AD7280ABSTZ datasheet(15/48 Pages) AD

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AD7280A

Rev. 0 | Page 15 of 48

THEORY OF OPERATION

CIRCUIT INFORMATION

The AD7280A is a lithium ion (Li-Ion) battery monitoring chip

that can monitor the voltage and temperature of four, five, or six

series-connected Li-Ion battery cells. The AD7280A also provides

an interface that can be used to control external transistors for

cell balancing.

The VDD and VSS supplies required by the AD7280A should

be taken from battery cells being monitored by the part. An

internal VREG rail is generated to provide power for the ADC

and the internal interface circuitry. This VREG voltage is available

on an output pin for use external to the AD7280A.

The AD7280A consists of a high voltage input multiplexer, a low

voltage input multiplexer, and a SAR ADC. The high voltage

multiplexer allows four, five, or six series-connected Li-Ion

battery cells to be measured. The low voltage multiplexer provides

the user with six single-ended ADC inputs that can be used in

combination with external thermistors to measure the tempera-

ture of each battery cell. The auxiliary ADC inputs can also be

used for external diagnostics in the application. Initiating conver-

sions on all 12 channels, that is, the six cell voltage channels and

the six auxiliary ADC channels, requires only a single CNVST

pulse. Alternatively, the conversion can be initiated through the

rising edge of CS. Each conversion result is stored in an individual

result register (see

Table 13

Each individual cell voltage and auxiliary ADC measurement

requires a minimum of 1 μs to acquire and complete a conver-

sion. Depending on the external components connected to the

analog inputs of the AD7280A, additional acquisition time may

be required. A higher acquisition time can be selected through

the control register. The AD7280A also provides a conversion

averaging option that can be selected through the control register.

This option allows the user to complete two, four, or eight averages

on each cell voltage and auxiliary ADC measurement. The aver-

aged conversion results are stored in the result registers. On

power-up, the default combined acquisition and conversion

time is 1 μs, with the averaging register set to 0, that is, a single

conversion per channel.

The results of the cell voltage and auxiliary ADC conversions

are read back via the 4-wire serial peripheral interface (SPI).

The SPI is also used to write to and read from the internal

registers.

The AD7280A features an alert function that can be triggered if

the voltage conversion results or the auxiliary ADC conversion

results exceed the maximum and minimum voltage thresholds

selected by the user. The alert modes and threshold levels are

selected by writing to internal registers.

The AD7280A provides six analog output voltages that can be

used to control external transistors as part of a cell balancing

circuit. Each cell balance output provides a 0 V or 5 V voltage,

with respect to the potential on the base of each individual cell,

that can be applied to the gate of the external cell balancing

transistors.

The AD7280A features a daisy-chain interface. Individual

AD7280A devices can monitor the cell voltages and tempera-

tures of six cells. A chain of AD7280As can be used to monitor

the cell voltages and temperatures of a larger number of cells. The

conversion data from each AD7280A in the chain passes to the

system controller via a single SPI interface. Control data can

similarly be passed via the SPI up the chain to each individual

AD7280A.

The AD7280A includes an on-chip 2.5 V reference. The

reference voltage is available for use external to the AD7280A.

The AD7280A also has a VDRIVE feature to control the voltage at

which the serial interface operates. VDRIVE allows the ADC to

easily interface to both 3 V and 5 V processors. For example, in

the recommended configuration, the AD7280A is operated with

a supply of 5 V; however, the VDRIVE pin can be powered from

a 3 V supply, allowing a large dynamic range with low voltage

digital processors.

CONVERTER OPERATION

The conversion paths of the AD7280A consist of a high voltage

input multiplexer or a low voltage input multiplexer and a SAR

ADC. The high voltage multiplexer selects the pair of analog

inputs, VIN0 to VIN6, that is to be converted. The voltage of

each individual cell is measured by converting the difference

between adjacent analog inputs, that is, VIN1 − VIN0, VIN2 −

VIN1, and so on (see Figure 22 and Figure 23). The low voltage

multiplexer selects the auxiliary ADC input, AUX1 to AUX6,

that is to be converted. The conversion results for each cell

voltage and auxiliary ADC input can be accessed tWAIT after

the programmed conversion sequence is completed.

VIN6

VIN5

VIN4

VIN3

VIN2

VIN1

VIN0

ADC VIN+

ADC VIN–

Figure 22. Mux Configuration During VIN1 to VIN0 Sampling

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