The ADT7461 is a dual-channel digital thermometer and low/high temperature alarm for use in PCs and thermal management systems. It is compatible with ADM1032. The ADT7461 has three additional features: series resistance cancellation (where resistance up to 3 K (typ) in series with the temperature monitoring diode is automatically removed from the temperature result, allowing noise filtering); configurable alarm output; and extended, switchable temperature measurement range. The ADT7461 can accurately measure remote thermal diode temperature ±1°C and ambient temperature ±3°c. The temperature measurement range defaults to 0°C to 127 °C, compatible with the ADM1032, but can be switched to a wider measurement range of -55° C to 150°c. The ADT7461 communicates via a 2-wire serial interface compatible with the System Management Bus (SMBus) standard. An alarm output signal when the chip or remote temperature is out of range. The Therm output is a comparator output that allows on/off control of the cooling fan. The alarm output can be reconfigured as a second thermal output if desired.

The SMBus address of the ADT7461 is 0x4C. Also available is the ADT7461-2, which uses SMBus address 0x4D.

feature

(1), chip and remote temperature sensor;

(2) 0.25°C resolution/1°C accuracy on the remote channel;

(3) 1°C resolution/3°C accuracy on the local channel;

(4), automatic elimination of up to 3 K (typ) resistors in series with remote diodes to allow noise filtering 8226 ; extended, switchable temperature measurement range; 0°C to +127°C (default) or -55°C to +150 degrees Celsius;

(5), pins and registers are compatible with ADM1032 • 2-wire SMBus serial interface that supports SMBus alarm • Provides two SMBus address versions: *ADT7461 SMBus address is 0x4C *ADT7461-2 SMBus address is 0x4D;

(6) Programmable ultra/low temperature limit;

(7) Offset register for system calibration;

(8), up to two overheating fail-safe thermal outputs;

(9), small 8-lead SOIC NB or 8-lead MSOP package;

(10), 170 A working current, 5.5 A standby current;

(11) These are lead-free devices.

Applications: Desktop and Notebook PCs; Industrial Controllers; Smart Batteries; Embedded Systems; Instrumentation.

Function description

The ADT7461 is a local and remote temperature sensor and over/under temperature alarm with additional functionality to automatically cancel the effects of a 3K (typ) resistor in series with a temperature monitoring diode. When the ADT7461 is operating normally, the on-board ADC operates in free-running mode. The analog input multiplexer alternately selects the on-chip temperature sensor to measure its local temperature or remote temperature sensor. The ADC digitizes these signals and the results are stored in local and remote temperature value registers.

Local and remote measurements are compared to corresponding high, low, and thermal temperature limits stored in eight on-chip registers. Comparisons that are out of limit generate flags that are stored in the status register. Exceeding the high temperature limit, low temperature limit or the result of external diode failure will cause the alarm output to remain low. Exceeding the thermal temperature limit will cause the thermal output to remain low. The alarm output can be reprogrammed as a second thermal output.

The limit registers can be programmed via the serial smbus, and the device can be controlled and configured. The contents of any register can also be read back via smbus.

Control and configuration functions include switching the device between normal operation and standby mode, selecting the temperature measurement scale, masking or enabling the alarm output, switching pin 6 between alarm and Therm2, and selecting the slew rate.

Series Resistance Elimination

The parasitic resistance of the D+ and D- inputs of the ADT7461 in series with the remote diode is caused by a number of factors, including PCB trace resistance and trace length. This series resistance shows up as a temperature offset in the remote sensor's temperature measurement. This error typically results in a 0.5°C offset per ohm of parasitic resistance in series with the remote diode.

The ADT7461 automatically removes the effect of this series resistance on temperature readings, giving more accurate results without requiring the user to characterize this resistance. The ADT7461 is designed to automatically eliminate resistors typically up to 3K. This is transparent to the user by using advanced temperature measurement methods. This feature allows resistors to be added to the sensor path to create filters, allowing the part to be used in noisy environments. See the Noise Filtering section for details.

temperature measurement method

A simple way to measure temperature is to take advantage of the negative temperature coefficient of a diode by measuring the base-emitter voltage (v) of a transistor operating at a constant current. However, this technique requires calibration to remove the effect of the absolute value of v, which varies from device to device. The technique used in the ADT7461 is to measure the change in V when the device is operated at three different currents. The previous device used only two operating currents, but it uses a third current, allowing automatic cancellation of the resistance in series with the external temperature sensor. Figure 15 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 could also be a discrete transistor. If a discrete transistor is used, the collector will not be grounded and should be connected to the base. To prevent ground noise from interfering with the measurement, the more negative terminal of the sensor is not referenced to ground, but is biased to ground by an internal diode at the D input. C1 can be added as a noise filter (1000 pF maximum recommended). However, in noisy environments, a better option is to add a filter, as described in the Noise Filtering section. See the Layout Considerations section for more information on C1.

