feature

10-Bit Temperature-to-Digital Converter; Temperature Range: -40°C to + 125 °C; Typical Accuracy of ±0.5°C at +40°C; SMBus/I2C® Compatible Serial Interface; 3μA Power-Down Current; Temperature Conversion Time: 29µs typical; space saving 6-lead ( AD7414 ) and 5-lead (AD7415); SOT-23 package; pin-selectable addressable via AS; overtemperature indicator (AD7414 only); SMBus alert function ( AD7414 only); 4 versions allow 8 I2C addresses (AD7414); 2 versions allow 6 I2C addresses (AD7415).

application

Hard Disk Drives; Personal Computers; Electronic Test Equipment; Office Equipment; Home Appliances; Process Control; Cell Phones.

General Instructions

The AD7414/AD7415 are complete temperature monitoring systems in 6-wire and 5-wire SOT-23 packages. They contain a bandgap temperature sensor and a 10-bit ADC to monitor and digitize temperature readings with a resolution of 0.25°C.

The AD7414/AD7415 provide a 2-wire serial interface compatible with SMBus and IC interfaces. The part is available in four versions: AD7414/AD7415-0, AD7414/AD7415-1, AD7414-2, 2, and AD7414-3. The AD7414/AD7415-0 and AD7414/AD7415-1 versions provide a choice of three different SMBus addresses for each version. All four AD7414 versions offer eight different IC addresses, while the two AD7415 versions allow up to six IC addresses.

The AD7414/AD7415's 2.7V supply voltage, low supply current, serial interface, and small package make it ideal for a variety of applications, including personal computers, office equipment, cell phones, and home appliances.

In the AD7414, on-chip registers can program the high and low temperature limits, and when programmed, the open circuit overtemperature indicator output (alert) becomes active beyond the limits. Configuration registers allow programming of the state of the alarm output (active high or active low). This output can be used as an interrupt or SMBus alert.

Product Highlights

1. On-chip temperature sensor. The sensor allows accurate measurement of ambient temperature. Temperature accuracy is ±0.5°C.

2. SMBus/IC compatible serial interface. This interface is for each version of the AD7414/AD7415, with a total of 8 address options for the AD7414 and 6 address options for the AD7415.

3. The supply voltage is 2.7 V to 5.5 V.

4. Space-saving 5-lead and 6-lead SOT-23 packages.

5. 10-digit temperature reading to 0.25°C resolution.

6. Overheating indicator. This indicator can be disabled by software. It is used as an interrupt for SMBus alerts.

7. One-time and automatic temperature conversion rate.

theory of operation

circuit information

The AD7414/AD7415 are stand-alone digital temperature sensors. On-chip temperature sensors allow accurate measurement of ambient device temperature. A 10-bit analog-to-digital converter converts the measured temperature to 2's complement format for storage in the temperature register. The ADC consists of a traditional successive approximation converter around a capacitor digital-to-analog converter (DAC). The serial interface is IC and SMBus compatible. The AD7414/AD7415 require a 2.7 V to 5.5 V supply. The operating measurement range of the temperature sensor is -40°C to +125°C. 2

Function description

There are two methods of temperature measurement. The first counts down with an internal clock of 800 ms and performs the conversion. The internal oscillator is the only circuit that energizes between transitions, and once it times out, a wake-up signal is sent every 800ms to energize the rest of the circuit. The monostable is activated at the beginning of the wake-up signal to ensure sufficient time for the power-up process. A monostable typically takes 4µs to time out. Then, it typically takes 25µs to complete each conversion. The new temperature value is loaded into the temperature value register and is ready to be read through the IC interface.

A temperature measurement is also initiated each time a one-shot method is used. This method requires the user to write a bit in the configuration register once when a temperature measurement is required. Setting the primary bit to 1 starts a temperature conversion directly after a write operation. In the stop condition, track and hold enter about 4 μs (monostable timeout), and then start converting. Usually after 25μs, the conversion is complete and the temperature value register is loaded with the new temperature value.

The measurement mode is compared to the high temperature limit stored in an 8-bit read/write register. This only applies to the AD7414 as the AD7415 has no alert pin and subsequently no thermal monitoring. If the measured value is greater than the upper limit, the alarm pin is activated (if enabled in the configuration register). There are two ways to deactivate the alarm pin again: when the alarm reset bit in the configuration register is set to 1 by a write operation, and when the measured temperature is less than the value in the T register. This alert pin is compatible with the SMBus SMBALERT option.

