Linear Technology LTC6804-1 - Page 72

LTC6804-1/LTC6804-2
72
680412fc
For more information www.linear.com/LTC6804-1
applicaTions inForMaTion
When an LTC6804-2 is not addressed, it will not transmit
data pulses. This scheme eliminates the possibility for col-
lisions since only the addressed device returns data to the
master
. Generally
, multi-drop systems are best confined
to compact assemblies where they can avoid excessive
isoSPI pulse-distortion and EMC pickup.
Basic Connection of the LTC6804-2 in a Multi-Drop
Configuration
In a multi-drop isoSPI bus, placing the termination at the
ends of the transmission line provides the best performance
(with 100Ω typically). Each of the LTC6804 isoSPI ports
should couple to the bus with a resistor network, as shown
in Figure 48a. Here again, a center-tapped transformer offers
the best performance and a common mode choke (CMC)
increases the noise rejection further, as shown in Figure
48b. Figure 48b also shows the use of an RC snubber at
the IC connections as a means to suppress resonances
(the IC capacitance provides sufficient out-of-band re
-
jection). When using a non-center-tapped transformer,
a virtual
CT can be generated by connecting a CMC as
a voltage-splitter. Series resistors are recommended to
decouple the LTC6804 and board parasitic capacitance
from the transmission line. Reducing these parasitics on
the transmission line will minimize reflections.
Connecting an MCU to an LTC6804-1 with an isoSPI
Data Link
The LTC6820 will convert standard 4-wire SPI into a
2-wire isoSPI link that can communicate directly with
the LTC6804. An example is shown in Figure 46. The
LTC6820 can be used in applications to provide isolation
between the microcontroller and the stack of LTC6804s.
The LTC6820 also enables system configurations that
have the BMS controller at a remote location relative to
the LTC6804 devices and the battery pack.
Configuring the LTC6804-2 in a Multi–Drop isoSPI
Link
The addressing feature of the LTC6804-2 allows multiple
devices to be connected to a single isoSPI master by dis
-
tributing them along one twisted pair, essentially creating
a large parallel SPI network. A basic multi-drop system is
shown in Figure 47
;
the twisted pair is terminated only at
the beginning (master) and the end of the cable. In between,
the additional LTC6804-2s are connected to short stubs
on the twisted pair. These stubs should be kept short, with
as little capacitance as possible, to avoid degrading the
termination along the isoSPI wiring.
10nF
GNDB
GNDB
10nF
GNDA
GNDA
10nF*
10nF*
10nF
GNDB
10nF*
1k 1k
1k 1k
•
•
GNDA
GNDB
•
•
GNDB
GNDA
49.9Ω
49.9Ω
49.9Ω
49.9Ω
IPA
IBIAS
ICMP
IBIAS
ICMP
LTC6804-1
IMA
49.9Ω
49.9Ω
IP
IM V
–
IPB
IMB
* IF TRANSFORMER BEING USED HAS A CENTER TAP, IT SHOULD BE BYPASSED WITH A 10nF CAP
680412 F46
LTC6820
V
–
Figure 46. Interfacing an LTC6804-1 with a µC Using an LTC6820 for Isolated SPI Control