DC/DC controller in automobile start/stop electronic system

The automatic start/stop function can turn off the engine every time the car comes to a complete stop, and automatically restart the engine, thus helping to reduce fuel consumption and exhaust emissions. Compared with cars that are not equipped with such systems, the fuel consumption savings in urban traffic environments can be as high as 8%. There is an additional benefit that can reduce its carbon dioxide emissions.

The automatic start/stop function can turn off the engine every time the car comes to a complete stop, and automatically restart the engine, thus helping to reduce fuel consumption and exhaust emissions. Compared with cars that are not equipped with such systems, the fuel consumption savings in urban traffic environments can be as high as 8%. There is an additional benefit that can reduce its carbon dioxide emissions.

The principle is simple: if the engine is not running, it will not consume fuel. When the engine is not required to work, the automatic start/stop system function will automatically shut down the engine. In traffic jams or even stop-and-go traffic conditions, simply place the car in the neutral position and remove your foot from the clutch to activate this function. A “Start/Stop” message ON the information Display will indicate “Engine has been shut down”. If you want to restart the engine, step on the clutch and put in gear, and the car will quickly return to working condition and you can continue driving immediately.

The automatic start/stop function does not affect driving comfort and safety. For example, this function will not be activated until the engine reaches a certain ideal operating temperature. This principle also applies to situations where the air conditioner has not adjusted the cabin to the desired temperature, the battery is not fully charged, or the driver has turned the steering wheel.

The automatic start/stop function is coordinated by a central control unit that monitors data from all relevant sensors, including the starter motor and the alternator. For comfort or safety needs, the control unit can also automatically restart the engine. For example: If the vehicle starts to drive, the battery charge drops to a low level or condensation forms on the windshield. In addition, most systems can also distinguish the difference between a short pause and the end of a journey. If the driver’s seat belt is loosened, or the door or trunk is opened, the system will not restart the engine. If necessary, pressing a button can completely cancel the automatic start/stop function.

However, when the engine is restarted and an infotainment system is on or there are any other Electronic devices that require a voltage above 5V, the 12V battery may be below 5V, which will cause such a system to reset. Some infotainment systems use a working input Voltage of 5V and 8.5V, and this voltage is fed by a step-down converter that runs on the car battery. If the input voltage drops below 5V during the engine restart, these systems will reset when the DC/DC converter can only step down the input voltage. Obviously, it would be unacceptable for users to automatically reset these audiovisual systems every time the car restarts during the process of watching videos or listening to CDs.

A new solution

Fortunately, Linear Technology has introduced a three-output DC/DC controller LTC3859A, which integrates a synchronous boost controller and two synchronous buck controllers in a single package. The output of the synchronous boost converter feeds the buck converter to maintain a sufficiently high voltage to avoid resetting of electronic systems that require a working voltage above 4V during the engine restart. In addition, when the input voltage from the car battery to the step-up converter is higher than its programmed output voltage, it will operate under a 100% duty cycle condition and simply transmit the input voltage directly to the step-down converter , Thereby greatly reducing the power loss. Figure 1 shows the schematic diagram of the LTC3859A. When the battery voltage drops below 10V, the synchronous step-up converter provides 10V to the synchronous step-down converter. In addition to powering the two step-down converters (5V/5A and 8.5V/3A in this example), the step-up converter can also be used as a “third output”, which can provide an additional 2A output.

DC/DC controller in automobile start/stop electronic system
Figure 1: Typical LTC3859A start/stop application circuit schematic

LTC3859A is a low quiescent current, current mode control, three-output synchronous DC/DC controller using all N-channel mosfets. When starting, LTC3859A works within the input voltage range of 4.5V to 38V, and keeps working until after starting As low as 2.5V. The two step-down controllers (channels 1 and 2) operate 180? out of phase and can generate an output voltage of 0.8V to 24V, which is ideal for powering navigation, infotainment systems, processors, and storage. The boost controller (channel 3) runs in phase with channel 1, and can generate an output voltage up to 60V. The powerful 1.1Ω built-in gate driver for each channel greatly reduces mosfet switching losses. The operating frequency can be set in the range of 50kHz to 900kHz, or synchronized to an external clock with a frequency range of 75kHz to 850kHz using an internal phase-locked loop. LTC3859A is different from LTC3859 in that it has an internal clamp circuit on the INTVCC pin. The clamp circuit provides a fail-safe way to avoid excessive voltage on the INTVCC pin when the user negligently uses a leakage Schottky limiting diode.

