FAN5240 Datasheet, PDF (14/19 Pages) Fairchild Semiconductor – Multi-Phase PWM Controller for AMD Mobile Athlon TM and Duron TM

FAN5240
PRODUCT SPECIFICATION
Design and Component Selection
Guidelines
As an initial step, define operating voltage range and mini-
mum and maximum load currents for the controller. For this
discussion,
IOUT Max
VIN
VOUT
25A
5.5 to 21 V
0.925 to 2 V
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 ripple current to be somewhere from
15% to 35% of the nominal current. At light load, the ripple
current also determines the point where the converter will
automatically switch to hysteretic mode of operation (IMIN)
to sustain high efficiency. The following equations help to
choose the proper value of the output filter inductor.
∆I
=
2 × I_MIN
=
-^∆----^V----^O----^U---^T-- ,
ESR
where ∆I is the inductor ripple current, which we will choose
for 20% of the full load current (12.5A in each phase) and
∆VOUT is the maximum output ripple voltage allowed.
L = ^V----_F-^I-^N--_S---_W^–-----V-_×---^O--_∆--^U--_I--^T- × -^V--_V--^O---_I-^UN---^T--
(13)
for this example we’ll use:
VIN = 20V, VOUT = 1.5V
∆I = 20% *12.5A (per phase) = 2.5A
FSW = 300KHz.
Therefore,
L ≈ 1.8µH
The inductor's current rating should be chosen per the
ILIMIT calculated above. Some transient currents over the
inductor current rating may be tolerable if the inductor’s
saturation
characteristic


d_-d---L_{-I-}
is
sufficiently
“soft”.
Output Capacitor Selection
The output capacitor serves two major functions in a switch-
ing power supply. Along with the inductor it filters the
sequence of pulses produced by the switcher, and it supplies
the load transient currents. The filtering requirements are a
function 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
decoupling requirements. Use only specialized low-ESR
electrolytic capacitors intended for switching-regulator
applications for the bulk capacitors. The bulk capacitor’s
ESR will determine the output ripple voltage and the initial
voltage drop after a transient. In most cases, multiple electro-
lytic capacitors of small case size perform better than a
single large case capacitor.
Input Capacitor Selection
The input capacitor should be selected by its ripple current
rating. For a 2 phase converter, the RMS currents is calcu-
lated:
I_RMS = ^I--^P-₂---^K- 2D – 4D²
(14)
This equation produces the worst case value at maximum
duty cycle. For our example, that occurs when VIN = 5.5V
and VOUT = 2V. For 25A maximum output the maximum
RMS current at CIN:
I_RMS(MAX) = 5.6A
Power MOSFET Selection
For the example in the following discussion, we will be
selecting components for:
VIN from 5V to 20V
VOUT = 1.5V @ ILOAD(MAX) = 12.5A/phase
The FAN5240 converter’s output voltage is very low with
respect to the input voltage, therefore the Lower MOSFET
(Q2) is conducting the full load current for most of the cycle.
Therefore, Q2 should be selected to be a MOSFET with low
RDS(ON) to minimize conduction losses.
In contrast, Q1 is on for a maximum of 20% (when VIN =
5V) of the cycle, and its conduction loss will have less of an
impact. Q1, however, sees most of the switching losses, so
Q1’s primary selection criteria should be gate charge
(QG(SW)).
14
REV. 1.1.7 8/29/02