LTM4620 View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LTM4620

Dual 13A or Single 26A DC/DC µModule Regulator

Linear Technology 

LTM4620

APPLICATIONS INFORMATION

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

=

VG0

–

Kt

 q

ln

 I0 

 ID

,

VT

=

kT

 q

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.

If we take this equation and differentiate it with respect to

temperature T, then:

dVD

dT

=

–

(

VG0

–

VD)

/

T

This dVD

dT

change as a function of temperature is the typical ~–2mV/°C.

This equation is simplified for the first order derivation.

Solving for T, T = –(VG0 – VD)/ dVD provide the temperature.

1st Example: Figure 4 for 27°C, or 300°C Kelvin the diode

voltage is 0.598V, thus, 300°C = –(1200mv – 598mv)/

–2mv/°C)

2nd Example: Figure 4 for 75°C, or 350°C Kelvin the di-

ode voltage is 0.50V, thus, 350°C = –(1200mv – 500mv)/

–2mv/°C)

Converting the Kelvin scale to Celsius is simply taking the

Kelvin temp and subtracting –273°C Kelvin from it.

A typical forward voltage is measured and placed in the

electrical characteristics section of the data sheet, and

Figure 8 is the plot of this forward voltage. Measure this

forward voltage at 27°C to establish a reference point.

Then use the above expression while measuring the

forward voltage over temperature will provide a general

temperature monitor.

0.8

ID = 100µA

0.7

0.6

0.5

0.4

0.3

–50 –25

0 25 50 75 100 125

TEMPERATURE (°C)

4620 F08

Figure 8. Diode Voltage VD vs Temperature

T(°C) for Different Bias Currents

The diode connected PNP transistor can be pulled up to

VIN with a resistor to set the current to 100µA for using

this diode connected transistor as a general temperature

monitor by monitoring the diode voltage drop with tem-

perature. See Figure 24 for an example.

Thermal Considerations and Output Current Derating

The thermal resistances reported in the Pin Configura-

tion section of the data sheet are consistent with those

parameters defined by JESD 51-12 and are intended for

use with finite element analysis (FEA) software modeling

tools that leverage the outcome of thermal modeling,

simulation, and correlation to hardware evaluation per-

formed on a µModule package mounted to a hardware

test board defined by JESD 51-9 (“Test Boards for Area

Array Surface Mount Package Thermal Measurements”).

The motivation for providing these thermal coefficients is

found in JESD 51-12 (“Guidelines for Reporting and Using

Electronic Package Thermal Information”).

For more information www.linear.com/LTM4620

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