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MAX6642ATT9A-T

±1°C, SMBus-Compatible Remote/Local Temperature Sensor with Overtemperature Alarm

Maxim Integrated 

MAX6642

±1°C, SMBus-Compatible Remote/

Local Temperature Sensor with

Overtemperature Alarm

III. A thermal diode on the substrate of an IC is normally

a PNP with its collector grounded. Connect the anode

(emitter) to DXP and the cathode to GND of the

MAX6642.

If a sense transistor with an ideality factor other than

1.008 is used, the output data is different from the data

obtained with the optimum ideality factor. Fortunately,

the difference is predictable.

Assume a remote-diode sensor designed for a nominal

ideality factor nNOMINAL is used to measure the tem-

perature of a diode with a different ideality factor n1.

The measured temperature TM can be corrected using:

TM

=

TACTUAL

⎛

⎜

⎝

n1

nNOMINAL

⎞

⎟

⎠

where temperature is measured in Kelvin and

nNOMIMAL for the MAX6642 is 1.008.

As an example, assume you want to use the MAX6642

with a CPU that has an ideality factor of 1.002. If the

diode has no series resistance, the measured data is

related to the real temperature as follows:

TACTUAL

=

TM

⎛

⎝⎜

nNOMINAL

n1

⎞

⎠⎟

=

TM

⎛

⎝⎜

1.008 ⎞

1.002 ⎠⎟

=

TM (1.00599)

For a real temperature of +85°C (358.15K), the mea-

sured temperature is +82.91°C (356.02K), an error of

-2.13°C.

Effect of Series Resistance

Series resistance in a sense diode contributes addition-

al errors. For nominal diode currents of 10µA and

100µA, the change in the measured voltage due to

series resistance is:

∆VM = RS (100µA - 10µA) = 90µA ✕ RS

Since +1°C corresponds to 198.6µV, series resistance

contributes a temperature offset of:

90 µV

Ω

198.6 µV

= 0.453 °C

Ω

°C

Assume that the diode being measured has a series

resistance of 3Ω. The series resistance contributes an

offset of:

Table 7. Remote-Sensor Transistor

Manufacturers

MANUFACTURER

MODEL NO.

Central Semiconductor (USA)

CMPT3906

Rohm Semiconductor (USA)

SST3906

Samsung (Korea)

KST3906-TF

Siemens (Germany)

SMBT3906

Zetex (England)

FMMT3906CT-ND

Note: Discrete transistors must be diode connected (base short-

ed to collector).

3Ω × 0.453 °C = +1.36°C

Ω

The effects of the ideality factor and series resistance

are additive. If the diode has an ideality factor of 1.002

and series resistance of 3Ω, the total offset can be cal-

culated by adding error due to series resistance with

error due to ideality factor:

1.36°C - 2.13°C = -0.77°C

for a diode temperature of +85°C.

In this example, the effect of the series resistance and

the ideality factor partially cancel each other.

Discrete Remote Diodes

When the remote-sensing diode is a discrete transistor,

connect its collector and base together. Table 7 lists

examples of discrete transistors that are appropriate for

use with the MAX6642.

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 at

the highest expected temperature must be greater than

0.25V at 10µA, and at the lowest expected tempera-

ture, the forward voltage must be less than 0.95V at

100µA. Large power transistors must not be used. Also,

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

specifications for forward current gain (50 < ß <150, for

example) indicate that the manufacturer has good

process controls and that the devices have consistent

VBE characteristics.

Manufacturers of discrete transistors do not normally

specify or guarantee ideality factor. This is normally not

a problem since good-quality discrete transistors tend

to have ideality factors that fall within a relatively narrow

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