To measure the voltage, the operating current through the sensor is switched between three related currents. Figure 15 shows different multiples of current n1×i and n2×i. The current through the temperature diode switches between i and n1×i, giving v, and then switches between i and n2×i, giving v. The temperature can then be calculated using the two v measurements. This method also eliminates the effect of any series resistance on the temperature measurement. BE1BE2 is the resulting v-waveform that goes through a 65khz low-pass filter to remove noise and then goes into a chopper-stabilized amplifier. This will amplify and rectify the waveform to produce a DC voltage proportional to V. The ADC digitizes this voltage and produces a temperature measurement. To reduce the effect of noise, digital filtering is performed by averaging the results over 16 measurement cycles at low slew rates. At rates of 16, 32 and 64 conversions per second, no numerical averaging was performed. The signal conditioning and measurement of the internal temperature sensor is done in the same way.

Temperature measurement results

The results of the local and remote temperature measurements are stored in the local and remote temperature value registers and compared to the limits programmed into the local and remote high and low limit registers.

The local temperature value is in Register 0x00 with a resolution of 1°C. The external temperature value is stored in two registers, the upper byte is in Register 0x01 and the lower byte is in Register 0x10. Only use the two msbs in the external temperature low byte. This enables external temperature measurements with 0.25°C resolution. Table 6 shows the data format of the external temperature low byte.

When reading the full external temperature value (high byte and low byte), both registers should be read consecutively. Reading one register does not lock the other, so both registers should be read before the next conversion is complete. In fact, there is enough time to read both registers, since transactions on the smbus are significantly faster than the conversion time.

Temperature measurement range

By default, the temperature measurement range for internal and external measurements is 0°C to +127°C. However, the ADT7461 can operate with an extended temperature range. It can measure the full temperature range of external diodes, from -55°C to +150°C. The user can switch between these two temperature ranges by setting or clearing Bit 2 in the configuration register. In the next measurement cycle after changing the temperature range, valid results are obtained.

In extended temperature mode, the upper and lower temperatures that the ADT7461 can measure is limited by the selection of the remote diode. The temperature register itself can have a value between -64°C and +191°C. However, most temperature sensing diodes have a maximum temperature range of -55°C to +150°C. Above 150°C, they may lose semiconducting properties and approximate conductors. This can cause the diode to short out. In this case, reading the temperature result register will give the last good temperature measurement. Users should be aware that temperature measurements on external channels may be inaccurate for temperatures outside the operating range of the remote sensor.

While local and remote temperature measurements can be made while the part is in extended temperature mode, the ADT7461 itself should not be exposed to temperatures above those specified in the Absolute Maximum Ratings section. In addition, the unit can only operate as specified at ambient temperatures from -40°C to +120°C.

temperature data format

The ADT7461 has two temperature data formats. When the temperature measurement range is 0°C to +127°C (default), the temperature data format for both internal and external temperature results is binary. When the measurement range is in extended mode, both internal and external results use the offset binary data format. Temperature values in offset binary data format are offset by 64°C. Temperature display in both data formats.

The user can switch between measurement ranges at any time. Switching the range also switches the data format. The next temperature result after switching will be reported back to the register in the new format. However, the contents of the limit registers have not changed. When the data format changes, the user must ensure that the limit registers are reprogrammed as needed. See the Limit Registers section for details.

ADT7461 registers

The ADT7461 contains a total of 22 8-bit registers. These registers are used to store remote and local temperature measurements, as well as high and low temperature limits, and to configure and control the device. The following is a description of these registers. See Tables 8 to 12 for additional details.

address pointer register

The address pointer register has no or no need for an address because the first byte of every write operation is automatically written to this register. The data in the first byte always contains the address of another register on the ADT7461, which is stored in the address pointer register. This register address is written by the second byte of a write operation, or used for a subsequent read operation.

The power-on default value of the address pointer register is 0x00. Therefore, if a read is performed immediately after power-up, without first writing to the address pointer, the local temperature value will be returned because its register address is 0x00.

