MAX6653 Просмотр технического описания (PDF) - Maxim Integrated

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MAX6653

Temperature Monitors and PWM Fan Controllers

Maxim Integrated 

Temperature Monitors and

PWM Fan Controllers

Remote Diode Considerations

The accuracy of the remote temperature measurements

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

(actually a transistor). The MAX6653/MAX6663/

MAX6664 are optimized for n = 1.008, which is the typi-

cal value for the Intel Pentium III. A thermal diode on

the substrate of an IC is normally a PNP with its collec-

tor grounded. DXP should be connected to the anode

(emitter) and DXN should be connected to the cathode

(base) of this PNP.

When the remote-sensing diode is a discrete transistor,

its collector and base should be connected together.

Table 15 lists examples of discrete transistors that are

appropriate for use with the MAX6653/MAX6663/

MAX6664.

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.

Table 15. Remote-Sensor Transistor

Manufacturers

MANUFACTURER

Central Semiconductor (USA)

Rohm Semiconductor (USA)

Samsung (Korea)

Siemens (Germany)

Zetex (England)

MODEL NO.

CMPT3904

SST3904

KST3904-TF

SMBT3904

FMMT3904CT-ND

Note: Discrete transistors must be diode connected (base

shorted to collector).

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. We have observed variations in remote tempera-

ture readings of less than ±2°C with a variety of dis-

crete transistors. Still, it is sound design practice to

verify good consistency of temperature readings with

several discrete transistors from any manufacturer

under consideration.

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