ISL6558 Datasheet, PDF (13/16 Pages) Intersil Corporation – Multi-Purpose Precision Multi-Phase PWM Controller With Optional Active Voltage Positioning

ISL6558
+5V_IN
C_BP
C_C
R_C
R_OS
R_FB
C_BP
+12V
VCC PVCC
BOOT
VCC
PWM
COMP FS/EN
ISL6558
R_T
FB
VSEN
R_ISEN
ISEN
PWM
PHASE
HIP6601B
USE INDIVIDUAL METAL RUNS
FOR EACH CHANNEL TO HELP
ISOLATE OUTPUT STAGES
C_BOOT
C_IN
L_O1
V_OUT
C_OUT
ISLAND ON POWER PLANE LAYER
ISLAND ON CIRCUIT PLANE LAYER
VIA CONNECTION TO GROUND PLANE
FIGURE 9. PRINTED CIRCUIT BOARD POWER PLANES AND ISLANDS
Component Selection Guidelines
OUTPUT CAPACITOR SELECTION
Output capacitors are required to filter the output inductor
current ripple and supply the load transient current. The
filtering requirements are a function of the channel switching
frequency and the output ripple current. The load transient
requirements are a function of the slew rate (di/dt) and the
magnitude of the transient load current. These requirements
are generally met with a mix of capacitors and careful layout.
Some modern microprocessors can produce transient load
rates above 200A/µs. High frequency capacitors are used to
supply the initial transient current and slow the rate-of-change
seen by the bulk capacitors. Bulk filter capacitor values are
generally determined by the ESR 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
device for any specific decoupling requirements.
Specialized low-ESR capacitors intended for switching
regulator applications are recommended 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 edge. Aluminum electrolytic capacitor ESR
values are related to case size with lower ESR available in
larger case sizes. However, the 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. Work with your capacitor
supplier and measure the capacitor’s impedance with
frequency to select a suitable component. In most cases,
multiple electrolytic capacitors of small case size perform
better than a single large case capacitor.
OUTPUT INDUCTOR SELECTION
The output inductor is selected to meet the voltage ripple
requirements and minimize the converter response time to a
load transient. In a multi-phase converter topology, the ripple
current of one active channel partially cancels with the other
active channels to reduce the overall ripple current. The
reduction in total output ripple current results in a lower
overall output voltage ripple.
The inductor selected for the power channels determines the
channel ripple current. Increasing the value of inductance
reduces the total output ripple current and total output
voltage ripple. However, increasing the inductance value will
slow the converter response time to a load transient.
One of the parameters limiting the converter’s response time
to a load transient is the time required to slew the inductor
current from its initial current level to the transient current
level. During this interval, the difference between the two
levels must be supplied by the output capacitance.
Minimizing the response time can minimize the output
capacitance required.
The channel ripple current is approximated by the following
equation:
DI _{C H}
=
V-----I--N------–_{----V----O------U----T-- x} V-----O-----U-----T--
FSWxL
VIN
(EQ. 10)
13
FN9027.12
June 21, 2005