LM87 datasheet(8/32 Pages) NSC

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Test Circuit

Typical Performance Characteristics

DAC Power Supply Sensitivity

10099536

Functional Description

1.0 GENERAL DESCRIPTION

The LM87 provides 7 analog inputs, an internal junction type

temperature sensor, two remote junction temperature sens-

ing channels, a Delta-Sigma ADC (Analog-to-Digital Con-

verter), a DAC output, 2 fan speed counters, WATCHDOG

registers, and a variety of inputs and outputs on a single

chip. A two wire SMBus Serial Bus interface is included. The

LM87 performs power supply, temperature, fan control and

fan monitoring for personal computers.

The analog inputs are useful for monitoring several power

supplies present in a typical computer. The LM87 includes

internal resistor dividers that scale external Vccp1, Vccp2,

+2.5V, +5.0 V, +12 V and internal +3.3V power supply volt-

ages to a 3/4 scale nominal ADC output. Two additional

inputs, +AIN1 and +AIN2 (2.5V full scale) are input directly

with no resistive dividers. The LM87 ADC continuously con-

verts the scaled inputs to 8-bit digital words. Measurement of

negative voltages (such as -5 V and -12 V power supplies)

can be accommodated with an external resistor divider ap-

plied to the +AIN1 or +AIN2 inputs. Internal and external

temperature is converted to 8-bit two’s-complement digital

words witha1˚C LSB.

Fan inputs measure the period of tachometer pulses from

the fans, providing a higher count for lower fan speeds. The

fan inputs are Schmitt-Trigger digital inputs with an accept-

able range of0VtoV

+ and a transition level of approxi-

mately V

+/2. Full scale fan counts are 255 (8-bit counter) and

this represents a stopped or very slow fan. Nominal speeds,

based on a count of 153, are programmable from 1100 to

10099506

FIGURE 3. Digital Output Load Test Circuitry

LM87 数据表(PDF) 8 Page - National Semiconductor (TI)

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