Power-Up Defaults

• Interrupt latch is cleared.

• ADC begins autoconverting at a 2Hz rate.

• Command byte is set to 01h to facilitate quick

remote receive-byte queries.

-55°C limits, respectively.

• ALERT and OVERT are reset to high-Z state.

• Device is in low-frequency PWM mode, 20Hz setting.

• PWM output is off (duty factor set to 0%).

• I/O1, I/O2 are high-Z (configured as inputs).

__________Applications Information

Remote Diode Selection

Temperature accuracy depends on having a good-

quality, diode-connected, small-signal transistor.

Accuracy has been experimentally verified for all of the

devices listed in Table 12. The MAX1669 can also

directly measure the die temperature of CPUs and

other ICs having on-board temperature-sensing diodes,

such as the Intel Pentium II.

The transistor must be a small-signal type having a rel-

atively high forward voltage; otherwise, the A/D input

voltage 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 must be <0.95V at 100µA; check to

ensure this is true at the lowest expected temperature.

Do not use large power transistors. Also, ensure that

the base resistance is <100

Ω. Tight specifications for

forward-current gain (+50 to +150, for example) indi-

cate that the manufacturer has good process controls

Series resistance causes +1/2°C error per ohm. When

monitoring the temperature of a remote unit’s internal

diode, ensure that trace series resistance does not

introduce significant error.

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 is required for high-accuracy

remote measurements in electrically noisy environ-

ments.

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

capacitance. Capacitance higher than 3300pF intro-

duces errors due to the rise time of the switched-cur-

rent source.

Nearly all noise sources tested cause the ADC mea-

surements to be higher than the actual temperature,

typically by +1°C to +10°C, depending on frequency

and amplitude (see Typical Operating Characteristics).

FAN Application Circuits

In PWM mode, the output impedance at FAN is <50

Ω,

enabling it to drive an N-channel MOSFET as shown in

the Typical Operating Circuit. Return the source of the

N-channel MOSFET to the system power ground, away

from the ground of the MAX1669. For 3.3V applications,

use low-threshold N-channel MOSFETs (Table 1).

In DAC mode, the FAN output can be linearly controlled

(Figure 5). Upon power-up, the fan is off. The N-channel

MOSFET is biased at the threshold of turning on. As

turned on at power-up, use the circuit shown in Figure 6.

Fan Controller and Remote Temperature Sensor

with SMBus Serial Interface

16

______________________________________________________________________________________

Table 12. Component Manufacturers

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

collector).

MMBT3906

KST3906

MANUFACTURER

MODEL NUMBER

SOT23 BJT

Central Semiconductor (USA)

CMPT3906

Fairchild Semiconductor (USA)

MMBT3906

Rohm Semiconductor (Japan)

SST3906

Samsung (Korea)

MOSFET N-CHANNEL

International Rectifier (USA)

IRF7201

MOSFET P-CHANNEL

International Rectifier (USA)

IRF7205

Motorola (USA)

MMBT3906

Fairchild Semiconductor (USA)

FDN359AN