Application Note 1789
For 12V V
IN
and V
OUT
more than 1.5V, the switching frequency
will need to be adjusted, as shown in Table 1. The resistor R
FSET
can be adjusted for the desired frequency. No frequency
adjustments are necessary for V
OUT
below 1.5V. For 5V V
IN
, the
frequency does not need to be adjusted and the module default
frequency can be used at any allowed V
OUT
. If the output voltage
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
TABLE 1. VALUE OF BOTTOM RESISTOR FOR DIFFERENT OUTPUT
VOLTAGES (R1 = 1k)
V
OUT
(V)
0.6
0.8
1.0
1.2
1.5
2.5
3.3
5.0
5.5
R2 /R64
(Ω)
0/0
3010/1500
1500750
1000/500
665/332
316/158
221/110
137/68.1
121/60.4
FREQUENCY
(kHz)
Default
Default
Default
Default
Default
650
800
950
950
R
FSET
(Ω)
(V
IN
= 12V)
Default
Default
Default
Default
Default
249k
124k
82.5k
82.5k
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
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.
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.
2
AN1789.0
December 3, 2012
INTERSIL [ INTERSIL CORPORATION ]
