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поискавой системы для электроныых деталей |
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поискавой системы для электроныых деталей |
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ADM9240ARU-REEL датащи(PDF) 11 Page - Analog Devices |
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ADM9240ARU-REEL датащи(HTML) 11 Page - Analog Devices |
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11 / 22 page  ADM9240 –11– REV. 0 MONITORING CYCLE TIME The monitoring cycle begins when a one is written to the Start Bit (Bit 0), and a zero to the INT_Clear Bit (Bit 3) of the Con- figuration Register. INT_Enable (Bit 1) should be set to one to enable the INT output. The ADC measures each analog input in turn, starting with VCCP2 and finishing with the on-chip tem- perature sensor. As each measurement is completed the result is automatically stored in the appropriate value register. This “round-robin” monitoring cycle continues until it is disabled by writing a 0 to Bit 0 of the Configuration Register. The counter controlling the multiplexer is driven by an on-chip clock of nominally 22.5 kHz, so the entire measurement sequence takes (nominally): 44.4 µs × 7 = 310.8 µs This rapid sampling of the analog inputs ensures a quick re- sponse in the event of any input going out of limits, unlike other monitoring chips that employ slower ADCs. When a monitoring cycle is started, monitoring of the fan speed inputs begins at the same time as monitoring of the analog in- puts. However, the two monitoring cycles are not synchronized in any way, and the monitoring cycle time for the fan inputs is dependent on fan speed and much slower than for the analog inputs. For more details see the Fan Speed Measurement section. INPUT SAFETY Scaling of the analog inputs is performed on-chip, so external attenuators are normally not required. However, since the power supply voltages will appear directly at the pins, it is advis- able to add small external resistors in series with the supply traces to the chip to prevent damaging the traces or power sup- plies should an accidental short such as a probe connect two power supplies together. As the resistors will form part of the input attenuators, they will affect the accuracy of the analog measurement if their value is too high. The analog input channels are calibrated assuming an external series resistor of 500 Ω, and the accuracy will remain within specification for any value from zero to 1 k Ω, so a stan- dard 510 Ω resistor is suitable. The worst such accident would be connecting –12 V to +12 V— a total of 24 V difference, with the series resistors this would draw a maximum current of approximately 24 mA. ANALOG OUTPUT The ADM9240 has a single analog output from an unsigned 8-bit DAC which produces 0 V–1.25 V. The analog output register defaults to FF during power-on reset, which produces maximum fan speed. The analog output may be amplified and buffered with external circuitry such as an op amp and transistor to provide fan speed control. A suitable drive circuit is given in Figure 5. Care must be taken when choosing the op amp to ensure that its input common-mode range and output voltage swing are suitable. The op amp may be powered from the +12 V rail alone or from ±12 V. If it is powered from +12 V then the input common- mode range should include ground to accommodate the mini- mum output voltage of the DAC, and the output voltage should swing below 0.6 V to ensure that the transistor can be turned fully off. If the op amp is powered from –12 V, precautions such as a clamp diode to ground may be needed to prevent the base- emitter junction of the transistor being reverse-biased in the unlikely event that the output of the op amp should swing nega- tive for any reason. The positive output swing of the op amp should be as close to +12 V as possible so that the maximum voltage can be obtained from the transistor. Even if the op amp swings to the rail, the maximum voltage from the emitter of the transistor will be about 11.4 V. typical values for this condition would be: Gain = 11.4/1.25 = 9.12 = 1 + R1/R2 R1 = 82 k Ω, R2 = 10 kΩ (nearest preferred value) giving an actual gain of 9.2. The transistor should have a reasonably high hFE to avoid its base current pulling down the output of the op amp, it must have an ICMAX greater than the maximum fan current and be capable of dissipating power due to the voltage dropped across it when the fan is not operating at full speed. Depending on the fan parameters, some suitable devices would be 2N2219A, 2N3019 or ZTX450. +12V R1 NTEST_IN/AOUT R2 Figure 5. Analog Output Driving Fan LAYOUT AND GROUNDING Analog inputs will provide best accuracy when referred to the GNDA pin. A separate, low impedance ground plane for analog ground, which provides a ground point for the voltage dividers and analog components, will provide best performance but is not mandatory. The power supply bypass, the parallel combination of 10 µF (electrolytic or tantalum) and 0.1 µF (ceramic) bypass capaci- tors connected between Pin 9 and ground, should also be lo- cated as close as possible to the ADM9240. |
Аналогичный номер детали - ADM9240ARU-REEL |
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Аналогичное описание - ADM9240ARU-REEL |
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