Detailed Description

The MAX6642 is a temperature sensor for local

and remote temperature-monitoring applications.

Communication with the MAX6642 occurs through the

SMBus-compatible serial interface and dedicated alert

pins. ALERT asserts if the measured local or remote

temperature is greater than the software-programmed

ALERT limit.

The MAX6642 converts temperatures to digital data

either at a programmed rate of eight conversions per

second or in single conversions. Temperature data is

represented by 8 data bits (at addresses 00h and 01h),

with the LSB equal to +1°C and the MSB equal to

+128°C. Two additional bits of remote temperature data

are available in the “extended” register at address 10h

and 11h (Table 2) providing resolution of +0.25

°C.

ADC and Multiplexer

The averaging ADC integrates over a 60ms period

(each channel, typ), 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 user can ignore the results of the unused chan-

nel. If the remote-diode channel is unused, connect

DXP to GND rather than leaving DXP open.

The conversion time per channel (remote and internal)

is 125ms. If both channels are being used, then each

channel is converted four times per second. If the

external conversion-only option is selected, then the

remote temperature is measured eight times per sec-

ond. The results of the previous conversion are always

available, even if the ADC is busy.

Low-Power Standby Mode

Standby mode reduces the supply current to less than

10µA by disabling the ADC and timing circuitry. Enter

standby mode by setting the RUN bit to 1 in the config-

uration byte register (Table 4). All data is retained in

memory, and the SMBus interface is active and listen-

ing for SMBus commands. Standby mode is not a shut-

down mode. With activity on the SMBus, the device

draws more supply current (see the

Typical Operating

Characteristics). In standby mode, the MAX6642 can

be forced to perform ADC conversions through the

one-shot command, regardless of the RUN bit status.

If a standby command is received while a conversion is

in progress, the conversion cycle is truncated, and the

data from that conversion is not latched into a tempera-

ture register. The previous data is not changed and

remains available.

Supply-current drain during the 125ms conversion peri-

od is 500µA (typ). In standby mode, supply current

drops to 3µA (typ).

SMBus Digital Interface

From a software perspective, the MAX6642 appears as

a set of byte-wide registers that contain temperature

data, alarm threshold values, and control bits. A stan-

dard SMBus-compatible 2-wire serial interface is used

to read temperature data and write control bits and

alarm threshold data.

The MAX6642 employs four standard SMBus protocols:

Write Byte, Read Byte, Send Byte, and Receive Byte.

(Figures 1, 2, and 3). The shorter Receive Byte protocol

allows quicker transfers, provided that the correct data

±1°C, SMBus-Compatible Remote/Local

Temperature Sensor with Overtemperature Alarm

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5

Pin Description

PIN

NAME

FUNCTION

Ω series resistor is

recommended but not required for additional noise filtering.

2

GND

Ground

3

DXP

Combined Remote-Diode Current Source and ADC Input for Remote-Diode Channel. Place a 2200pF

capacitor between DXP and GND for noise filtering.

4

SCLK

5

SDA

6

ALERT

SMBus Alert (Interrupt) Output, Open Drain.

ALERT asserts when temperature exceeds user-set limits. See

the

ALERT Interrupts section.

—EP

Exposed Pad. Internally connected to GND. Connect to a PCB ground pad for optimal performance. Not

intended as an electrical connection point.