[

]

TJ = TA + TJA (VDDIQ) + (VDD - VO) IL

LM94021

www.ti.com

SNIS138E – FEBRUARY 2005 – REVISED JUNE 2013

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

temperature ranges of interest.

MOUNTING AND THERMAL CONDUCTIVITY

The LM94021 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 LM94021 die is directly attached to the GND pin (Pin

2). The temperatures of the lands and traces to the other leads of the LM94021 will also affect the temperature

reading.

Alternatively, the LM94021 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 LM94021 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 LM94021 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 (

θJA) is the parameter used to calculate the rise of a device junction

temperature due to its power dissipation. The equation used to calculate the rise in the LM94021's die

temperature is

(6)

where TA is the ambient temperature, IQ is the quiescent current, IL is the load current on the output, and VO is

the output voltage. For example, in an application where TA = 30°C, VDD = 5 V, IDD = 9 μA, Gain Select = 11,

VOUT = 2.231 mV, and IL = 2 μA, the junction temperature would be 30.021°C, showing a self-heating error of

only 0.021°C. Since the LM94021's junction temperature is the actual temperature being measured, care should

be taken to minimize the load current that the LM94021 is required to drive. Table 3 shows the thermal

resistance of the LM94021.

Table 3. LM94021 Thermal Resistance

DEVICE NUMBER

PACKAGE NUMBER

THERMAL RESISTANCE (

θJA)

LM94021BIMG

DCK0005A

415°C/W

NOISE CONSIDERATIONS

The LM94021 has excellent noise rejection (the ratio of the AC signal on VOUT to the AC signal on VDD). During

bench tests, sine wave rejection of

−54 dB or better was observed over 200 Hz to 10 kHz; Also, −28 dB or better

was observed from 10 kHz to 1 MHz. A load capacitor on the output can help filter noise; for example, a 1 nF

load capacitor resulted in

−51 dB or better from 200 Hz to 1 MHz.

There is no specific requirement for the use of a bypass capacitor close to the LM94021 because it does not

draw transient currents. For operation in very noisy environments, some bypass capacitance may be required.

The capacitance does not need to be in close proximity to the LM94021. The LM94021 has been bench tested

successfully with a bypass capacitor as far as 6 inches away. In fact, it can be powered by a properly-bypassed

logic gate.

CAPACITIVE LOADS

The LM94021 handles capacitive loading well. In an extremely noisy environment, or when driving a switched

sampling input on an ADC, it may be necessary to add some filtering to minimize noise coupling. Without any

precautions, the LM94021 can drive a capacitive load less than or equal to 1100 pF as shown in Figure 14. For

capacitive loads greater than 1100 pF, a series resistor may be required on the output, as shown in Figure 15.

Copyright © 2005–2013, Texas Instruments Incorporated

Submit Documentation Feedback

13

Product Folder Links: LM94021