AD7280 datasheet(19/33 Pages) AD

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Preliminary Technical Data

AD7280

Rev. PrD | Page 19 of 33

together it is recommended that series resistors be placed

between the CB outputs of the AD7280 and the gates of the

external Cell Balancing transistors. These are recommended to

protect the AD7280s in the event that the external cell balancing

transistors are damaged during the initial connection of the

monitoring circuitry to the battery stack.

An example of how this could occur would be a connection

sequence which first provides the system ground, that is the

ground supply to the master AD7280 on the daisychain,

followed by a connection from any of the battery cells at a

potential high enough to exceed the VGS of the cell balancing

transistor, for example 40V. If these two connections are the

only battery connections made in the system then this will

result in 40V being applied to one of the Vin pins of the

AD7280, which is also connected to the source input of one of

the cell balancing transistors. However, because no power has

been supplied to the VDD pin of the AD7280 all the CB outputs

will be 0V. This will result in a reverse voltage of 40V across the

VGS of the external transistor which may damage the device.

In the event that the external transistor is damaged, the AD7280

may be protected by the use of 10kOhm series resistors on each

of the CB output pins. Consideration should also be given to

the protection of these external transistors during the initial

connection of the monitoring circuitry to the battery stack.

Vin0

Vin1

Vin2

Vin3

Vin4

Vin5

Vin6

CB6

CB5

CB4

CB3

CB2

CB1

AD7280

10k

Ω

10k

Ω

10k

Ω

10k

Ω

10k

Ω

10k

Ω

Figure 19. Cell Balancing Configuration

The AD7280 offers 6 Cell Balance timer registers which allow

the on-time of each CB output to be programmed. These are

referred to as the CB TIMER registers. The CB timers can be

set to a value between 0 and 30 minutes. The resolution of the

CB Timer is 1 minute. At the end of the programmed CB Time

the 6 CB outputs will return to their default state of 0V. The

default value of the CB TIMER registers on power up is 0h.

As noted in the Power Down section a power down timer may

be programmed to allow cell balancing to occur for a set time

before powering down the AD7280. If no power down timer has

been set, that is if the PD TIMER register is at its default value

of 0h, then a falling edge on the PD pin, or the setting of bit D8

in the CONTROL register to 1, will switch off the CB outputs

and power down the AD7280. If a power down time has been

set the CB outputs will be powered down when the

programmed power down timer has elapsed and the AD7280 is

powered down.

ALERT OUTPUT

The Alert output on the AD7280 may be used to indicate if any

of the following faults have occurred:

•

Over-Voltage

•

Under-Voltage

•

Over-Temperature

•

Under-Temperature

Following each completed conversion the cell voltage and

temperature measurement results are compared to the fault

thresholds. The fault thresholds can be set by writing to the

OVER VOLTAGE. UNDER VOLTAGE, OVER TEMP and

UNDER TEMP registers. An ALERT output is generated if the

cell voltage or temperature results are outside the programmed

fault thresholds.

The Alert output can be defined as a static or a dynamic output,

this is set by writing to the ALERT register. The static Alert

output is a high signal which is pulled low in the event of a over

or under voltage or temperature. The dynamic Alert is a square

wave which can be programmed to a frequency of 100Hz or

1kHz. The Alert output may be used as part of a daisy chain in

which case the AD7280 at the top of the chain, that is furthest

away from the DSP/µP should be programmed to generate the

initial Alert output and each AD7280 in the chain will either

pass that output through or pull the Alert signal low to indicate

that there is a fault with that particular device. At the end of the

daisy chain the master AD7280, that is the AD7280 which is

connected to the DSP/µP will take the Alert signal from the

chain and pass it, in standard digital voltage format to the

DSP/µP. The functionality of the fault detection circuit, which

generates the Alert output may be programmed through bits D7

to D4 of the ALERT register.

As outlined previously (See Conversion of less then 6 Voltage

cells) some applications may require less than 6 voltage

measurements. As shown in Figure 17 it is recommended that

the channels which are not being used on the AD7280 be

shorted to the channel below them. To prevent the incorrect

triggering of the Alert output in this application the AD7280

allows the user to select up to 2 voltage channels which may be

taken out of the fault detection circuit. This may be

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