Other features of this device include built-in LDO for IC power supply and gate drive, programmable soft start, power good signal and external VCC control. VREF accuracy is 1% in the operating temperature range of -40℃ to 85℃. LTC3859A adopts 38-pin SSOP package or 38-pin 5mmx7mm QFN package.

Extend battery working time

For any battery-powered system that requires a “always on” power bus when the rest of the system is off, saving battery energy is a must. This state is usually called “sleep”, “standby” or “idle” mode, and only requires the system to have a very low quiescent current.May include many electrical circuits

In most automotive applications, it is particularly important to require low quiescent current in order to save battery energy. In standby mode, the total current consumption of such systems must be as low as possible; moreover, as the operation of cars increasingly relies on electronic systems, the pressure on car manufacturers to save battery energy continues to increase.

In sleep mode (boost converter and one of the two buck converters are in the on state), the LTC3859A only draws a mere 75μA of current. When all three channels are turned on and are in sleep mode, the absorption current of LTC3859A is only 100μA, which significantly extends the battery’s working time in idle mode. This is achieved by configuring the device to enter a high-efficiency Burst Mode operating state. In this operating mode, the LTC3859A delivers a short current pulse to the output Capacitor, followed by a sleep cycle. The output capacitor transfers the output power to the load. Figure 2 shows a conceptual timing diagram illustrating its working principle.

DC/DC controller in automobile start/stop electronic system
An effective way to save automobile fuel: “Start/Stop” system (Electronic Engineering Special)
Figure 2: LTC3859A burst mode operating voltage line diagram

The burst mode output ripple has nothing to do with the load, the only thing that will change is the length of the sleep interval. In the sleep mode, most of the internal circuits are turned off, except for the critical circuits used to achieve fast response, thereby further reducing its quiescent current. When the output voltage drop is large enough, the sleep signal level goes low, and the controller resumes the standard burst mode operation by turning on the external MOSFET at the top. On the other hand, there are also situations where users expect the device to work in forced continuous mode or constant frequency pulse skipping mode under light load current conditions. The configuration of these two modes is very easy, their quiescent current is higher and the peak-to-peak output ripple is lower.

Load dump / efficiency / solution size

The term “load dump” refers to the inductive shock that occurs after the starter motor is turned off. For a 12V lead-acid battery system for automobiles, this surge voltage is generally clamped at 36V (large value). This surge requires the controller, MOSFET, and associated components to work under the clamp voltage. These higher voltage devices (for example: 40V MOSFET) will lead to a drop in efficiency, and care must be taken to minimize this adverse effect. When the circuit in Figure 1 is used, the efficiency of each voltage rail is higher than 92% (as shown in Figure 3). For clarity, the efficiency of each step-down and step-up section is shown separately. In addition, Figure 4 also shows the layout and size of this circuit, which has a high part of 4.8mm.

DC/DC controller in automobile start/stop electronic system
Figure 3: The relationship curve between LTC3859A efficiency and load current (for different converter parts)
An effective way to save automobile fuel: “Start/Stop” system (Electronic Engineering Special)

Startup and shutdown

The three channels of LTC3859A can be individually shut down using RUN1, RUN2 and RUN3 pins. Pulling any of these pins below 1.2V will shut down the main control loop for the corresponding channel. Pulling all three pins below 0.7V will disable all controllers and most internal circuits, including the built-in LDO. In this state, the LTC3859A only absorbs 8μA of quiescent current.