Temperature value register

The ADT7461 has three registers to store the results of local and remote temperature measurements. These registers can only be written by adc and can be read by user via smbus. The local temperature value register is located at address 0x00. The external temperature value high byte register is located at Address 0x01 and the low byte register is located at Address 0x10. The power-on default value for all three registers is 0x00.

configuration register

The configuration register is address 0x03 when read and address 0x09 when written. Its power-on default value is 0x00. Only four bits of the configuration register are used. Bits 0, 1, 3 and 4 are reserved and should not be written by the user. Bit 7 of the configuration register is used to mask the alarm output. If bit 7 is 0, the alarm output is enabled. This is the power-on default. If bit 7 is set to 1, the alarm output is disabled. This only applies if pin 6 is configured as an alarm. If pin 6 is configured as therm2, the value of bit 7 has no effect.

If bit 6 is set to 0 (default power on), the device is in the working mode of ADC conversion. If bit 6 is set to 1, the device is in standby mode and the ADC is not converting. However, smbus remains active in standby mode, so values can be read or written from adt7461 via smbus in this mode. The alarm and heat output are also active in standby mode. Changes made to registers in standby mode can affect the thermal or alarm outputs, causing these signals to be updated. Bit 5 determines the configuration of pin 6 on the ADT7461. If bit 5 is 0 (default), then pin 6 is configured as an alarm output. If bit 5 is 1, then pin 6 is configured as therm2 output. Bit 7, the alarm mask bit, is only active when pin 6 is configured as an alarm output. Bit 7 has no effect if pin 6 is set as therm2 output. Bit 2 sets the temperature measurement range. If bit 2 is 0 (default), the temperature measurement range is set between 0°C and +127°C. Setting bit 2 to 1 means that the measurement range is set to the extended temperature range.

Slew rate register

The slew rate register is at Address 0x04 when reading and at Address 0x0A when writing. The lowest four bits of this register are used to program the slew rate by dividing the internal oscillator clock by 1, 2, 4, 8, 16, 32, 64, 128, 256 , 512 , or 1024 to give a change from 15.5 ms (code 0x0a) to 16 s (code 0x00) transition time. For example, a conversion rate of 8 conversions per second means starting at 125 ms intervals; the device performs conversions on the internal and external temperature channels. This register can be written to and read via smbus. The upper four bits of this register are unused and must be set to 0. The default value of this register is 0x08, which is 16 conversions per second. As shown in Table 9, using a slower transition time greatly reduces device power consumption.

limit register

The ADT7461 has eight limit registers: high, low, and thermal temperature limits for local and remote temperature measurements. The remote temperature upper and lower limits span two registers, each register containing an upper and a lower limit byte. There is also a thermal hysteresis register. All limit registers can be written to and read via smbus. Address details about limit registers and their power-on defaults.

When pin 6 is configured as an alarm output, the high-limit register performs a > comparison, while the low-limit register performs a ≤ comparison. For example, if the upper limit register is programmed to 80°C, measuring 81°C would result in an out of limit condition, setting a flag in the status register. If the lower limit register is programmed to 0°C, measuring 0°C or lower will result in an out of limit condition.

Exceeding the local or remote thermal limit indicates a thermal low. When pin 6 is configured as Therm2, exceeding the local or remote high limit will assert Therm2 low. The default hysteresis value of 10°C applies to both thermal channels. This hysteresis value can be reprogrammed to any value after power-up (register address 0x21).

It must be remembered that the temperature limit data format is the same as the temperature measurement data format. Therefore, if the temperature measurement uses the default binary, the temperature limit also uses the binary scale. However, if the temperature measurement scale is switched, the temperature limit is not switched automatically. The user must reprogram the limit registers to the desired value in the correct data format. For example, if the remote lower limit is set to 10°C, and the default binary scale is used, the limit register value should be 0000 1010B. If the scale is switched to offset binary, the value in the low temperature limit register should be reprogrammed to 0100 1010B.

status register

The Status Register is a read-only register at address 0x02. It contains status information for the ADT7461. Bit 7 of the status register indicates that the ADC is busy converting when it is high. Other bits in this register flag out-of-limit temperature measurements (bits 6 to 3, bits 1 to 0) and remote sensor open (bit 2).

