Zakładka z wyszukiwarką danych komponentów |
|
ADM1032ARM Arkusz danych(PDF) 11 Page - Analog Devices |
|
ADM1032ARM Arkusz danych(HTML) 11 Page - Analog Devices |
11 / 12 page REV. 0 ADM1032 –11– A PPLICATIONS INFORMATION FACTORS AFFECTING ACCURACY Remote Sensing Diode The ADM1032 is designed to work with substrate transistors built into processors’ CPUs or with discrete transistors. Sub- strate transistors will generally be PNP types with the collector connected to the substrate. Discrete types can be either PNP or NPN transistor connected as a diode (base shorted to collector). If an NPN transistor is used, the collector and base are connected to D+ and the emitter to D–. If a PNP transistor is used, the collector and base are connected to D– and the emitter to D+. Substrate transistors are found in a number of CPUs. To reduce the error due to variations in these substrate and discrete transistors, a number of factors should be taken into consideration: 1. The ideality factor, nf, of the transistor. The ideality factor is a measure of the deviation of the thermal diode from ideal behavior. The ADM1032 is trimmed for an nf value of 1.008. The following equation may be used to calculate the error introduced at a temperature T °C when using a transistor whose nf does not equal 1.008. Consult the processor datasheet for nf values. ∆T n Kelvin T natural = () ×+ () – . . . 1 008 1 008 273 15 This value can be written to the offset register and is automati- cally added to or subtracted from the temperature measurement. 2. Some CPU manufacturers specify the high and low current levels of the substrate transistors. The high current level of the ADM1032, IHIGH, is 230 A and the low level current, ILOW, is 13 A. If the ADM1032 current levels do not match the levels of the CPU manufacturers, then it may become necessary to remove an offset. The CPU’s datasheet will advise whether this offset needs to be removed and how to calculate it. This offset may be programmed to the offset register. It is important to note that if accounting for two or more offsets is needed, then the algebraic sum of these offsets must be programmed to the Offset Register. If a discrete transistor is being used with the ADM1032 the best accuracy will be obtained by choosing devices according to the following criteria: • Base-emitter voltage greater than 0.25 V at 6 mA, at the highest operating temperature. • Base-emitter voltage less than 0.95 V at 100 mA, at the lowest operating temperature. • Base resistance less than 100 Ω. • Small variation in hFE (say 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. THERMAL INERTIA AND SELF-HEATING Accuracy depends on the temperature of the remote-sensing diode and/or the internal temperature sensor being at the same temperature as that being measured, and a number of factors can affect this. Ideally, the sensor should be in good thermal contact with the part of the system being measured, for example the processor. If it is not, the thermal inertia caused by the mass of the sensor will cause a lag in the response of the sensor to a temperature change. In the case of the remote sensor this should not be a problem, as it will either be a substrate transistor in the processor, or can be a small package device such as SOT-23 placed in close proximity to it. The on-chip sensor, however, will often be remote from the processor, and will only be monitoring the general ambient temperature around the package. The thermal time constant of the SO-8 package in still air is about 140 seconds, and if the ambient air temperature quickly changed by 100 degrees, it would take about 12 minutes (5 time constants) for the junction temperature of the ADM1032 to settle within 1 degree of this. In practice, the ADM1032 package will be in electrical, and hence thermal, contact with a printed circuit board, and may also be in a forced airflow. How accurately the temperature of the board and/or the forced airflow reflect the temperature to be measured will also affect the accuracy. Self-heating due to the power dissipated in the ADM1032 or the remote sensor, causes the chip temperature of the device or remote sensor to rise above ambient. However, the current forced through the remote sensor is so small that self-heating is negligible. In the case of the ADM1032, the worst-case condition occurs when the device is converting at 16 conversions per second while sinking the maximum current of 1 mA at the ALERT and THERM output. In this case, the total power dissipation in the device is about 11 mW. The thermal resistance, θ JA, of the SO-8 package is about 121 °C/W. In practice, the package will have electrical and hence thermal connection to the printed circuit board, so the temperature rise due to self-heating will be negligible. LAYOUT CONSIDERATIONS Digital boards can be electrically noisy environments, and the ADM1032 is measuring very small voltages from the remote sensor, so care must be taken to minimize noise induced at the sensor inputs. The following precautions should be taken: 1. Place the ADM1032 as close as possible to the remote sensing diode. Provided that the worst noise sources, i.e., clock gen- erators, data/address buses, and CRTs, are avoided, this distance can be 4 to 8 inches. 2. Route the D+ and D– tracks close together, in parallel, with grounded guard tracks on each side. Provide a ground plane under the tracks if possible. 3. Use wide tracks to minimize inductance and reduce noise pickup. 10 mil track minimum width and spacing is recommended. 10MIL 10MIL 10MIL 10MIL 10MIL 10MIL 10MIL GND D+ D– GND Figure 6. Arrangement of Signal Tracks 4. Try to minimize the number of copper/solder joints, which can cause thermocouple effects. Where copper/solder joints are used, make sure that they are in both the D+ and D– path and at the same temperature. |
Podobny numer części - ADM1032ARM |
|
Podobny opis - ADM1032ARM |
|
|
Link URL |
Polityka prywatności |
ALLDATASHEET.PL |
Czy Alldatasheet okazała się pomocna? [ DONATE ] |
O Alldatasheet | Reklama | Kontakt | Polityka prywatności | Linki | Lista producentów All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |