LTM4630 datasheet(21/34 Pages) LINER

21 / 34 page

LTM4630

4630fa

For more information www.linear.com/LTM4630

value is shown in the Pin Configuration section and should

accurately equal the

θJA value because approximately

100% of power loss flows from the junction through the

board into ambient with no airflow or top mounted heat

sink. Each system has its own thermal characteristics,

therefore thermal analysis must be performed by the user

in a particular system.

The LTM4630 module has been designed to effectively

remove heat from both the top and bottom of the pack-

age. 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.

Figures 10 and 11 show temperature plots of the LTM4630

with no heat sink and 200LFM airflow.

These plots equate to a paralleled 12V to 1.0V at 36A

design operating at 84.5% efficiency, and 12V to 1.2V at

36A design operating at 86% efficiency.

Safety Considerations

The LTM4630 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

to be provided to protect each unit from catastrophic

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.

APPLICATIONS INFORMATION

Power Derating

The 1.0V and 1.5V power loss curves in Figures 13 and 14

can be used in coordination with the load current derating

curves in Figures 15 to 22 for calculating an approximate

ΘJA thermal resistance for the LTM4630 with various heat

sinking and airflow conditions. The power loss curves are

taken at room temperature, and are increased with a 1.35

to 1.4 multiplicative factor at 125°C. These factors come

from the fact that the power loss of the regulator increases

about 45% from 25°C to 150°C, thus a 50% spread over

125°C delta equates to ~0.35%/°C loss increase. A 125°C

maximumjunctionminus25°Croomtemperatureequates

to a 100°C increase. This 100°C increase multiplied by

0.35%/°C equals a 35% power loss increase at the 125°C

junction, thus the 1.35 multiplier.

The derating curves are plotted with CH1 and CH2 in

parallel single output operation starting at 36A of load

with low ambient temperature. The output voltages are

1.0V and 1.5V. These are chosen to include the lower and

higher output voltage ranges for correlating the thermal

resistance. Thermal models are derived from several

temperature measurements in a controlled temperature

chamber along with thermal modeling analysis.

Figure 10. Thermal Image 12V to 1.0V,

36A with 200LFM without Heat Sink

Figure 11. Thermal Image 12V to 1.2V,

36A with 200LFM without Heat Sink

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