The following applies if pin 6 is configured as an alarm output. If the local temperature measurement exceeds its limit, a bit 6 (upper limit) or bit 5 (lower limit) assertion of the status register flags this condition. Bit 4 (upper limit) or bit 3 (lower limit) is asserted if the remote temperature measurement exceeds its limit. Bit 2 asserts to flag an open circuit condition on the remote sensor. The five flags are not together, so if any of them are high, the alert interrupt latch is set and the alert output goes low. Reading the status register will clear the five flag bits 6 through 2, provided the error condition that caused the flag to be set has disappeared. The flag bits can only be reset when the corresponding value register contains the measured value within limits or the sensor is good.

The alert interrupt latch is not reset by reading the status register. When the alarm output is serviced by a host reading the device address, it will reset if the error condition disappears and the status register flag bit is reset; when flag 1 and/or flag 0 are set, the thermal output goes low to Indicates that the temperature measurement exceeds the programmed limit. Unlike the alarm output, the thermal output does not need to be reset. Once the measured value is within the limits, the corresponding status register bit will automatically reset and the thermal output will go high. The user can add hysteresis by programming Register 0x21. The thermal output will only reset when the temperature drops to the limit minus the hysteresis; only the high temperature limit is relevant when pin 6 is configured as Therm2. If Flag 6 and/or Flag 4 are set, the Therm2 output goes low to indicate that the temperature measurement exceeds the programmed limit. Mark 5 and Mark 3 have no effect on therm2. Otherwise, therm2 behaves the same as therm.

offset register

Offset errors can be introduced into remote temperature measurements through clock noise or thermal diodes located away from hot spots. To achieve the specified accuracy on this channel, these offsets must be removed.

The offset value is stored as a 10-bit two's complement value in registers 0x11 (high byte) and 0x12 (low byte, left justified). Only the upper 2 bits of register 0x12 are used. The msb of register 0x11 is the sign bit. The programmable minimum offset is -128°C and the maximum offset is +127.75°C. The value in the offset register will be added to the remote temperature measurement.

The offset register powers up with a default value of 0°C and has no effect unless the user writes a different value to it.

one-time register

When the ADT7461 is in standby mode, one register is used to initiate a conversion and compare cycle, after which the device returns to standby. Writing to the one-time register address (0x0f) causes the ADT7461 to perform conversions and comparisons on the internal and external temperature channels. This is not such a data register; a write to address 0x0f results in a one-time conversion. Data written to this address is irrelevant and will not be stored.

Continuous Alarm Register

The value written to this register determines how many out-of-limit measurements must occur before an alarm can be generated. The default is to generate an alert for a metric that exceeds the limit. The maximum value that can be selected is 4. The purpose of this register is to allow the user to do some filtering of the output. This is especially useful at the fastest three conversion rates, where no averaging is done. This register is located at address 0x22.

serial bus interface

Control of the ADT7461 is performed over the serial bus. Under the control of the master device, the adt7461 is connected to this bus as a slave device. After the conversion sequence is complete, there should be no SMBus transaction to the ADT7461 for at least one conversion time to allow the next conversion to complete. The conversion time depends on the value programmed in the conversion rate register.

The ADT7461 has an SMBus timeout feature. When this option is enabled, SMBus typically times out after 25ms of inactivity. However, this feature is not enabled by default. Bit 7 of the Continuous Alerts Register (Address = 0x22) should be set to enable it.

addressing device

Typically, every smbus device has a 7-bit device address, except for some devices that have extended 10-bit addresses. When a master sends a device address over the bus, the slave with that address responds. The ADT7461 has a device address of 0x4C (1001 100B). The ADT7461-2 also provides a device address 0x4D (1001 101B)

The serial bus protocol operates as follows:

1. The master initiates a data transfer by establishing a start condition, defined as a high-to-low transition on the serial data line sdata, while the serial clock line sclk remains high. This means that the address/data stream will follow. All slave peripherals connected to the serial bus respond to the start condition and shift in the next 8 bits consisting of the 7-bit address (msb first) plus the r/w bit , the r/w bit determines the direction of the data transfer, i.e. whether the data will be written to the slave device or read from it. The peripheral whose address corresponds to the address sent responds by pulling the data line low during the low cycle before the ninth clock pulse (called the acknowledge bit). All other devices on the bus are now idle while the selected device is waiting to read or write data from it. If the r/w bit is 0, the master writes to the slave. If the r/w bit is 1, the master reads from the slave.

