NXP RDA777T2 Battery Junction Box Reference Design

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IMPORTANT NOTICE
For engineering development or evaluation purposes only

NXP provides this evaluation product under the following conditions:
Evaluation kits or reference designs are intended solely for technically qualified professionals, specifically for use in research and development environments to facilitate evaluation purposes. This evaluation kit or reference design is not a finished product, nor is it intended to be a part of a finished product. Any software or software tools provided with an evaluation product are subject to the applicable terms that accompany such software or software tool.

The evaluation kit or reference design is provided as a sample IC pre-soldered to a printed circuit board to make it easier to access inputs, outputs, and supply terminals. This evaluation kit or reference design may be used with any development system or other source of I/O signals by connecting it to the host MCU or computer board via off-the-shelf cables. Final device in an application will be heavily dependent on proper printed circuit board layout and heat sinking design as well as attention to supply filtering, transient suppression, and I/O signal quality. This evaluation kit or reference design provided may not be complete in terms of required design, marketing, and or manufacturing related protective considerations, including product safety measures typically found in the end device incorporating the evaluation product. Due to the open construction of the evaluation product, it is the responsibility of the user to take all appropriate precautions for electric discharge. To minimize risks associated with the customers’ applications, adequate design and operating safeguards must be provided by the customer to minimize inherent or procedural hazards. For any safety concerns, contact NXP sales and technical support services.

WARNING

Lethal voltage and fire ignition hazard
The non-insulated high voltages that are present when operating this product, constitute a risk of electric shock, personal injury, death and/or ignition of fire. This product is intended for evaluation purposes only. It shall be operated in a designated test area by personnel qualified according to local requirements and labor laws to work with non-insulated mains voltages and high-voltage circuits. This product shall never be operated unattended.

 Introduction

NXP provides a BJB reference design (RD) to showcase the MC33777A. The reference design is used to quickly prototype the hardware and software of a high-voltage battery management system. The reference design showcases the latest generation of BJB controller IC. This document describes the reference design features.

Getting to know the hardware

Board features
The reference design offers the following features:

• Four current measurement channels with external shunt (from −300 mV to +300 mV)
• Eight positive high-voltage measurement inputs (from 0 V to +1000 V)
• Two bipolar high-voltage measurement inputs (from −1000 V to +1000 V)
• Isolation monitoring between high-voltage domains and low-voltage domains
• Two temperature measurement channels with an external negative temperature coefficient (NTC) resistor
• One isolated crash signal monitoring input
• Two pyrotechnic switch control outputs with independent energy reservoir capacitor
• One EEPROM for calibration data storage
• Galvanically isolated electrical transport protocol link (ETPL) for communication
• Galvanically isolated DC-DC converter to supply the board from the low-voltage section
• Printed-circuit board (PCB) designed according to IEC 60664 (pollution degree 2, material group IIIa)

Connectors
Figure 1 shows the location of the connectors interfacing the reference design with a power supply, an emulator, or external instruments.

Table 1. Connector description

Table 2 lists the reference of the connectors and their mating part number.

Table 2. Connector part number

 LEDs
The battery junction box embeds two LEDs: D10 and D11. They are switched on when the MC33777A is in active mode.

Kit contents
Table 3 lists the components included in the kit.

Table 3. Kit contents

 Extra hardware
The RDA777T2 requires an external +12 V power supply (see Section 3.1). The following equipment can also ease the evaluation:

• ETPL communication board (KIT-PC2TPLEVB)
• Battery junction box emulator to emulate the high voltages, the battery current, and the pyrotechnic switch controllers (PACK-BJBEMUL)
• High-voltage source
• High-current source coupled with a shunt resistor

 Configure the hardware
This section describes the typical setup to configure the RDA777T2 and to evaluate the MC33777A key features. It uses a PACK-BJBEMUL to emulate the voltages, the battery current, and the pyrotechnic switches. Any other external equipment can replace the optional board. The setup shows a KIT-PC2TPLEVB board to interface the MC33777A with the computer via NXP software tools (for example, BMS Script GUI).

Table 4 lists the material required to set up the test.

Table 4. Bill of materials

Feature description

Power supply
The reference design usually receives power from the battery management unit (BMU) on the connector J12. The power supply must follow the characteristics described in Table 5.

Table 5. Power supply characteristics

The BMU is in the low-voltage domain, whereas the BJB is in the high-voltage domain. Therefore, the RDA777T2 embeds an isolated DC-DC converter to power the MC33777A and the external circuitry. The converter provides a 1.5 kV isolation.

