LTM4644 datasheet(19/32 Pages) LINER

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LTM4634

4634f

For more information www.linear.com/LTM4634

applicaTions inForMaTion

The derating curves are plotted with the output current

starting at 15A (5A/CH) and the ambient temperature at

~40°C. The 15A comes from each of the three channels

operating at 5A each. This simplifies the loading for this

thermal testing. The output voltages are 3.3V, and 5V

when all three channels are loaded together in parallel.

Channel 1 and Channel 2 are designed to operate with

outputs up to 5V, and Channel 3 is designed for 12V. The

power loss curve values at a particular output voltage and

output current for each output are taken and multiplied by

1.4 for increased power loss at 120°C junction. 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 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 with increasing

ambient temperature. The decreased output current will

decrease the internal module loss as ambient tempera-

ture 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 Figure 17 the 5.0V load current is derated

to ~12.6A at ~71°C with no air and with no heat sink. In

Figure 10, the 12V to 5.0V power loss at 4.2A per chan-

nel is 1.25W. The total loss would be 3 times 1.25W for

3.75W total power loss. The 3.75W is then multiplied by

the 1.4 multiplier for 120°C junction. This 5.25W value is

used with the total temperature rise of 120°C minus the

71°C ambient to calculate

θJA thermal resistance. If the

71°C ambient temperature is subtracted from the 120°C

junction temperature, then the difference of 49°C divided

5.25W equals a 9.3°C/W

θJA thermal resistance. Table 2

specifies a 9.0°C/W value which is very close. Tables 2

and 3 provide equivalent thermal resistances for 3.3V and

5V outputs with and without air flow and heat sinking. The

derivedthermalresistancesinTables 2and3forthevarious

conditions can be multiplied by the calculated power loss

as a function of the 120°C maximum junction tempera-

ture to determine if the temperature rise plus ambient is

belowthe120°Cmaximumjunctiontemperature.Thermal

measurements or infrared analysis should be performed

to validate the values. Ambient temperature power loss

can be derived from the power loss curves in Figures 8 to

13 and adjusted with the 1.4 multiplier. 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 in Table 3.

Temperature Monitoring (TEMP1 and TEMP2)

Diode connected PNP transistors are used for the TEMP1,

TEMP2monitoringfunctionsincethediodeforwardvoltage

varies with temperature. The temperature dependence of

the diodes can be understood in the equation:

VD =nVT ln









where VT is the thermal voltage (kT/q), and n, the ideality

factor, is 1 for the two diode connected PNPs being used

in the LTM4634. IS is expressed by the typical empirical

equation:

IS =I0 exp

–VG0









where I0 is a process and geometry-dependent current (I0

is typically around 20 orders of magnitude larger than IS

at room temperature), and VG0 is the band gap voltage of

1.2V extrapolated to absolute zero or –273°C.

If we take the IS equation and substitute into the VD equa-

tion, then we get:

VD = VG0 –

















, VT =

The expression shows that the diode voltage decreases

(linearly if I0 were constant) with increasing temperature

and constant diode current. Figure 5 shows a plot of VD

vs Temperature over the operating temperature range of

the LTM4634.

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