11

FN7456.2

September 8, 2005

The QFN Package Requires Additional PCB Layout

Rules for the Thermal Pad

The thermal pad is electrically connected to V- supply

through the high resistance IC substrate. Its primary function

is to provide heat sinking for the IC. However, because of the

connection to the V- supply through the substrate, the

thermal pad must be tied to the V- supply to prevent

unwanted current flow to the thermal pad. Do not tie this pin

to GND. Connecting this pin to GND could result in large

back biased currents flowing between GND and V-. The

ISL59445 uses the package with pad dimensions of

D2 = 2.48mm and E2 = 3.4mm.

Maximum AC performance is achieved if the thermal pad is

attached to a dedicated de-coupled layer in a multi-layered

PC board. In cases where a dedicated layer is not possible,

AC performance may be reduced at upper frequencies.

The thermal pad requirements are proportional to power

dissipation and ambient temperature. A dedicated layer

eliminates the need for individual thermal pad area. When a

dedicated layer is not possible a 1” x 1” pad area is sufficient

for the ISL59445 that is dissipating 0.5W in +50°C ambient.

Pad area requirements should be evaluated on a case by

case basis.

ISL59424, ISL59445