Data Sheet

LTM4640

analog.com

Rev. 0

24 of 38

To obtain a linear voltage proportional to the temperature, we cancel the IS variable in the natural logarithm term

to remove the IS dependency from Equation 15. This is accomplished by measuring the diode voltage at two

currents: I1 and I2, where I1 = 10 × I2, and subtracting is given in Equation 16.

∆������������ = ������(������������������������������������) × ������������ × ������������

������1

������������

− ������(������������������������������������) × ������������ × ������������

������2

������������

(16)

Combining like terms, and then simplifying the natural log terms yields Equation 17.

∆������������ = ������(������������������������������������) × ������������ × ������������(10)

(17)

and redefining the constant is given in Equation 18.

������′������ = ������������ × ������������(10) =

198������������

������

(18)

yields Equation 19.

∆������������ = ������′������ × ������(������������������������������������)

(19)

Equation 20 solves for temperature.

������(������������������������������������) =

∆������������

������′������

������(°������������������������������������������) = ������(������������������������������������) − 273.15

(20)

Where 300.15K = 27°C is an example.

If we take the difference in voltage across the diode measured at two currents with a ratio of 10, the resulting

voltage is 198μV per Kelvin of the junction with a zero intercept at 0 Kelvin.

The internal diode-connected NPN transistor between TSENSE+ and TSENSE– pins is used to monitor the internal

temperature of the LTM4640.

Thermal Considerations and Output Current Derating

The thermal resistances reported in the Pin Configurations and Function Descriptions section are consistent with

those parameters defined by JESD5112 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

performed on a µModule package mounted to a hardware test board. The motivation for providing these thermal

coefficients is found in JESD5112 (Guidelines for Reporting and Using Electronic Package Thermal Information).

Many designers may use laboratory equipment and a test vehicle, such as an evaluation (demo) board, to

anticipate the µModule regulator’s thermal performance in their application at various electrical and

environmental operating conditions to complement any FEA activities. Without FEA software, the thermal

resistances reported in the Pin Configurations and Function Descriptions section are, in and of themselves, not

relevant to providing guidance on thermal performance; instead, the derating curves provided in Figure 29 through

Figure 34 can be used in a manner that yields insight and guidance about the user’s application and can be adapted

to correlate thermal performance to the user’s application.