LM87CIMT/NOPB National Semiconductor, LM87CIMT/NOPB Datasheet - Page 18

• Manufacturer Part Number: LM87CIMT/NOPB
• Description: IC INTERFACE SER MONITOR 24TSSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM87CIMT/NOPB

• Function: Hardware Monitor
• Topology: ADC (Sigma Delta), Comparator, Fan Speed Control, Register Bank
• Sensor Type: External & Internal
• Sensing Temperature: -40°C ~ 125°C, External Sensor
• Output Type: I²C™/SMBus™
• Output Alarm: No
• Output Fan: Yes
• Voltage - Supply: 2.8 V ~ 3.8 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 24-TSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *LM87CIMT; *LM87CIMT/NOPB; LM87CIMT

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Noise on the digital lines, overshoot greater than V
dershoot less than GND, may prevent successful SMBus
communication with the LM87. SMBus no acknowledge is the
most common symptom, causing unnecessary traffic on the
bus. Although, the SMBus maximum frequency of communi-
cation is rather low (400 kHz max) care still needs to be taken
to ensure proper termination within a system with multiple
parts on the bus and long printed circuit board traces. A low-
pass filter, in series with the SMBCLK and SMBData, has
event that noise does couple to the diode lines it would
be ideal if it is coupled common mode. That is equally to
the D+ and D− lines.
Avoid routing diode traces in close proximity to power
supply switching or filtering inductors.
Avoid running diode traces close to or parallel to high
speed digital and bus lines. Diode traces should be kept
at least 2 cm. apart from the high speed digital traces.
FIGURE 9. Recommended Diode Trace Layout
CC
and un-
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been added internally to the LM87 for noise immunity. The
lowpass filter has a typical cutoff frequency of 20MHz. Addi-
tional noise immunity can be achieved by placing a resistor
(4.7k to 5.1k Ohms) in series with the SMBCLK input as close
to the LM87 as possible. This resistance, in conjunction with
the IC input capacitance, reduces high frequency noise seen
at the SMBCLK input and increases the reliability of commu-
nications.
If it is necessary to cross high speed digital traces, the
diode traces and the high speed digital traces should
cross at a 90 degree angle.
The ideal place to connect the LM87's GND pin is as
close as possible to the Processors GND associated with
the sense diode. For the Pentium II this would be pin A14.
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