8.1.1 Capacitive Loads

The LMT89 device handles capacitive loading well. Without any precautions, the LMT89 device can drive any capacitive load less than 300 pF as shown in Figure 2. The specified temperature range the LMT89 device has a maximum output impedance of 160 Ω. In an extremely noisy environment it may be necessary to add some filtering to minimize noise pickup. TI recommends that 0.1 μF be added from V⁺ to GND to bypass the power supply voltage, as shown in Figure 2. In a noisy environment it may even be necessary to add a capacitor from the output to ground with a series resistor as shown in Figure 2. A 1-μF output capacitor with the 160-Ω maximum output impedance and a 200-Ω series resistor will form a 442-Hz lowpass filter. Because the thermal time constant of the LMT89 device is much slower, the overall response time of the LMT89 device will not be significantly affected.

In situations where a transient load current is placed on the circuit output, the series resistance value may be increased to compensate for any ringing that may be observed.

Figure 2. LMT89 No Decoupling Required for Capacitive Loads Less Than 300 pF

Figure 3. LMT89 With Filter for Noisy Environment and Capacitive Loading Greater Than 300 pF

8.2.1 Full-Range Centigrade Temperature Sensor

Figure 4. Full-Range Celsius (Centigrade) Temperature Sensor (−55°C to 130°C) Operating from a Single Li-Ion Battery Cell

8.2.1.2 Detailed Design Procedure

The LMT89 device output is shown in Equation 4.

Equation 4. V_O = (−3.88 × 10⁻⁶ × T²) + (−1.15 × 10⁻² × T) + 1.8639

Solve for T as shown in Equation 5:

Equation 5.

where

• T is temperature, and V_O is the measured output voltage of the LMT89 device. Equation 5 is the most accurate equation that can be used to calculate the temperature of the LMT89 device.

An alternative to the quadratic equation a second order transfer function can be determined using the least squares method shown in Equation 6.

Equation 6. T = (−2.3654 × V_O²) + (−78.154 × V_O ) + 153.857

where

• T is temperature express in °C and V_O is the output voltage expressed in volts.

A linear transfer function can be used over a limited temperature range by calculating a slope and offset that give best results over that range. A linear transfer function can be calculated from the parabolic transfer function of the LMT89 device. The slope of the linear transfer function can be calculated using Equation 7.

Equation 7. m = −7.76 × 10⁻⁶× T − 0.0115,

where

• T is the middle of the temperature range of interest and m is in V/°C. For example for the temperature range of T_MIN = −30 to T_MAX = 100°C

Equation 8. T = 35°C

and

Equation 9. m = −11.77 mV/°C

The offset of the linear transfer function can be calculated using Equation 10.

Equation 10. b = (V_OP(T_MAX) + V_OP(T) − m × (T_MAX+T)) / 2

where

• V_OP(T_MAX) is the calculated output voltage at T_MAX using the parabolic transfer function for V_O.
• V_OP(T) is the calculated output voltage at T using the parabolic transfer function for V_O.

The best fit linear transfer function for many popular temperature ranges was calculated in Table 3. As shown in Table 3, the error introduced by the linear transfer function increases with wider temperature ranges.

Table 3. First Order Equations Optimized for Different Temperature Ranges

TEMPERATURE RANGE		LINEAR EQUATION	MAXIMUM DEVIATION OF LINEAR EQUATION FROM PARABOLIC EQUATION (°C)
Tmin (°C)	Tmax (°C)
−55	130	VO = −11.79 mV/°C × T + 1.8528 V	±1.41
−40	110	VO = −11.77 mV/°C × T + 1.8577 V	±0.93
−30	100	VO = −11.77 mV/°C × T + 1.8605 V	±0.70
-40	85	VO = −11.67 mV/°C × T + 1.8583 V	±0.65
−10	65	VO = −11.71 mV/°C × T + 1.8641 V	±0.23
35	45	VO = −11.81 mV/°C × T + 1.8701 V	±0.004
20	30	VO = –11.69 mV/°C × T + 1.8663 V	±0.004

8.2.1.3 Application Curve

Figure 5. Output Voltage vs Temperature

8.2.2 Centigrade Thermostat

Figure 6. Centigrade Thermostat

8.2.2.2 Detailed Design Procedure

The threshold values can be calculated using Equation 11 and Equation 12.

Equation 11.

Equation 12.

8.2.2.3 Application Curve

Figure 7. Thermostat Output Waveform

8.3.1 Conserving Power Dissipation With Shutdown

The LMT89 device draws very little power therefore it can simply be shutdown by driving its supply pin with the output of an logic gate as shown in Figure 8.

Figure 8. Conserving Power Dissipation With Shutdown

8.3.2 Analog-to-Digital Converter Input Stage

Most CMOS ADCs found in ASICs have a sampled data comparator input structure that is notorious for causing problems for analog output devices, such as the LMT89 and many op amps. The cause of this difficulty is the requirement of instantaneous charge of the input sampling capacitor in the ADC. This requirement is easily accommodated by the addition of a capacitor. Because not all ADCs have identical input stages, the charge requirements will vary necessitating a different value of compensating capacitor. This ADC is shown as an example only. If a digital output temperature is required, refer to devices such as the LM74 device.

Figure 9. Suggested Connection to a Sampling Analog-to-Digital Converter Input Stage