The ever-increasing number of electronic circuits in vehicles has led to a substantial increase in the amount of battery. power that needs to be consumed . To support features such as remote keyless entry and security, the battery continues to supply power even when the car is parked or turned off.
Since all vehicles run on limited battery power, a way must be found to add more functionality (especially when designing a car's front-end power system) without significantly increasing power consumption. The need to comply with stringent electromagnetic compatibility ( EMC ) standards (for example, ISO7637 of the International Organization for Standardization and the LV 124 standard formulated by German car manufacturers ) directly affects the overall design of the front-end battery reverse protection system. Some OEMs specify total current consumption when the vehicle is parked to less than 100µA per electronic control unit (ECU) in a 12V battery powered system and less than 500µA in a 24V battery powered system .
In this article, I will describe three methods for designing low quiescent current (I Q ) automotive battery reverse protection systems.
Use T15 as ignition or wake-up signal
The first way to design a low- IQ battery reverse protection system is to use T15 as an ignition or wake-up signal. T15 is a terminal that disconnects from the battery when the vehicle ignition is turned off. Using T15 as an external wake-up signal is a traditional way to run an ECU in sleep or active mode . Figure 1 is an example.
Figure 1 : Reverse battery protection in automotive ECU using T15 as wake-up signal
When the ignition switch is turned on, T15 is connected to the battery voltage (V BATT ) potential, causing the ideal diode 's enable pin to be at a logic high level. Ideal diode controller in active mode that actively controls external FETs for ideal diode operation while enabling charge pump, control, and field effect transistor (FET) driver circuits . When the vehicle is stopped, T15 drops to 0V and the ideal diode controller responds with an off state, which causes the charge pump and control block to shut down, keeping the I Q consumption below 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 power 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 system's communication channel enables a low- IQ shutdown mode. Figure 2 shows an example system design using this approach.
Figure 2 : Low IQ reverse battery protection with enable control using MCU and CAN wake - up signal
CAN transceivers in the vehicle translate messages from the communication bus to the respective controller (usually an MCU ). The transceiver can indicate when a function is not required by issuing a command to enter standby mode until it is woken up. The relay message at this point instructs the controller to deliver an instruction to put the system into a low power state, which is accomplished by having the ideal diode controller's enable signal at a logic low level. With more advanced transceivers and system basis chips, one device can handle multiple functions of this process and transition to low-power states or wake up.
This scheme requires internal control signals from the MCU (controlled via CAN ).
Using a normally open ideal diode controller
A third method is to use a normally open ideal diode controller. You can imagine a system design that does not require control signals to enter a low-power state. In this design, no additional wiring or reliance on system software is required to keep the ideal diode controller enabled at all times, even in sleep mode. This type of system design can be implemented using a low- IQ ideal diode controller, such as the LM74720 -Q1 , LM74721-Q1 , or LM74722-Q1 , as shown in Figure 3 . These devices integrate all necessary control blocks for EMC -compliant reverse battery protection designs and a boost regulator to drive a high-side external MOSFET , resulting in an I Q of 27µA during normal operation . For more information, see the application note " Ideal Diode Basics ".
Figure 3 : Reverse-Battery Protection Using Normally-On Low- IQ Ideal Diode Controller Without External Enable Control
These ideal diode controllers support reverse battery protection with active rectification, as well as load-disconnect FET control using a back-to-back FET topology to protect downstream during system faults such as overvoltage events, as shown in Figure 4 .
Figure 4 : Reverse battery protection in a 24V automotive ECU using the 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 24V car battery input based systems design.
The LM74720-Q1 and LM74721-Q1 provide fast response comparators with 0.45µs reverse current and 1.9µs forward current, as well as a powerful 30mA boost regulator for automotive AC superposition testing at frequencies up to 100kHz supports and implements flexible and efficient active rectification. The LM74722-Q1 rectifies twice as fast as the LM74720-Q1 and LM74721-Q1 devices, with a forward comparator response current of 0.8µs , enabling active rectification frequencies up to 200KHz . The LM74721-Q1 features an integrated drain-source voltage (VDS) clamp that enables reverse-battery protection designs without a transient voltage suppressor ( TVS ) , resulting in a more compact system solution. To learn more about active rectification, read our application report " Active Rectification and Its Benefits in Automotive ECU Design. "
Epilogue
With the LM74720-Q1 , LM74721-Q1 , and LM74722 -Q1 low- IQ normally-on ideal diode controllers, you can design automotive reverse battery protection systems without the need for an external enable control. These ideal diode controllers feature low I Q , back-to-back FET drive capability, and overvoltage protection, allowing downstream components such as capacitors with lower voltage ratings to be used in the design and reducing printed circuit board size for space-constrained ECUs size of.