ADT7463ARQZ-R7 datasheet(25/52 Pages) ONSEMI

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REV. C

ADT7463

–25–

Configuring the ADT7463 THERM Pin as an Output

In addition to the ADT7463 being able to monitor THERM as

an input, the ADT7463 can optionally drive THERM low as an

output. The user can preprogram system critical thermal limits.

If the temperature exceeds a thermal limit by 0.25

°C, THERM

asserts low. If the temperature is still above the thermal limit on

the next monitoring cycle, THERM stays low. THERM remains

asserted low until the temperature is equal to or below the

thermal limit. Since the temperature for that channel is mea-

sured only every monitoring cycle, once THERM asserts it is

guaranteed to remain low for at least one monitoring cycle.

The THERM pin can be configured to assert low if the

Remote 1, Local, or Remote 2 Temperature THERM limits get

exceeded by 0.25

°C. The THERM limit registers are at loca-

tions 0x6A, 0x6B, and 0x6C, respectively. Setting Bit 3 of

Registers 0x5F, 0x60, and 0x61 enables the THERM output

feature for the Remote 1, Local, and Remote 2 Temperature

channels, respectively. Figure 28 shows how the THERM pin

asserts low as an output in the event of a critical overtemperature.

THERM LIMIT

+0.25�C

THERM LIMIT

TEMP

THERM

ADT7463

MONITORING

CYCLE

Figure 28. Asserting THERM as an Output, Based

on Tripping THERM Limits

FAN DRIVE USING PWM CONTROL

The ADT7463 uses pulse-width modulation (PWM) to control

fan speed. This relies on varying the duty cycle (or on/off ratio)

of a square wave applied to the fan to vary the fan speed. The

external circuitry required to drive a fan using PWM control is

extremely simple. A single NMOSFET is the only drive device

required. The specifications of the MOSFET depend on the

maximum current required by the fan being driven. Typical

notebook fans draw a nominal 170 mA, and so SOT devices can

be used where board space is a concern. In desktops, fans can

typically draw 250 mA to 300 mA each. If you drive several fans

in parallel from a single PWM output or drive larger server fans,

the MOSFET needs to handle the higher current requirements.

The only other stipulation is that the MOSFET should have a

gate voltage drive, VGS < 3.3 V for direct interfacing to the

PWM_OUT pin. VGS can be greater than 3.3 V as long as the

pull-up on the gate is tied to 5 V. The MOSFET should also have

a low on resistance to ensure that there is not significant voltage

drop across the FET. This reduces the voltage applied across

the fan and therefore the maximum operating speed of the fan.

Figure 29 shows how a 3-wire fan may be driven using PWM

control.

ADT7463

TACH/AIN

PWM

4.7k�

10k�

3.3V

12V

FAN

NDT3055L

TACH

1N4148

Figure 29. Driving a 3-Wire Fan Using an

N-Channel MOSFET

Figure 29 uses a 10 k

Ω pull-up resistor for the TACH signal. This

assumes that the TACH signal is open-collector from the fan. In

all cases, the TACH signal from the fan must be kept below 5 V

maximum to prevent damaging the ADT7463. If in doubt as to

whether the fan used has an open-collector or totem pole TACH

output, use one of the input signal conditioning circuits shown

in the Fan Speed Measurement section of the data sheet.

Figure 30 shows a fan drive circuit using an NPN transistor

such as a general-purpose MMBT2222. While these devices are

inexpensive, they tend to have much lower current handling

capabilities and higher on resistance than MOSFETs. When

choosing a transistor, care should be taken to ensure that it

meets the fan’s current requirements.

Ensure that the base resistor is chosen such that the transistor is

saturated when the fan is powered on.

ADT7463

TACH/AIN

PWM

4.7k�

10k�

470�

3.3V

12V

FAN

MMBT2222

1N4148

TACH

Figure 30. Driving a 3-Wire Fan Using an NPN Transistor

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