LTM4644 datasheet(24/38 Pages) LINER

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LTM4650A

4650afb

For more information www.linear.com/LTM4650A

APPLICATIONS INFORMATION

Figure 11. Thermal Image 12V to 1V, 50A

with No Airflow without Heat Sink

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

with 200LFM Airflow without Heat Sink

Safety Considerations

The LTM4650A 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

The 1V, 1.8V, 3.3V and 5V power loss curves in Figures 14

to 17 can be used in coordination with the load current

derating curves in Figures 18 to 31 for calculating an ap-

proximate θJA thermal resistance for the LTM4650A with

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

Figure22,theloadcurrentisderatedto~25Aat~90°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 90°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

equivalent thermal resistances for 1V to 5V outputs with

and without airflow and heat sinking.

LTM4644 Datasheet(PDF) 24 Page - Linear Technology

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