ISL6315 Datasheet, PDF (18/20 Pages) Intersil Corporation – Two-Phase Multiphase Buck PWM Controller with Integrated MOSFET Drivers

ISL6315
plane and make all critical component ground connections
with vias to this layer. Dedicate another solid layer as a power
plane and break this plane into smaller islands of common
voltage levels. Keep the metal runs from the PHASE terminal
to inductor L_OUT short. The power plane should support the
input power and output power nodes. Use 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 IC to the MOSFETs’ gates
and sources should be sized to carry at least one ampere of
current (0.02” to 0.05”).
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 a microprocessor impresses 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.
Should the load be characterized by high slew rates, attention
should be particularly paid to the selection and placement of
high-frequency decoupling capacitors (MLCCs, typically -
multi-layer ceramic capacitors). 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 capacitance requirements.
High frequency decoupling capacitors should be placed as
close to the power pins of the load, or for that reason, to any
decoupling target they are meant for, as physically possible.
Attention should be paid as 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/or measure the capacitor’s impedance with
frequency to help 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. In a multiphase converter, small inductors reduce
the response time with less impact to the total output ripple
current (as compared to single-phase converters).
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 approximated by:
I_{L, P-P}
=
-^V----^I--^N-----–----^V-----^O----^U----^T--
F_SW ⋅ L
×
V_{-----O----U----T--}
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)
FIGURE 11. RIPPLE CURRENT vs DUTY CYCLE
The current from multiple channels tend to cancel each other
and reduce the total ripple current. The total output ripple
current can be determined using the curve in Figure 11; it
provides the total ripple current as a function of duty cycle
and number of active channels, normalized to the parameter
K_NORM at zero duty cycle.
K_NORM
=
---V----^O----^U-----^T----
L ⋅ F_SW
where L is the channel inductor value.
Find the intersection of the active channel curve and duty
cycle for your particular application. The resulting ripple
current multiplier from the y-axis is then multiplied by the
normalization factor, K_NORM, to determine the total output
ripple current for the given application.
∆I_TOTAL = K_NORM ⋅ K_CM
Input Capacitor Selection
The important parameters for the bulk input capacitors are
the voltage rating and the RMS current rating. For reliable
operation, select bulk input capacitors with voltage and
current ratings above the maximum input voltage and
largest RMS current required by the circuit. The capacitor
voltage rating should be at least 1.25 times greater than the
18
FN9222.0
February 10, 2006