ADT7490(2016) Просмотр технического описания (PDF) - ON Semiconductor

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ADT7490
(Rev.:2016)

Remote Thermal Monitor and Fan Controller with PECI Interface

ON Semiconductor 

ADT7490

the following equation to calculate the error introduced

at a temperature T (°C) when using a transistor whose

nf does not equal 1.008. Refer to the data sheet for the

related CPU to obtain the nf values.

DT + (nf * 1.008)ń1.008 ǒ273.15 K ) TǓ (eq. 4)

To factor this in, the user can write the DT value to the

offset register. The ADT7490 automatically adds it to

or subtracts it from the temperature measurement.

• Some CPU manufacturers specify the high and low

current levels of the substrate transistors. The high

current level of the ADT7490, IHIGH, is 192 mA and the

low level current, ILOW, is 12 mA. If the ADT7490

current levels do not match the current levels specified

by the CPU manufacturer, it may be necessary to

remove an offset. The CPU’s data sheet advises

whether this offset needs to be removed and how to

calculate it. This offset can be programmed to the offset

register. It is important to note that if more than one

offset must be considered, the algebraic sum of these

offsets must be programmed to the offset register.

If a discrete transistor is used with the ADT7490, the best

accuracy is obtained by choosing devices according to the

following criteria:

• Base-emitter voltage greater than 0.25 V at 12 mA at the

highest operating temperature.

• Base-emitter voltage less than 0.95 V at 192 mA at the

lowest operating temperature.

• Base resistance less than 100 W.

• Small variation in hFE (such as 50 to 150) that indicates

tight control of VBE characteristics.

Transistors, such as 2N3904, 2N3906, or equivalents in

SOT−23 packages, are suitable devices to use.

Reading Temperature from the ADT7490

It is important to note that temperature can be read from

the ADT7490 as an 8-bit value (with 1°C resolution) or as

a 10-bit value (with 0.25°C resolution). If only 1°C

resolution is required, the temperature readings can be read

back at any time and in no particular order.

If the 10-bit measurement is required, it involves a

2-register read for each measurement. The Extended

Resolution 2 register (0x77) should be read first. This causes

all temperature reading registers to be frozen until all

temperature reading registers have been read from. This

prevents an MSB reading from being updated while its two

LSBs are being read and vice versa.

Nulling Out Temperature Errors

As CPUs run faster, it becomes more difficult to avoid

high frequency clocks when routing the D+/D− traces

around a system board. Even when recommended layout

guidelines are followed, some temperature errors may still

be attributable to noise coupled onto the D+/D− lines.

Constant high frequency noise usually attenuates or

increases temperature measurements by a linear, constant

value.

The ADT7490 has temperature offset registers at

Address 0x70, Address 0x71, and Address 0x72 for the

Remote 1, local, and Remote 2 temperature channels,

respectively. By performing a one-time calibration of the

system, the user can determine the offset caused by system

board noise and null it out using the offset registers. The

offset registers automatically add a twos complement 8-bit

reading to every temperature measurement.

The temperature offset range and resolution is selected by

setting Bit 1 of Register 0x7C. This ensures that the readings

in the temperature measurement registers are as accurate as

possible. Setting this bit to 0 means the LSBs add 0.5°C

offset to the temperature reading, so the 8-bit register

effectively allows temperature offsets from −63°C to +64°C

with a resolution of 0.5°C. Setting this bit to 1 means the

LSBs add 1°C offset to the temperature reading, so the 8-bit

register effectively allows temperature offsets of up to

−63°C to +127°C with a resolution of 1°C. For the PECI

offset registers, the resolution is always 1°C.

Table 19. TEMPERATURE OFFSET REGISTERS

Register

0x70

0x71

0x72

0x94

0x95

0x96

0x97

Description

Remote 1 Temperature Offset

Local Temperature Offset

Remote 2 Temperature Offset

PECI0 Temperature Offset

PECI1 Temperature Offset

PECI2 Temperature Offset

PECI3 Temperature Offset

Default

0x00 (0°C)

0x00 (0°C)

0x00 (0°C)

0x00 (0°C)

0x00 (0°C)

0x00 (0°C)

0x00 (0°C)

Temperature Measurement Limit Registers

Associated with each temperature measurement channel

are high and low limit registers. Exceeding the programmed

high or low limit causes the appropriate status bit to be set.

Exceeding either limit can also generate SMBALERT

interrupts (depending on the way the interrupt mask register

is programmed and assuming that SMBALERT is set as an

output on the appropriate pin).

Additional ADC Functions for Temperature

Measurement

A number of other functions are available on the

ADT7490 to offer the system designer increased flexibility.

Turn-off Averaging

For each temperature measurement read from a value

register, 16 readings have actually been made internally, and

the results averaged, before being placed into the value

register. Sometimes it is necessary to take a very fast

measurement. Setting Bit 4 of Configuration Register 2

(0x73) turns averaging off. The default round-robin cycle

time with averaging off is a maximum of 23 ms.

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