ADM1067ASU datasheet(13/32 Pages) AD

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ADM1067

Rev. B | Page 13 of 32

POWERING THE ADM1067

The ADM1067 is powered from the highest voltage input on

either the positive-only supply inputs (VPn) 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 (discussed in the Supply Supervision section). A

VDD arbitrator on the device chooses which supply to use. The

arbitrator can be considered an OR’ing of five 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

ADM1067 from a supply as low as 3.0 V. Note that the supply

on the VXn 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 18. The capacitor has

another use during brownouts (momentary loss of power).

Under these conditions, when the input supply (VPn 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.

Note that 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

OUT

4.75V

LDO

OUT

4.75V

LDO

OUT

4.75V

LDO

OUT

4.75V

LDO

OUT

4.75V

LDO

VP4

VP3

VP2

VP1

VDDCAP

INTERNAL

DEVICE

SUPPLY

Figure 18. VDD Arbitrator Operation

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