Configuration features include:

(1) Switch between normal operation and complete power failure;

(2), enable or disable SCL and SDA filters;

(3), enable or disable the alarm function;

(4), set the alarm pin polarity.

measurement technology

A common method of measuring temperature is to use the negative temperature coefficient of a diode or the base-emitter voltage of a transistor, operating at a constant current. Unfortunately, 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 AD7414/AD7415 is to measure the change in V when the device is operated at two different currents. This is made by becoming:

where: K is the Boltzmann constant. q is the charge of the electron (1.6 x 10-19 coulombs). T is the absolute temperature in Kelvins. N is the ratio of the two currents.

Figure 7 shows the method used by the AD7414/AD7415 to measure ambient device temperature. To measure ΔV, the sensor (substrate transistor) switches between I and N×I operating currents. The resulting waveform is passed through a chopper-stabilized amplifier, which performs the functions of amplifying and rectifying the waveform to produce a DC voltage proportional to ΔV. This voltage is measured by the ADC to give the temperature output as a 10-bit, 2's complement format.

temperature data format

The temperature resolution of the ADC is 0.25°C, which corresponds to 1 LSB of the ADC. The ADC can theoretically measure a temperature range of 255 °C, and the minimum practical value is limited to 40 °C due to device maximum ratings. Class A can measure temperatures from -40°C to +125°C.

The grade temperature conversion formula is as follows:

Note that DB9 is removed from the ADC code in the negative temperature equation.

Internal register structure

The AD7414 has five internal registers, as shown in Figure 8. Four are data registers and one is an address pointer register.

The AD7415 has three internal registers, as shown in Figure 9. Two are data registers and one is an address pointer register.

Each data register has an address that the address pointer register points to when communicating with it. The temperature value register is the only read-only data register.

address pointer register

The address pointer register is an 8-bit register that stores an address to one of the four data registers of the AD7414 and one of the two data registers of the AD7415. The first byte of each serial write operation to the AD7414/AD7415 is the address of one of the data registers, which is stored in the address pointer register and selects the data register to which subsequent data bytes are written. Only the 2 LSBs of this register are used to select the data register.

Configuration Register (Address 0X01)

The configuration register is an 8-bit read/write register that sets the operating mode of the AD7414/AD7415. In the AD7414, the six main distribution boards (D7 to D2) are used to set the operating mode (see Table 10). D0 and D1 are for factory settings and must be written with zeros during normal operation.

In the AD7415, only three bits (D7, D6, and D2) are used to set the operating mode (see Table 12). D0, D1, and D3 to D5 are used for factory settings and must be written with zeros during normal operation.

If the AD7414/AD7415 are in power-down mode (D7=1), a temperature conversion can still be initiated with a single operation. This involves writing to the configuration register and setting the one-time bit to 1 (D2 = 1), which causes the AD7414/AD7415 to power up, perform a single conversion, and power down again. This is a very energy efficient mode.

Temperature Value Register (Address 0X00)

The temperature value register is a 10-bit read-only register that stores the temperature data read from the ADC in two's complement format. Reading data from this register requires two reads. Table 13 shows the contents of the first byte to be read, while Table 14 and Table 15 show the contents of the second byte to be read from the AD7414 and AD7415, respectively. In Table 14, D3 to D5 of the second byte are used as flag bits and are obtained from other internal registers. Their functions are as follows:

ALERT_Flag: The state of this bit is the same as the state of the ALERT pin.

TúU Logo: High

This flag is set to 1 when the measured temperature exceeds the T limit. It is reset when the second temperature byte (Table 14) is read. If the temperature is still above the T limit after the read operation, the flag appears again.

TúU flag: low

This flag is set to 1 when the measured temperature is below the T limit. It is reset when the second temperature byte (Table 14) is read. If the temperature is still below the T limit after the read operation, the flag is set again.

The full theoretical range of the ADC is 255°C, but in practice, the temperature measurement range is limited to the device's operating range, -40°C to +125°C for Class A.

AD7414 T Register (Address 0X02)

The T register (see Table 16) is an 8-bit read/write register that stores the upper limit of the active alarm output. Therefore, if the value in the temperature value register is greater than the value in the T register, the alert pin is activated (ie, if the alert is enabled in the configuration register). Because it is an 8-bit register, the temperature resolution is 1°C.

AD7414 T Register (Address 0X03)

The T register (see Table 17) is an 8-bit read/write register that stores the lower limit of the deasserted output. Therefore, if the value in the temperature value register is less than the value in the T register, the alert pin will be deactivated (ie, if the alert is enabled in the configuration register).

Because it is an 8-bit register, the temperature resolution is 1°C.

serial interface

Control of the AD7414/AD7415 is performed over an ICcompatible serial bus. The AD7414/AD7415 are connected to this bus as slave devices under the control of a master device such as a processor.

serial bus address

Like all IC-compatible devices, the AD7414/AD7415 have a 7-bit serial address. The four msbs of this address for the AD7414/AD7415 are set to 1001. The AD7414/AD7415 are available in four versions: AD7414/AD7415-0, AD7414/AD7415-1, AD7414-2, and AD7414-3. The first two versions have three different IC addresses that can be selected by tying the AS pin to GND, V, or leaving the pin floating (see Table 4). By giving the four versions different addresses, up to eight AD7414s or six AD7415s can be connected to a single serial bus, or these addresses can be set to avoid conflicts with other devices on the bus.

