ADT7483AARQZ-R7 datasheet(20/24 Pages) ONSEMI

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ADT7483A

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registers are now outside the new limits, an ALERT is

generated, even though the ADT7483A is still in standby.

SENSOR FAULT DETECTION

The ADT7483A has internal sensor fault detection circuitry

located at its D+ input. This circuit can detect situations where

a remote diode is not connected, or is incorrectly connected, to

the ADT7483A. A simple voltage comparator trips if the voltage

at D+ exceeds VDD − 1 V (typical), signifying an open circuit

between D+ and D−. The output of this comparator is checked

when a conversion is initiated. Bit 2 (D1 OPEN flag) of the

Status Register 1 (Address 0x02) is set if a fault is detected on

the Remote 1 channel. Bit 2 (D2 OPEN flag) of the Status

Remote 2 channel. If the ALERT pin is enabled, setting this flag

will cause ALERT to assert low.

If a remote sensor is not used with the ADT7483A, then the D+

and D− inputs of the ADT7483A need to be tied together to

prevent the OPEN flag from being continuously set.

Most temperature sensing diodes have an operating temperature

range of −55°C to +150°C. Above 150°C, they lose their

semiconductor characteristics and approximate conductors

instead. This results in a diode short, setting the OPEN flag. The

remote diode in this case no longer gives an accurate temperature

measurement. A read of the temperature result register will give

the last good temperature measurement. The user should be

aware that, while the diode fault is triggered, the temperature

measurement on the remote channels may not be accurate.

INTERRUPT SYSTEM

The ADT7483A has two interrupt outputs, ALERT and

THERM. Both outputs have different functions and behavior.

ALERT is maskable and responds to violations of software

programmed temperature limits or an open-circuit fault on the

remote diode. THERM is intended as a fail-safe interrupt

output that cannot be masked.

If the Remote 1, Remote 2, or local temperature exceeds the

programmed high temperature limits, or equals or exceeds the

low temperature limits, the ALERT output is asserted low. An

open-circuit fault on the remote diode also causes ALERT to

assert. ALERT is reset when serviced by a master reading its

device address, provided the error condition has gone away and

the status register has been reset.

Similarly, the THERM output asserts low if the Remote 1,

Remote 2, or local temperature exceeds the programmed

THERM limits. The THERM temperature limits should

normally be equal to or greater than the high temperature

limits. THERM is automatically reset when the temperature

falls back within the (THERM − Hysteresis) limit. The local and

remote THERM limits are set by default to 85°C. An hysteresis

value can be programmed, in which case, THERM resets when

the temperature falls to the limit value minus the hysteresis

value. This applies to both local and remote measurement

channels. The power-on hysteresis default value is 10°C, but this

may be reprogrammed to any value after power-up.

The hysteresis loop on the THERM outputs is useful when

THERM is used for on/off control of a fan. The user’s system

can be set up so that when THERM asserts, a fan can be

switched on to cool the system. When THERM goes high again,

the fan can be switched off. Programming an hysteresis value

protects from fan jitter, wherein the temperature hovers around

the THERM limit and the fan is constantly being switched.

Table 17. THERM Hysteresis

THERM Hysteresis

Binary Representation

0°C

0 000 0000

1°C

0 000 0001

10°C

0 000 1010

Figure 20 shows how the THERM and ALERT outputs operate.

The ALERT output can be used as an SMBALERT to signal to

the host via the SMBus that the temperature has risen. If the

temperature continues to increase, the THERM output can be

used to turn on a fan to cool the system. This method ensures

that there is a fail-safe mechanism to cool the system, without

the need for host intervention.

THERM LIMIT

HIGH TEMP LIMIT

THERM LIMIT-HYSTERESIS

RESET BY MASTER

ALERT

THERM

100°C

TEMPERATURE

90°C

80°C

70°C

60°C

50°C

40°C

Figure 20. Operation of the ALERT and THERM Outputs

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