ISL6521 Просмотр технического описания (PDF) - Renesas Electronics
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ISL6521 Datasheet PDF : 14 Pages
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ISL6521
+5VIN
LIN
+
CIN
VOUT2
+
COUT2
+12V
CVCC
VCC GND
OCSET
UGATE
PHASE
DRIVE2
Q3
LGATE
COCSET
ROCSET
Q1
LOUT
VOUT1
COUT1+
Q2
CR1
VOUT3
+ COUT3
Q4
ISL6521
VOUT4
DRIVE3 DRIVE4
+
COUT4
PGND
Q5
+3.3VIN
KEY
ISLAND ON POWER PLANE LAYER
ISLAND ON CIRCUIT OR POWER PLANE LAYER
VIA CONNECTION TO GROUND PLANE
FIGURE 7. PRINTED CIRCUIT BOARD POWER PLANES AND
ISLANDS
Component Selection Guidelines
Output Capacitor Selection
The output capacitors for each output have unique
requirements. In general, the output capacitors should be
selected to meet the dynamic load regulation requirements.
Additionally, the PWM converters require an output capacitor to
filter the current ripple. The load transient for some embedded
processors requires high quality capacitors to supply the high
slew rate (di/dt) current demands.
PWM Output Capacitors
High performance embedded processors can produce
transient load rates above 1A/ns. High frequency capacitors
initially supply the 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. 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. 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.
Linear Output Capacitors
The output capacitors for the linear regulators provide dynamic
load current. The linear controllers use dominant pole
compensation integrated into the error amplifier and are
insensitive to output capacitor selection. Output capacitors
should be selected for transient load regulation.
PWM Output Inductor Selection
The PWM converter requires an output inductor. The output
inductor is selected to meet the output voltage ripple
requirements and sets the converter’s response time to a load
transient. The inductor value determines the converter’s ripple
current and the ripple voltage is a function of the ripple current.
The ripple voltage and current are approximated by the
following equations:
I = V-----I--N-F----–S----V-----O-L---U----T-- -V---V-O---I-U-N---T--
VOUT = I ESR
Increasing the value of inductance reduces the ripple current
and voltage. However, the large inductance values increase
the converter’s response time to a load transient.
One of the parameters limiting the converter’s response to a
load transient is the time required to change the inductor
current. Given a sufficiently fast control loop design, the
ISL6521 will provide either 0% or 100% duty cycle in response
to a load transient. The response time is the time interval
required to slew the inductor current from an initial current
value to the post-transient current level. During this interval the
difference between the inductor current and the transient
current level must be supplied by the output capacitor(s).
Minimizing the response time can minimize the output
capacitance required.
The response time to a transient is different for the application
of load and the removal of load. The following equations give
the approximate response time interval for application and
removal of a transient load:
tRISE = -VL---O-I--N-----–---I-V-T---R-O----AU---N-T--
tFALL = L----O----V----O--I--T-U---R-T---A----N--
FN9148 Rev 2.00
Feb 8, 2005
Page 10 of 14
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