3 ways to design a low quiescent current (Iq) reverse protection system for automotive batteries

The number of Electronic circuits in vehicles continues to increase, so that the amount of battery power that needs to be consumed has also increased substantially. In order to support remote keyless entry and security functions, the battery must continue to supply power even when the car is parked or turned off.

The number of electronic circuits in vehicles continues to increase, so that the amount of battery power that needs to be consumed has also increased substantially. In order to support remote keyless entry and security functions, the battery must continue to supply power even when the car is parked or turned off.

Since all vehicles use limited battery power, it is necessary to find a way to add more functions ON the one hand (especially when designing the car’s front-end power system) without significantly increasing power consumption. Whether it is necessary to comply with strict electromagnetic compatibility (EMC) standards (for example, ISO7637 of the International Organization for Standardization and LV 124 standards formulated by German automakers) directly affects the overall design of the front-end battery reverse protection system. Some original equipment manufacturers stipulate the total current consumption when the vehicle is parked as: each electronic control unit (ECU) is less than 100µA in a 12V battery-powered system, and less than 500µA in a 24V battery-powered system.

In this article, I will introduce the design of low quiescent current (IQ) Three methods of car battery reverse protection system.

Use T15 as ignition or wake-up signal

Design low IQ The first method of the battery reverse protection system is to use T15 as an ignition or wake-up signal. T15 is a terminal block. When the ignition switch of the vehicle is turned off, it will be disconnected from the battery. Using T15 as an external wake-up signal is a traditional method of running the ECU in sleep or active mode. Figure 1 is an example.

3 ways to design a low quiescent current (Iq) reverse protection system for automotive batteries
Figure 1: Battery reverse protection in automotive ECU using T15 as a wake-up signal

When the ignition switch is turned on, T15 will be connected to the battery voltage (VBATT) Potential, so that the enable pin of the ideal diode is at a logic high level. An ideal diode controller in active mode that actively controls external FETs to achieve ideal diode operation while enabling charge pump, control, and field effect Transistor (FET) driver circuits. When the vehicle is parked, T15 drops to 0V, and the ideal diode controller responds with the off state, which will cause the charge pump and the control block to turn off, thus making the IQ The consumption is less than 3µA. In this mode of operation, the external FET is turned off, and the body diode of the FET forms a forward conduction path to supply power to the load. This solution requires additional wiring to the ECU.

Use the system’s MCU and CAN wake-up signal

The second method is to use the system’s microcontroller (MCU) and controller area network (CAN) to wake up. In many cases, the communication channel of the system makes low IQ Shutdown mode becomes possible. Figure 2 shows an example system design using this method.

3 ways to design a low quiescent current (Iq) reverse protection system for automotive batteries
Figure 2: Use MCU and CAN wake-up signal to achieve low I enable controlQ Battery reverse protection

The CAN transceiver in the vehicle converts the message from the communication bus to the respective controller (usually an MCU). The transceiver can indicate when the relevant function is not needed by issuing a command to enter the standby mode until it is awakened. At this time, the Relay message instructs the controller to deliver an instruction to place the system in a low power consumption state. The implementation is to make the enable signal of the ideal diode controller at a logic low level. With more advanced transceivers and system basis chips, a device can handle multiple functions of this process and transition to a low-power state or wake up.

This solution requires internal control signals from the MCU (control via CAN).

Use normally open ideal diode controller

The third method is to use a normally open ideal diode controller. You can imagine this system design that does not require a control signal to enter a low-power state. In this design, no additional wiring or system software is required, so that the ideal diode controller is always enabled, even in sleep mode. This type of system design can use low IQ Ideal diode controller to achieve, such as LM74720-Q1, LM74721-Q1 or LM74722-Q1, as shown in Figure 3. These devices integrate all the necessary control blocks for the EMC-compliant battery reverse protection design, and integrate a boost Regulator for driving the high-side external mosfet, so that the IQ It is 27µA. For more information, please refer to the application note “Basic Knowledge of Ideal Diodes”.

3 ways to design a low quiescent current (Iq) reverse protection system for automotive batteries
Figure 3: Using normally open low I without external enable controlQ Ideal diode controller for reverse battery protection

These ideal diode controllers support reverse battery protection with active rectification, as well as load-off FET control using a back-to-back FET topology to protect downstream during system failures (such as overvoltage events), as shown in Figure 4.

3 ways to design a low quiescent current (Iq) reverse protection system for automotive batteries
Figure 4: Battery reverse protection in 24V automotive ECU using LM74720-Q1

With adjustable overvoltage protection, you can use 50V rated downstream filter Capacitors (instead of 80V to 100V rated capacitors) and 40V rated DC/DC converters (instead of 65V rated converters) for system design based on 24V car battery input .

LM74720-Q1 and LM74721-Q1 provide 0.45µs reverse current fast response comparator and 1.9µs forward current fast response comparator, as well as a powerful 30mA boost regulator for superimposing tests on automotive AC frequencies up to 100kHz Supports and implements flexible and efficient active rectification. The rectification speed of LM74722-Q1 is twice as fast as that of LM74720-Q1 and LM74721-Q1 devices. The forward comparator response current is 0.8µs and can achieve an active rectification frequency up to 200KHz. LM74721-Q1 has an integrated drain-source Voltage (VDS) clamp, which can realize a reverse battery protection design without a transient voltage suppressor (TVS), thereby making the system solution more compact. To learn more about active rectification, please read our application report “Active Rectification and Its Advantages in Automotive ECU Design”.

Concluding remarks

Low I with LM74720-Q1, LM74721-Q1 and LM74722-Q1Q Normally open ideal diode controller, you can design a car battery reverse protection system without external enable control. These ideal diode controllers have low IQ, Back-to-back FET drive capability and overvoltage protection characteristics, so downstream components such as capacitors with lower rated voltages can be used in the design, and the size of the printed circuit board can be reduced for space-constrained ECUs.

