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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

OUT

4.75V

LDO

OUT

4.75V

LDO

OUT

4.75V

LDO

OUT

4.75V

LDO

VP3

VP2

VP1

VDDCAP

INTERNAL

DEVICE

SUPPLY

Figure 21. VDD Arbitrator Operation

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