MAX1253 Datasheet, PDF(26/29 Page) Maxim Integrated Products – Stand-Alone, 10-Channel, 12-Bit System Monitors with Internal Temperature Sensor and VDD Monitor

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MAX1253 Datasheet, PDF (26/29 Pages) Maxim Integrated Products – Stand-Alone, 10-Channel, 12-Bit System Monitors with Internal Temperature Sensor and VDD Monitor

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Stand-Alone, 10-Channel, 12-Bit System Monitors

with Internal Temperature Sensor and VDD Monitor

Automatic Reference Shutdown

The MAX1253/MAX1254 enter an automatic shutdown

mode when in reference mode 00 or when the sample

wait is greater than 80Âµs in reference mode 01. Using

either of these reference modes and a sample wait

period as long as the application allows results in the

lowest power consumption.

Temperature Measurement

The MAX1253/MAX1254 support both single-ended

and differential temperature measurements. The design

decision between the two types of measurements

depends on the desired level of accuracy and on type

and/or number of temperature sensors. The superior

common-mode rejection and lower noise of the differ-

ential mode reduces measurement errors and provides

higher accuracy, while single-ended measurements

require a lower number of connections, resulting in a

simpler implementation and a higher number of moni-

tored points for each MAX1253/MAX1254.

Differential Temperature Measurement

Connect the anode of a diode-connected transistor to

the even input channel and the cathode to the odd

input channel of an input pair configured for differential

temperature measurement (AIN0/AIN1, AIN2/AIN3,

AIN4/AIN5, or AIN6/AIN7). Run the two sensor connec-

tion lines parallel to each other with minimum spacing.

This improves temperature measurement accuracy by

minimizing the differential noise between the two lines,

since they have equal exposure to most sources of

noise. For further improved noise rejection, shield the

two sensor connections by running them between

ground planes, when available.

Configure the MAX1253/MAX1254 inputs for differential

temperature measurement in the input configuration

channel number in the channel enable register (see

Table 4).

Table 16. Remote Sensor Transistor

Manufacturers

MANUFACTURER

MODEL NUMBER

Central Semiconductor (USA) CMPT3904

Fairchild Semiconductors (USA) MMBT3904

Motorola (USA)

MMBT3904

Rohm Semiconductor (Japan) SST3904

Siemens (Germany)

SMB3904

Zetex (England)

FMMT3904CT-ND

Diodes Inc.

MMBT3904

Single-Ended Temperature Measurement

Connect the anode of a diode-connected transistor to

the input channel and the cathode to ground. Choose

ground connections for sensors away from high-current

return paths to avoid the introduction of errors caused by

voltage drops in the board/system ground, which is the

main drawback for single-ended measurements.

Practical options for better accuracy are the use of a

star-configured subsystem ground or a signal ground

plane; to isolate the anode sensor connection trace away

from board and system noise sources; or to shield it with

ground lines and ground planes (when available) to pre-

vent accuracy degradation in the temperature measure-

ments caused by magnetic/electric noise induction.

Configure the MAX1253/MAX1254 input used for single-

ended temperature measurement in the input configura-

tion register (see Tables 9 and 10) and enable the analog

input in the channel-enable register (see Table 4).

Remote Temperature Sensor Selection

Temperature-sensing accuracy depends on having a

good-quality, diode-connected, small-signal transistor

as a sensor. Accuracy has been experimentally verified

for 2N3904-type devices. The transistor must be a

small-signal type with low base resistance. Tight speci-

fications for forward current gain (+50 to +150, for

example) indicate that the manufacturer has good

process controls and that the devices have consistent

VBE characteristics. CPU on-board sensors and other

ICsâ on-board temperature-sensing devices can also

be used (see Table 16 for recommended devices).

OUTPUT CODE

11....111

11....110

11....101

FULL-SCALE

TRANSITION

FS = VREF

ZS = 0

00....011

00....010

1 LSB = VREF

4096

00....001

00....000

0123

INPUT VOLTAGE (LSB)

FS = 3/2 LSB

Figure 10. Unipolar Transfer Function, Full Scale (FS) = VREF

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