Soft start or tracking

The TRACK/SS1 and TRACK/SS2 pins of the two step-down controllers can be used to adjust the soft start on time or to “coincide” or “proportional” tracking two or more power supplies during startup. These correlation curves are shown in Figure 5, and a Resistor divider is also placed between the TRACK/SS pins of the main power supply and the slave power supply.

DC/DC controller in automobile start/stop electronic system
Figure 5: LTC3859A output voltage tracking: (a) coincidence tracking (b) proportional tracking

Protective function

The LTC3859A can be configured to use DCR (Inductor Resistance) or a sense Resistor to detect the output current. As to which of the two current detection schemes to choose, to a large extent depends on the comprehensive trade-offs of cost, power consumption, and accuracy. DCR is becoming more and more popular because it can eliminate expensive current-sensing resistors and is more efficient, especially in high-current applications. The LTC3859A has a current foldback function for the step-down channel to help limit the load current when the output is shorted to ground.

The built-in comparator is responsible for monitoring the step-down output voltage, and indicates an overvoltage condition when the output is greater than 10% of its nominal output voltage. When this condition is detected, the top MOSFET is turned off and the bottom MOSFET is turned on until the overvoltage condition is cleared. As long as the overvoltage condition persists, the bottom MOSFET will continue to be turned on. If the output voltage returns to a safe level, normal operation is automatically restored.

Under higher temperature conditions, or internal power consumption causes excessive self-heating inside the chip, the overheat shutdown circuit will shut down the LTC3859A. When the junction temperature exceeds approximately 170°C, the overheating protection circuit will disable the built-in bias LDO, causing the bias supply to drop to 0V and effectively shut down the entire LTC3859A in an orderly manner. Once the junction temperature drops to about 155°C, the LDO will turn on again.

in conclusion

Fuel-saving automobile start/stop systems will continue to develop in the next few years. Caution must be exercised for the power supply of in-vehicle infotainment and navigation systems, as well as the power supply of disk drives that require voltages up to or even more than 5V to achieve correct operation. This type of system resets when the input voltage drops outside the regulated range due to the restart of the engine. LTC3859A provides a solution that can use its built-in synchronous boost controller to raise the battery voltage to a safe operating level. LTC3859A integrates a synchronous step-up controller with two synchronous step-down controllers, which is very suitable for powering many devices and can maintain the regulation of all output voltages when the engine is restarted.

The automatic start/stop function can turn off the engine every time the car comes to a complete stop, and automatically restart the engine, thus helping to reduce fuel consumption and exhaust emissions. Compared with cars that are not equipped with such systems, the fuel consumption savings in urban traffic environments can be as high as 8%. There is an additional benefit that can reduce its carbon dioxide emissions.

The principle is simple: if the engine is not running, it will not consume fuel. When the engine is not required to work, the automatic start/stop system function will automatically shut down the engine. In traffic jams or even stop-and-go traffic conditions, simply place the car in the neutral position and remove your foot from the clutch to activate this function. A “Start/Stop” message on the information display will indicate “Engine has been shut down”. If you want to restart the engine, step on the clutch and put in gear, and the car will quickly return to working condition and you can continue driving immediately.

The automatic start/stop function does not affect driving comfort and safety. For example, this function will not be activated until the engine reaches a certain ideal operating temperature. This principle also applies to situations where the air conditioner has not adjusted the cabin to the desired temperature, the battery is not fully charged, or the driver has turned the steering wheel.

The automatic start/stop function is coordinated by a central control unit that monitors data from all relevant sensors, including the starter motor and the alternator. For comfort or safety needs, the control unit can also automatically restart the engine. For example: If the vehicle starts to drive, the battery charge drops to a low level or condensation forms on the windshield. In addition, most systems can also distinguish the difference between a short pause and the end of a journey. If the driver’s seat belt is loosened, or the door or trunk is opened, the system will not restart the engine. If necessary, pressing a button can completely cancel the automatic start/stop function.

