FAN5231 Datasheet, PDF (13/17 Pages) Fairchild Semiconductor – Precision Dual PWM Controller And Linear Regulator for Notebook CPUs

FAN5231
Locate small critical components like soft start capacitor and
current sense resistors as close, as possible to the respective
pins of the IC.
high frequency de coupling for the Pentium Pro processor to
be composed of at least fourty 1uF ceramic capacitors in the
1206 surface-mount package.
Design Procedure and Component
Selection Guidelines
As an initial step, define operating voltage range, maximum
load current in performance mode,
Output Capacitor Selection
An output capacitor serves two major functions in a switch-
ing power supply. Along with an inductor it filters the
sequence of pulses produced by the switcher and supply the
load transient currents. The filtering requirements are a func-
tion 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. For example, Intel recommends that the
Use only specialized low-ESR electrolytic capacitors
intended for switching-regulator applications for the bulk
capacitors. The bulk capacitor’s ESR will determine the out-
put ripple voltage and the initial voltage drop after a tran-
sient. In most cases, multiple electrolytic capacitors of small
case size perform better than a single large case capacitor.
Output Inductor Selection
The minimum practical output inductor value is the one that
keeps inductor current just on the boundary of continuous
conduction at some minimum load. The industry standard
practice is to choose the minimum current some where from
10% to 25% of the nominal current. At light load, FAN5231
PWM controllers switch to a hysteretic mode of operation to
sustain high efficiency operation. It is suggested that transi-
tion to the hysteretic mode occurred before inductor current
becomes discontinuous. Following equations help to choose
proper value of the output filter inductor..
∆I = 2 ⋅ I_min
∆I = ^∆---_E--^V--_S--^o--_R-^u---^t
L = ^V-----^iF--ⁿ--_s--–--_×---^V--_∆--^o--_I-^u---^t × ^V--_V---^o-_i--^un---^t
Table 2.
Component
Maximum CPU Current
Inductor
Output Capacitor
High-Side MOSFET
Low-Side MOSFET
Current-Input Resistor for ~6%
Droop @ Vo=1.6V
Circuit 1
8.0A
2.0µH
Panasonic
ETQP6F2R0BFA
3x270µF
Panasonic
EEFUE0D271R
or
Sanyo
4x2R5TPC220M
FDS6690A
2x
FDS6670A
1.27kΩ
Circuit 2
12.0A
1.0µH
Panasonic
ETQP6F2R0BFA
5x270µF
Panasonic
EEFUE0D271R
or
Sanyo 6x2R5TPC220M
FDS6690A
2x
FDS6670A
1.00kΩ
Circuit 3
18.0A
0.8µH
Panasonic
ETQP6F2R0BFA
6x270µF
Panasonic
EEFUE0D271R
2x
FDS6690A
2x
FDS6670A
1.50kΩ
REV. 1.1.1 8/15/01
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