ADBMS1818CSWAZ-R7 datasheet(86/92 Pages) AD

86 / 92 page

Data Sheet

ADBMS1818

APPLICATIONS INFORMATION

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Rev. B | 86 of 92

isoSPI Layout Guidelines

The layout of the isoSPI signal lines also plays a significant role in

maximizing the noise immunity of a data link. The following layout

guidelines are recommended:

1. The transformer must be placed as close to the isoSPI cable

connector as possible. The distance must be kept less than 2

cm. The ADBMS1818 must be placed close to but at least 1 cm

to 2 cm away from the transformer to help isolate the IC from

magnetic field coupling.

2. A V– ground plane must not extend under the transformer,

the isoSPI connector, or in between the transformer and the

connector.

3. The isoSPI signal traces must be as direct as possible while

isolated from adjacent circuitry by ground metal or space. No

traces must cross the isoSPI signal lines, unless separated by a

ground plane on an inner layer.

System Supply Current

The ADBMS1818 has various supply current specifications for the

different states of operation. The average supply current depends

on the control loop in the system. It is necessary to know which

commands are being executed each control loop cycle, and the

duration of the control loop cycle. With this information, it is possible

to determine the percentage of time the ADBMS1818 is in the

measure state versus the low power sleep state. The amount

of isoSPI or SPI communication also affects the average supply

current.

Calculating Serial Throughput

For any given ADBMS1818, the calculation to determine communi-

cation time is simple: it is the number of bits in the transmission

multiplied by the SPI clock period being used. The control protocol

of the ADBMS1818 is uniform. Therefore, almost all commands can

be categorized as a write or read operation. Table 76 can be used

to determine the number of bits in a given ADBMS1818 command.

ENHANCED APPLICATIONS

Using the ADBMS1818 with Fewer than 18

Cells

Cells can be connected in a conventional bottom (C1) to top (C18)

sequence with all unused C inputs either shorted to the highest

connected cell or left open. The unused S pins can simply be left

disconnected.

Alternatively, to optimize measurement synchronization in applica-

tions with fewer than 18 cells, the unused C pins can be equally

distributed between the top of the third mux (C18), the top of the

second mux (C12) and the top of the first mux (C6) (see Figure

108). If the number of cells being measured is not a multiple

of three, the top mux(es) must have fewer cells connected. The

unused cell inputs must be tied to the other unused inputs on the

same mux and connected to the battery stack through a 100 Ω

resistor. The unused inputs result in a reading of 0.0 V for those

cells.

Current Measurement with a Hall-Effect Sensor

The ADBMS1818 auxiliary ADC inputs (GPIO pins) may be used

for any analog signal, including active sensors with 0 V to 5

V analog outputs. For battery current measurements, Hall-effect

sensors provide an isolated, low power solution. Figure 109 shows

schematically a typical Hall-effect sensor that produces two outputs

that proportion to the VCC provided. The sensor in Figure 109 has

two bidirectional outputs centered at half of VCC. CH1 is a 0 A to 50

A low range and CH2 is a 0 A to 200 A high range. The sensor is

powered from a 5 V source and produces analog outputs that are

connected to the GPIO pins or inputs of the mux application shown

in Figure 111. The use of GPIO1 and GPIO2 as the ADC inputs

has the possibility of being digitized within the same conversion

sequence as the cell inputs (using the ADCVAX command), thus

synchronizing cell voltage and cell current measurements.

Table 76. Daisy Chain Serial Time Equations

Command

Type

CMD Bytes

+ CMD PEC

Data Bytes

+ Data PEC

per IC

Total Bits

Communication

Time

Read

(4 + (8 × #ICs)) × 8 Total bits × clock

period

Write

(4 + (8 × #ICs)) × 8 Total bits × clock

period

Operation

4 × 8 = 32

32 × clock period

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