 Current measurement

• The RDA777T2 measures up to four currents.
• For typical use cases, two channels are sufficient to measure redundantly the battery current to meet ASIL D safety goals.
• For more complex systems (for example, switched battery packs with two separate current measurements), the reference design offers two extra current measurement channels.

Current measurement characteristics
The user can connect to current measurement inputs to:

• A shunt resistor to measure the current flowing in it
• An external voltage source emulating the shunt resistor voltage drop

Table 6 lists the current measurement input characteristics.

Table 6. Current measurement characteristics

The board follows the MC33777A data sheet regarding the required external components.

 Current measurement connection
The RDA777T2 measures the current on the following MC33777A inputs:

Table 7. Current measurement channel allocation

The current measurement connector also offers MC33777A ground connections. The ground lines are separated from the measurement lines. It removes the current in the positive/negative lines and improves the accuracy.

The user can evaluate the current measurement with two methods:

• Applying a current in a shunt resistor
• Using an external voltage source

Figure 3 shows an example of a connection on a shunt resistor.

• The positive and negative lines are connected on both sides of the shunt sensing element. Inverting the orientation of the lines simply inverts the polarity of the current measurement.
• In the given example, a battery discharge current gives a positive measurement. A battery charge current gives a negative measurement.
• The user must connect the ground line to any side of the shunt resistors. It ensures that the shunt common mode voltage meets the MC33777A input range.

Figure 4 shows an example of a connection with a voltage source.

• To evaluate the current measurement, the user can also connect a voltage source to the inputs. Then, there is no need for a high-voltage battery or a high-current source.
• The positive and negative lines are connected on both sides of the voltage source. Inverting the orientation of the lines simply inverts the polarity of the current measurement.
• The user must connect the ground line to any side of the voltage source. It ensures that the common mode voltage meets the MC33777A input range.

Current measurement conversion

• The MC33777A automatically converts the input voltage measurement to a current value (more information is available in the device reference manual).
• The user must configure the sensor current to voltage ratio in a register (for example, the external shunt conductance).

Temperature measurement

• The RDA777T2 measures two temperatures with external NTC resistors. The user has the possibility to place the NTC resistor close to the shunt resistor to measure its temperature.

 Temperature measurement characteristics
Table 8 describes the characteristics of the temperature measurement feature.

Table 8. Temperature measurement characteristics

Temperature measurement circuit description

• The user can directly connect the external NTC resistor between the two dedicated pins on the current measurement connector.
• The temperature measurement circuitry follows the MC33777A data sheet recommendations.

The RDA777T2 measures the temperature on the following inputs:

Table 9. Temperature measurement channel allocation

The MC33777A outputs a 5 V source to bias the NTC resistor. To improve the accuracy of the measurement, the user must configure the analog input for ratiometric measurements.

Temperature measurement conversion
With the analog input configured for ratiometric measurements, the MC33777A returns a ratio of the biasing voltage.

The system controller can calculate the NTC value using the following equation:

Where:

• RNTC is the result of the NTC calculation in Ω
• RTC is the pullup resistor in Ω
• RESULT is the result of the analog-to-digital converter (ADC) measurement (16-bit value)
• Δres(ratio-io) is the ratiometric measurement resolution (see MC33777A data sheet)

Then, the controller can compute the temperature by using the NTC resistor data sheet (for example: calculation with β coefficient, or with look-up table).

High-voltage measurement
The RDA777T2 measures several high voltages in the system. The BMU can compute the result and proceed, for instance, to contactor monitoring.

 High-voltage measurement characteristic
Table 10 describes the high-voltage measurement characteristics.

Table 10. High-voltage measurement characteristics

 High-voltage measurement circuit description
The RDA777T2 measures up to ten high voltages in the system.
The eight positive inputs typically monitor the voltage across the high-side contactors and high-side fuses (for example, a contactor between the battery positive terminal and the inverter positive terminal). These inputs accept high voltages meeting Vhv+. Two inputs (one primary and another secondary) can monitor the same point to provide redundancy and increase the overall safety integrity level. The two bipolar inputs typically monitor the voltage across the low-side contactors (for example, a contactor between the battery negative terminal and the charger negative terminal). These inputs accept high voltages meeting Vhv-.

Figure 5 describes the circuitry of positive and bipolar high-voltage measurement paths.

