MAX6654MEE Maxim Integrated Products, MAX6654MEE Datasheet - Page 8

• Manufacturer Part Number: MAX6654MEE
• Description: Board Mount Temperature Sensors
• Manufacturer: Maxim Integrated Products

Specifications of MAX6654MEE

• Full Temp Accuracy: +/- 5 C
• Package / Case: QSOP-16
• Digital Output - Bus Interface: Serial (2-Wire, I2C)
• Digital Output - Number Of Bits: 10 bit + Sign
• Maximum Operating Temperature: + 125 C
• Minimum Operating Temperature: - 55 C
• Output Type: Digital
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

1°C Accurate Remote/Local Temperature
Sensor with SMBus Serial Interface
measure ambient temperature; when measuring local
temperature, it senses the temperature of the PC board
to which it is soldered. The leads provide a good ther-
mal path between the PC board traces and the
MAX6654’s die. Thermal conductivity between the
MAX6654’s die and the ambient air is poor by compari-
son. Because the thermal mass of the PC board is far
greater than that of the MAX6654, the device follows
temperature changes on the PC board with little or no
perceivable delay.
When measuring temperature with discrete remote sen-
sors, the use of smaller packages, such as SOT23s,
yields the best thermal response times. Take care to
account for thermal gradients between the heat source
and the sensor, and ensure that stray air currents
across the sensor package do not interfere with mea-
surement accuracy. When measuring the temperature
of a CPU or other IC with an on-chip sense junction,
thermal mass has virtually no effect; the measured tem-
perature of the junction tracks the actual temperature
within a conversion cycle.
Self-heating does not significantly affect measurement
accuracy. Remote-sensor self-heating due to the diode
current source is negligible. For the local diode, the
worst-case error occurs when autoconverting at the
fastest rate and simultaneously sinking maximum cur-
rent at the ALERT output. For example, at an 8Hz rate
and with ALERT sinking 1mA, the typical power dissi-
pation is V
A is about 150°C/Ω, so with V
PC board heat sinking, the resulting temperature rise is:
Even with these contrived circumstances, it is difficult
to introduce significant self-heating errors.
The ADC is an integrating type with inherently good
noise rejection, especially of low-frequency signals such
as 60Hz/120Hz power-supply hum. Micropower opera-
tion places constraints on high-frequency noise rejection;
therefore, careful PC board layout and proper external
noise filtering are required for high-accuracy remote
measurements in electrically noisy environments.
High-frequency EMI is best filtered at DXP and DXN with
an external 2200pF capacitor. This value can be
increased to about 3300pF (max), including cable
capacitance. Capacitance >3300pF introduces errors
due to the rise time of the switched current source.
Nearly all noise sources tested cause the ADC measure-
ments to be higher than the actual temperature, typically
by +1°C to +10°C, depending on the frequency and
amplitude (see Typical Operating Characteristics).
8
_______________________________________________________________________________________
CC
∆T = 2.7mW x 150°C/W = 0.4°C
x 450µA + 0.4V x 1mA. Package theta J-
ADC Noise Filtering
CC
= 5V and no copper
1) Place the MAX6654 as close as practical to the
2) Do not route the DXP-DXN lines next to the deflec-
3) Route the DXP and DXN traces in parallel and in
4) Connect guard traces to GND on either side of the
5) Route through as few vias and crossunders as pos-
6) When introducing a thermocouple, make sure that
7) Use wide traces. Narrow traces are more inductive
Figure 2. Recommended DXP/DXN PC Traces
10MILS
10MILS
remote diode. In a noisy environment, such as a
computer motherboard, this distance can be 4
inches to 8 inches (typ) or more, as long as the
worst noise sources (such as CRTs, clock genera-
tors, memory buses, and ISA/PCI buses) are avoid-
ed.
tion coils of a CRT. Also, do not route the traces
across a fast memory bus, which can easily intro-
duce +30°C error, even with good filtering.
Otherwise, most noise sources are fairly benign.
close proximity to each other, away from any high-
voltage traces, such as +12VDC. Leakage currents
from PC board contamination must be dealt with
carefully since a 20MΩ leakage path from DXP to
ground causes about +1°C error.
DXP-DXN traces (Figure 2). With guard traces in
place, routing near high-voltage traces is no longer
an issue.
sible to minimize copper/solder thermocouple
effects.
both the DXP and the DXN paths have matching
thermocouples. In general, PC-board-induced ther-
mocouples are not a serious problem. A copper-
solder thermocouple exhibits 3µV/°C, and it takes
about 200µV of voltage error at DXP-DXN to cause
a +1°C measurement error. So, most parasitic ther-
mocouple errors are swamped out.
and tend to pick up radiated noise. The 10mil
widths and spacings recommended in Figure 2
aren’t absolutely necessary (as they offer only a
GND
DXN
GND
DXP
PC Board Layout
10MILS
MINIMUM
10MILS