Application Note 1790

2

AN1790.0

December 3, 2012

will need to be adjusted, as shown in Table 1; no frequency

frequency does not need to be adjusted and the module default

is set to more than 1.8V, the output current will need to be

derated to allow for safe operation at elevated ambient

temperatures. Please refer to the derating curves in the

ISL8225M datasheet.

Also, it is preferred that the EN/FF voltage be over 1.5V in order

to achieve better stability.

Evaluation Board Information

The evaluation board size is 3 inch x 3 inch. It is a 4-layer board,

containing 2-ounce copper on the top and bottom layers and

1-ounce copper on all internal layers. The board can be used as a

30A reference design. Refer to the “Layout” section beginning on

page 4. The board is made up of FR4 material and all

components, including the solder attachment, are lead-free.

Thermal Considerations and Current Derating

For high current applications, board layout is very critical in order

to make the module operate safely and deliver maximum

allowable power. To carry large currents, the board layout needs

to be carefully designed to maximize thermal performance. To

achieve this, select enough trace width, copper weight and the

proper connectors.

This evaluation board is designed for running 30A @ 1.2V at

room temperature without additional cooling systems needed.

However, if the output voltage is increased or the board is

operated at elevated temperatures, then the available current is

derated. Refer to the derated current curves in the datasheet to

determine the output current available.

For layout of designs using the ISL8225M, the thermal

performance can be improved by adhering to the following

design tips:

1. Use the top and bottom layers to carry the large current.

VOUT1, VOUT2, Phase 1, Phase 2, PGND, VIN1 and VIN2

should have large, solid planes. Place enough thermal vias to

connect the power planes in different layers under and

around the module.

2. Phase 1 and Phase 2 pads are switching nodes that generate

switching noise. Keep these pads under the module. For

noise-sensitive applications, it is recommended to keep

phase pads only on the top and inner layers of the PCB; do not

place phase pads exposed to the outside on the bottom layer

of the PCB. To improve the thermal performance, the phase

pads can be extended in the inner layer, as shown in Phase 1

and Phase 2 pads on layer 3 (Figure 6) for this 30A evaluation

board. Make sure that layer 2 and layer 4 have the GND layers

to cover the extended areas of phase pads at layer 3 to avoid

noise coupling.

3. To avoid noise coupling, we recommend adding 470pF

capacitors on all COMP pins of each module for multiple

module operations.

4. If the ambient temperature is high or the board space is

limited, airflow is needed to dissipate more heat from the

modules. A heat sink can also be applied to the top side of the

module to further improve the thermal performance (heat

sink recommendation: Aavid Thermalloy, part number

375424B00034G, www.aavid.com).

TABLE 1. RESISTANCE SETTING FOR DIFFERENT OUTPUT VOLTAGES

AND OPERATING FREQUENCY (R3 = 1k)

(V)

(

Ω)

FREQUENCY

(kHz)

1.0

1500

Default

Default

1.2

Default

Default

Default

1.5

665

Default

Default

2.5

316

650

249k

3.3

221

800

124k

5.0

137

950

82.5k

5.5

121

950

82.5k