ZHCSTD7A 数据表 | 德州仪器 TI.com.cn

ZHCSTD7A December 2021 – November 2023 BQ79631-Q1

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9.2.1.2.12.2 Devices Connected on Different PCBs

For devices that are daisy-chained to different PCBs through a pair of twisted cables, all three isolation types can be used for daisy-chain isolation. It is also possible to use one type of isolation on one side of the daisy-chain (for example, transformer isolation on COMLP/N to the Battery Management Unit) while using a different type of isolation for the other side of the daisy-chain (for example, capacitor isolation on COMH/N to another BQ796xx Unit).

Option 1: Capacitor Isolation

Table 9-11 Components for Capacitor Isolation on Different PCBs

Components	Value	Description (Capacitor Isolation Between PCBs)
R_TERM	1 kΩ	Termination resistor
R_SERIES	49 Ω	Filter resistor and impedance matching resistor. The connection between devices must be ~120 Ω (~50 Ω on each end of the signal connection of the device plus 10-Ω internal resistance).
C_BYPASS	220 pF/50 V	Bypass capacitor
C_ISO	2.2 nF	Isolation capacitor Voltage rating depends on application requirement. It is common to select 2x of module voltage rating to provide standoff margin in the event of a fault in the system.
ESD diode	TVS diode	The ESD protector should provide protection to the communication interface pins during hot-plug events and also for absorption of high-voltage transients during service disconnect or reconnect. Select the ESD diodes to limit the maximum voltage on the COM* bus to below the maximum rating. A voltage rating close to the maximum voltage to provide the highest possible common-mode voltage range is recommended for best EMC performance. The capacitance must be low compared to the coupling capacitance (if using capacitor coupling).

Figure 9-13 Capacitor Isolation on Different PCB

Figure 9-13 shows the capacitor isolation circuit for devices connecting between PCBs. Similar to the capacitor isolation on the same PCB case, the capacitor must be rated with a high enough voltage to provide standoff margin in the event of a fault in the system that exposes the device to a local hazardous voltage. The voltage is determined by the application requirement but it is common to select 2x of the module voltage.

The capacitance on the daisy-chain bus has a direct effect on performance. All parasitic capacitances from the support components and cabling must be taken into consideration when designing for communication robustness to EMC. Capacitance from the cables, ESD diodes, bypass capacitance, and chokes form a capacitive divider with the isolation capacitors that may affect performance. Additionally, the amount of capacitance on the bus has a direct impact to the operating current during communication (the capacitor charging or discharging).

Option 2: Capacitor Plus Common-Mode Choke Isolation

Table 9-12 Components for Capacitor Plus Common-Mode Choke Isolation

Components	Value	Description (Capacitor Plus Choke Isolation Between PCBs)
R_TERM	1 kΩ	Termination resistor
R_SERIES	49 Ω	Filter resistor and impedance matching resistor. The connection between devices must be ~120 Ω (~50 Ω on each end of the signal connection of the device plus 10-Ω internal resistance).
C_BYPASS	220 pF/50 V	Bypass capacitor
C_ISO	2.2 nF	Isolation capacitor Voltage rating depends on application requirement. It is common to select 2x of module voltage rating to provide a standoff margin in the event of a fault in the system.
Common-mode choke	100 μH to 500 μH	Common-mode choke An inductance range of 100 μH to 500 μH is a general guidance value, not a specified range as there are many parameters affecting the performance of a common-mode choke. When selecting a recommended part, refer to the SLVAEP4 BQ79616-Q1 Daisy Chain Communications Application Report. The user shall perform the through test in their environment.
ESD diode (optional)	TVS diode	Optional ESD protection depends on pcb level ESD requirement

Figure 9-14 Capacitor Plus Choke Isolation

Longer cable lengths, or abnormally noisy applications may require the use of a common-mode choke filter. A capacitor plus choke isolation has better noise immunity than capacitor only. For these applications, use an automotive grade from 100-μH to 500-μH common-mode filter minimum for proper operation. To achieve the best performance in noisy environments, use dual common-mode filters (470 μH). The recommended impedance of the choke is at least 1 kΩ from 1 MHz to 100 MHz and above 300 Ω for higher frequencies.

Option 3: Transformer Isolation

Table 9-13 Components for Transformer Isolation

Components	Value	Description (Capacitor Plus Choke Isolation Between PCBs)
R_TERM	1 kΩ	Termination resistor
R_SERIES	49 Ω	Filter resistor and impedance matching resistor. The connection between devices must be ~120 Ω (~50 Ω on each end of the signal connection of the device plus 10-Ω internal resistance)
C_BYPASS	220 pF/50 V	Bypass capacitor
Transformer	Inductance: 150 μH to 1400 μH	The inductance range 150 μH to 1400 μH is a general guidance value, not a specified range as there are many parameters affecting the performance of the transformer. When selecting a recommended part, refer to the SLVAEP4 BQ79616-Q1 Daisy-Chain Communications Application Report. The user shall perform the through test in their environment.
ESD diode (optional)	TVS diode	Optional ESD protection depends on pcb level ESD requirement

Figure 9-15 Transformer Isolation

Transformer isolation is supported and can be implemented as above. For example, transformer isolation can be used between the low-voltage and high-voltage boundary for galvanic isolation.

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9.2.1.2.12.2 Devices Connected on Different PCBs