ADBMS2950BCCSZ datasheet(78/97 Pages) AD

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ADBMS2950B

Data Sheet

Rev. 0 | Page 78 of 97

not trigger until both the OC1 and OC2 channels report

overcurrent events. Either way, the safety function of two-

channel redundancy for safe pyrofuse ignition is not degraded.

However, with only two channels, the active one must

compromise, whether in the case of a single-point fault in the

OCxADC channels, the ignition needs to be forced or

prevented.

With the 3-channel concept, only the majority voters and all the

following function blocks can cause unintended ignition

because of single-point faults. If two channels are only used, and

the OC3TH is set to 0, additionally, a single point fault

preceding the majority voters can cause unintended ignition.

On the other hand, if the OC3TH is set to 0x7F, a single point

fault preceding the majority voters can prevent ignition. In any

case, the fault is reported to the serial controller through read

commands or through the PWM overcurrent output mode

coding.

Also, in case only two current sense pairs for the shunt interface

are used, the OC3ADC inputs should still be connected as

shown in Figure 60 with the recommended external filter

components to have support for the OC3ADC measurements

including the minimum and maximum tracking registers

OC3MIN, OC3MAX. None of this functionality is affected if

the overcurrent detection of the 3rd channel is always asserted

or de-asserted through the OC3TH setting.

Shunt Redundancy

In addition to Figure 51 showing a single shunt resistor

connected to the redundant inputs of the ADBMS2950/52, the

following figures give ideas of dual-shunt implementations in

which two separate sense elements are used. The shunt current

measurement being a low-side measurement requires the shunt

resistor to be grounded one side. To minimize the common

mode signal on the current sense inputs, the ground connection

is preferably done in the middle of the two sensing parts.

Figure 61. Preferred Dual-Shunt Ground Connection

Figure 62 shows a dual series sense element shunt resistor with

separated pads on the PCB footprint that connects all three

sense pairs and the GND separately. GND and NTCs are

connected to the center tap. The center tap must be as small as

possible to minimize its impedance (R and L), and in result, the

common mode signal. The resistors seen at the (over-) current

channels are:

•

I1, OC1: Rsns1

•

I2, OC2: Rsns2

•

OC3: Rsns2

Ideally, if the mechanical design is symmetric, both sense

elements have roughly the same nominal value. The

temperature gradient across the shunt (across the Y-axis in

Figure 62) must be minimized to reduce thermocouple voltages.

ADBMS2950BCCSZ 数据表(PDF) 78 Page - Analog Devices

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