ADBMS1818ASWAZ-R7 datasheet(83/92 Pages) AD

83 / 92 page

Data Sheet

ADBMS1818

APPLICATIONS INFORMATION

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Rev. B | 83 of 92

Figure 106. Capacitive Isolation Coupling for ADBMS1818s on the Same PCB

Connecting an MCU to an ADBMS1818 with an

isoSPI Data Link

The LTC6820 converts a standard 4-wire SPI into a 2-wire iso-

SPI link that can communicate directly with the ADBMS1818. An

example is shown in Figure 107. The LTC6820 can be used in

applications to provide isolation between the microcontroller and

the stack of ADBMS1818 devices. The LTC6820 also enables

system configurations that have the battery management system

(BMS) controller at a remote location relative to the ADBMS1818

devices and the battery pack.

Transformer Selection Guide

As shown in Figure 103, a transformer or pair of transformers

isolates the isoSPI signals between two isoSPI ports. The isoSPI

signals have programmable pulse amplitudes up to 1.6 V p-p and

pulse widths of 50 ns and 150 ns. To be able to transmit these

pulses with the necessary fidelity, the system requires that the

transformers have primary inductances above 60 μH and a 1:1

turns ratio. It is also necessary to use a transformer with less

than 2.5 μH of leakage inductance. In terms of pulse shape, the

primary inductance mostly affects the pulse droop of the 50 ns

and 150 ns pulses. If the primary inductance is too low, the pulse

amplitude begins to droop and decay over the pulse period. When

the pulse droop is severe enough, the effective pulse width seen

by the receiver drops substantially, reducing noise margin. Some

droop is acceptable as long as it is a relatively small percentage of

the total pulse amplitude. The leakage inductance primarily affects

the rise and fall times of the pulses. Slower rise and fall times

effectively reduce the pulse width. Pulse width is determined by

the receiver as the time the signal is above the threshold set at

the ICMP pin. Slow rise and fall times cut into the timing margins.

Generally, it is best to keep pulse edges as fast as possible. When

evaluating transformers, it is also worth noting the parallel winding

capacitance. While transformers have very good CMRR at a low

frequency, this rejection degrades at higher frequencies, largely due

to the winding to winding capacitance. When choosing a transform-

er, it is best to pick one with less parallel winding capacitance when

possible.

When choosing a transformer, it is equally important to pick a

device that has an adequate isolation rating for the application.

The working voltage rating of a transformer is a key specification

when selecting a device for an application. Interconnecting daisy-

chain links between ADBMS1818 devices see <60 V stress in

typical applications. Ordinary pulse and local area network (LAN)

type transformers suffice. Connections to the LTC6820, in general,

may need much higher working voltage ratings for good long-term

reliability. Usually, matching the working voltage to the voltage of

the entire battery stack is conservative. Unfortunately, transformer

vendors often only specify one-second high voltage testing, and

this is not equal to the long-term (permanent) rating of the device.

For example, according to most safety standards, a 1.5 kV rated

transformer is expected to handle 230 V continuously, and a 3

kV device is capable of 1100 V long-term, though manufacturers

may not always certify to those levels (refer to actual vendor data

for specifics). Usually, the higher voltage transformers are called

high-isolation or reinforced insulation types by the suppliers. Table

74 shows a list of transformers that have been evaluated in isoSPI

links.

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