Remote Temperature Sensor

with SMBus Serial Interface

6

_______________________________________________________________________________________

Pin Description

Detailed Description

The MAX1618 is a temperature sensor designed to

work in conjunction with an external microcontroller

(µC) or other intelligence in thermostatic, process-con-

trol, or monitoring applications. The µC is typically a

power-management or keyboard controller, generating

SMBus serial commands by “bit-banging” general-pur-

pose input-output (GPIO) pins or through a dedicated

SMBus interface block.

Essentially an 8-bit serial analog-to-digital converter

(ADC) with a sophisticated front end, the MAX1618

contains a switched-current source, a multiplexer, an

ADC, an SMBus interface, and the associated control

logic (Figure 1). Temperature data from the ADC is

loaded into a data register, where it is automatically

compared with data previously stored in over/under-

temperature alarm threshold registers. The alarm

threshold registers can be set for hysteretic fan control.

ADC and Multiplexer

The averaging ADC integrates over a 30ms period (typ)

with excellent noise rejection. The ADC converts at a

rate of 16Hz. The multiplexer automatically steers bias

currents through the remote diode, measures the for-

ward voltage, and computes the temperature.

The DXN input is biased at 0.65V above ground by an

internal diode to set up the analog-to-digital (A/D)

inputs for a differential measurement. The worst-case

DXP-DXN differential input voltage range is 0.25V to

0.95V.

Excess resistance in series with the remote diode

causes about +1/2°C error/

Ω. A 200µV offset voltage at

DXP-DXN causes about +1°C error.

A/D Conversion Sequence

If a Start command is written (or generated automatical-

ly in the free-running autoconvert mode), the result of

the measurement is available after the end of conver-

sion. A BUSY status bit in the status byte shows that the

device is performing a new conversion. The result of the

previous conversion is always available even when the

ADC is busy.

SMBus Serial-Data Input/Output. Open drain.

SMBDATA

9

SMBus Alert (Interrupt) Output. Open drain.

ALERT

10

Combined Current Source and A/D Positive Input. Do not leave DXP floating. Place a 2200pF capacitor

between DXP and DXN for noise filtering.

DXP

5

Supply Voltage Input. Bypass to GND with a 0.1µF capacitor.

6

Hardware-Standby Input. Temperature and comparison threshold data are retained in standby mode.

Low = standby mode. High = operating mode.

STBY

7

SMBus Serial-Clock Input

SMBCLK

8

Combined Current Sink and A/D Negative Input. DXN is normally biased to a diode voltage above

ground.

DXN

4

Ground

GND

3

PIN

SMBus Slave Address Select Input. (See Table 6.) ADD0 and ADD1 are sampled upon power-up. Excess

capacitance (>50pF) at the address pins when floating may cause address-recognition problems.

ADD1

2

SMBus Slave Address Select Input. (See Table 6.) ADD0 and ADD1 are sampled upon power-up. Excess

capacitance (>50pF) at the address pins when floating may cause address-recognition problems.

ADD0

1

FUNCTION

NAME