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ADuM2251ARIZ-RL Datasheet(PDF) 11 Page - Analog Devices |
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ADuM2251ARIZ-RL Datasheet(HTML) 11 Page - Analog Devices |
11 / 16 page Data Sheet ADuM2250/ADuM2251 Rev. A | Page 11 of 16 STARTUP Both the VDD1 and VDD2 supplies have an under voltage lockout feature that prevents the signal channels from operating unless certain criteria is met. This feature is to avoid the possibility of input logic low signals from pulling down the I2C bus inadver- tently during power-up/power-down. Criteria that must be met for the signal channels to be enabled are as follows: Both supplies must be at least 2.5 V. At least 40 μs must elapse after both supplies exceed the internal start-up threshold of 2.0 V. Until both of these criteria are met for both supplies, the ADuM2250/ADuM2251 outputs are pulled high thereby ensuring a startup that avoids any disturbances on the bus. Figure 8 and Figure 9 illustrate the supply conditions for fast and slow input supply slew rates. SUPPLY VALID MINIMUM RECOMMENDED OPERATING SUPPLY, 3.0V MINIMUM VALID SUPPLY, 2.5V INTERNAL STARTUP THRESHOLD, 2.0V 40µs Figure 8. Start-Up Condition, Supply Slew Rate < 12.5 V/ms SUPPLY VALID MINIMUM RECOMMENDED OPERATING SUPPLY, 3.0V MINIMUM VALID SUPPLY, 2.5V INTERNAL STARTUP THRESHOLD, 2.0V 40µs Figure 9. Start-Up Condition, Supply Slew Rate > 12.5 V/ms MAGNETIC FIELD IMMUNITY The ADuM2250/ADuM2251 are extremely immune to external magnetic fields. The limitation on the magnetic field immunity of the ADuM2250/ADuM2251 is set by the condition in which induced voltage in the receiving coil of the transformer is suffi- ciently large to either falsely set or reset the decoder. The following analysis defines the conditions under which this may occur. The 3 V operating condition of the ADuM2250/ADuM2251 is examined because it represents the most susceptible mode of operation. The pulses at the transformer output have an amplitude greater than 1.0 V. The decoder has a sensing threshold at about 0.5 V, therefore establishing a 0.5 V margin in which induced voltages can be tolerated. The voltage induced across the receiving coil is given by N n r dt d V n ... , 2 , 1 ; ) / β ( 2 where: β is the magnetic flux density (gauss). rn is the radius of the nth turn in the receiving coil (cm). N is the number of turns in the receiving coil. Given the geometry of the receiving coil in the ADuM2250/ ADuM2251 and an imposed requirement that the induced voltage be at most 50% of the 0.5 V margin at the decoder, a maximum allowable magnetic field is calculated, as shown in Figure 10. MAGNETIC FIELD FREQUENCY (Hz) 100 0.001 1M 10 0.01 1k 10k 10M 0.1 1 100M 100k Figure 10. Maximum Allowable External Magnetic Flux Density For example, at a magnetic field frequency of 1 MHz, the maximum allowable magnetic field of 0.2 kgauss induces a voltage of 0.25 V at the receiving coil. This is about 50% of the sensing threshold and does not cause a faulty output transition. Similarly, if such an event were to occur during a transmitted pulse (and had the worst-case polarity), it would reduce the received pulse from >1.0 V to 0.75 V (still well above the 0.5 V sensing threshold of the decoder). |
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