FAN5069 Fairchild Semiconductor, FAN5069 Datasheet - Page 10

• Manufacturer Part Number: FAN5069
• Description: The FAN5069 combines a high efficiency Pulse-Width-Modulated PWM controller and a LDO (Low DropOut) linear regulator controller
• Manufacturer: Fairchild Semiconductor

FAN5069 Rev. 1.1.5
© 2005 Fairchild Semiconductor Corporation
Block Diagram
Detailed Operation Description
FAN5069 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 VCC pin facilitates the controller
operation from either a 5V or 12V power source.
V
FAN5069 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
VCC pin is capable of sinking 150mA of current to
ensure that the controller’s internal V
5.6V maximum.
CC
V
CC
IN
. When the 12V supply is used, a resistor R
CC
R
Bias Supply
ILIM
R(RAMP)
R
. When 5V supply is used for V
RAMP
COMP
ILIM
Vcc
SS
EN
FB
Vref
Ramp
Generator
Shunt Reg
10μA
Enable
10μA
Error
Amplifier
Vcc
Internal Vcc 5.6V Max.
CC
Summing
Amplifier
CC
Σ
is maintained at
, no resistor is
PWM
Comparator
Figure 20. Block Diagram
Current Limit
Comparator
OSC
CC,
VCC
as
Current
Sense
Amplifier
is
10
R
S
Choose a resistor such that:
■
■
R
The selection of R
■
■
■
■
■
Calculate R
the V
For a typical example, where: Vin
Q
398.65Ω.
PWM Section
The FAN5069’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
FET
Internal
VCC
It is rated to handle the power dissipation.
Current sunk within the controller is minimized to
prevent IC temperature rise.
Variation of the 12V supply
Gate charge of the top and bottom FETs (Q
Switching frequency (F
Shunt regulator minimum current (1mA)
Quiescent current of the IC (I
PW M
CC
= 30nC, F
Selection (IC)
=
Gate Drive
Adaptive
:
Circuit
----------------------------------------------------------------------------------------- -
(
I
Q
VCC
+
1 10
SW
based on the minimum input voltage for
•
VCC
DS_ON
= 300KHz, R
Vin
–
3
is dependent on:
+
BOOT
PGND
MIN
HDRV
SW
LDRV
Q
to generate the PWM signals.
SW
FET
–
)
5.6
•
Q
C
V
)
F
VCC
BOOT
IN
MIN
SW
= 11.5V, I
is calculated to be
•
1.2
)
L
www.fairchildsemi.com
O
FET
Q
)
= 3mA,
(EQ. 1)
Vout
C
O