MAX1619 View Datasheet(PDF) - Maxim Integrated

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MAX1619

Remote/Local Temperature Sensor with Dual Alarm Outputs and SMBus Serial Interface

Maxim Integrated 

Remote/Local Temperature Sensor with Dual-

Alarm Outputs and SMBus Serial Interface

A/D Conversion Sequence

If a Start command is written (or generated automatical-

ly in the free-running auto-convert mode), both channels

are converted, and the results of both measurements

are available after the end of conversion. A BUSY status

bit in the status byte shows that the device is actually

performing a new conversion; however, even if the ADC

is busy, the results of the previous conversion are

always available.

Remote-Diode Selection

Temperature accuracy depends on having a good-qual-

ity, diode-connected small-signal transistor. Accuracy

has been experimentally verified for all the devices list-

ed in Table 1. The MAX1619 can also directly measure

the die temperature of CPUs and other integrated cir-

cuits having on-board temperature-sensing diodes.

The transistor must be a small-signal type with a rela-

tively high forward voltage; otherwise, the A/D input

voltage range can be violated. The forward voltage

must be greater than 0.25V at 10µA; check to ensure

this is true at the highest expected temperature. The

forward voltage must be less than 0.95V at 100µA;

check to ensure this is true at the lowest expected

temperature. Large power transistors don’t work. Also,

ensure that the base resistance is less than 100Ω. Tight

specifications 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.

For heatsink mounting, the 500-32BT02-000 thermal

sensor from Fenwal Electronics is a good choice. This

device consists of a diode-connected transistor, an

aluminum plate with screw hole, and twisted-pair cable

(Fenwal Inc., Milford, MA, 508-478-6000).

Thermal Mass and Self-Heating

Thermal mass can seriously degrade the MAX1619’s

effective accuracy. The thermal time constant of the

QSOP-16 package is about 4sec in still air. To settle to

within +1°C after a sudden +100°C change, the

MAX1619 junction temperature requires about five time

constants. The use of smaller packages for remote sen-

sors, such as SOT23s, improves the situation. Take

care to account for thermal gradients between the heat

source and the sensor, and ensure that stray air cur-

rents across the sensor package do not interfere with

measurement accuracy.

Self-heating does not significantly affect measurement

accuracy. Remote-sensor self-heating due to the diode

current source is negligible. For the local diode, the

Table 1. Remote-Sensor Transistor

Manufacturers

MANUFACTURER

Central Semiconductor (USA)

Fairchild Semiconductor (USA)

Motorola (USA)

Rohm Semiconductor (Japan)

Siemens (Germany)

Zetex (England)

MODEL NUMBER

CMPT3904

MMBT3904

MMBT3904

SST3904

SMBT3904

FMMT3904CT-ND

Note: Transistors must be diode-connected (base shorted to

collector).

worst-case error occurs when auto-converting at the

fastest rate and simultaneously sinking maximum cur-

rent at the ALERT and OVERT outputs. For example, at

an 8Hz rate and with ALERT and OVERT each sinking

1mA, the typical power dissipation is:

(VCC)(450µA) + 2(0.4V)(1mA)

Package θJA is about 120°C/W, so with VCC = 5V and

no copper PC board heatsinking, the resulting tempera-

ture rise is:

∆T = 3.1mW(120°C/W) = 0.36°C

Even with these contrived circumstances, it is difficult

to introduce significant self-heating errors.

ADC Noise Filtering

The ADC is an integrating type with inherently good

noise rejection, especially of low-frequency signals such

as 60Hz/120Hz power-supply hum. Micropower opera-

tion places constraints on high-frequency noise rejection;

therefore, careful PC board layout and proper external

noise filtering are required for high-accuracy remote

measurements in electrically noisy environments.

High-frequency EMI is best filtered at DXP and DXN

with an external 2200pF capacitor. This value can be

increased to about 3300pF (max), including cable

capacitance. Capacitance higher than 3300pF intro-

duces errors due to the rise time of the switched cur-

rent source.

Nearly all noise sources tested cause the ADC measure-

ments to be higher than the actual temperature, typically

by +1°C to +10°C, depending on the frequency and

amplitude (see Typical Operating Characteristics).

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