the slew rate seen by the bulk capacitors. The
bulk capacitor values are generally determined by the total
allowable ESR rather than actual capacitance requirements.
High frequency decoupling capacitors should be placed as
close to the processor power pins as physically possible.
Consult with the processor manufacturer for specific decou-
pling requirements. Use only specialized low-ESR electro-
lytic capacitors intended for switching-regulator applications
for the bulk capacitors. The bulk capacitor’s ESR will deter-
mine the output ripple voltage and the initial voltage drop
after a transient. In most cases, multiple electrolytic capaci-
tors of small case size perform better than a single large case
capacitor.
Power MOSFET Selection
For the example in the following discussion, we will be
selecting components for:
V
V
The FAN5250 converter's output voltage is very low with
respect to the input voltage, therefore the Lower MOSFET
(Q2) is conducting the full load current for most of the cycle.
Therefore, Q2 should be selected to be a MOSFET with low
R
In contrast, Q1 is on for a maximum of 20% (when V
5V) of the cycle, and its conduction loss will have less of an
impact. Q1, however, sees most of the switching losses, so
Q1’s primary selection criteria should be gate charge
(Q
14
OUT
IN
DS(ON)
G(SW)
from 5V to 20V
= 1.2V @ I
).
to minimize conduction losses.
LOAD(MAX)
= 7A
IN
=
High-Side Losses:
Assuming switching losses are about the same for both the
rising edge and falling edge, Q1’s switching losses, as can be
seen by Figure 14, are given by:
Where R
(rise or fall time) and is predominantly the sum of t2, t3
(Figure 14), a function of the impedance of the driver and the
Q
= V
driver to simplify the calculation of t
G(SW)
SP
C
ISS
V
V
we can use a constant current assumption for the
V
DS
GS
V
I
of the MOSFET. Since most of t
TH
D
DS(ON)
SP
= C
5V
Figure 14. Switching Losses and Q
t
Figure 15. Drive Equivalent Circuit
R
GS
S
P
P
P
D
UPPER
SW
COND
=
|| C
is @T
t1
-------------------- -
I
Q
DRIVER
=
C
GD
G SW
Q
ISS
(


GS
=
=
V
---------------------
J(MAX)
19
20
DS
V
--------------
P
t2
)
V
OUT
SW
2
≈
SW
HDRV
IN
×
Q
----------------------------------------------------- -


I
----------------------------------------------- -
R
+
L
G(SW)
×
and t
DRIVER
P
×
I
C
COND
2
OUT
VDD V
Q
2 t
RSS
GD
Q
t3
×
S
G
G SW
is the switching period
S
×
S
(
:
–
 F

+
R
R
DS ON
R
S
GATE
SW
)
SP
C
REV. 1.1.6 3/12/03
GATE
occurs when V
GS
(
C
t4
GD
C
)
ISS


G
t5
4.5V
VIN
(14a)
(14b)
(14c)
(15)
GS