
12
FN9062.2
April 13, 2004
pins of the control IC, and connect them to ground through a 
via placed close to the ground pad. Minimize any leakage 
current paths from the SS node, as the internal current 
source is only 10
μ
A (typical).
A multi-layer printed circuit board is recommended. 
Figure 11 shows the connections to most of the components 
in the circuit. Note that the individual capacitors shown each 
could represent numerous physical capacitors. Dedicate one 
solid layer for a ground plane and make all critical 
component ground connections through vias placed as close 
to the component terminal as possible. Dedicate another 
solid layer as a power plane and break this plane into 
smaller islands of common voltage levels. Ideally, the power 
plane should support both the input power and output power 
nodes. Use copper filled polygons on the top and bottom 
circuit layers to create power islands connecting the filtering 
components (output capacitors) and the loads. Use the 
remaining printed circuit layers for small signal wiring.
Component Selection Guidelines
Output Capacitors Selection
The output capacitors should be selected to allow the output 
voltage to meet the dynamic regulation requirements of 
active state operation (S0, S1). The load transient for the 
various microprocessor system’s components may require 
high quality capacitors to supply the high slew rate (di/dt) 
current demands. Thus, it is recommended that the output 
capacitors be selected for transient load regulation, paying 
attention to their parasitic components (ESR, ESL).
Also, during the transition between active and sleep states 
on the 3.3V
DUAL
/3.3V
SB
 and 5V
DUAL
 outputs, there is a 
short interval of time during which none of the power pass 
elements are conducting - during this time the output 
capacitors have to supply all the output current. The output 
voltage drop during this brief period of time can be easily 
approximated with the following formula:
×
=
, where
V
OUT
 - output voltage drop
ESR
OUT
 - output capacitor bank ESR
I
OUT
 - output current during transition
C
OUT
 - output capacitor bank capacitance
t
t
 - active-to-sleep or sleep-to-active transition time (10
μ
s typ.)
The output voltage drop is heavily dependent on the ESR 
(equivalent series resistance) of the output capacitor bank, 
the choice of capacitors should be such as to maintain the 
output voltage above the lowest allowable regulation level.
Input Capacitors Selection
The input capacitors for an ISL6504/A application must have 
a sufficiently low ESR so as not to allow the input voltage to 
dip excessively when energy is transferred to the output 
capacitors. If the ATX supply does not meet the 
specifications, certain imbalances between the ATX’s 
outputs and the ISL6504/A’s regulation levels could have as 
a result a brisk transfer of energy from the input capacitors to 
the supplied outputs. At the transition between active and 
sleep states, such phenomena could be responsible for the 
5V
SB
 voltage drooping excessively and affecting the output 
regulation. The solution to such a potential problem is using 
larger input capacitors with a lower total combined ESR.
Transistor Selection/Considerations
The ISL6504/A usually requires one P-Channel (or bipolar 
PNP), two N-Channel MOSFETs, and one bipolar NPN 
transistors.
One important criteria for selection of transistors for all the 
linear regulators/switching elements is package selection for 
efficient removal of heat. The power dissipated in a linear 
regulator or an ON/OFF switching element is 
Select a package and heatsink that maintains the junction 
temperature below the rating with the maximum expected 
ambient temperature.
L
VOUT1
CHF1
L
FIGURE 11. PRINTED CIRCUIT BOARD ISLANDS
VOUT3
Q1
Q2
Q3
CSS
+12VIN
+5VSB
CIN
VIA CONNECTION TO GROUND PLANE
ISLAND ON POWER PLANE LAYER
ISLAND ON CIRCUIT/POWER PLANE LAYER
ISL6504/A
VOUT4
SS
GND
5VDLSB
3V3DLSB
KEY
5VSB
DLA
Q4
CBULK4
L
C5VSB
L
CHF3
CHF4
5VDL
+5VIN
+3.3VIN
3V3DL
3V3
1V5SB
CBULK2
CHF2
CBULK1
1V2VID
CBULK3
VOUT2
V
OUT
I
OUT
ESR
OUT
t
OUT
---------------
+
P
LINEAR
I
O
V
IN
V
OUT
–
(
)
×
=
ISL6504, ISL6504A