LM81CIMT-3 National Semiconductor, LM81CIMT-3 Datasheet - Page 16

• Manufacturer Part Number: LM81CIMT-3
• Manufacturer: National Semiconductor

Specifications of LM81CIMT-3

• Monitored Voltage 1 (typ): 2.5/3.3/5/12V
• Battery Backup Switching: No
• Watchdog Timer: Yes
• Chip Enable Signals: No
• Manual Reset: No
• Package Type: TSSOP
• Operating Supply Voltage (min): 2.8V
• Operating Supply Voltage (max): 3.8V
• Operating Temp Range: -40C to 125C
• Operating Temperature Classification: Automotive
• Power Fail Detection: No
• Mounting: Surface Mount
• Pin Count: 24
• Lead Free Status / Rohs Status: Not Compliant

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Functional Description
equal to its LOW limit will cause a flag to be set in its
Interrupt Status Register. This flag will activate the INT out-
put when its mask bit is set low. Mask bits are found in the
Interrupt Mask Registers.
5.0 LAYOUT AND GROUNDING
A separate, low-impedance ground plane for analog ground,
which provides a ground point for both GND pins, voltage
dividers and other analog components, will provide best
performance but is not mandatory. Analog components such
as voltage dividers should be located physically as close as
possible to the LM81.
The power supply bypass, the parallel combination of 10 µF
(electrolytic or tantalum) and 0.1 µF (ceramic) bypass ca-
pacitors connected between pin 12 and ground, should also
be located as close as possible to the LM81.
6.0 FAN INPUTS
The FAN1 and FAN2 inputs accept signals from fans
equipped with tachometer outputs. These are logic-level
inputs with an approximate threshold of V
tioning in the LM81 accommodates the slow rise and fall
times typical of fan tachometer outputs. The maximum input
signal range is 0 to V
from fan outputs which exceed 0 to V
division or diode clamping must be included to keep inputs
within an acceptable range, as shown in Figure 7 . R2 is
selected so that it does not develop excessive voltage due to
input leakage. R1 is selected based on R2 to provide a
minimum input of 2V and a maximum of V
low as possible to provide the maximum possible input up to
V
ence or zener diode to clamp the input level.
+
for best noise immunity. Alternatively, use a shunt refer-
+
. In the event these inputs are supplied
(Continued)
+
+
+
. R1 should be as
, either resistive
/2. Signal condi-
16
If fans can be powered while the power to the LM81 is off,
the LM81 inputs will provide diode clamping. Limit input
current to the Input Current at Any Pin specification shown in
the ABSOLUTE MAXIMUM RATINGS section. In most
cases, open collector outputs with pull-up resistors inher-
ently limit this current. If this maximum current could be
exceeded, either a larger pull up resistor should be used or
resistors connected in series with the fan inputs.
The Fan Inputs gate an internal 22.5 kHz oscillator for one
period of the Fan signal into an 8-bit counter (maximum
count = 255). The default divisor, located in the VID/Fan
Divisor Register, is set to 2 (choices are 1, 2, 4, and 8)
providing a nominal count of 153 for a 4400 rpm fan with two
pulses per revolution. Typical practice is to consider 70% of
normal RPM a fan failure, at which point the count will be
219.
Determine the fan count according to:
Note that Fan 1 and Fan 2 Divisors are programmable via
the VID/Fan Divisor Register.
Fan tachometer outputs that provide one pulse per revolu-
tion should use a divisor setting twice that of outputs that
provide two pulses per revolution. For example, a 4400 RPM
fan that provides one pulse per revolution should have the
divisor set to 4 such that the nominal counter output is 153.