ADT7421ARMZ-2RL View Datasheet(PDF) - ON Semiconductor

Part Name

Description

MFG CO.

ADT7421ARMZ-2RL

Precision Temperature Sensor with Beta Compensation (for <45 nm Geometries)

ON Semiconductor 

ADT7421

Theory of Operation

The ADT7421 is a local and remote temperature sensor

and over/undertemperature alarm, with the added ability to

automatically cancel the effect of beta variations in

embedded thermal transistors in small geometry CPU’s.

When the ADT7421 is operating normally, the on−board

ADC operates in a free running mode. The analog input

multiplexer alternately selects either the on−chip

temperature sensor to measure its local temperature or the

remote temperature sensor. The ADC digitizes these signals

and the results are stored in the local and remote temperature

value registers.

The local and remote measurement results are compared

with the corresponding high, low, and THERM temperature

limits, stored in eight on−chip registers. Out−of−limit

comparisons generate flags that are stored in the status

register. A result that exceeds the high temperature limit or

the low temperature limit causes the ALERT output to

assert. The ALERT output also asserts if an external

transistor fault is detected. Exceeding the THERM

temperature limits causes the THERM output to assert low.

The ALERT output can be reprogrammed as a second

THERM output.

The limit registers are programmed and the device

controlled and configured via the serial SMBus. The

contents of any register are also read back via the SMBus.

Control and configuration functions consist of switching

the device between normal operation and standby mode,

selecting the temperature measurement range, masking or

enabling the ALERT output, switching Pin 6 between

ALERT and THERM2, and selecting the conversion rate.

Beta Variation Cancellation

The ADT7421 includes a new temperature sensing

method which cancels out the effect of varying Beta factors

being observed when different currents are applied to the

embedded thermal transistor in small geometry processes.

This method also ensure consistent and accurate

temperature measurements between CPU’s.

Series Resistance Cancellation

Parasitic resistance to the D+ and D− inputs to the

ADT7421, seen in series with the remote transistor, is

caused by a variety of factors, including PCB track

resistance and track length. This series resistance appears as

a temperature offset in the remote sensor’s temperature

measurement. This error typically causes a 0.5°C offset per

ohm of parasitic resistance in series with the remote

transistor.

The ADT7421 automatically cancels the effect of this

series resistance on the temperature reading, giving a more

accurate result, without the need for user characterization of

this resistance. The ADT7421 is designed to automatically

cancel typically up to 50 W of resistance. By using an

advanced temperature measurement method, this process is

transparent to the user.

Temperature Measurement Method

A simple method of measuring temperature is to exploit

the negative temperature coefficient of a transistor,

measuring the base emitter voltage (VBE) of a transistor

operated at constant current. However, this technique

requires calibration to null the effect of the absolute value of

VBE, which varies from device to device.

The technique used in the ADT7421 measures the change

in VBE when the device operates at three different currents.

Previous devices used only two operating currents, but it is

the use of a third current that allows automatic cancellation

of resistances in series with the external temperature sensor.

Figure 9 shows the input signal conditioning used to

measure the output of an external temperature sensor. This

figure shows the external sensor as a substrate transistor, but

it can equally be a discrete transistor. If a discrete transistor

is used, the collector is not grounded but is linked to the base.

To prevent ground noise interfering with the measurement,

the more negative terminal of the sensor is not referenced to

ground, but is biased above ground by an internal transistor

at the D− input. C1 may be added as a noise filter (a

recommended maximum value of 2200 pF).

To measure DVBE, the operating current through the

sensor is switched among three related currents. As shown

in Figure 9, N1 × I and N2 × I are different multiples of the

current, I. The currents through the temperature transistor

are switched between I and N1 × I, giving VBE1; and then

between I and N2 × I, giving DVBE2. The temperature is then

calculated using the two DVBE measurements. This method

also cancels the effect of any series resistance on the

temperature measurement.

The resulting DVBE waveforms are passed through a

65 kHz low−pass filter to remove noise and then to a

chopper−stabilized amplifier. This amplifies and rectifies

the waveform to produce a dc voltage proportional to DVBE.

The ADC digitizes this voltage producing a temperature

measurement. To reduce the effects of noise, digital filtering

is performed by averaging the results of 16 measurement

cycles for low conversion rates. At rates of 10, 20, and

36 conversions per second, no digital averaging occurs.

Signal conditioning and measurement of the internal

temperature sensor are performed in the same manner.

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