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ADM1069ASTZ Datasheet(PDF) 13 Page - Analog Devices |
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ADM1069ASTZ Datasheet(HTML) 13 Page - Analog Devices |
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13 / 32 page 
ADM1069 Rev. C | Page 13 of 32 POWERING THE ADM1069 The ADM1069 is powered from the highest voltage input on either the positive-only supply inputs (VPx) or the high voltage supply input (VH). This technique offers improved redundancy because the device is not dependent on any particular voltage rail to keep it operational. The same pins are used for supply fault detection (see the Supply Supervision section). A VDD arbitrator on the device chooses which supply to use. The arbitrator can be considered an OR’ing of four low dropout regulators (LDOs) together. A supply comparator chooses the highest input to provide the on-chip supply. There is minimal switching loss with this architecture (~0.2 V), resulting in the ability to power the ADM1069 from a supply as low as 3.0 V. Note that the supply on the VXx pins cannot be used to power the device. An external capacitor to GND is required to decouple the on- chip supply from noise. This capacitor should be connected to the VDDCAP pin, as shown in Figure 21. The capacitor has another use during brownouts (momentary loss of power). Under these conditions, when the input supply (VPx or VH) dips transiently below VDD, the synchronous rectifier switch immediately turns off so that it does not pull VDD down. The VDD capacitor can then act as a reservoir to keep the device active until the next highest supply takes over the powering of the device. A 10 μF capacitor is recommended for this reservoir/decoupling function. The VH input pin can accommodate supplies up to 14.4 V, which allows the ADM1069 to be powered using a 12 V backplane supply. In cases where this 12 V supply is hot swapped, it is recommended that the ADM1069 not be connected directly to the supply. Suitable precautions, such as the use of a hot swap controller, should be taken to protect the device from transients that could cause damage during hot swap events. When two or more supplies are within 100 mV of each other, the supply that first takes control of VDD keeps control. For example, if VP1 is connected to a 3.3 V supply, VDD powers up to approximately 3.1 V through VP1. If VP2 is then connected to another 3.3 V supply, VP1 still powers the device unless VP2 goes 100 mV higher than VP1. SUPPLY COMPARATOR IN EN OUT 4.75V LDO IN EN OUT 4.75V LDO IN EN OUT 4.75V LDO IN EN OUT 4.75V LDO VH VP3 VP2 VP1 VDDCAP INTERNAL DEVICE SUPPLY Figure 21. VDD Arbitrator Operation |
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