LM87CIMT National Semiconductor, LM87CIMT Datasheet - Page 16

• Manufacturer Part Number: LM87CIMT
• Description: Temperature Sensor IC
• Manufacturer: National Semiconductor

Specifications of LM87CIMT

• Peak Reflow Compatible (260 C): No
• Ic Function: Temperature Sensor IC
• Supply Voltage Max: 3.8V
• Leaded Process Compatible: No
• Mounting Type: Surface Mount
• Operating Temperature Range: -40°C To +125°C
• Temperature Sensor Function: Temp Sensor
• Interface Type: Serial (2-Wire)
• Output Type: Digital
• Package Type: TSSOP
• Operating Temperature (min): -40C
• Operating Temperature (max): 125C
• Operating Temperature Classification: Automotive
• Operating Supply Voltage (typ): 3.3V
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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Divide by 1
Divide by 2
Divide by 4
Divide by 8
Mode Select
Counts are based on 2 pulses per revolution tachometer out-
puts.
7.0 DAC OUTPUT
The LM87 provides an 8-bit DAC (Digital-to-Analog Convert-
er) with an output range of 0 to 2.5 volts (9.80 mV LSB). This
DAC can be used in any way, but in most applications of the
LM87 the DAC will be used for fan control. Typically the DAC
output would be amplified to provide the up to 12 volt drive
required by the fan. At power-on the DAC provides full output,
insuring that full fan speed is the default condition. Care
should be taken such that the analog circuitry tied to this pin
does not drive this pin above 2.5 V. Doing so will place the
LM87 in NAND tree test mode which will make all pins inputs.
After the first SMBus communication with the LM87, it will
leave NAND tree test mode and all inputs/outputs will function
normally.
Fans do not start reliably at reduced voltages, so operation at
a reduced voltage should be preceded by a brief (typically 1
second) excursion to full operating voltage, then reduce the
voltage. Most fans do not operate at all below 5 to 7 volts. At
those lower voltages the fan will simply consume current, dis-
sipate power, and not operate, and such conditions should be
avoided.
The output of the amplifier can be configured to provide a high
or low side pass transistor. A high side pass transistor sim-
plifies the coupling of tachometer outputs to the tachometer
(c) Fan with Tach Pull-Up to +12V and Diode Clamp
RPM
4400
3080
2640
Nominal RPM
8800
4400
2200
1100
Time per Revolution
13.64 ms
19.48 ms
22.73 ms
Time per Revolution
13.64 ms
27.27 ms
54.54 ms
6.82 ms
FIGURE 7. Alternatives for Fan Inputs
10099514
Given Speed in Decimal
16
(d) Fan with Strong Tach Pull-Up or Totem Pole Output and
Counts for “Divide by 2”
Counts for the
inputs of the LM87 since the fan remains grounded. Low side
drive will require AC coupling along with clamping at the LM87
input to prevent negative excursions.
A typical circuit for fan drive is shown in
8.0 TEMPERATURE MEASUREMENT SYSTEM
The LM87 temperature sensor(s) and ADC produce 8-bit
two's-complement temperature data. One internal diode junc-
tion temperature, and up to two external junction tempera-
tures can be monitored. A digital comparator compares the
temperature data to the user-programmable High, Low, and
Hardware Limit setpoints, and Hysteresis values.
(Default) in Decimal
(maximum counts)
153
153
153
153
153 counts
219 counts
255 counts
Diode Clamp
70% RPM
6160
3080
1540
770
Time per Revolution
Figure
Typical RPM
for 70% RPM
Comments
70% RPM
60% RPM
19.48 ms
38.96 ms
77.92 ms
9.74 ms
13.
10099515