MAX1617MEE+T【PDF 8/20 页】IC REMOTE/LOCAL TEMP SNSR 16QSOP

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Remote/Local Temperature Sensor

with SMBus Serial Interface

MAX1617

Maxim Integrated

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. See Table 1

for a recommended list of diode-connected small-signal

transistors. The MAX1617 can also directly measure the

die temperature of CPUs and other integrated circuits

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 10礎; check to ensure

this is true at the highest expected temperature. The

forward voltage (V

DXP

- V

DXN

) must be less than 0.95V

at 100礎; additionally, ensure the maximum V

DXP

(DXP

voltage) d (0.78 x V

- 1.1) volts over your expected

range of temperature. Large power transistors dont

work at all. Also, ensure that the base resistance is less

than 100? Tight specifications for forward-current gain

(+50 to +150, for example) indicate that the manufac-

turer has good process controls and that the devices

have consistent VBE characteristics.

For heat-sink 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 MAX1617s

effective accuracy. The thermal time constant of the

QSOP-16 package is about 140sec in still air. For the

MAX1617 junction temperature to settle to within +1癈

after a sudden +100癈 change requires about five time

constants or 12 minutes. The use of smaller packages

for remote sensors, such as SOT23s, improves the situ-

ation. Take care to account for thermal gradients

between the heat source and the sensor, and ensure

that stray air currents 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

worst-case error occurs when auto-converting at the

fastest rate and simultaneously sinking maximum cur-

rent at the ALERT output. For example, at an 8Hz rate

and with ALERT sinking 1mA, the typical power dissi-

pation is V

x 450礎 plus 0.4V x 1mA. Package theta

J-A is about 150癈/W, so with V

= 5V and no copper

PCB heat-sinking, the resulting temperature rise is:

dT = 2.7mW x 150癈/W = 0.4癈

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 rejec-

tion; therefore, careful PCB 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. Higher capacitance 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癈 to +10癈, depending on the frequency and

amplitude (see Typical Operating Characteristics).

CMPT3904

Central Semiconductor (USA)

MMBT3904

Motorola (USA)

MMBT3904

SST3904

Rohm Semiconductor (Japan)

KST3904-TF

Samsung (Korea)

FMMT3904CT-ND

Zetex (England)

MANUFACTURER

MODEL NUMBER

SMBT3904

Siemens (Germany)

Table 1. Remote-Sensor Transistor

Manufacturers

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

collector).

National Semiconductor (USA)

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