Applications Information

Figures 3–6 show various configurations.

Remote-Diode Considerations

When using an external thermal diode, temperature

accuracy depends upon having a good-quality, diode-

connected, small-signal transistor. Accuracy has been

experimentally verified for a variety of discrete small-

signal transistors, some of which are listed in Table 5.

The MAX6643/MAX6644/MAX6645 can also directly

measure the die temperature of CPUs and other ICs

with on-board temperature-sensing diodes.

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

tively high forward voltage. This ensures that the input

voltage is within the ADC input voltage range. The for-

ward voltage must be greater than 0.25V at 10µA at the

highest expected temperature. The forward voltage

must be less than 0.95V at 100µA at the lowest expect-

ed temperature. The base resistance has to be less

than 100

Ω. Tight specification of forward-current gain

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

turer has good process control and that the devices

have consistent characteristics.

Effect of Ideality Factor

The accuracy of the remote temperature measurements

depends on the ideality factor (n) of the remote diode

(actually a transistor). The MAX6643/MAX6644/MAX6645

are optimized for n = 1.01, which is typical of many dis-

crete 2N3904 and 2N3906 transistors. It is also near the

ideality factors of many widely available CPUs, GPUs, and

FPGAs. However, any time a sense transistor with a differ-

ent ideality factor is used, the output data is different.

Fortunately, the difference is predictable. Assume a

remote-diode sensor designed for a nominal ideality fac-

where temperature is measured in Kelvin.

As mentioned above, the nominal ideality factor of the

MAX6643/MAX6644/MAX6645 is 1.01. As an example,

assume the MAX6643/MAX6644/MAX6645 are config-

ured with a CPU that has an ideality factor of 1.008. If

the diode has no series resistance, the measured data

is related to the real temperature as follows:

For a real temperature of +60°C (333.15K), the mea-

sured temperature is 59.33°C (332.49K), which is an

error of -0.66°C.

TT

n

n

T

1.01

1.008

T

ACTUAL

M

NOMINAL

1

MM

=

⎛

⎝⎜

⎞

⎠⎟

=

⎛

⎝⎜

⎞

⎠⎟

=

()

.

1 00198

TT

n

n

M

ACTUAL

1

NOMINAL

=

⎛

⎝⎜

⎞

⎠⎟

Automatic PWM Fan-Speed Controllers with

Overtemperature Output

8

_______________________________________________________________________________________

MANUFACTURER

MODEL NO.

Central Semiconductor (USA)

CMPT3906

Rohm Semiconductor (USA)

SST3906

Samsung (Korea)

KST3906-TF

Siemens (Germany)

SMBT3906

Table 5. Remote-Sensor Transistor

Manufacturers

OT2

OT1

L SUFFIX

00

60

0

High-Z

65

01

70

High-Z

0

75

High-Z

High-Z

80

High-Z

1

85

10

90

1

High-Z

95

1

1

100

Table 4. Setting the Overtemperature

Table 3. Configuring the FAN_IN_ Inputs with TACHSET

VDD

GND

UNCONNECTED

TACHSET

FAN_IN1

FAN_IN2

FAN_IN1

FAN_IN2

FAN_IN1

FAN_IN2

MAX6643

Tachometer

Tachometer

Do not connect

to GND

Do not connect

to GND

Disables fan-

failure detection

Disables fan-

failure detection

MAX6644

Tachometer

Tachometer

Current sense

Current sense

Locked rotor

Locked rotor

MAX6645

Tachometer

Tachometer

Current sense

Current sense

Locked rotor

Locked rotor

High-Z = high impedance