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LTM4645

4645f

For more information www.linear.com/LTM4645

applicaTions inForMaTion

The LTM4645 has been designed to effectively remove

heat from both the top and bottom of the package. The

bottom substrate material has very low thermal resistance

to the printed circuit board. An external heat sink can be

applied to the top of the device for excellent heat sinking

with airflow. Basically all power dissipating devices are

mounted directly to the substrate and the top exposed

metal. This provides two low thermal resistance paths to

remove heat.

Figures10and11showthethermalimagesoftheLTM4645

with no heat sink and no airflow running at 1V/25A and

1.8V/25A.

Safety Considerations

The LTM4645 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 needs

tobeprovidedtoprotecteachunitfromcatastrophicfailure.

The fuse or circuit breaker should be selected to limit the

current to the regulator during overvoltage in case of an

internal top MOSFET fault. If the internal top MOSFET fails,

then turning it off will not resolve the overvoltage, thus

the internal bottom MOSFET will turn on indefinitely trying

to protect the load. Under this fault condition, the input

voltage will source very large currents to ground through

the failed internal top MOSFET and enabled internal bot-

tom MOSFET. This can cause excessive heat and board

damage depending on how much power the input voltage

can deliver to this system. A fuse or circuit breaker can be

used as a secondary fault protector in this situation. The

device does support over current protection. The TEMP+

and TEMP– pins are provided for monitoring internal tem-

perature, and can be used to detect the need for thermal

shutdown that can be done by controlling the HIZB pin.

Output Current Derating

The 1V, 1.5V power loss curves in Figures 12 to 13 can

be used in coordination with the load current derating

curves in Figures 14 to 21 for calculating an approximate

θJA thermal resistance for the LTM4645 with various heat

sinking and airflow conditions. The power loss curves

are taken at room temperature and are increased with a

multiplicativefactoraccordingtothejunctiontemperature,

which is 1.3 for 120°C. The derating curves are plotted

with the output current starting at 25A and the ambient

temperature at ~30°C. The output voltages are 1V and

1.5V. These are chosen to include the lower and higher

outputvoltagerangesforcorrelatingthethermalresistance.

Thermal models are derived from several temperature

measurementsinacontrolledtemperaturechamberalong

withthermalmodelinganalysis.Thejunctiontemperatures

are monitored while ambient temperature is increased

with and without airflow. The power loss increase with

Figure 10. LTM4645 12VIN to 1VOUT at 25A with No Air Flow

and No Heat Sink

Figure 11. LTM4645 12VIN to 1.8VOUT at 25A with No Air

Flow and No Heat Sink

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