6

simultaneously. The DLA pin is forced to a high impedance

state which allows the 12V rail to enhance the two N-

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

3V3AUX connection is properly sized to carry 1A without

exhibiting significant resistive losses at the load end.

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

5VATX

+3.3VIN

3V3AUX

5VDUAL

3V3DUAL

EPAD

ISL6506, ISL6506A, ISL6506B