MAX1617A View Datasheet(PDF) - Maxim Integrated

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MAX1617A

Remote/Local Temperature Sensor with SMBus Serial Interface

Maxim Integrated 

MAX1617A

Remote/Local Temperature Sensor

with SMBus Serial Interface

Excess resistance in series with the remote diode causes

about +1/2°C error per ohm. Likewise, 200μV of offset

voltage forced on DXP–DXN causes about 1°C error.

A/D Conversion Sequence

If a Start command is written (or generated automatically

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-quality,

diode-connected small-signal transistor. See Table 1 for a

recommended list of diode-connected small-signal tran-

sistors. The MAX1617A 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 volt-

age 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

(VDXP - VDXN) must be less than 0.95V at 100μA; addi-

tionally, ensure the maximum VDXP (DXP voltage) ≤ (0.78

x VCC - 1.1) volts over your expected range of tempera-

ture. Large power transistors don’t work at all. Also ensure

that the base resistance is less than 100Ω. Tight specifica-

tions 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 sen-

sor 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 MAX1617A’s

effective accuracy. The thermal time constant of the

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

MAX1617A junction temperature to settle to within +1°C

after a sudden +100°C change requires about five time

constants or 12 minutes. The use of smaller packages

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

tion. 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.

Table 1. Remote-Sensor Transistor

Manufacturers

MANUFACTURER

Central Semiconductor (USA)

Motorola (USA)

National Semiconductor (USA)

Rohm Semiconductor (Japan)

Samsung (Korea)

Siemens (Germany)

Zetex (England)

MODEL NUMBER

CMPT3904

MMBT3904

MMBT3904

SST3904

KST3904-TF

SMBT3904

FMMT3904CT-ND

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

collector).

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 fast-

est rate and simultaneously sinking maximum current at

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

ALERT sinking 1mA, the typical power dissipation is VCC

x 450μA plus 0.4V x 1mA. Package theta J-A is about

150°C/W, so with VCC = 5V and no copper PC board

heatsinking, the resulting temperature rise is:

dT = 2.7mW x 150°C/W = 0.4°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 operation

places constraints on high-frequency noise rejection;

therefore, careful PC board layout and proper external

noise filtering are required for high-accuracy remote mea-

surements 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 capaci-

tance. Higher capacitance than 3300pF introduces errors

due to the rise time of the switched current 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 ampli-

tude (see Typical Operating Characteristics).

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