ISL6442 INTERSIL [Intersil Corporation], ISL6442 Datasheet - Page 15

• Manufacturer Part Number: ISL6442
• Manufacturer: INTERSIL [Intersil Corporation]

Strategy
Layout Considerations
As in any high frequency switching converter, layout is very
important. Switching current from one power device to another
can generate voltage transients across the impedances of the
interconnecting bond wires and circuit traces. These
interconnecting impedances should be minimized by using
wide, short printed circuit traces. The critical components
should be located as close together as possible using ground
plane construction or single point grounding.
Figure 17 shows the critical power components of the
converter. To minimize the voltage overshoot, the
interconnecting wires indicated by heavy lines should be part
of ground or power plane in a printed circuit board. The
components shown in Figure 17 should be located as close
together as possible. Please note that the capacitors C
and C
Locate the ISL6442 within 1 inch of the MOSFETs, Q1 and
Q2. The circuit traces for the MOSFETs’ gate and source
connections from the ISL6442 must be sized to handle up to
2A peak current.
1. The output capacitor of the linear regulator circuit must be
2. The voltage divider can be chosen to sink 250µA to
3. Compute the pole and zero for the linear regulator circuit
4. Make:
5. Fix R300 at 100Ω. Solve for C300. Use an MLCC, COG
chosen such that the ESR Zero is less than 200kHz:
1.5mA of sense current, but this is simply a guideline, not
a rule. The values should be chosen such that
where I
and 0.6V is the internal voltage reference that LCFB will
equal.
from Equations 31 and 32.
or NPO type capacitor for this value.
F
R301
R302
F
FIGURE 16. LINEAR COMPENSATION COMPONENTS
Z1
P2
OUT
<
=
LCDR
LCFB
200kHz
=
=
each represent numerous physical capacitors.
F
--------- -
sen
10
V
------------------------------------- -
0.6V
------------
Z1
I
sen
OUT3
= is the current through the resistor divider,
I
sen
–
0.6V
R307
C300
R300
R302
15
R301
Q300
C
OUT3
VIN3
C
V
IN3
OUT3
(EQ. 30)
(EQ. 31)
(EQ. 32)
(EQ. 33)
IN
ISL6442
Figure 18 shows the circuit traces that require additional
layout consideration. Use single point and ground plane
construction for the circuits shown. Locate the RT resistor as
close as possible to the RT pin and the SGND pin. Provide
local decoupling between VCC and GND pins.
For each switcher, minimize any leakage current paths on
the SS/EN pin and locate the capacitor, C
SS/EN pin because the internal current source is only 30µA.
All of the compensation network components for each
switcher should be located near the associated COMP and
FB pins. Locate the capacitor, C
to the BOOT and PHASE pins (but keep the noisy PHASE
plane away from the IC (except for the PHASE pin
connection).
The OCSET circuits (see Figure 2) should have a separate
trace from the upper FET to the OCSET R and C; that will
more accurately sense the VIN at the FET than just tying
them to the VIN plane. The OCSET R and C should be
placed near the IC pins.
R
FIGURE 17. PRINTED CIRCUIT BOARD POWER AND
FIGURE 18. PRINTED CIRCUIT BOARD POWER AND
RT
ISL6442
C
SGND
SS
C
VCC
UGATE
PHASE
LGATE
PGND
GROUND PLANES OR ISLANDS
GROUND PLANES OR ISLANDS
SS
RT
VCC
SGND
ISL6442
PHASE
BOOT
PGND
VIN
RETURN
V
Q2
C
Q1
IN
BOOT
+V
IN
BOOT
C
C
VIN
IN
as close as practical
C
IN
SS
L
PGND
Q1
+V
OUT
Q2
C
IN
close to the
OUT
C
L
OUT
OUT
October 31, 2008
V
OUT
FN9204.2
V
OUT