ISL6529 Intersil Corporation, ISL6529 Datasheet - Page 10

• Manufacturer Part Number: ISL6529
• Description: Dual Regulator.Synchronous Rectified Buck PWM and Linear Power Controller
• Manufacturer: Intersil Corporation

plane and make all critical component ground connections
through 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 the output inductor short. The power
plane should support the input and output power nodes. Use
copper filled polygons on the top and bottom circuit layers for
the phase node. Use the remaining printed circuit layers for
small signal wiring. The wiring traces from the UGATE pin to
the MOSFET gate should be kept short and wide enough to
easily handle the 1A of drive current.
The switching components should be placed close to the
ISL6529 first. Minimize the length of the connections
between the input capacitors, C
by placing them nearby. Position both the ceramic and bulk
input capacitors as close to the upper MOSFET drain as
possible. Position the output inductor and output capacitors
between the upper MOSFET and lower diode and the load.
The critical small signal components include any bypass
capacitors, feedback components, and compensation
FIGURE 8. PRINTED CIRCUIT BOARD POWER PLANES
+3.3 V
ISL6529
IN
KEY
DRIVE2
LGATE
UGATE
AND ISLANDS
12VCC
PGND
COMP
5VCC
GND
VIA CONNECTION TO GROUND PLANE
FB2
ISLAND ON POWER PLANE LAYER
ISLAND ON CIRCUIT PLANE LAYER
FB
C
C
+12 VCC
BP
BP
+5 VCC
R
C
2
2
PHASE
R6
R4
10
R5
C
IN
1
C
, and the power switches
3
R1
Q1
Q2
+3.3 V
R
3
C
C
L
OUT2
Q3
OUT
IN
IN
C
OUT1
V
OUT2
V
OUT1
ISL6529
components. Position the bypass capacitors, C
the VCC pin with a via directly to the ground plane. Place the
PWM converter compensation components close to the FB
and COMP pins. The feedback resistors for both regulators
should also be located as close as possible to the relevant
FB pin with vias tied straight to the ground plane as required.
Component Selection Guidelines
Output Capacitor Selection
Output capacitors are required to filter the output and supply
the load transient current. The filtering requirements are a
function of switching frequency and output current ripple.
The load transient requirements are a function of the
transient load current slew rate (di/dt) and magnitude. These
requirements are generally met with a mix of capacitors and
careful layout.
PWM Regulator Output Capacitors
Modern digital ICs can produce high transient load slew
rates. High frequency capacitors initially supply the transient
current and slow the load rate-of-change seen by the bulk
capacitors. The bulk filter capacitor selection is generally
determined by the effective series resistance (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 on
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, tantalum,
and special polymer capacitor ESR values are 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.
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 response time to a load
transient. The inductor value determines the converter’s
ripple current and the ripple voltage is also a function of the
BP
, close to