+145°C Precision SMBus-Compatible Remote/

Local Sensors with Overtemperature Alarms

Detailed Description

The MAX6646/MAX6647/MAX6649 are temperature sen-

sors designed to work in conjunction with a microproces-

sor or other intelligence in thermostatic, process-control,

or monitoring applications. Communication with the

MAX6646/MAX6647/MAX6649 occurs through the

SMBus-compatible serial interface and dedicated alert

and overtemperature outputs. ALERT asserts if the mea-

sured local or remote temperature is greater than the

software-programmed ALERT high limit or less than the

ALERT low limit in the MAX6649. ALERT also asserts, in

the MAX6649, if the remote-sensing diode pins are short-

ed or unconnected. The overtemperature alarm, OVERT,

asserts if the software-programmed OVERT limit is

exceeded. OVERT can be connected to fans, a system

shutdown, a clock throttle control, or other thermal-man-

agement circuitry.

The MAX6646/MAX6647/MAX6649 convert temperatures

to digital data either at a programmed rate or in single

conversions. Temperature data is represented as 11 bits,

with the LSB equal to 0.125°C. The “main” temperature

data registers (at addresses 00h and 01h) are 8-bit regis-

ters that represent the data as 8 bits with the full-scale

reading indicating the diode fault status (Table 1). The

remaining 3 bits of temperature data are available in the

“extended” registers at addresses 11h and 10h (Table 2).

ADC and Multiplexer

The averaging ADC integrates over a 60ms period

(each channel, typically), with excellent noise rejection.

The multiplexer automatically steers bias currents

through the remote and local diodes. The ADC and

associated circuitry measure each diode’s forward volt-

age and compute the temperature based on this volt-

age. Both channels are automatically converted once

the conversion process has started, either in free-run-

ning or single-shot mode. If one of the two channels is

not used, the device still performs both measurements,

and the results of the unused channel can be ignored.

If the remote-diode channel is unused, connect DXP to

DXN rather than leaving the inputs open.

Pin Description

PIN

NAME

FUNCTION

1

series resistor is

recommended but not required for additional noise filtering.

2

DXP

Combined Remote-Diode Current Source and A/D Positive Input for Remote-Diode Channel. DO NOT

LEAVE DXP UNCONNECTED; connect DXP to DXN if no remote diode is used. Place a 2200pF

capacitor between DXP and DXN for noise filtering.

3

DXN

Combined Remote-Diode Current Sink and A/D Negative Input. DXN is internally connected to ground.

4

OVERT

Overtemperature Alert/Interrupt Output, Open Drain.

OVERT is logic low when the temperature is above

the software-programmed threshold.

5

GND

Ground

6

ALERT

SMBus Alert (Interrupt) Output, Open Drain.

ALERT asserts when temperature exceeds limits (high or

low temperature).

ALERT stays asserted until acknowledged by either reading the status register or by

successfully responding to an alert response address, provided that the fault condition no longer

exists. See the

ALERT Interrupts section.

7

SDA

SMBus Serial-Data Input/Output, Open Drain

8

SCLK

SMBus Serial-Clock Input

5

TEMP (

°C)

DIGITAL OUTPUT

+145

1001 0001

+130

1000 0010

+128

1000 0000

+25

0001 1001

0

0000 0000

<0

0000 0000

Diode fault

(short or open)

1111 1111

Table 1. Main Temperature Data Register

Format (00h, 01h)