LTM4630 datasheet(22/34 Pages) LINER

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LTM4630

4630fa

For more information www.linear.com/LTM4630

The junction temperatures are monitored while ambient

temperature is increased with and without airflow. The

power loss increase with ambient temperature change

is factored into the derating curves. The junctions are

maintained at ~120°C maximum while lowering output

current or power while increasing ambient temperature.

The decreased output current will decrease the internal

module loss as ambient temperature is increased.

The monitored junction temperature of 120°C minus

the ambient operating temperature specifies how much

module temperature rise can be allowed. As an example in

Figure15,theloadcurrentisderatedto~25Aat~86°Cwith

no air or heat sink and the power loss for the 12V to 1.0V

at 25A output is a ~5.5W loss. The 5.5W loss is calculated

with the ~4.1W room temperature loss from the 12V to

1.0V power loss curve at 25A, and the 1.35 multiplying

factor at 125°C ambient. If the 86°C ambient temperature

is subtracted from the 120°C junction temperature, then

the difference of 34°C divided 5.5W equals a 6.2°C/W

ΘJA

thermal resistance. Table 2 specifies a 7°C/W value which

is pretty close. The airflow graphs are more accurate due

to the fact that the ambient temperature environment is

controlled better with airflow. As an example in Figure 19,

the load current is derated to ~30A at ~72°C with 200LFM

of airflow and the power loss for the 12V to 1.5V at 30A

output is a ~7.9W loss. The 7.9W loss is calculated with

the ~5.9W room temperature loss from the 12V to 1.5V

power loss curve at 22A, and the 1.35 multiplying factor

at 125°C ambient. If the 72°C ambient temperature is

subtracted from the 120°C junction temperature, then

the difference of 48°C divided 7.9W equals a 6.0°C/W

θJA thermal resistance. Table 2 specifies a 6.0°C/W value

which is pretty close. Tables 2 and 3 provide equivalent

thermal resistances for 1.0V and 1.5V outputs with and

without airflow and heat sinking.

The derived thermal resistances in Tables 2 and 3 for the

various conditions can be multiplied by the calculated

power loss as a function of ambient temperature to derive

temperature rise above ambient, thus maximum junction

temperature.Roomtemperaturepowerlosscanbederived

from the efficiency curves and adjusted with the above

ambient temperature multiplicative factors. The printed

circuit board is a 1.6mm thick four layer board with two

ounce copper for the two outer layers and one ounce

copper for the two inner layers. The PCB dimensions are

101mm × 114mm. The BGA heat sinks are listed in Table 3.

Layout Checklist/Example

The high integration of LTM4630 makes the PCB board

layoutverysimpleandeasy.However,tooptimizeitselectri-

cal and thermal performance, some layout considerations

are still necessary.

• Use large PCB copper areas for high current paths,

including VIN, GND, VOUT1 and VOUT2. It helps to mini-

mize the PCB conduction loss and thermal stress.

• Place high frequency ceramic input and output capaci-

tors next to the VIN, PGND and VOUT pins to minimize

high frequency noise.

• Place a dedicated power ground layer underneath the

unit.

• To minimize the via conduction loss and reduce module

thermal stress, use multiple vias for interconnection

between top layer and other power layers.

• Do not put via directly on the pad, unless they are

capped or plated over.

• Use a separated SGND ground copper area for com-

ponents connected to signal pins. Connect the SGND

to GND underneath the unit.

• For parallel modules, tie the VOUT, VFB, and COMP pins

together. Use an internal layer to closely connect these

pins together. The TRACK pin can be tied a common

capacitor for regulator soft-start.

• Bring out test points on the signal pins for monitoring.

Figure 12 gives a good example of the recommended

layout. LGA and BGA PCB layouts are identical with the

exceptionofcirclepadsforBGA(seePackageDescription).

APPLICATIONS INFORMATION

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