Datasheet TMP35, TMP36, TMP37 (Analog Devices) - 8

TMP35/TMP36/TMP37 Data Sheet FUNCTIONAL DESCRIPTION
An equivalent circuit for the TMP35/TMP36/TMP37 micropower,
+VS
centigrade temperature sensors is shown in Figure 22. The core
SHUTDOWN
of the temperature sensor is a band gap core that comprises
25µA
transistors Q1 and Q2, biased by Q3 to approximately 8 µA. The band gap core operates both Q1 and Q2 at the same col ector
3X 2X
current level; however, because the emitter area of Q1 is 10 times that of Q2, the VBE of Q1 and the VBE of Q2 are not equal by the following relationship:
Q2 1X Q4
 A 
R1
∆V = V ×  E,Q1  BE T l  n 
Q1
 A
10X
E,Q2 
R3
Resistors R1 and R2 are used to scale this result to produce
VOUT R2
the output voltage transfer characteristic of each temperature
7.5µA Q3
sensor and, simultaneously, R2 and R3 are used to scale the VBE of
2X
Q1 as an offset term in V
6X
006 OUT. Table 4 summarizes the differences in the output characteristics of the three temperature sensors.
GND
00337- Figure 22. Temperature Sensor Simplified Equivalent Circuit The output voltage of the temperature sensor is available at the emitter of Q4, which buffers the band gap core and provides load current drive. The current gain of Q4, working with the
Table 4. TMP35/TMP36/TMP37 Output Characteristics
available base current drive from the previous stage, sets the
Offset Output Voltage Output Voltage
short-circuit current limit of these devices to 250 µA.
Sensor Voltage (V) Scaling (mV/°C) at 25°C (mV)
TMP35 0 10 250 TMP36 0.5 10 750 TMP37 0 20 500 Rev. H | Page 8 of 19