AD7280ABSTZ datasheet(27/48 Pages) AD

27 / 48 page

AD7280A

Rev. 0 | Page 27 of 48

ALERT OUTPUT

The alert output on the AD7280A can be used to indicate

whether any of the following faults has occurred:

•

Cell overvoltage

•

Cell undervoltage

•

Auxiliary ADC overvoltage

•

Auxiliary ADC undervoltage

Following each completed conversion, the cell voltage and

auxiliary ADC measurement results are compared to the alert

thresholds. The alert thresholds are set by writing to the cell

overvoltage, cell undervoltage, AUX ADC overvoltage, and

AUX ADC undervoltage registers. An alert output is generated

if the cell voltage and/or the auxiliary ADC results are outside

the programmed alert thresholds.

The alert output can be configured as a static or dynamic output

by writing to the alert register. The static alert output is a high

signal that is pulled low in the event of an overvoltage or under-

voltage on the cell voltage or auxiliary ADC input conversions.

The dynamic alert is a square wave that can be programmed to

a frequency of 100 Hz, 1 kHz, or 10 kHz. The alert output can

be used as part of a daisy chain, in which case the AD7280A at

the top of the chain, that is, farthest away from the DSP/micro-

processor, should be programmed to generate the initial alert

output, and all other devices in the chain should be programmed

to allow the alert signal to pass through. If a conversion result

outside the programmed thresholds occurs, either on the device

generating the initial alert signal or on any device in the chain,

the alert signal is pulled low to indicate that an alert condition

has occurred. At the end of the daisy chain, the master AD7280A,

which is connected to the DSP/microprocessor, takes the alert

signal from the chain and passes it in standard digital voltage

format to the DSP/microprocessor. The configuration settings

for the alert register are described in Table 11 and Table 12.

Table 11. Alert Register Settings, Bits[D7:D4]1

Bits[D7:D6]

Bits[D5:D4]

Action

No alert signal generated or

passed (default)

Generates a static (high) alert

signal to be passed down the

daisy chain

Generates a 100 Hz square wave

alert signal to be passed down

the daisy chain

Generates a 1 kHz square wave

alert signal to be passed down

the daisy chain

Generates a 10 kHz square wave

alert signal to be passed down

the daisy chain

Reserved

Passes an alert signal from the

AD7280A at higher potential in

the daisy chain

1 X is don’t care.

Table 12. Alert Register Settings, Bits[D3:D0]1

Bits[D3:D2]

Bits[D1:D0]

Action

Includes all six voltage channels in

alert detection (default)

Removes VIN5 from alert

detection

Removes VIN5 and VIN4 from alert

detection

Reserved

Includes all AUX ADC channels

selected for conversion in alert

detection2 (default)

Removes AUX5 from alert

detection3

Removes AUX5 and AUX3 from

alert detection3

Reserved

1 X is don’t care.

2 Includes six auxiliary ADC channels in the alert detection if conversions on

six auxiliary ADC channels are selected in the control register; includes three

auxiliary ADC channels in the alert detection if conversions on three auxiliary

ADC channels are selected in the control register.

3 To remove AUX5 or AUX5 and AUX3 from the alert detection, conversions on

three auxiliary ADC input channels only must be selected in the control register.

Some applications require fewer than six voltage measurements

(see the Connection of Fewer Than Six Voltage Cells section).

As shown in Figure 35, it is recommended that a channel that

is not being used on the AD7280A be shorted to the channel

below it. To prevent the incorrect triggering of the alert output

in this application, the AD7280A allows the user to select up to

two voltage channels that can be taken out of the overvoltage/

undervoltage detection circuit. This is programmed through

Bits[D3:D2] of the alert register. The user can also remove all

or selected auxiliary ADC channels from the detection circuit.

This is programmed through Bits[D1:D0] of the alert register in

combination with Bits[D15:D14] of the control register.

The operation of the alert output can be verified by initiating a

self-test conversion. The self-test conversion converts the band

gap reference voltage, 1.2 V, which triggers an alert output if the

cell undervoltage threshold is set higher than 1.2 V. To test the

alert output, a self-test conversion should be initiated on the

AD7280A farthest away from the DSP/microprocessor.

The operation of the alert output can also be verified by increas-

ing or decreasing the thresholds around a known input voltage

to trigger an alert condition. The alert operation of each device

in the daisy chain of AD7280As can be verified by, for example,

decreasing the cell overvoltage threshold of that device below the

value of the input voltage on the cells. Initiating a conversion on

all devices in the daisy chain pulls the alert signal low as it passes

through that device. The relevant threshold on that device can

then be returned to its previous value and the process repeated

on the next device in the daisy chain.

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