The serial bus protocol operates as follows.

The master initiates a data transfer by establishing a start condition, defined as a high-to-low transition on the serial data line SDA, while the serial clock line SCL remains high. This means that an address/data stream follows. All slave peripherals connected to the serial bus respond to the start condition and shift in the next 8 bits, including the 7-bit address (MSB first) plus an R/W bit that determines the data The direction of the transfer and whether the data is written to or read from the device.

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). While the selected device is waiting to read or write data from the bus, all other devices on the bus remain idle. If the R/W bit is 0, the master device writes to the slave device. If the R/W bit is 1, the master device reads data from the slave device.

Data is sent over the serial bus in a sequence of 9 clock pulses, 8 bits of data followed by an acknowledgment bit from the data receiver. Transitions on the data line must occur during the low period of the clock signal and remain stable during the high period because a low-to-high transition while the clock is high can be interpreted as a stop signal.

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 10th clock pulse. In read mode, the master device pulls the data line high for a low period 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 10th clock pulse and then high during the 10th clock pulse.

Any byte of data can be transferred over the serial bus in one operation, but reads and writes cannot be mixed in one operation. The type of action is determined at the beginning and cannot be changed without starting a new action.

write mode

The AD7414/AD7415 can be written in two different ways, depending on the register being written to.

Write to the address pointer register for subsequent reads

In order to read data from a particular register, the address pointer register must contain the address of that register. If not, the correct address must be written to the address pointer register by performing a single-byte write operation, as shown in Figure 10. A write operation consists of the serial bus address and address pointer bytes. No data is written to any data registers. Then perform a read operation to read the register.

Write a single byte of data to the configuration register, T high register, or T low register. All three registers are 8-bit registers, so only one byte of data can be written to each register. Writing a single byte of data to one of these registers consists of the serial bus address, the address of the data register written to the address pointer register, and the byte of data written to the selected data register. As shown in Figure 11.

read mode

Reading data from the AD7414/AD7415 is a 1-byte or 2-byte operation. Reading the contents of a configuration register, T register, or T register is a single-byte read operation, as shown in Figure 12. The register address was previously set by a single-byte write operation to the address pointer register. Once a register address is set, any number of reads can be performed from that register without writing to the address pointer register again. To read from another register, the address pointer register must be written again to set the associated register address.

Reading data from the temperature value register is a 2-byte operation, as shown in Figure 13. The same rule applies to 2-byte reads and 1-byte reads.

SMBUS Alert

The AD7414 alert output is an SMBus interrupt line for devices that want to trade the capabilities of a master device for an extra pin. The AD7414 is a slave-only device that uses an SMBus alert to signal the host device that it wants to talk. The SMBus alarm on the AD7414 acts as an overheat indicator.

The ALERT pins have an open-drain configuration that allows the ALERT outputs of several AD7414s to be tied together when the ALERT pin is active low. Use D4 of the configuration register to set the active polarity of the alarm output. The power-on default is active low. The alarm function can be disabled or enabled by setting D5 of the configuration register to 1 or 0, respectively.

The host device can handle the alarm interrupt and simultaneously access all SMBus alarm devices via the alarm response address. Only the device that pulls the alarm low will acknowledge the Alarm Response Address (ARA). If multiple devices pull the ALERT pin low, during a slave address transfer, the highest priority (lowest address) device wins communication rights through standard IC arbitration.

The alarm output activates when the value in the temperature value register exceeds the value in the T register. It is reset when a write to the configuration register sets D3 to 1 or when the temperature falls below the value stored in the T register.

The alert output requires an external pull-up resistor. This can be connected to a different voltage than V, as long as the maximum voltage rating of the ALERT output pin is not exceeded. The value of the pull-up resistor is application dependent, but should be as large as possible to avoid excessive leakage current at the alarm output, which would heat the chip and affect temperature readings.

boot default

The AD7414/AD7415 always power up with the following default: address pointer register to temperature value register.

The T register is loaded with 7Fh.

The T register is loaded with 80h.

The configuration registers are loaded with 40h.

Note that the AD7415 does not have any T or T registers.

Operating mode

Mode 1

This is the power-on default mode for the AD7414/AD7415. In this mode, the AD7414/AD7415 perform a temperature conversion every 800 ms and then partially power down until the next conversion occurs.

If an operation is performed between automatic conversions (D2 of the configuration register is set to 1), the conversion is started immediately after the write operation. After this transition, the widget will go back to performing transitions every 800 milliseconds.

