LMT01-Q1_17 Datasheet, PDF (22/37 Pages) Texas Instruments – Accurate 2-Pin Digital Output Temperature Sensor with Pulse Count Interface

LMT01-Q1
SNIS192A – NOVEMBER 2016 – REVISED APRIL 2017
System Examples (continued)
VP
LMT01
VN
3.3V
34 and
125 µA 7.5k
MMBT3906
VDD
MCU/
FPGA/
ASIC
GPIO
100k
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Note: the VN of the LMT01-Q1 must be connected to the MCU GND.
Figure 28. Common Ground With High Side Signal
9 Power Supply Recommendations
Because the LMT01-Q1 is only a 2-pin device the power pins are common with the signal pins, thus the LMT01-
Q1 has a floating supply that can vary greatly. The LMT01-Q1 has an internal regulator that provides a stable
voltage to internal circuitry.
Care must be taken to prevent reverse biasing of the LMT01-Q1 as exceeding the absolute maximum ratings
may cause damage to the device.
Power supply ramp rate can effect the accuracy of the first result transmitted by the LMT01-Q1. As shown in
Figure 29 with a 1ms rise time the LMT01-Q1 output code is at 1286 which converts to 30.125°C. The scope
photo shown in Figure 30 reflects what happens when the rise time is too slow. As can be seen the power supply
(yellow trace) is still ramping up to final value while the LMT01-Q1 (red trace) has already started a conversion.
This causes the output pulse count to decrease from the 1286, shown previously, to 1282 or 29.875°C. Thus, for
slow ramp rates TI recommends that the first conversion be discarded. For even slower ramp rates more than
one conversion may have to be discarded as TI recommends that either the power supply be within final value
before a conversion is used or that ramp rates be faster than 2.5 ms.
Yellow trace = 1 V/div, Red trace = 100 mV/div, Time Base = 20
ms/div
T_A= 30°C LMT01 Pulse Count = 1286
VP-VN = 3.3 V
Rise Time = 1 ms
Figure 29. Output Pulse Count With Appropriate Power
Supply Rise Time
Yellow trace = 1V/div, Red trace = 100 mV/div, Time base = 20
ms/div
T_A=30°C
LMT01 Pulse Count = 1282
VP-VN=3.3 V
Rise Time = 100 ms
Figure 30. Output Pulse Count With Slow Power Supply
Rise Time
22
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