LM80

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SNIS102F – JUNE 1999 – REVISED MARCH 2013

A typical sequence of events upon power on of the LM80 would consist of:

1. Set WATCHDOG Limits

2. Set Interrupt Masks

3. Start the LM80 monitoring process

ANALOG INPUTS

The 8-bit ADC has a 10 mV LSB, yielding a 0V to 2.55V (2.56 - 1LSB) input range. This is true for all analog

inputs. In PC monitoring applications these inputs would most often be connected to power supplies. The 2.5,

3.3, ± 5 and ± 12 volt inputs should be attenuated with external resistors to any desired value within the input

A typical application, such as is shown in Figure 8, might select the input voltage divider to provide 1.9V at the

analog inputs of the LM80. This is sufficiently high for good resolution of the voltage, yet leaves headroom for

upward excursions from the supply of about 25%. To simplify the process of resistor selection, set the value of

R2 first. Select a value for R2 or R4 between 10 k

Ω and 100 kΩ This is low enough to avoid errors due to input

leakage currents yet high enough to both protect the inputs under overdrive conditions as well as minimize

loading of the source. Then select R1 or R3 to provide a 1.9V input as show in Figure 8.

Resistor values shown in table provide approximately 1.9V at the analog inputs.

Figure 8. Input Examples

For positive input voltages the equation for calculating R1 is as follows:

(1)

For negative input voltages the equation for Calculating R3 is as follows:

(2)

The analog inputs have internal diodes that clamp inputs exceeding 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 k

Ω should be added in series with the inputs to limit the

input current.

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