
6
simultaneously. The DLA pin is forced to a high impedance 
state which allows the 12V rail to enhance the two N-
MOSFETs (Q
1
 and Q
3
) that connect the ATX rails to the 
3.3V
DUAL
 and 5V
DUAL
 rails. The 5VDLSB pin is forced to a 
high impedance state which will turn the P-MOSFET (Q
2
) 
off. Finally, the internal LDO which regulates the 3.3V
DUAL
rail in sleep states is put in standby mode.
Internal Linear Regulator Undervoltage Protection
The undervoltage protection on the internal linear regulator 
is only active during sleep states and after the initial soft start 
ramp of the 3.3V linear regulator. The undervoltage trip point 
is set at 25% below nominal, or 2.475V.
When an undervoltage is detected, the 3.3V linear regulator 
is disabled. One soft start interval later, the 3.3V linear 
regulator is retried with a soft start ramp. If the linear 
regulator is retried 3 times and a fourth undervoltage is 
detected, then the 3.3V linear regulator is disabled and can 
only be reset through a POR reset.
Internal Linear Regulator Over Current Protection
When an overcurrent condition is detected, the gate voltage 
to the internal NMOS pass element is reduced which causes 
the output voltage of the linear regulator to be reduced. 
When the output voltage is reduced to the undervoltage trip 
point, the undervoltage protection is initiated and the output 
will shutdown.
Layout Considerations
The typical application employing an ISL6506 is a fairly 
straight forward implementation. Like with any other linear 
regulator, attention has to be paid to the few potentially 
sensitive small signal components, such as those connected 
to sensitive nodes or those supplying critical bypass current.
The power components (pass transistors) and the controller 
IC should be placed first. The controller should be placed in 
a central position on the motherboard, not excessively far 
from the 3.3V
DUAL
 island or the I/O circuitry. Ensure the 
3V3AUX connection is properly sized to carry 1A without 
exhibiting significant resistive losses at the load end. 
Similarly, the input bias supply (5V
SB
) carries a similar level 
of current - for best results, ensure it is connected to its 
respective source through an adequately sized trace and is 
properly decoupled. The pass transistors should be placed 
on pads capable of heatsinking matching the device’s power 
dissipation. Where applicable, multiple via connections to a 
large internal plane can significantly lower localized device 
temperature rise.
Placement of the decoupling and bulk capacitors should 
reflect their purpose. As such, the high-frequency 
decoupling capacitors should be placed as close as possible 
to the load they are decoupling; the ones decoupling the 
controller close to the controller pins, the ones decoupling 
the load close to the load connector or the load itself (if 
embedded). Even though bulk capacitance (aluminum 
electrolytics or tantalum capacitors) placement is not as 
critical as the high-frequency capacitor placement, having 
these capacitors close to the load they serve is preferable.
Locate all small signal components close to the respective 
pins of the control IC, and connect them to ground, if 
applicable, through a via placed close to the ground pad.
A multi-layer printed circuit board is recommended. 
Figure 5 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. The EPAD should be 
tied to the ground plane with three to five vias for good 
thermal management. 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.
FIGURE 5. PRINTED CIRCUIT BOARD ISLANDS
Q2
Q3
12VATX
CIN
VIA CONNECTION TO GROUND PLANE
ISLAND ON POWER PLANE LAYER
ISLAND ON CIRCUIT/POWER PLANE LAYER
ISL6506/A/B
GND
5VDLSB
KEY
VCC
5VSB
DLA
Q4
C
5V
L
C
5VSB
L
C
HF3V
C
HF5V
5VATX
+3.3VIN
3V3AUX
C
3V
5VDUAL
3V3DUAL
EPAD
ISL6506, ISL6506A, ISL6506B