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LTM4650A-1

4650a1f

For more information www.linear.com/LTM4650A-1

APPLICATIONS INFORMATION

Figure 13. Thermal Image 12V to 1.0V,

50A with No Airflow without Heat Sink

Figure 14. Thermal Image 12V to 5V, 50A

with 200LFM Airflow without Heat Sink

Figure 13 shows the thermal image of the LTM4650A-1,

without airflow, without heat sink, running paralleled

from 12V to 1V at 50A with around 87.3% efficiency and

7.2W power loss. Figure 14 shows the thermal image of

the LTM4650A-1, with 200LFM airflow and external heat

sink, running paralleled form 12V to 5V at 50A with around

95% efficiency and 13W power loss.

Safety Considerations

The LTM4650A-1 modules do not provide isolation from

VIN to VOUT. There is no internal fuse. If required, a slow

blow fuse with a rating twice the maximum input current

needstobeprovidedtoprotecteachunitfromcatastrophic

failure. The device does support over current protection.

A temperature diode is provided for monitoring internal

temperature,andcanbeusedtodetecttheneedforthermal

shutdown that can be done by controlling the RUN pin.

Power Derating

The1.0V,1.8V,3.3Vand5VpowerlosscurvesinFigures 16

to 19 can be used in coordination with the load current

derating curves in Figures 20 to 33 for calculating an ap-

proximateθJAthermalresistancefortheLTM4650A-1with

variousheatsinkingandairflowconditions.Thepowerloss

curves are taken at room temperature, and are increased

with a 1.2 multiplicative factor at 125°C.

ThederatingcurvesareplottedwithCH1andCH2inparallel

single output operation starting at 50A of load with low

ambient temperature. The output voltages are 1V to 5V.

These are chosen to include the lower and higher output

voltage ranges for correlating the thermal resistance.

Thermal models are derived from several temperature

measurementsinacontrolledtemperaturechamberalong

with thermal modeling analysis.

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

Figure24,theloadcurrentisderatedto~25Aat~40°Cwith

no air or heat sink and the power loss for the 12V to 1.8V

at 25A output is a ~4.4W loss. The 4.4W loss is calculated

with the ~3.7W room temperature loss from the 12V to

1.8V power loss curve at 25A, and the 1.2 multiplying

factor at 120°C ambient. If the 80°C ambient temperature

is subtracted from the 120°C junction temperature, then

the difference of 30°C divided 4.4W equals a 6.8°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. Tables 2 to 5 provide equiva-

lent thermal resistances for 1.0V to 5V outputs with and

without airflow and heat sinking.

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