Preliminary Technical Data
AD7280
results exceed the maximum and minimum voltage thresholds
selected by the user. The threshold levels are selected by writing
to the internal registers.
THEORY OF OPERATION
CIRCUIT INFORMATION
The AD7280 provides 6 analog output voltages which can be
used to control external transistors as part of a cell balancing
circuit. Each Cell Balance output provides a 0V or 5V voltage,
with respect to the potential on base of each individual cell,
which can be applied to the gate of the external cell balancing
transistors.
The AD7280 is a Lithium Ion battery monitoring chip with the
ability to monitor the voltage and temperature of 6 series
connected battery cells. The AD7280 also provides an interface
which can be used to control transistors for cell balancing.
The VDD and VSS supplies required by the AD7280 can be taken
from the upper and lower voltages of the series connected
battery cells. An internal VREG rail is generated from the supply
voltage which provides power for the ADC and the internal
interface circuitry. This VREG voltage is available on an output
pin for use external to the AD7280.
The AD7280 features a daisy chain interface. Individual
AD7280s can monitor the cell voltages and temperatures of 6
cells, a chain of AD7280s can be used to monitor the cell
voltages and temperatures of a larger number of cells. The
conversion data from each AD7280 in the chain passes to the
system controller via a single standard serial interface. Control
data can similarly be passed via the standard serial interface up
the chain to each individual AD7280s
The AD7280 consists of a high voltage input multiplexer, a low
voltage input multiplexer and a 12 bit ADC. The high voltage
multiplexer allows up to 6 series connected Lithium Ion battery
cells to be measured. The low voltage multiplexer allows the
The AD7280 includes an on-chip 2.5V reference. The reference
voltage is available for use external to the AD7280.
temperature of each cell to be measured. A single
CNVST
signal is required to initiate conversions on all 12 channels, that
is 6 voltage and 6 temperature channels. Alternatively the
CONVERTER OPERATION
conversion can be initiated through the rising edge of
on the
CS
SPI interface. Each conversion result is stored in a results
register (See Register section). On power-up, the
The AD7280 consists of a high voltage input multiplexer, a low
voltage input multiplexer and a 12 bit ADC.
signal
CNVST
The high voltage multiplexer selects which pair of analog
inputs, Vin0 to Vin6, are 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, etc. This is illustrated in Figure 4 and Figure 5. The
conversion results for each cell may be accessed after the
programmed conversion sequence is complete.
is the default option, this can be changed by writing to the
CONTROL register. The default sequence of conversions
completed following the
signal, or software convert
CNVST
start, is all 6 voltage channels followed by all 6 temperature
channels. Two further conversion sequences may be selected by
the user, 6 voltage channels followed by 3 temperature channels
or just 6 voltage channels. The conversion sequence may be
selected by writing to the CONTROL register.
The second multiplexer selects which voltage temperature
input, VT1 to VT6, is to be converted. The conversion results
for each cell may be accessed after the programmed conversion
sequence is complete.
Each voltage and temperature measurement requires a
minimum of 1us to acquire and complete a conversion.
Depending on the external components connected to the
analog inputs of the AD7280 additional acquisition time may be
required. A higher acquisition time may be selected through the
CONTROL register. For increased accuracy in a noisy
environment the user may also select the averaging option
through the CONTROL register. This option allows the user to
complete 2, 4 or 8 averages on each cell voltage and cell
temperature measurement. The averaged conversion results are
stored in the results registers. On power-up the default
combined acquisition and conversion time will be 1us, with the
averaging register set to zero, that is, a single conversion per
channel.
Vin6
Vin5
Vin4
Vin3
Vin2
ADC Vin+
ADC Vin-
Vin1
Vin0
Figure 4. MUX Configuration During Vin1-Vin0 Sampling
The results of the voltage and temperature conversions are read
back via the 4 wire Serial Peripheral Interface. The SPI interface
is also used to write to and read data from the internal registers.
The AD7280 features an ALERT function which is triggered if
the voltage conversion results or the temperature conversion
Rev. PrF | Page 10 of 38
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