LM80CIMT-3 National Semiconductor, LM80CIMT-3 Datasheet - Page 13

• Manufacturer Part Number: LM80CIMT-3
• Manufacturer: National Semiconductor

Specifications of LM80CIMT-3

• Watchdog Timer: Yes
• Chip Enable Signals: No
• Package Type: TSSOP
• Operating Supply Voltage (min): 2.8V
• Operating Supply Voltage (max): 5.75V
• Operating Temp Range: -25C to 125C
• Operating Temperature Classification: Commercial
• Power Fail Detection: No
• Mounting: Surface Mount
• Pin Count: 24
• Battery Backup Switching: No
• Manual Reset: No
• Lead Free Status / RoHS Status: Not Compliant

imize loading of the source. Then select R1 or R3 to provide
a 1.9V input as show in
For positive input voltages the equation for calculating R1 is
as follows:
For negative input voltages the equation for Calculating R3 is
as follows:
The analog inputs have internal diodes that clamp inputs ex-
ceeding the power supply and ground. Exceeding any analog
input has no detrimental effect on other channels. The input
diodes will also clamp voltages appearing at the inputs of an
un-powered LM80. External resistors should be included to
limit input currents to the values given in the ABSOLUTE
MAXIMUM RATINGS for Input Current At Any Pin. Inputs with
the attenuator networks will usually meet these requirements.
If it is possible for inputs without attenuators to be turned on
while LM80 is powered off, additional resistors of about 10
kΩ should be added in series with the inputs to limit the input
current.
5.0 LAYOUT AND GROUNDING
Analog inputs will provide best accuracy when referred to the
AGND pin or a supply with low noise. A separate, low-
+2.5V
+3.3V
+5.0V
+12V
table provide approximately 1.9V at the analog inputs.
FIGURE 6. Input Examples. Resistor values shown in
Measure-
Voltage
ments
−12V
(V
−5V
S
)
R3 = [ (V
23.7 kΩ
22.1 kΩ
24 kΩ
160 kΩ
R1 or R3 R2 or R4
R1 = [ (V
160 kΩ
36 kΩ
S
Figure
− V
S
75 kΩ
30 kΩ
14.7 kΩ
30.1 kΩ
− V
IN
35.7 kΩ
16.2 kΩ
) / (V
6.
IN
) / V
IN
− 5V)] R4
IN
] R2
+1.9V
+1.9V
+1.9V
+1.9V
( ADC code 190)
Analog Inputs
Voltage
+1.9V
+1.9V
at
10004011
13
impedance ground plane for analog ground, which provides
a ground point for the voltage dividers and analog compo-
nents, will provide best performance but is not mandatory.
Analog components such as voltage dividers should be lo-
cated physically as close as possible to the LM80.
The power supply bypass, the parallel combination of 10 μF
(electrolytic or tantalum) and 0.1 μF (ceramic) bypass capac-
itors connected between pin 9 and ground, should also be
located as close as possible to the LM80.
6.0 FAN INPUTS
Inputs are provided for signals from fans equipped with
tachometer outputs. These are logic-level inputs with an ap-
proximate threshold of V
accommodates the slow rise and fall times typical of fan
tachometer outputs. The maximum input signal range is 0 to
V
which exceed 0 to V+, either resistive division or diode clamp-
ing must be included to keep inputs within an acceptable
range, as shown in
develop excessive error voltage due to input leakage. R1 is
selected based on R2 to provide a minimum input of 2V and
a maximum of V
the maximum possible input up to V
Alternatively, use a shunt reference or zener diode to clamp
the input level.
If fans can be powered while the power to the LM80 is off, the
LM80 inputs will provide diode clamping. Limit input current
to the Input Current at Any Pin specification shown in the AB-
SOLUTE MAXIMUM RATINGS section. In most cases, open
collector outputs with pull-up resistors inherently limit this cur-
rent. 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
Fan Divisor/RST_OUT/OS Register.
FAN1 and FAN2 inputs can also be programmed to be level
sensitive digital inputs.
Fans that provide only one pulse per revolution would require
a divisor set twice as high as fans that provide two pulses,
thus maintaining a nominal fan count of 153. Therefore the
divisor should be set to 4 for a fan that provides 1 pulse per
revolution with a nominal RPM of 4400.
+
. In the event these inputs are supplied from fan outputs
+
. R1 should be as low as possible to provide
Figure
+
/2. Signal conditioning in the LM80
7. R2 is selected so that it does not
+
for best noise immunity.
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