However, when the engine is restarted and an infotainment system is on or there are any other electronic devices that require a voltage above 5V, the 12V battery may be below 5V, which will cause such a system to reset. Some infotainment systems use a working input voltage of 5V and 8.5V, and this voltage is fed by a step-down converter that runs on the car battery. If the input voltage drops below 5V during the engine restart, these systems will reset when the DC/DC converter can only step down the input voltage. Obviously, it would be unacceptable for users to automatically reset these audiovisual systems every time the car restarts during the process of watching videos or listening to CDs.

A new solution

Fortunately, Linear Technology has introduced a three-output DC/DC controller LTC3859A, which integrates a synchronous boost controller and two synchronous buck controllers in a single package. The output of the synchronous boost converter feeds the buck converter to maintain a sufficiently high voltage to avoid resetting of electronic systems that require a working voltage above 4V during the engine restart. In addition, when the input voltage from the car battery to the step-up converter is higher than its programmed output voltage, it will operate under a 100% duty cycle condition and simply transmit the input voltage directly to the step-down converter , Thereby greatly reducing the power loss. Figure 1 shows the schematic diagram of the LTC3859A. When the battery voltage drops below 10V, the synchronous step-up converter provides 10V to the synchronous step-down converter. In addition to powering the two step-down converters (5V/5A and 8.5V/3A in this example), the step-up converter can also be used as a “third output”, which can provide an additional 2A output.

DC/DC controller in automobile start/stop electronic system
Figure 1: Typical LTC3859A start/stop application circuit schematic

LTC3859A is a low quiescent current, current mode control, three-output synchronous DC/DC controller using all N-channel MOSFETs. When starting, LTC3859A works within the input voltage range of 4.5V to 38V, and keeps working until after starting As low as 2.5V. The two step-down controllers (channels 1 and 2) operate 180? out of phase and can generate an output voltage of 0.8V to 24V, which is ideal for powering navigation, infotainment systems, processors, and storage. The boost controller (channel 3) runs in phase with channel 1, and can generate an output voltage up to 60V. The powerful 1.1Ω built-in gate driver for each channel greatly reduces MOSFET switching losses. The operating frequency can be set in the range of 50kHz to 900kHz, or synchronized to an external clock with a frequency range of 75kHz to 850kHz using an internal phase-locked loop. LTC3859A is different from LTC3859 in that it has an internal clamp circuit on the INTVCC pin. The clamp circuit provides a fail-safe way to avoid excessive voltage on the INTVCC pin when the user negligently uses a leakage Schottky limiting diode.

Other features of this device include built-in LDO for IC power supply and gate drive, programmable soft start, power good signal and external VCC control. VREF accuracy is 1% in the operating temperature range of -40℃ to 85℃. LTC3859A adopts 38-pin SSOP package or 38-pin 5mmx7mm QFN package.

Extend battery working time

For any battery-powered system that requires a “always on” power bus when the rest of the system is off, saving battery energy is a must. This state is usually called “sleep”, “standby” or “idle” mode, and only requires the system to have a very low quiescent current.May include many electrical circuits

In most automotive applications, it is particularly important to require low quiescent current in order to save battery energy. In standby mode, the total current consumption of such systems must be as low as possible; moreover, as the operation of cars increasingly relies on electronic systems, the pressure on car manufacturers to save battery energy continues to increase.

In sleep mode (boost converter and one of the two buck converters are in the on state), the LTC3859A only draws a mere 75μA of current. When all three channels are turned on and are in sleep mode, the absorption current of LTC3859A is only 100μA, which significantly extends the battery’s working time in idle mode. This is achieved by configuring the device to enter a high-efficiency Burst Mode operating state. In this operating mode, the LTC3859A delivers a short current pulse to the output capacitor, followed by a sleep cycle. The output capacitor transfers the output power to the load. Figure 2 shows a conceptual timing diagram illustrating its working principle.

DC/DC controller in automobile start/stop electronic system
An effective way to save automobile fuel: “Start/Stop” system (Electronic Engineering Special)
Figure 2: LTC3859A burst mode operating voltage line diagram

The burst mode output ripple has nothing to do with the load, the only thing that will change is the length of the sleep interval. In the sleep mode, most of the internal circuits are turned off, except for the critical circuits used to achieve fast response, thereby further reducing its quiescent current. When the output voltage drop is large enough, the sleep signal level goes low, and the controller resumes the standard burst mode operation by turning on the external MOSFET at the top. On the other hand, there are also situations where users expect the device to work in forced continuous mode or constant frequency pulse skipping mode under light load current conditions. The configuration of these two modes is very easy, their quiescent current is higher and the peak-to-peak output ripple is lower.