2. Data is sent over the serial bus as a series of unclocked pulses, 8 bits of data followed by an acknowledgment bit from the slave device. The transition on the data line must occur during the low period to remain stable during the high period, because a transition from low to high when the clock is high can be interpreted as a stop signal. The number of bytes of data that can be transferred over the serial bus in a single read or write operation is limited only by what the master and slave devices can handle.

3. A stop condition is established when all data bytes are read or written. In write mode, the master asserts a stop condition by pulling the data line high during the tenth clock pulse. In read mode, the master overwrites the acknowledge bit by pulling the data line high in the low cycle before the ninth clock pulse. This is called non-recognition. The master then asserts the stop condition by taking the data line low during the low period before the tenth clock pulse and then high during the tenth clock pulse.

Any amount of data can be transferred over the serial bus in one operation, but reads and writes cannot be mixed in one operation because the operation type is determined at the beginning and cannot be followed without starting a new operation Change. For the ADT7461, a write operation consists of one or two bytes, and a read operation consists of one byte.

To write data to or read data from one of the device data registers, the address pointer register must be set up so that the correct data register is addressed. The first byte of a write operation always contains the effective address stored in the address pointer register. If data is to be written to the device, the write operation consists of writing the second data byte of the register selected by the address pointer register.

As shown in Figure 16. The device address is sent over the bus and then R/W is set to 0. Followed by two data bytes. The first data byte is the address of the internal data register to be written, which is stored in the address pointer register. The second data byte is the data to be written to the internal data register. The examples shown in Figure 16 through Figure 18 use the ADT7461 SMBus address 0x4C.

When reading data from a register, there are two possibilities.

1. If the address pointer register value of the ADT7461 is unknown or not the desired value, it must be set to the correct value before data can be read from the desired data register. This is done by writing to the ADT7461 as before, but only sending the data byte containing the address to read from the register, as the data will not be written to the register, as shown in Figure 17.

A read operation is then performed, including the serial bus address, the r/w bit is set to 1, and the data byte is read from the data register, as shown in Figure 18.

2. If the address pointer register is known to be at the desired address, data can be read from the corresponding data register without first writing to the address pointer register, and the bus transaction shown in Figure 17 can be omitted.

While it is possible to read data bytes from the data register without first writing to the address pointer register, it is not possible to write data without writing to the address pointer register if the address pointer register is already at the correct value register, since the first data byte written is always written to the addreSS pointer register.

Also, some registers have different read and write addresses. If data is to be written to a register, the register's write address must be written to the address pointer, but data may not be read from that address. The read address of a register must be written to the address pointer before data can be read from that register.

Alarm Output

This applies if pin 6 is configured as an alarm output. The alarm output will go low whenever an out-of-limit measurement is detected or if the remote temperature sensor is open circuited. It is an open drain output and needs to be pulled up to V. Several alarm outputs can be wired together, so if one or more of the alarm outputs go low, the common line goes low. The DD alert output can be used as an interrupt signal for the processor or as a smbalert. Slaves on the SMBus typically cannot signal to the bus master that they want to talk, but the SMBAlert feature allows them to do so. One or more alert outputs can be connected to a common smbalert line connected to the main server. The process shown in Figure 19 occurs when one of the devices pulls the smbalert line low.

1. smbalert is pulled low.

2. The host initiates a read operation and sends an alarm response address (ara=0001 100). This is a general calling address that cannot be used as a specific device address.

3. The device whose alarm output is low responds to the alarm response address, and the host reads its device address. Since the device address is 7 bits, the lsb of 1 is added. The address of the device is now known and can be queried in the usual way.

4. If the alarm output on multiple devices is low, the device with the lowest device address has priority according to normal SMBus arbitration.

5. Once the ADT7461 responds to the AlertResponse address, it will reset its alert output, provided the error condition that caused the alert no longer exists. If the smbalert line remains low, the master sends ara again; this sequence continues until all devices with the alert output low have responded.

Low power standby mode

The ADT7461 can enter a low-power standby mode by setting Bit 6 of the configuration register. When bit 6 is low, the ADT7461 operates normally. When bit 6 is high, the ADC is disabled and any conversion in progress is terminated without writing the result to the corresponding value register. SMBus is still enabled. Power consumption in standby mode is reduced to less than 10 A if there is no SMBus activity, and to 100 A with clock and data signals on the bus.

