ADM1266ACPZ-R7 datasheet(14/62 Pages) AD

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ADM1266

Data Sheet

Rev. A | Page 14 of 62

THEORY OF OPERATION

POWERING THE ADM1266

The ADM1266 is powered from the highest voltage input on VH1

or VH2. This technique, called supply arbitration, offers improved

redundancy because the device is not dependent on any one

particular voltage rail to keep it operational. The AVDD_CAP

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

can be considered an OR’ing of two LDO regulators together. A

supply comparator chooses the highest input to provide the

on-chip supply. It is not recommended to connect both VH1

and VH2 to the same voltage levels because the ripple on the two

voltages may cause the arbitrator circuit to constantly toggle.

This architecture has minimal voltage drop, resulting in the ability

to power the ADM1266 from a supply as low as 3 V. A 10 μF bypass

capacitor and 0.1 μF decoupling capacitors are needed on both

the VH1 and VH2 pins. Additionally, these capacitors ensure a

successful arbitration when switching from VH1 to VH2 and vice

versa. In a system with multiple ADM1266 devices, it is important

that all the devices are powered from the same voltage rail.

An external capacitor from AVDD_CAP to GND is required to

decouple the on-chip supply from noise, as shown in Figure 7.

The capacitor has another use during brownouts (momentary

loss of power). Under these conditions, when all the input

supplies (VHx pins) fall below AVDD_CAP, the LDO regulators

immediately turn off so that the VHx power supply does not

pull AVDD_CAP down. The AVDD_CAP capacitor can then

act as a reservoir to keep the ADM1266 active until the next

highest supply takes over the powering of the device. A capacitor

with a minimum value of 68 μF is recommended for this

reservoir/decoupling function.

If all supplies fail, the value of the AVDD_CAP capacitor can be

increased if it is necessary to guarantee that a complete fault

record is written into EEPROM.

The VHx input pins can accommodate supplies of up to 15 V,

which allows the ADM1266 to be powered using a 12 V backplane

supply. In cases where this 12 V supply is hot swapped, it is

recommended that the ADM1266 not be connected directly to the

supply. Take suitable precautions, such as the use of a hot swap

controller or RC filter network, to protect the device from

transients that may cause damage during hot swap events.

When two or more supplies are within the VH1/VH2 arbitration

hysteresis value of each other, the supply that first takes control

of AVDD_CAP keeps control. For example, if VH1 is connected

to a 5.0 V supply, AVDD_CAP powers up to 3.3 V (typical)

through VH1. If VH2 is then connected to another 5.0 V supply,

VH1 still powers the device, unless VH2 goes approximately

317 mV higher than VH1.

ARBITRATOR

SUPPLY

SELECT

AVDD_CAP

3.3V

LDO

3.3V

LDO

VH1

VH2

68µF

10µF

0.1µF

10µF

0.1µF

Figure 7. AVDD_CAP Arbitrator Operation

During power-up, the ADM1266 checks the main boot loader,

the main firmware, the main configuration, and the backup

configuration to ensure that the data in these sections is correct.

If multiple devices are connected on the same IDB, all the

devices individually check the main and backup configurations

and send this information back to the master. Then, the master

decides to run the correct configuration. The boot up time from

VH1 or VH2 crossing 3 V to the device ready to execute State 1

varies based on the size of the configuration. On the top right

corner of the GUI, an icon displays the size of the configuration

memory in a percentage. Use this percentage in the following

equation to calculate the boot up time:

Typical Boot Up Time (ms) = 1.142 × Percentage + 192

For example, if 27% of the memory is used, then,

Boot Up Time = 1.142 × 27 +192

Boot Up Time = 223 ms

Figure 8. GUI Icon Showing Configuration Memory Size

ADM1266ACPZ-R7 数据表(PDF) 14 Page - Analog Devices

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