The number of electronic circuits in vehicles continues to increase, so that the amount of battery power that needs to be consumed has also increased substantially. In order to support remote keyless entry and security functions, the battery must continue to supply power even when the car is parked or turned off.

Since all vehicles use limited battery power, it is necessary to find a way to add more functions on the one hand (especially when designing the car’s front-end power system) without significantly increasing power consumption. Whether it is necessary to comply with strict electromagnetic compatibility (EMC) standards (for example, ISO7637 of the International Organization for Standardization and LV 124 standards formulated by German automakers) directly affects the overall design of the front-end battery reverse protection system. Some original equipment manufacturers stipulate the total current consumption when the vehicle is parked as: each electronic control unit (ECU) is less than 100µA in a 12V battery-powered system, and less than 500µA in a 24V battery-powered system.

In this article, I will introduce the design of low quiescent current (IQ) Three methods of car battery reverse protection system.

Use T15 as ignition or wake-up signal

Design low IQ The first method of the battery reverse protection system is to use T15 as an ignition or wake-up signal. T15 is a terminal block. When the ignition switch of the vehicle is turned off, it will be disconnected from the battery. Using T15 as an external wake-up signal is a traditional method of running the ECU in sleep or active mode. Figure 1 is an example.

3 ways to design a low quiescent current (Iq) reverse protection system for automotive batteries
Figure 1: Battery reverse protection in automotive ECU using T15 as a wake-up signal

When the ignition switch is turned on, T15 will be connected to the battery voltage (VBATT) Potential, so that the enable pin of the ideal diode is at a logic high level. An ideal diode controller in active mode that actively controls external FETs to achieve ideal diode operation while enabling charge pump, control, and field effect Transistor (FET) driver circuits. When the vehicle is parked, T15 drops to 0V, and the ideal diode controller responds with the off state, which will cause the charge pump and the control block to turn off, thus making the IQ The consumption is less than 3µA. In this mode of operation, the external FET is turned off, and the body diode of the FET forms a forward conduction path to supply power to the load. This solution requires additional wiring to the ECU.

Use the system’s MCU and CAN wake-up signal

The second method is to use the system’s microcontroller (MCU) and controller area network (CAN) to wake up. In many cases, the communication channel of the system makes low IQ Shutdown mode becomes possible. Figure 2 shows an example system design using this method.

3 ways to design a low quiescent current (Iq) reverse protection system for automotive batteries
Figure 2: Use MCU and CAN wake-up signal to achieve low I enable controlQ Battery reverse protection

The CAN transceiver in the vehicle converts the message from the communication bus to the respective controller (usually an MCU). The transceiver can indicate when the relevant function is not needed by issuing a command to enter the standby mode until it is awakened. At this time, the relay message instructs the controller to deliver an instruction to place the system in a low power consumption state. The implementation is to make the enable signal of the ideal diode controller at a logic low level. With more advanced transceivers and system basis chips, a device can handle multiple functions of this process and transition to a low-power state or wake up.

This solution requires internal control signals from the MCU (control via CAN).

Use normally open ideal diode controller

The third method is to use a normally open ideal diode controller. You can imagine this system design that does not require a control signal to enter a low-power state. In this design, no additional wiring or system software is required, so that the ideal diode controller is always enabled, even in sleep mode. This type of system design can use low IQ Ideal diode controller to achieve, such as LM74720-Q1, LM74721-Q1 or LM74722-Q1, as shown in Figure 3. These devices integrate all the necessary control blocks for the EMC-compliant battery reverse protection design, and integrate a boost regulator for driving the high-side external MOSFET, so that the IQ It is 27µA. For more information, please refer to the application note “Basic Knowledge of Ideal Diodes”.

3 ways to design a low quiescent current (Iq) reverse protection system for automotive batteries
Figure 3: Using normally open low I without external enable controlQ Ideal diode controller for reverse battery protection

These ideal diode controllers support reverse battery protection with active rectification, as well as load-off FET control using a back-to-back FET topology to protect downstream during system failures (such as overvoltage events), as shown in Figure 4.

3 ways to design a low quiescent current (Iq) reverse protection system for automotive batteries
Figure 4: Battery reverse protection in 24V automotive ECU using LM74720-Q1

With adjustable overvoltage protection, you can use 50V rated downstream filter capacitors (instead of 80V to 100V rated capacitors) and 40V rated DC/DC converters (instead of 65V rated converters) for system design based on 24V car battery input .

LM74720-Q1 and LM74721-Q1 provide 0.45µs reverse current fast response comparator and 1.9µs forward current fast response comparator, as well as a powerful 30mA boost regulator for superimposing tests on automotive AC frequencies up to 100kHz Supports and implements flexible and efficient active rectification. The rectification speed of LM74722-Q1 is twice as fast as that of LM74720-Q1 and LM74721-Q1 devices. The forward comparator response current is 0.8µs and can achieve an active rectification frequency up to 200KHz. LM74721-Q1 has an integrated drain-source voltage (VDS) clamp, which can realize a reverse battery protection design without a transient voltage suppressor (TVS), thereby making the system solution more compact. To learn more about active rectification, please read our application report “Active Rectification and Its Advantages in Automotive ECU Design”.

Concluding remarks

Low I with LM74720-Q1, LM74721-Q1 and LM74722-Q1Q Normally open ideal diode controller, you can design a car battery reverse protection system without external enable control. These ideal diode controllers have low IQ, Back-to-back FET drive capability and overvoltage protection characteristics, so downstream components such as capacitors with lower rated voltages can be used in the design, and the size of the printed circuit board can be reduced for space-constrained ECUs.

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