When the device is in standby mode, one conversion of both channels can still be initiated by writing to the register (address 0x0f) once, and the device returns to standby. It doesn't matter what is written to the one-shot register, because all data written to the register will be ignored. A new value can also be written to the limit register while in standby. If the value stored in the temperature value register now exceeds the new limit, an alert will be generated even if the ADT7461 is still in standby.

Sensor failure detection

On its D+ input, the ADT7461 contains internal sensor fault detection circuitry. This circuit detects when an external remote diode is not connected or is not properly connected to the ADT7461. If the voltage at D+ exceeds V-1 V (typ), a simple voltage comparator trips, indicating an open circuit between D+ and D-. Check the output of the comparator at the start of the conversion. If a fault is detected, bit 2 of the status register (open flag) is set. If the alert pin is enabled, setting this flag will cause the alert to be asserted low. If the user does not wish to use an external sensor with the ADT7461, the user should connect the D+ and D- inputs together in order to prevent continuous setting of the open flag.

ADT7461 Interrupt System

The ADT7461 has two interrupt outputs, Alert and Therm. Both have different functions and behaviors. Alerts are maskable in response to software-programmed temperature limit violations or external diode open faults. therm is a failsafe interrupt output that cannot be masked.

If the external or local temperature exceeds the programmed high temperature limit or equals or exceeds the low temperature limit, the alarm output is asserted low. An open circuit fault on the external diode can also cause an alarm. If the error condition disappears and the status register has been reset, the alarm will reset when the host reads its device address.

The thermal output is asserted low if the external or local temperature exceeds the programmed temperature limit. The temperature limit should normally be equal to or greater than the high temperature limit. Thermal resets automatically when the temperature falls back within thermal limits. By default, the external limit is set to 85°C, which is the local temperature limit. The hysteresis value can be programmed so that the thermal reset occurs when the temperature drops to the limit minus the hysteresis value. This applies to both local and remote measurement channels. Power-on hysteresis defaults to 10°C, but it can be reprogrammed to any value after power-up. A hysteresis loop on the thermal output is useful when using a thermal switch to control the fan. The user's system can be set up so that when therm asserts, the fans can be turned on to cool the system. When the heat rises again, the fan can be turned off. Setting the hysteresis value prevents fan chatter from hovering around the temperature limit, with fans constantly switching.

application information

noise filtering

For temperature sensors operating in noisy environments, industry standard practice is to place a capacitor on the D+ and D- pins to help cancel the effects of noise. However, large capacitance can affect the accuracy of temperature measurement, so the recommended maximum capacitance value is 1000 pf. While this capacitor reduces noise, it does not cancel noise, which makes the sensor difficult to use in very noisy environments.

The adt7461 has a major advantage over other devices in eliminating the effects of noise on external sensors. The series resistance cancellation feature allows a filter to be constructed between the external temperature sensor and the component. The effect of any filter resistance in series with the remote sensor is automatically removed from the temperature results.

The structure of the filter allows the ADT7461 and the remote temperature sensor to operate in noisy environments. Figure 22 shows a low pass rcr filter with the following values: R=100 and C=1nF

This filtering reduces both common-mode noise and differential noise.

Remote sensing diode

The ADT7461 is designed for substrate transistors or discrete transistors built into processors. Substrate transistors are usually of the pnp type, with the collector connected to the substrate. The discrete type can be a PNP or NPN transistor connected as a diode (base to collector shorted). If using an npn transistor, the collector and base are connected to d+ and the emitter is connected to d-. If a PNP transistor is used, the collector and base are connected to D- and the emitter is connected to D+.

To reduce errors due to variations in substrate transistors and discrete transistors, several factors should be considered:

(1) The ideality factor nf of a transistor is a measure of the thermal diode's deviation from ideal behavior. ADT7461 was trimmed to a NF value of 1.008. When using transistors with nf not equal to 1.008, the error introduced at temperature t (°C) can be calculated using the following formula. T(NF 1.008) 1.008 (273.15 Kelvin T) (Equation 1) To take this into account, the user can write the value of t to the offset register. The temperature measurement is then automatically added or subtracted by the ADT7461.

(2) Some CPU manufacturers specify high and low current levels for substrate transistors. The ADT7461 has a high current level of 96 A and a low current level of 6 A. If the current level of the ADT7461 does not match the current level specified by the CPU manufacturer, the offset may need to be removed. The CPU datasheet suggests if this offset needs to be removed and how to calculate it. This offset is programmable into the offset register. The caveat is that if multiple offsets must be considered, the algebraic sum of those offsets must be programmed into the offset register.

