LTM4615EVPBF datasheet(15/24 Pages) LINER

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LTM4615

4615f

TYPICAL APPLICATIONS

Thermal Considerations and Output Current Derating

The power loss curves in Figures 5 and 6 can be used

in coordination with the load current derating curves in

Figures 7 to10 for calculating an approximate

θJA thermal

resistance for the LTM4615 with various heat sinking and

airﬂow conditions. Both of the LTM4615 outputs are at full

4A load current, and the power loss curves in Figures 5

and 6 are combined power losses plotted for both output

voltages up to 4A each. The VLDO regulator is set to have

a power dissipation of a 0.5W since it is generally used

with dropout voltages of 0.5V or less. For example: 1.2V

to 1V, 1.5V to 1V, 1.5V to 1.2V and 1.8V to 1.5V. Other

drop voltages can be supported at VLDO maximum load,

but further thermal analysis will be required for the VLDO.

The 4A output voltages are 1.2V and 3.3V. These voltages

are chosen to include the lower and higher output voltage

ranges for correlating the thermal resistance. Thermal

models are derived from several temperature measure-

ments in a controlled temperature chamber along with

thermal modeling analysis. The junction temperatures are

monitored while ambient temperature is increased with and

without airﬂow. The junctions are maintained at ~120°C

while lowering output current or power while increasing

ambient temperature. The 120°C is chosen to allow for

a 5°C margin window relative to the maximum 125°C.

The decreased output current will decrease the internal

module loss as ambient temperature is increased. The

Figure 5. 1.2V Power Loss

Figure 6. 3.3V Power Loss

power loss curves in Figures 5 and 6 show this amount of

power loss as a function of load current that is speciﬁed

for both channels. 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 Figure 7 the load current is derated to 3A for

each channel with 0LFM at ~90°C and the power loss for

both channels at 5V to 1.2V at 3A output are ~1.4W, then

include the VDLO power loss of 0.5W to equal 1.9W. If the

90°C ambient temperature is subtracted from the 120°C

maximum junction temperature, then the difference of

30°C divided 1.9W equals a 15.7°C/W thermal resistance.

Table 2 speciﬁes a 15°C/W value which is very close. Table

2 and Table 3 provide equivalent thermal resistances for

1.2V and 3.3V outputs with and without air ﬂow and heat

sinking. The combined power loss for the two 4A outputs

plus the VLDO power loss can be summed together and

multiplied by the thermal resistance values in Tables 2

and 3 for module temperature rise under the speciﬁed

conditions. The printed circuit board is a 1.6mm thick

four layer board with two ounce copper for the two outer

layers and 1 ounce copper for the two inner layers. The

PCB dimensions are 95mm

× 76mm. The BGA heat sinks

are listed below Table 3. The data sheet lists the

θJP (Junc-

tion to pin) and

θJC (Junction to case) thermal resistances

under the Pin Conﬁguration diagram.

LOAD CURRENT (A)

1.0

1.5

4615 F05

0.5

2.5

2.0

VIN = 5V

LOAD CURRENT (A)

0.5

1.0

1.5

2.0

2.5

3.0

4615 F06

VIN = 5V

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