MAX6699EE34+【PDF 13/19 页】IC TEMP MONITOR 5CH 16-QSOP

sensor designed for a nominal ideality factor n

NOMINAL

is used to measure the temperature of a diode with a

different ideality factor n1. The measured temperature

T

M

can be corrected using:

where temperature is measured in Kelvin and

n

NOMIMAL

for the MAX6699 is 1.008. As an example,

assume you want to use the MAX6699 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:

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

sured temperature is +82.87癈 (356.02K), an error of

-2.13癈.

Series Resistance Cancellation

Some thermal diodes on high-power ICs can have

excessive series resistance, which can cause tempera-

ture-measurement errors with conventional remote tem-

perature sensors. Channel 1 of the MAX6699 has a

series resistance cancellation feature (enabled by bit 3

of the Configuration 1 register) that eliminates the effect

of diode series resistance. Set bit 3 to 1 if the series

resistance is large enough to affect the accuracy of

channel 1. The series resistance cancellation function

increases the conversion time for channel 1 by 125ms.

This feature cancels the bulk resistance of the sensor

and any other resistance in series (wire, contact resis-

tance, etc.). The cancellation range is from 0 to 100&.

Discrete Remote Diodes

When the remote-sensing diode is a discrete transistor,

its collector and base must be connected together.

Table 10 lists examples of discrete transistors that are

appropriate for use with the MAX6699. The transistor

must be a small-signal type with a relatively 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 10礎, and at

the lowest expected temperature, the forward voltage

must be less than 0.95V at 100礎. Large power transis-

tors must not be used. Also, ensure that the base resis-

tance is less than 100&. Tight specifications for forward

current gain (50 < ?<150, for example) indicate that the

manufacturer has good process controls and that the

devices have consistent V

BE

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

range. A variety of discrete transistors have variations in

remote temperature readings of less than ?癈. Still, it is

good design practice to verify good consistency of tem-

perature readings with several discrete transistors from

any manufacturer under consideration.

T

n

T

ACTUAL M

NOMINAL

M

=

?/DIV>

=

?/DIV>

=

1

1 008

1 002

1 00599

.

( .

)

T T

n

M

ACTUAL

NOMINAL

=

?/DIV>

1

5-Channel Precision Temperature Monitor

______________________________________________________________________________________ 13

Table 6. Configuration 3 Register

BIT

NAME

POR

STATE

FUNCTION

7(MSB)

Reserved

0

6

Reserved

0

5

Reserved

4

Reserved

3

Mask OVERT 4

0

Channel 4 Remote-Diode OVERT Mask Bit. Set to logic 1 to mask channel 4

OVERT.

2

Reserved

0

1

Reserved

0

Mask OVERT 1

0

Channel 1 Remote-Diode OVERT Mask Bit. Set to logic 1 to mask channel 1

OVERT.

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