• To reduce the leakage current in the resistors when there is no measurement, a high-voltage switch can disconnect the bridge. An MC33777A digital output controls this switch.
• A voltage divider divides the high voltage down to the device input voltage range. The resistors forming RH must withstand the high voltage.
• To avoid leakages due to the high voltage, the board has to ensure a big enough creepage distance between the different nodes. The cuttings in the PCB increase the creepage distance. The cuttings are optional when using coating or with lower voltages.
• For bipolar voltage measurement, the MC33777A outputs a 2.5 V reference. It shifts the output of the resistor bridge to half of the MC33777A input voltage range. The device can do a differential measurement between the output of the divider and the reference.
• An analog anti-aliasing filter improves the noise performance. Due to the filter and the switch circuitry response time, the controller must wait ts before starting a voltage measurement.

The MC33777A measures the divided voltage. To improve the accuracy, the user must configure the analog input as:

• Absolute mode (for positive measurements)
• Differential mode versus 2.5 V reference (for bipolar measurements)

 High-voltage measurement channel allocation
Table 11 describes the RDA777T2 high-voltage measurement channel allocation.

Table 11. Channel allocation

Positive-voltage measurement conversion
For positive-voltage measurements, the voltage divider is referenced to the MC33777A ground. The device directly measures the output voltage of the divider as:

Then, the controller can compute the high-voltage measurement as:

With:

• VHV is the high voltage to measure in V
• VSENSE is the output of the voltage divider in V
• VADC is the device measurement in V
• RL is the low-side divider resistor equal to 10 kΩ
• RH is the high-side divider resistor equal to 2.1 MΩ
• RESULT is the device measurement result (16-bit number)
• Vres(abs-io) is the device measurement resolution equal to 154 µV/LSB

Bipolar-voltage measurement conversion
For bipolar-voltage measurements, the voltage divider is referenced to the MC33777A 2.5 V reference. The output voltage of the divider is:

The device runs a differential measurement between the output of the divider and the 2.5 V reference:

Then, the controller can calculate the high-voltage measurement as:

With:

• VHV is the high voltage to measure in V
• VSENSE is the output of the voltage divider in V
• VADC is the device measurement in V
• RL is the low-side divider resistor equal to 4.7 kΩ
• RH is the high-side divider resistor equal to 2.1 MΩ
• RESULT is the device measurement result (16-bit number)
•  Vres(v2v5-io) is the device measurement resolution equal to 154 µV/LSB

 Adapting circuitry for low-voltage measurements

• Using a low-voltage source can ease the RDA777T2 evaluation. However, as the board typically measures high voltages, the user can adapt the circuitry.
• The simplest solution is to change the low-side divider resistor (RL). By choosing a higher value resistor, the divider ratio increases.
• The time constant of the anti-aliasing filter depends on the divider impedance. To keep the same cut-off frequency, the user can adapt the capacitor of the filter (CAAF) along with RL.
• Table 12 presents typical values for RL and CAAF to measure low voltage. Following these values ensures meeting the MC33777A measurement range.

Table 12. Component value to measure low voltage

The user must clearly identify the modified boards. Applying high voltage to a modified board can lead to injuries and permanent damage to the board.

Isolation monitoring
The RDA777T2 is in between the low-voltage section (car chassis, +12 V battery) and the high-voltage section (high-voltage battery, inverter) of the car. The board embeds the circuitry to monitor the isolation between the two sections. It helps to detect any isolation failure that could put the car user in danger.

Note: The RDA777T2 provides the isolation monitoring circuitry as an example to demonstrate the usage. The example has not undergone extensive testing in production. NXP advises the user to evaluate its suitability for their specific use cases.

Isolation monitoring characteristics
Table 13 describes the characteristics of the isolation monitoring feature.

Table 13. Isolation monitoring characteristics

 Isolation monitoring circuit description
Figure 7 describes the isolation monitoring circuitry.

This feature aims to evaluate the value of the equivalent resistance between:

•  The battery positive terminal and the chassis (RISO+)
• The battery negative terminal and the chassis (RISO-)

A high-voltage switch (SW3) connects the chassis to the circuit before measuring. As the measurement resistors are high enough, closing SW3 does not lead to an isolation failure and does not put the car user in danger. Another high-voltage switch (SW1) disconnects the resistor bridge to reduce the leakage current on the high‑voltage battery when there is no measurement.

The circuit has to measure two resistors (RISO+ and RISO-). Two voltage measurements are necessary to solve this two-unknown equation. The first measurement involves R1, R2, and RL. Enabling R3 (with SW2) allows getting a second voltage measurement. Section 3.5.3 describes the measurement sequence.

The output voltage (VSENSE) depends on the measurement circuitry (R1, R2, RL, and R3 if enabled), the battery voltage, and the isolation resistors. The MC33777A measures this voltage. To improve the accuracy, the user must configure the analog input for absolute measurements.

