FAN5099 Fairchild Semiconductor, FAN5099 Datasheet - Page 10

• Manufacturer Part Number: FAN5099
• Description: PWM and ULDO Controller Combo
• Manufacturer: Fairchild Semiconductor

FAN5099 Rev. 1.0.1
© 2006 Fairchild Semiconductor Corporation
V
Block Diagram
Detailed Operation Description
FAN5099 combines a high-efficiency, fixed-frequency
PWM controller designed for single-phase synchronous
buck Point-Of-Load converters with an integrated LDO
controller to support GTL-type loads. This controller is
ideally suited to deliver low-voltage, high-current power
supplies needed in desktop computers, notebooks,
workstations, and servers. The controller comes with an
integrated boot diode which helps reduce component
cost and increase space savings. With this controller, the
input to the power supply can be varied from 3V to 24V
and the output voltage can be set to regulate at 0.8V to
15V on the switcher output. The LDO output can be con-
figured to regulate between 0.8V to 3V and the input to
the LDO can be from 1.5V to 5V, respectively. An internal
shunt regulator at the V
operation from either a 5V or 12V power source.
V
FAN5099 can be configured to operate from 5V or 12V
for V
required to be connected between the supply and the
V
connected between the 12V supply and the V
shown in Figure 1. The internal shunt regulator at the V
pin is capable of sinking 150mA of current to ensure the
controller’s internal V
Choose a resistor such that:
■
■
IN
CC
CC
It is rated to handle the power dissipation.
Current sunk within the controller is minimized to
prevent IC temperature rise.
R
. When the 12V supply is used, a resistor R
ILIM
R(RAMP)
R
CC
Bias Supply
RAMP
COMP
. When 5V supply is used for V
ILIM
Vcc
SS
EN
FB
Vref
Ramp
Generator
CC
Shunt Reg
is maintained at 5.6V maximum.
CC
10µA
Enable
pin facilitates the controller
10µA
Error
Amplifier
Vcc
Internal Vcc 5.6V Max.
Summing
Amplifier
CC
, no resistor is
PWM
Comparator
Figure 21. Block Diagram
Current Limit
Comparator
OSC
CC,
VCC
CC
as
Current
Sense
Amplifier
is
10
R
S
R
The selection of R
■
■
■
■
■
Calculate R
the V
For a typical example, where:
Vin
R
PWM Section
The FAN5099’s PWM controller combines the conven-
tional voltage mode control and current sensing through
lower MOSFET R
This method of current sensing is loss-less and cost
effective. For more accurate current sense requirements,
an optional external resistor can be connected with the
bottom MOSFET in series.
R
Boot Diode
Q
VCC
VCC
Internal
VCC
Variation of the 12V supply
Sum of gate charges of top and bottom FETs (Q
Switching frequency (F
Shunt regulator minimum current (1mA)
Quiescent Current of the IC (I
MIN
PW M
CC
is calculated to be 398.65Ω.
Selection (IC)
=
= 11.5V, I
Gate Drive
.:
Adaptive
Circuit
----------------------------------------------------------------------------------------- -
(
I
Q
VCC
+
1 10
based on the minimum input voltage for
Q
•
VCC
DS_ON
= 3mA, Q
Vin
–
3
is dependent on:
+
MIN
BOOT
HDRV
PGND
SW
LDRV
Q
to generate the PWM signals.
SW
FET
–
)
FET
5.6
•
Q
= 30nC, F
F
C
V
)
BOOT
SW
IN
•
1.2
SW
)
www.fairchildsemi.com
L
O
= 300kHz,
FET
Vout
)
C