FAN5099MTCX(2006) データシートの表示（PDF） - Fairchild Semiconductor
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FAN5099MTCX Datasheet PDF : 24 Pages
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Block Diagram
Vcc
RILIM ILIM
Internal Vcc 5.6V Max.
Shunt Reg
10μA
Current Limit
Comparator
Internal
Boot Diode
COMP
FB
Vref
Error
Amplifier
Vcc
PWM
Comparator
10μA
OSC
SS
VIN RRAMP
R(RAMP)
EN
Ramp
Generator
Enable
Summing
Amplifier
PW M
RQ
S
Adaptive
Gate Drive
Circuit
Current
Sense
Amplifier
CBOOT
BOOT
VIN
HDRV
SW
LDRV
PGND
LO
Vout
CO
Figure 21. 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 VCC pin facilitates the controller
operation from either a 5V or 12V power source.
VCC Bias Supply
FAN5099 can be configured to operate from 5V or 12V
for VCC. When 5V supply is used for VCC, no resistor is
required to be connected between the supply and the
VCC. When the 12V supply is used, a resistor RVCC is
connected between the 12V supply and the VCC, as
shown in Figure 1. The internal shunt regulator at the VCC
pin is capable of sinking 150mA of current to ensure the
controller’s internal VCC is maintained at 5.6V maximum.
Choose a resistor such that:
■ It is rated to handle the power dissipation.
■ Current sunk within the controller is minimized to
prevent IC temperature rise.
RVCC Selection (IC)
The selection of RVCC is dependent on:
■ Variation of the 12V supply
■ Sum of gate charges of top and bottom FETs (QFET)
■ Switching frequency (FSW)
■ Shunt regulator minimum current (1mA)
■ Quiescent Current of the IC (IQ)
Calculate RVCC based on the minimum input voltage for
the VCC:
RVCC = (---I--Q-----+-----1-----•----1---0--V--–--3-I-N--+-M----Q-I-N---F--–--E---5T---.-•-6----F---S----W------•----1---.--2----)
For a typical example, where:
VINMIN = 11.5V, IQ = 3mA, QFET = 30nC, FSW = 300kHz,
RVCC is calculated to be 398.65Ω.
PWM Section
The FAN5099’s PWM controller combines the conven-
tional voltage mode control and current sensing through
lower MOSFET RDS_ON to generate the PWM signals.
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.
© 2006 Fairchild Semiconductor Corporation
FAN5099 Rev. 1.1.3
10
www.fairchildsemi.com
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