Table 14 describes the allocation of the MC33777A inputs and outputs for isolation monitoring.

Table 14. Isolation monitoring channel allocation

Due to the switch circuitry response time, the BMU must wait ts before starting each voltage measurement. After running the sequence, the BMU computes the voltage measurements to determine the isolation resistors as explained in Section 3.5.4.

 Isolation monitoring sequence
Table 15 describes the steps of the isolation monitoring sequence.

Table 15. Isolation monitoring sequence

 Isolation monitoring conversion
During the isolation monitoring sequence, the MC33777A proceeds to voltage measurements. The IC returns a 16-bit. The controller computes the result in V following below equation:

Where:

• Vmeas is the MC33777A input voltage, measured by the ADC, in V
• RESULT is the result of the ADC conversion
• Vres(abs-io) is the device measurement resolution equal to 154 µV/LSB

Once the sequence is over, the controller computes the measurements to calculate the isolation resistors. To ease the calculation, the formula uses the conductance instead of the resistance.

The below equation describes the relationship between resistance and conductance.

Where:

• YX is the conductance in S
• RX is the resistance in Ω

The formula expressing the isolation resistances in function of the measurements is as follows:

Where:

• YISO+ is the conductance of the positive isolation resistance in S
• YISO- is the conductance of the negative isolation resistance in S
• VBAT is the converted high-voltage measurement of the battery in V
• V1 is the first converted voltage measurement of the sequence in V
• V2 is the second converted voltage measurement of the sequence in V
• YL, Y1, Y2, and Y3 are the conductances of the measurement resistors in S Table 16 describes the conversion parameters of the RDA777T2.

Table 16. Isolation measurement conversion parameters

 Crash signal monitoring
The RDA777T2 monitors an isolated digital signal. It can be a crash signal coming from the low-voltage section. Then, the MC33777A can trigger a reaction (for example, pyrotechnic switch controller) based on the signal state.

 Crash signal monitoring characteristics
Table 17 describes the crash signal monitoring characteristics.

Table 17. Crash signal monitoring characteristics

Crash signal monitoring circuitry
Figure 8 describes the crash signal monitoring circuitry.

The circuitry accepts any voltage meeting Vin. An optocoupler isolates the signal and forwards the information to the MC33777A. The device outputs a 5 V biasing voltage and monitors the signal on a digital input.

The circuitry inverts the signal:

• An input signal lower than Vth results in a high-level reading
• An input signal higher than Vth results in a low-level reading Table 18 describes the channel allocation.

Table 18. Crash signal monitoring channel allocation

 Pyrotechnic switch control
The RDA777T2 supports the driving of two pyrotechnic switches, with the MC33777A independent pyrotechnic switch controllers.

Pyrotechnic switch control characteristics
Table 19 describes the pyrotechnic switch control characteristics.

Table 19. Pyrotechnic switch control characteristics

 Pyrotechnic switch control circuitry
The two MC33777A pyrotechnic switch controllers are available on two connectors. The user can connect:

• Both controllers to a single pyrotechnic switch (redundant driving)
• Each controller to a different pyrotechnic switch (independent driving)

Both pyrotechnic switch controllers have an independent energy reservoir capacitor (CER). If there is a power supply loss, the capacitors store the energy for the firing and keep the device active. By default, the resistors connected on the pin PSC_CFG configures the capacitor charge current to charge.

Communication
The RDA777T2 communicates with the BMU with ETPL. A transformer galvanically isolates both boards. The MC33777A data sheet describes the required circuitry for the communication.

 References
NXP Semiconductors provides online resources for this evaluation board and its supported devices on http://www.nxp.com. The information page for the MC33777A is http://nxp.com/mc33777. The page provides overview information, documentation, software and tools, parametrics, ordering information and a getting started tab.

Revision history

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Document feedback

• Date of release: 21 March 2025
• Document identifier: UM12056

Frequently Asked Questions

1. Is the RDA777T2 board suitable for high-voltage applications?
   Yes, the RDA777T2 board is designed for high-voltage battery management systems and can handle up to 800 V.
2. Can the reference design be used as a standalone product?
   No, the reference design is intended for evaluation and prototyping purposes only, not as a finished product.
3. What precautions should be taken when operating the product?
   Ensure that qualified personnel operate the product in designated test areas due to the risk of electric shock and fire hazards associated with high voltages.

Documents / Resources

NXP RDA777T2 Battery Junction Box Reference Design [pdf] User Guide RDA777T2 Battery Junction Box Reference Design, RDA777T2, Battery Junction Box Reference Design, Junction Box Reference Design, Box Reference Design, Reference Design

References

• User Manual