ISL8502 Intersil Corporation, ISL8502 Datasheet - Page 18

• Manufacturer Part Number: ISL8502
• Description: 2.5A Synchronous Buck Regulator
• Manufacturer: Intersil Corporation

Layout Considerations
Layout is very important in high frequency switching
converter design. With power devices switching efficiently
between 500kHz and 1.2MHz, 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 lower MOSFET. 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 ISL8502
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.
A multi-layer printed circuit board is recommended. Figure
38 shows the connections of the critical components in the
converter. Note that capacitors CIN and COUT could 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
PHASE 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 phase nodes. Use the remaining printed
circuit layers for small signal wiring. The wiring traces from
the GATE pins to the MOSFET gates should be kept short
and wide enough to easily handle the 1A of drive current.
FIGURE 37. ASYMPTOTIC BODE PLOT OF CONVERTER GAIN
100
-20
-40
-60
80
60
40
20
0
10
(R
20LOG
MODULATOR
2
/R
1
GAIN
)
100
1K
F
Z1
F
FREQUENCY (Hz)
LC
F
Z2
10K
18
F
F
P1
ESR
(V
100K
IN
20LOG
F
/ΔV
P2
OSC
OPEN LOOP
ERROR AMP GAIN
1M
)
COMPENSATION
CLOSED LOOP
10M
GAIN
GAIN
ISL8502
In order to dissipate heat generated by the internal V
LDO, the ground pad, pin 29, should be connected to the
internal ground plane through at least five 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
ISL8502 first. Minimize the length of the connections
between the input capacitors, CIN, and the power switches
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 and lower MOSFETs and the load. Make
the PGND and the output capacitors as short as possible.
The critical small signal components include any bypass
capacitors, feedback components, and compensation
components. Place the PWM converter compensation
components close to the FB and COMP pins. The feedback
resistors should be located as close as possible to the FB
pin with vias tied straight to the ground plane as required.
5V
C
FIGURE 38. PRINTED CIRCUIT BOARD POWER PLANES AND
BP1
R
BP
KEY
C
BP2
ISLAND ON POWER PLANE LAYER
ISLAND ON CIRCUIT AND/OR POWER PLANE LAYER
VIA CONNECTION TO GROUND PLANE
PVCC
VCC
ISLANDS
GND PAD
ISL8502
PHASE
COMP
PGND
VIN
FB
V
IN
R
C
2
2
R
4
C
L
IN
C
1
C
1
OUT1
C
3
R
1
R
V
January 17, 2007
OUT1
3
TT
FN6389.0