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LTM4636-1

46361fa

For more information www.linear.com/LTM4636-1

where VD appears to increase with temperature. It is com-

mon knowledge that a silicon diode biased with a current

source has an approximate –2mV/°C temperature rela-

tionship (Figure 7), which is at odds with the equation. In

fact, the IS term increases with temperature, reducing the

ln(ID/IS) absolute value yielding an approximate –2mV/°C

composite diode voltage slope.

APPLICATIONS INFORMATION

To obtain a linear voltage proportional to temperature

we cancel the IS variable in the natural logarithm term to

remove the IS dependency from the equation 1. This is

accomplished by measuring the diode voltage at two cur-

rents I1, and I2, where I1 = 10 • I2) and subtracting we get:

∆VD T(KELVIN)•KD •IN

– T(KELVIN)•KD •IN

Combining like terms, then simplifying the natural log

terms yields:

∆VD = T(KELVIN) • KD • lN(10)

and redefining constant

K'D KD •IN(10)

198µV

yields

∆VD = K'D • T(KELVIN)

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

46361 F07

125

Solving for temperature:

T(KELVIN)

∆VD

K'D

( CELSIUS) T(KELVIN) – 273.15

where

300°K = 27°C

means that is we take the difference in voltage across the

diode measured at two currents with a ratio of 10, the

resulting voltage is 198V per Kelvin of the junction with

a zero intercept at 0 Kelvin.

The diode connected NPN transistor at the TEMP pin

can be used to monitor the internal temperature of the

LTM4636-1.

Figure 8. The Two Images Show the LTM4636-1 Operating at 1V

at 40A and 3.3V at 35A from a 12V Input. Both Images Reflect

Only a 40°C to 45°C Rise Above Ambient at Full Load Current

with 200LPM

(8b)

(8a)

VIN

VOUT

IOUT

AIR FLOW

200 LFM

VIN

VOUT

IOUT

AIR FLOW

3.3

200 LFM

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