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LTM4644/LTM4644-1

4644fe

For more information www.linear.com/LTM4644

APPLICATIONS INFORMATION

Temperature Monitoring

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 voltage

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 diode connected PNP transistor be-

ing used in the LTM4644. 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 20k 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 –

⎛

⎝

⎜

⎞

⎠

⎟ln

⎛

⎝

⎜

⎞

⎠

⎟, VT =

The expression shows that the diode voltage decreases

(linearly if I0 were constant) with increasing temperature

and constant diode current. Figure 6 shows a plot of VD

vs Temperature over the operating temperature range of

the LTM4644.

If we take this equation and differentiate it with respect to

temperature T, then:

dVD

= –

VG0 – VD

This dVD/dT term is the temperature coefficient equal to

about –2mV/K or –2mV/°C. The equation is simplified for

the first order derivation.

Solving for T, T = –(VG0 – VD)/(dVD/dT) provides the

temperature.

1st Example: Figure 7 for 27°C, or 300K the diode

voltage is 0.598V, thus, 300K = –(1200mV – 598mV)/

–2.0 mV/K)

2nd Example: Figure 7 for 75°C, or 350K the diode

voltage is 0.50V, thus, 350K = –(1200mV – 500mV)/

–2.0mV/K)

Converting the Kelvin scale to Celsius is simply taking the

Kelvin temp and subtracting 273 from it.

A typical forward voltage is given in the electrical charac-

teristics section of the data sheet, and Figure 7 is the plot

of this forward voltage. Measure this forward voltage at

27°C to establish a reference point. Then using the above

expression while measuring the forward voltage over

temperature will provide a general temperature monitor.

Connect a resistor between TEMP and VIN to set the cur-

rent to 100µA. See Figure 35 for an example.

Figure 7. Diode Voltage VD vs Temperature T(°C)

TEMPERATURE (°C)

–50 –25

0.3

0.5

0.8

0.4

0.7

0.6

100

4637 F07

125

ID = 100µA

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

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