Application Note 1789

2

AN1789.0

December 3, 2012

can be adjusted for the desired frequency. 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. Please refer to the derating

curves in the ISL8225M datasheet.

Board Setting

If low current applications are needed, this 90A evaluation board

can be easily programmed to 30A and 60A use.

30A Application (1 Module)

EN -- Open, EN2-- OFF, EN3 -- OFF

In this mode, only module 1 is running and modules 2 and 3 are

disabled.

60A Application (2 Modules)

EN -- Open, EN2-- ON, EN3 -- OFF

Or:

EN -- Open, EN2-- OFF, EN3 -- ON

In this mode, only modules 1 and 2 (or 3) are running and

module 3 (or 2) is disabled.

90A Application (3 Modules)

EN -- Open, EN2-- ON, EN3 -- ON

In this mode, all modules are running.

Disable All Modules and Use the EN Pin to

Start the Modules

EN -- Connected

In this mode, all modules are disabled and EN can be used to

control all modules to startup.

Evaluation Board Information

The evaluation board size is 150mm x 130mm. It is a 6-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 90A reference design. Refer to the “Layout” section

beginning on page 7. The board is made of FR4 material and all

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

Current Sharing Check

The evaluation board allows the user to measure the current

sharing accuracy. Four zero ohm resistors (i.e. R59~R62 for M1

channel 2 in Figure 2) are put serially on each output with two on

each side of the evaluation board. To measure the output current

of each phase, please remove all four resistors and put looped

wires or sensing resistors on correct positions.

Although the assembled resistors have zero resistance, there is

still small resistance (< 50mΩ) on each resistor. At large output

current, the efficiency can be decreased by 1~3% due to the

power loss on those zero ohm resistors. The efficiency curves are

shown in Figures 16 and 17 with zero ohm resistors, while

Figures 18 and 19 show the efficiency curves by replacing those

resistors with short copper straps.

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 designed carefully to maximize thermal performance. To

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

proper connectors.

This evaluation board is designed for running 90A @ 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 2 pads on layer 3 (Figure 11) for this 90A 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.

TABLE 1. VALUE OF BOTTOM RESISTOR FOR DIFFERENT OUTPUT

VOLTAGES (R1 = 1k)

(V)

R2 /R64

(Ω)

FREQUENCY

(kHz)

0.6

0/0

Default

Default

0.8

3010/1500

Default

Default

1.0

1500750

Default

Default

1.2

1000/500

Default

Default

1.5

665/332

Default

Default

2.5

316/158

650

249k

3.3

221/110

800

124k

5.0

137/68.1

950

82.5k

5.5

121/60.4

950

82.5k