J

A

JA

DD S

DD

OUT

L

T = T +

(V I ) + (V

- V

) I

T

ª

º

¬

¼

(-13.6 mV /

oC) u

V =

T + 2637 mV

(-13.6 mV /

oC) u

V - 2365 mV =

(T - 20

oC)

1958 mV - 2365 mV

50

oC - 20oC

u

¹

·

V - 2365 mV =

(T - 20

oC)

11

LMT87

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SNIS170E – JANUARY 2014 – REVISED OCTOBER 2017

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(1)

For information on self-heating and thermal response time see section Mounting and Thermal Conductivity.

For example, if the user wanted to resolve this equation, over a temperature range of 20°C to 50°C, they would

proceed as follows:

(4)

(5)

(6)

Using this method of linear approximation, the transfer function can be approximated for one or more

temperature ranges of interest.

8.4 Device Functional Modes

8.4.1 Mounting and Thermal Conductivity

The LMT87 can be applied easily in the same way as other integrated-circuit temperature sensors. It can be

glued or cemented to a surface.

To ensure good thermal conductivity, the backside of the LMT87 die is directly attached to the GND pin. The

temperatures of the lands and traces to the other leads of the LMT87 will also affect the temperature reading.

Alternatively, the LMT87 can be mounted inside a sealed-end metal tube, and can then be dipped into a bath or

screwed into a threaded hole in a tank. As with any IC, the LMT87 and accompanying wiring and circuits must be

kept insulated and dry, to avoid leakage and corrosion. This is especially true if the circuit may operate at cold

temperatures where condensation can occur. If moisture creates a short circuit from the output to ground or VDD,

the output from the LMT87 will not be correct. Printed-circuit coatings are often used to ensure that moisture

cannot corrode the leads or circuit traces.

The thermal resistance junction to ambient (RθJA or θJA) is the parameter used to calculate the rise of a device

junction temperature due to its power dissipation. Use Equation 7 to calculate the rise in the LMT87 die

temperature:

where

•

TA is the ambient temperature,

•

IS is the supply current,

•

ILis the load current on the output,

•

and VO is the output voltage.

(7)

For example, in an application where TA = 30°C, VDD = 5 V, IS = 5.4 μA, VOUT = 2231 mV, and IL = 2 μA, the

junction temperature would be 30.014°C, showing a self-heating error of only 0.014°C. Because the junction

temperature of the LMT87 is the actual temperature being measured, take care to minimize the load current that

the LMT87 is required to drive. Thermal Information(1) shows the thermal resistance of the LMT87.

8.4.2 Output Noise Considerations

A push-pull output gives the LMT87 the ability to sink and source significant current. This is beneficial when, for

example, driving dynamic loads like an input stage on an analog-to-digital converter (ADC). In these applications

the source current is required to quickly charge the input capacitor of the ADC. The LMT87 is ideal for this and

other applications which require strong source or sink current.

The LMT87 supply-noise gain (the ratio of the AC signal on VOUT to the AC signal on VDD) was measured during

bench tests. The typical attenuation is shown in Figure 8 found in the Typical Characteristics section. A load

capacitor on the output can help to filter noise.

For operation in very noisy environments, some bypass capacitance should be present on the supply within

approximately 5 centimeters of the LMT87.