13
FN9003.3
February 11, 2005
copper filled polygons on the top and bottom circuit layers for
the phase nodes. Use the remaining printed circuit layers for
small signal wiring. The wiring traces from the driver IC to the
MOSFET gate and source should be sized to carry at least
one ampere of current.
Component Selection Guidelines
Output Capacitor Selection
The output capacitor is selected to meet both the dynamic
load requirements and the voltage ripple requirements. The
load transient for the microprocessor CORE is characterized
by high slew rate (di/dt) current demands. In general,
multiple high quality capacitors of different size and dielectric
are paralleled to meet the design constraints.
Modern microprocessors produce severe transient load rates.
High frequency capacitors supply the initially transient current
and slow the load rate-of-change seen by the bulk capacitors.
The bulk filter capacitor values are generally determined by
the ESR (effective series resistance) and voltage rating
requirements rather than actual capacitance requirements.
High frequency decoupling capacitors should be placed as
close to the power pins of the load as physically possible. Be
careful not to add inductance in the circuit board wiring that
could cancel the usefulness of these low inductance
components. Consult with the manufacturer of the load on
specific decoupling requirements.
Use only specialized low-ESR capacitors intended for
switching-regulator applications for the bulk capacitors. The
bulk capacitor’s ESR determines the output ripple voltage
and the initial voltage drop following a high slew-rate
transient’s edge. In most cases, multiple capacitors of small
case size perform better than a single large case capacitor.
Bulk capacitor choices include aluminum electrolytic, OS-
Con, Tantalum and even ceramic dielectrics. An aluminum
electrolytic capacitor’s ESR value is related to the case size
with lower ESR available in larger case sizes. However, the
equivalent series inductance (ESL) of these capacitors
increases with case size and can reduce the usefulness of
the capacitor to high slew-rate transient loading.
Unfortunately, ESL is not a specified parameter. Consult the
capacitor manufacturer and measure the capacitor’s
impedance with frequency to select a suitable component.
Output Inductor Selection
One of the parameters limiting the converter’s response to a
load transient is the time required to change the inductor
current. Small inductors in a multi-phase converter reduces
the response time without significant increases in total ripple
current.
The output inductor of each power channel controls the
ripple current. The control IC is stable for channel ripple
current (peak-to-peak) up to twice the average current. A
single channel’s ripple current is approximately:
The current from multiple channels tend to cancel each other
and reduce the total ripple current. Figure 12 gives the total
ripple current as a function of duty cycle, normalized to the
parameter at zero duty cycle. To determine
the total ripple current from the number of channels and the
duty cycle, multiply the y-axis value by .
Small values of output inductance can cause excessive
power dissipation. The ISL6554 is designed for stable
operation for ripple currents up to twice the load current.
However, for this condition, the RMS current is 115% above
the value shown in the following MOSFET Selection and
Considerations section. With all else fixed, decreasing the
inductance could increase the power dissipated in the
MOSFETs by 30%.
50 10010 20 200 500 1,000 5,000 10,0002,000
1
2
5
10
20
50
100
200
500
1,000
R
T
(k)
CHANNEL OSCILLATOR FREQUENCY, F
SW
(kHz)
FIGURE 10. RESISTANCE R
T
vs FREQUENCY
I
V
IN
V
OUT
–
F
SW
L
--------------------------------
V
OUT
V
IN
----------------
=
1.0
0.8
0.6
0.4
0.2
0
0
0.1 0.2 0.3 0.4 0.5
DUTY CYCLE (V
O
/V
IN
)
RIPPLE CURRENT (A
PEAK-PEAK
)
V
O
/ (L
X
F
SW
)
SINGLE
CHANNEL
2 CHANNEL
3 CHANNEL
4 CHANNEL
FIGURE 11. RIPPLE CURRENT vs DUTY CYCLE
ISL6554
