LTM4637 datasheet(20/32 Pages) LINER

20 / 32 page

LTM4633

4633f

For more information www.linear.com/LTM4633

applicaTions inForMaTion

loss curves in Figures 8 to 10 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 one ounce copper for the two inner layers. The PCB

dimensions are 95mm x 76mm. The BGA heat sinks are

listed below Table 4.

Temperature Monitoring (TEMP1 and TEMP2)

A diode connected PNP transistor is used for the TEMP

monitor function by monitoring its voltage over tempera-

ture. The temperature dependence of this diode can be

understood in the equation:

= 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 LTM4633. Since ID has an exponential temperature

dependence that can be understood from the typical em-

pirical equation for IS:

IS =IOexp

–VGO









whereIOissomeprocessandgeometry-dependentcurrent

(IO is typically around 20 orders of magnitude larger than

IS at room temperature, so IO is much larger than typical

values of ID), and VG0 is the band gap voltage of 1.2V

extrapolated to absolute zero of –273°C Kelvin. Figure 4

shows a plot of the diode temperature characteristic of

the diode connected PNP transistor biased with a 100ua

current source. This plot would extend to the left back to

1.2V at –273°C Kelvin. This curve is stop at –55°C due to

the test system limits.

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

tion, then we get:

VD = VGO –

















, VT =

The expression shows that the junction voltage of the PNP

connected diode decreases linearly if IO were constant

from a value VG0 of 1.2V at absolute zero to a decreasing

value with increased temperature.

Figure 4. TEMP Pin Diode Voltage vs Temperature

Figure 5. Thermal Plot for 12V Input to 1.8V at 10A Output 1,

1.8V at 10A Output 2, and 5V at 8A Output 3 with 200LFM of

Airflow

TEMPERATURE (°C)

–50 –25

0.3

0.5

0.8

0.4

0.7

0.6

100

4633 F04

125

ID = 100µA

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