FAN5018BMTCX Fairchild Semiconductor, FAN5018BMTCX Datasheet - Page 15

• Manufacturer Part Number: FAN5018BMTCX
• Description: IC CTRLR DC-DC MULTIPH 28TSSOP
• Manufacturer: Fairchild Semiconductor

Specifications of FAN5018BMTCX

• Applications: Controller, High-Current, Implementing Low-Voltage CPU Core Power Circuits
• Voltage - Input: 12V
• Number Of Outputs: 1
• Voltage - Output: 0.5 ~ 3.5 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-TSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

PRODUCT SPECIFICATION
General Description
Note: The information in this section is intended to assist
users in their design and understanding of the FAN5018B
functionality. For clarity and ease of understanding, device
parameters have been included in the text. In the event there
are discrepancies between values stated in this section and
the actual specification tables, the specification tables shall
be deemed correct.
Theory of Operation
The FAN5018B combines a multi-mode, fixed-frequency
PWM control with multi-phase logic outputs for use in 2, 3
and 4 phase synchronous buck CPU core supply power con-
verters. If VID5 is pulled up to a voltage greater than
VTBLSEL, then the DAC code corresponds to VRM9.
Multi-phase operation is important for producing the high
currents and low voltages demanded by today’s micropro-
cessors. Handling the high currents in a single-phase con-
verter would place high thermal demands on the components
in the system, such as the inductors and MOSFETs. The
internal 6-bit VID DAC conforms to Intel’s VRD/VRM 10
specifications.
The multi-mode control of the FAN5018B ensures a stable,
high-performance topology for:
• Balancing currents and thermals between phases
• High speed response at the lowest possible switching
• Minimizing thermal switching losses due to lower
• Tight load line regulation and accuracy
• High current output from having up to 4 phase operation
• Reducing output ripple due to multi-phase cancellation
• PC board layout noise immunity
• Ease of use and design due to independent component
• Flexibility in operation for tailoring design to low cost or
Number of Phases
The number of operational phases and their phase relation-
ship is determined by internal circuitry which monitors the
PWM outputs. Normally, the FAN5018B operates as a 4-
phase PWM controller. Grounding the PWM4 pin programs
a 3-phase operation; grounding the PWM3 and PWM4 pins
programs a 2-phase operation.
When the FAN5018B is initially enabled, the controller out-
puts a voltage on PWM3 and PWM4 of approximately
550mV. An internal comparator checks each pin’s voltage
versus a threshold of 400mV. If the pin is grounded, then it
will be below the threshold and the phase will be disabled.
The output impedance of the PWM pin is approximately
5k
PWM pin should not be less than 25k
operation. The phase detection is made prior to starting nor-
REV. 1.0.0 Jul/15/05
Ω
frequency and output decoupling
frequency operation
selection
high performance
. Any external pull-down resistance connected to the
Ω
to ensure proper
mal operation. After this time, if the PWM output was not
grounded, then it will operate normally. If the PWM output
was grounded, then it will remain off.
The PWM outputs become logic-level output devices once
normal operation starts, and are intended for driving external
gate drivers. Since each phase is monitored independently,
operation approaching 100% duty cycle is possible. Also,
more than one output can be on at a time for overlapping
phases.
Master Clock Frequency
The clock frequency of the FAN5018B is set with an exter-
nal resistor connected from the RT pin to ground. The fre-
quency follows the graph shown in TPC 1. To determine the
frequency per phase, the clock is divided by the number of
phases in use. If PWM4 is grounded, then divide the master
clock by 3 and if both PWM3 and 4 are grounded, then
divide by 2. If all phases are in use, divide by 4.
Output Voltage Differential Sensing
The FAN5018B provides a high accuracy VID DAC and
error-amplifier to maintain a ±14 mV output setpoint toler-
ance over temperature. Output voltage is differentially
sensed between the FB and FBRTN pins. FB should be con-
nected through a resistor to the regulation point, usually the
remote sense pin of the microprocessor. FBRTN should be
connected directly to the remote sense ground point. The
internal VID DAC and precision reference are referenced to
FBRTN, which has a typical current of 150µA, to allow
accurate remote sensing. The internal error amplifier com-
pares the output of the DAC to the FB pin to regulate the out-
put voltage.
Output Current Sensing
The FAN5018B provides a dedicated current sense amplifier
(CSA) to monitor the total output current for proper voltage
positioning versus load current and for current limit detec-
tion. Sensing the load current at the output gives the total
average current being delivered to the load, which is an
inherently more accurate method than peak current detection
or sampling the current across a sense element such as the
low side MOSFET. There are several ways of configuring
this amplifier depending on the objectives of the system:
• Output inductor DCR sensing without thermistor for
• Output inductor DCR sensing with thermistor for
• Sense resistors for highest accuracy measurements
The positive input of the CSA is connected to the CSREF
pin, which is connected to the output voltage. The inputs to
the amplifier are summed together through resistors from the
sensing element (such as the switch node side of the output
inductors) to the inverting input, CSSUM. The feedback
resistor between CSCOMP and CSSUM sets the gain of the
lowest cost
improved accuracy with tracking of inductor temperature
FAN5018B
15