isl8540 Intersil Corporation, isl8540 Datasheet - Page 15

• Manufacturer Part Number: isl8540
• Description: Dc/dc Power Switching Regulator
• Manufacturer: Intersil Corporation

between 100kHz and 600kHz, the resulting current
transitions from one device to another cause voltage spikes
across the interconnecting impedances and parasitic circuit
elements. These voltage spikes can degrade efficiency,
radiate noise into the circuit, and lead to device overvoltage
stress. Careful component layout and printed circuit board
design minimizes these voltage spikes.
As an example, consider the turn-off transition of the control
MOSFET. Prior to turn-off, the MOSFET is carrying the full
load current. During turn-off, current stops flowing in the
MOSFET and is picked up by the freewheeling Schottky
diode. Any parasitic inductance in the switched current path
generates a large voltage spike during the switching interval.
Careful component selection, tight layout of the critical
components, and short, wide traces minimizes the
magnitude of voltage spikes.
There are two sets of critical components in the ISL8540
switching converter. The switching components are the most
critical because they switch large amounts of energy, and
therefore tend to generate large amounts of noise. Next are
the small signal components which connect to sensitive
nodes or supply critical bypass current and signal coupling.
FIGURE 30. PRINTED CIRCUIT BOARD POWER PLANES AND
C
BP2
KEY
VCC5
PGND
ISLAND ON POWER PLANE LAYER
ISLAND ON CIRCUIT AND/OR POWER PLANE LAYER
VIA CONNECTION TO GROUND PLANE
ISLANDS
GND PAD
ISL8540
COMP
VIN
FB
LX
15
V
IN
R
C
D
4
6
R
3
C
L
IN
C
C
10
OUT1
C
7
R
2
R
V
OUT1
6
ISL8540
A multi-layer printed circuit board is recommended.
Figure 30 shows the connections of the critical components
in the converter. Note that capacitors C
each represent numerous physical capacitors. Dedicate one
solid layer, usually a middle layer of the PC board, for a
ground 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 LX terminals to the output inductor 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 LX nodes. Use the remaining printed
circuit layers for small signal wiring.
In order to dissipate heat generated by the internal LDO and
other power components, the ground pad at the bottom of
the device should be connected to the ground plane through
at least nine vias. This allows the heat to move away from
the IC and also ties the pad to the ground plane through a
low impedance path.
The switching components should be placed close to the
ISL8540 first. Minimize the length of the connections
between the input capacitors, C
output capacitors as short as possible.
NOTE: It is recommended that any applications with input
voltage greater than 30VDC should be polymer coated to
meet Intersil’s IPC-2221 creepage and clearance
specification.
IN
. Make the PGND and the
IN
and C
September 18, 2007
OUT
could
FN6495.4