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LTM4650-1

46501fc

For more information www.linear.com/LTM4650-1

applicaTions inForMaTion

and by definition must always be less than ID. Combining

all of the constants into one term:

KD =

η•k

where KD = 8.62 • 10−5, and knowing ln(ID/IS) is always

positive because ID is always greater than IS, leaves us

with the equation that:

VD =T KELVIN

(

)•KD •In

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

mon knowledge that a silicon diode biased with a current

sourcehasanapproximate–2mV/°Ctemperaturerelation-

ship (Figure 10), 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.

its impedance is equal to the resistor at the ring frequency.

Calculatedby:ZC=1/(2πfC).Thesevaluesareagoodplace

to start with. Modification to these components should

be made to attenuate the ringing with the least amount

of power loss.

Temperature Monitoring

Measuring the absolute temperature of a diode is pos-

sible due to the relationship between current, voltage

and temperature described by the classic diode equation:

ID =IS •e

η• VT

⎛

⎝

⎜

⎞

⎠

⎟

VD =η• VT •In

where ID is the diode current, VD is the diode voltage, η

is the ideality factor (typically close to 1.0) and IS (satura-

tion current) is a process dependent parameter. VT can

be broken out to:

VT =

k • T

where T is the diode junction temperature in Kelvin, q is

the electron charge and k is Boltzmann’s constant. VT is

approximately 26mV at room temperature (298K) and

scales linearly with Kelvin temperature. It is this linear

temperature relationship that makes diodes suitable tem-

perature sensors. The IS term in the previous equation is

the extrapolated current through a diode junction when

the diode has zero volts across the terminals. The IS term

varies from process to process, varies with temperature,

Figure 10. Diode Voltage VD vs Temperature T(°C)

TEMPERATURE (°C)

–50 –25

0.3

0.5

0.8

0.4

0.7

0.6

100

46501 F10

125

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