Design Article
Tackling power conversion in auto electronics
By Jeff Greutter, Linear Technology Corp.
10/24/2012 10:45 AM EDT
Disconnect problems
Load Dump
“Load-Dump” is a condition where the battery cables are disconnected while the alternator is still charging the battery. This can occur when a battery cable is loose while the car is operating, or when a battery cable breaks while the car is running. Such an abrupt disconnection of the battery cable can produce transient voltage spikes up to 60V as the alternator is attempting a full-charge of an “absent” battery. Transorbs on the alternator usually clamp the bus voltage somewhere between 30V and 34V and absorb the majority of the surge; however DC/DC converters downstream of the alternator are subjected to transient voltage spikes as high as 36V as shown in Figure 1. These converters are not only expected to survive, but must also continually regulate an output voltage through this transient event.
“Always-On” Systems Need Ultralow Supply Current
Many electronic subsystems are required to operate in “standby” or “keep alive” mode drawing minimal quiescent current at a regulated voltage while in this state. These circuits can be found in most navigation, safety, security, and engine management electronic power systems. Each of these subsystems can use several microprocessors and microcontrollers. Most luxury cars have over 100 of these DSPs onboard and approximately 20% of these require always on operation. In these systems, the power conversion ICs must operate in two different modes. First, when the car is running, the power supplies that power these DSPs will generally operate at full current fed by the battery and charging system. However, when the car ignition is turned off, the microprocessors in these systems must remain “alive,” requiring their power ICs to provide a constant voltage while drawing minimal current from the battery. Since there can be upwards of 20 of these always-on processors operating at once, there is an significant power demand on the battery even when the ignition is turned off. Collectively, hundreds of milliamps (mA) of supply current can be required to power these always-on processors, which could completely drain a battery in a matter of days.
In order to better manage these requirements, several automotive manufacturers created a low quiescent current target of <10uA for each always-on DC/DC converter. Until recently, systems manufacturers were required to connect a low quiescent current LDO in parallel with a step-down converter, and switch between the two to reduce the current draw from the battery when the car’s engine is not running. This created expensive, bulky and relatively inefficient solutions.
A New Alternative
The voltage of automotive battery bus can vary from under 4V to over 40V as it is exposed to different transient scenarios. With the aggressive adoption of stop-start operation, the power bus will be subjected to low voltage transient conditions many times during an average trip. The need for well regulated voltage rails through these conditions will be of paramount importance for automotive electronics. As the growth of electronic content in automobiles continues to accelerate with ECUs used for security, safety, navigation, chassis control and engine/transmission management so will the need for high voltage power management ICs which can offer high efficiency, low quiescent current, high frequency switching with very robust protection features and reliability.
Linear Technology’s LT8610 is the first of a family of high voltage synchronous buck regulators. Its 3.4V to 42V input voltage range makes it ideal for automotive applications that are subjected to both low voltage transients such as cold-crank or stop-start scenarios as well as high voltage transients encountered during a load dump scenario. Its 2.5A continuous output current capability and ability to deliver outputs from VIN-200mV to 0.97V makes it well suited for many automotive rails that run directly from the battery bus. Its very compact and simple solution footprint eliminates the need for any external diodes and can be seen in Figure 2.
Next: Tackling transients
Load Dump
“Load-Dump” is a condition where the battery cables are disconnected while the alternator is still charging the battery. This can occur when a battery cable is loose while the car is operating, or when a battery cable breaks while the car is running. Such an abrupt disconnection of the battery cable can produce transient voltage spikes up to 60V as the alternator is attempting a full-charge of an “absent” battery. Transorbs on the alternator usually clamp the bus voltage somewhere between 30V and 34V and absorb the majority of the surge; however DC/DC converters downstream of the alternator are subjected to transient voltage spikes as high as 36V as shown in Figure 1. These converters are not only expected to survive, but must also continually regulate an output voltage through this transient event.
“Always-On” Systems Need Ultralow Supply Current
Many electronic subsystems are required to operate in “standby” or “keep alive” mode drawing minimal quiescent current at a regulated voltage while in this state. These circuits can be found in most navigation, safety, security, and engine management electronic power systems. Each of these subsystems can use several microprocessors and microcontrollers. Most luxury cars have over 100 of these DSPs onboard and approximately 20% of these require always on operation. In these systems, the power conversion ICs must operate in two different modes. First, when the car is running, the power supplies that power these DSPs will generally operate at full current fed by the battery and charging system. However, when the car ignition is turned off, the microprocessors in these systems must remain “alive,” requiring their power ICs to provide a constant voltage while drawing minimal current from the battery. Since there can be upwards of 20 of these always-on processors operating at once, there is an significant power demand on the battery even when the ignition is turned off. Collectively, hundreds of milliamps (mA) of supply current can be required to power these always-on processors, which could completely drain a battery in a matter of days.
In order to better manage these requirements, several automotive manufacturers created a low quiescent current target of <10uA for each always-on DC/DC converter. Until recently, systems manufacturers were required to connect a low quiescent current LDO in parallel with a step-down converter, and switch between the two to reduce the current draw from the battery when the car’s engine is not running. This created expensive, bulky and relatively inefficient solutions.
A New Alternative
The voltage of automotive battery bus can vary from under 4V to over 40V as it is exposed to different transient scenarios. With the aggressive adoption of stop-start operation, the power bus will be subjected to low voltage transient conditions many times during an average trip. The need for well regulated voltage rails through these conditions will be of paramount importance for automotive electronics. As the growth of electronic content in automobiles continues to accelerate with ECUs used for security, safety, navigation, chassis control and engine/transmission management so will the need for high voltage power management ICs which can offer high efficiency, low quiescent current, high frequency switching with very robust protection features and reliability.
Linear Technology’s LT8610 is the first of a family of high voltage synchronous buck regulators. Its 3.4V to 42V input voltage range makes it ideal for automotive applications that are subjected to both low voltage transients such as cold-crank or stop-start scenarios as well as high voltage transients encountered during a load dump scenario. Its 2.5A continuous output current capability and ability to deliver outputs from VIN-200mV to 0.97V makes it well suited for many automotive rails that run directly from the battery bus. Its very compact and simple solution footprint eliminates the need for any external diodes and can be seen in Figure 2.
Figure 2. LT8610 Typical Automotive Schematic for a 5V, 2.5A Output
Next: Tackling transients
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cupster
10/25/2012 3:41 PM EDT
You mentioned that no diodes are needed. I assume you were referring to flyback diodes since you have a synchronous output. However, automotive also has a reverse battery requirement which means adding a diode in the battery feed, unless the LT8610 has reverse voltage protection. This adds a diode drop at all voltages and means that the low end battery voltage is now 3.7Volts (assuming we can use a schottky diode). USB charging and data application require 5Volts. We really need a buck-boost supply.
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JefW
11/3/2012 6:57 AM EDT
You don't need an input diode for reverse battery protection. You can use a fet. This is a no-brainer if your low-battery-voltage operating-current is above half an amp or so.
Sepic seems easier to keep rf-clean. You have to use a boost controller rather than buck but with modern ceramic caps it's fairly plain sailing.
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