Load dump / efficiency / solution size

The term “load dump” refers to the inductive shock that occurs after the starter motor is turned off. For a 12V lead-acid battery system for automobiles, this surge voltage is generally clamped at 36V (large value). This surge requires the controller, MOSFET, and associated components to work under the clamp voltage. These higher voltage devices (for example: 40V MOSFET) will lead to a drop in efficiency, and care must be taken to minimize this adverse effect. When the circuit in Figure 1 is used, the efficiency of each voltage rail is higher than 92% (as shown in Figure 3). For clarity, the efficiency of each step-down and step-up section is shown separately. In addition, Figure 4 also shows the layout and size of this circuit, which has a high part of 4.8mm.

DC/DC controller in automobile start/stop electronic system
Figure 3: The relationship curve between LTC3859A efficiency and load current (for different converter parts)
An effective way to save automobile fuel: “Start/Stop” system (Electronic Engineering Special)

Startup and shutdown

The three channels of LTC3859A can be individually shut down using RUN1, RUN2 and RUN3 pins. Pulling any of these pins below 1.2V will shut down the main control loop for the corresponding channel. Pulling all three pins below 0.7V will disable all controllers and most internal circuits, including the built-in LDO. In this state, the LTC3859A only absorbs 8μA of quiescent current.

Soft start or tracking

The TRACK/SS1 and TRACK/SS2 pins of the two step-down controllers can be used to adjust the soft start on time or to “coincide” or “proportional” tracking two or more power supplies during startup. These correlation curves are shown in Figure 5, and a resistor divider is also placed between the TRACK/SS pins of the main power supply and the slave power supply.

DC/DC controller in automobile start/stop electronic system
Figure 5: LTC3859A output voltage tracking: (a) coincidence tracking (b) proportional tracking

Protective function

The LTC3859A can be configured to use DCR (Inductor Resistance) or a sense resistor to detect the output current. As to which of the two current detection schemes to choose, to a large extent depends on the comprehensive trade-offs of cost, power consumption, and accuracy. DCR is becoming more and more popular because it can eliminate expensive current-sensing resistors and is more efficient, especially in high-current applications. The LTC3859A has a current foldback function for the step-down channel to help limit the load current when the output is shorted to ground.

The built-in comparator is responsible for monitoring the step-down output voltage, and indicates an overvoltage condition when the output is greater than 10% of its nominal output voltage. When this condition is detected, the top MOSFET is turned off and the bottom MOSFET is turned on until the overvoltage condition is cleared. As long as the overvoltage condition persists, the bottom MOSFET will continue to be turned on. If the output voltage returns to a safe level, normal operation is automatically restored.

Under higher temperature conditions, or internal power consumption causes excessive self-heating inside the chip, the overheat shutdown circuit will shut down the LTC3859A. When the junction temperature exceeds approximately 170°C, the overheating protection circuit will disable the built-in bias LDO, causing the bias supply to drop to 0V and effectively shut down the entire LTC3859A in an orderly manner. Once the junction temperature drops to about 155°C, the LDO will turn on again.

in conclusion

Fuel-saving automobile start/stop systems will continue to develop in the next few years. Caution must be exercised for the power supply of in-vehicle infotainment and navigation systems, as well as the power supply of disk drives that require voltages up to or even more than 5V to achieve correct operation. This type of system resets when the input voltage drops outside the regulated range due to the restart of the engine. LTC3859A provides a solution that can use its built-in synchronous boost controller to raise the battery voltage to a safe operating level. LTC3859A integrates a synchronous step-up controller with two synchronous step-down controllers, which is very suitable for powering many devices and can maintain the regulation of all output voltages when the engine is restarted.

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