Depending on where the serial port is accessed during the conversion, the conversion may be aborted. If the conversion completes before the part recognizes the serial port access, the temperature register will be updated with the new conversion. If the conversion completes after the part recognizes the serial port access, internal logic prevents the temperature register from being updated because corrupt data could be read.

A temperature conversion can start at any time during a serial port access (except for one operation), but the result of the conversion is loaded into the temperature register only when the serial port access is not active at the end of the conversion.

Mode 2

The only other mode in which the AD7414/AD7415 operates is full power down mode. This mode is typically used when temperature measurements need to be made at a very slow rate. In this mode, the power consumption of the part can be greatly reduced by writing to the part so that it is completely powered down. A full power down is initiated immediately after D7 of the configuration register is set to 1.

When a temperature measurement is required, a write operation can be performed to power up the part and place it in one-shot mode (setting D2 of the configuration register to 1). It takes about 4µs to power up, then transitions, and reverts to full power outage. Temperature values can be read in full power-down mode because the serial interface is still powered.

Power and Throughput

The two modes of operation of the AD7414/AD7415 yield different power and throughput performance. Mode 2 is the sleep mode of the part, which achieves the best power performance.

Mode 1

In this mode, continuous conversions are performed at a rate of approximately 1 every 800 milliseconds. Figure 14 shows the time and current associated with this mode of operation for a 5 V supply. At 5v, the current consumption of this part is typically 1.1ma when converting, and the quiescent current is typically 188µA. The 25µs transition time plus the typically 4µs power-on time contributes 199.3nw to the total power dissipation in the following way:

(29 μs/800 ms) × (5 × 1.1 mA) = 199.3 nW

The remaining time contributes 939.96 μW to the total power consumption.

(799.97 ms/800 ms) × (5 × 1.1 μA) = 199.3 μW

So the total power dissipated in each cycle is: 199.3 nW + 939.96 μW = 940.16 μW

Mode 2

In this mode, the part is completely powered down. All circuits except the serial interface are turned off. In this mode, the most power-efficient method is to use the one-shot method. Write the configuration register and set the one shot bit to 1. The part then performs the conversion at about 4µs power. After the conversion is complete, the device is powered down again until the PD bit in the configuration register is set to 0 or the one-shot bit is set to 1. Figure 15 shows the same timing as Figure 14 in Mode 1; it starts every 800 milliseconds. If we set the voltage to 5 V, we can calculate the power consumption in the following way. The current draw of the part when switching is typically 1.1mA, and the quiescent current is typically 800mA. The 25µs transition time plus the typically 4µs power-on time contributes 199.3nw to the total power dissipation in the following way:

(29 μs/800 ms) × (5 V × 1.1 mA) = 199.3 nW

The remaining time contributes 3.9 μW to the total power dissipation.

(799.971 ms/800 ms) × (5 V × 800 nA) = 3.9 μW

Therefore, the total power dissipated in each cycle is: 199.3 NW + 3.9 μW = 940.16 μW

Install the AD7414/AD7415

The AD7414/AD7415 can be used in surface or air temperature sensing applications. If a thermally conductive adhesive is used to bond the device to the surface, the mold temperature is within 0.1°C of the surface temperature due to the low power consumption of the device. If the ambient air temperature is different from the surface temperature being measured, care should be taken to insulate the back of the device and the leads from the air.

The ground pins provide the best thermal path to the die, so the temperature of the die is close to the temperature of the printed circuit ground track. Care should be taken to ensure good thermal contact with the surface being tested.

As with any integrated circuit, the AD7414/AD7415 and its associated wiring and circuitry must be protected from moisture to prevent leakage and corrosion, especially in cold conditions where condensation is more likely to occur. Water-resistant varnishes and conformal coatings can be used for protection. The small size of the AD7414/AD7415 package allows them to fit within a hermetically sealed metal probe, which provides a safe environment for the device.

Power decoupling

The AD7414/AD7415 should be separated from at least a 0.1µF ceramic capacitor between V and GND. This is especially important if the AD7414/AD7415 are mounted away from the power supply.

Temperature Accuracy and Supply

Guaranteed temperature accuracy specifications are only available for 3 V and 5.5 V. Figure 16 shows typical performance characteristics of large sample parts over the full voltage range of 2.7 V to 5.5 V. Figure 17 shows the typical performance characteristics of a part over a voltage range of 2.7 volts to 5.5 volts.

Typical Temperature Error Graph

Figure 18 shows a VDD device at 3.3 V and 5.5 V.

Figure 19 shows a histogram of temperature error for ambient temperature (40°C) over 6000 units. Figure 19 shows that over 70% of the AD7414/AD7415 devices tested have temperature errors within ±0.3°C.