If discrete transistors are used with the ADT7461, select the device according to the following criteria for best accuracy:

(1) At the highest operating temperature, the base-emitter voltage at 6 A is greater than 0.25 V.

(2) At the lowest operating temperature, the base-emitter voltage at 100 A is less than 0.95 V.

(3) The basic resistance is less than 100.

(4), small changes in H (50 to 150), indicating tight control of VBE characteristics. Iron transistors, such as 2N3904, 2N3906, or equivalents in SOT-23 packages are suitable devices to use.

Thermal inertia and self-heating

Accuracy depends on the temperature of the remote sensing diode and/or internal temperature sensor being the same as the temperature of the environment being measured; many factors can affect this. Ideally, the sensor should maintain good thermal contact with the system component under test. Otherwise, thermal inertia caused by the mass of the sensor can cause a lag in the sensor's response to temperature changes. For a remote sensor, this shouldn't be a problem, since it's either the substrate transistor in the processor, or a small packaged device placed near it, like a SOT-23.

However, on-chip sensors are usually located far from the processor and only monitor the general ambient temperature around the package. The thermal time constant of the SOIC-8 package in still air is about 140 seconds, and if the ambient air temperature changes rapidly by 100 degrees, it will take about 12 minutes (5 time constants) for the junction temperature of the ADT7461 to stabilize within 1 degree. In fact, the adt7461 package is in electrical contact with the pcb and thus is also hot and may also be in forced airflow. How accurately the temperature of the circuit board and/or forced airflow reflects the temperature to be measured can also affect accuracy. Self-heating due to power dissipation in the ADT7461 or the remote sensor causes the die temperature of the device or remote sensor to be higher than ambient. However, the current through the remote sensor is so small that self-heating is negligible. For the ADT7461, the worst-case scenario occurs when the device converts at a rate of 64 conversions per second while sinking a maximum current of 1 mA at both the alarm and thermal outputs. In this case, the total power dissipation of the device is about 4.5mW. The thermal resistance of the soic-8 package is about 121°c/w.

Layout Considerations

A digital circuit board can be an electrically noisy environment, and the ADT7461 measures very small voltages from a remote sensor, so care must be taken to minimize noise at the sensor input. The following precautions should be taken:

1. Place the ADT7461 as close to the remote sensing diode as possible. This distance can be anywhere from 4 inches to 8 inches if the worst noise sources such as clock generators, data/address buses, and CRTs are avoided.

2. Arrange the D+ and D- rails together in parallel, with grounding protection rails on each side. To reduce inductance and reduce noise pickup, a track width and spacing of 5 mil is recommended. If possible, provide a ground plane under the track.

Figure 23. Typical Layout of Signal Lines

3. Minimize the number of copper/solder joints that could lead to thermocouple effects. Where copper/solder joints are used, make sure they are in the D+ and D- paths and at the same temperature. Thermocouple effects should not be a major issue, as 1°C corresponds to about 200 mV and the thermocouple voltage is about 3 mV/°C of temperature difference. Unless there are two thermocouples with a large temperature difference, the thermocouple voltage should be much less than 200mV.

4. Place a 0.1F bypass capacitor near VPIN. In extremely noisy environments, the input filter capacitors can be placed close to D+ and D- of the ADT7461. This capacitance affects temperature measurements, so care must be taken to ensure that any capacitance seen at D+ and D- is DD

1000 pf max. this maximum

Include the filter capacitance plus any cable or stray capacitance between the pin and the sensor diode. 5. If the distance to the remote sensor is more than 8 inches, twisted pair cable is recommended. This can reach 6 to 12 feet. For very long distances (up to 100 feet), use shielded twisted pair cable such as Belden No. 8451 microphone cable. Connect the twisted pair to D+ and D- and the shield to GND close to the ADT7461. Leave the far end of the shield unconnected to avoid ground loops.

Since the measurement technique uses switched current sources, excessive cable or filter capacitance can affect the measurement. When using long cables, filter capacitance can be reduced or eliminated.

application circuit

Figure 24 shows a typical application circuit for the ADT7461 using discrete sensor transistors connected through shielded twisted pairs. Pull-ups on SCLK, SData, and Alert are only required if they are not already provided elsewhere in the system.

The SCLK and SData pins of the ADT7461 can be directly connected to the SMBus of an I/O controller such as the Intel820 chipset.