Reliable in-vehicle power management design solves load dump and cold start problems

In-vehicle power management requirements are becoming more demanding, requiring power supplies to operate over a wider input voltage range, higher currents , and higher temperature extremes. These requirements will make switch-mode power supply designs mainstream because of their greater flexibility, better configurability, and higher thermal efficiency.

The core component of a switch-mode power supply is the DC-DC converter. Today's automotive converters must be able to support a variety of operating conditions, such as low-voltage operation (i.e., cold crank) and positive transient survivability (i.e., suppressed or unsuppressed load dump conditions). The higher load demands brought about by the emergence of automotive subsystems have made the design of these data more complicated. This article will give designers a brief introduction to automotive power requirements and introduce a new DC-DC converter recently launched by TI, the TPIC74100.

Transient protection

Load Dump

Virtually all electronic components and circuits that are connected directly to the vehicle battery require protection from suppression, transient voltages (up to 60V), and reverse voltage conditions. It is also a common requirement for these electronic circuits to be able to withstand a certain level of overvoltage on the power line. This is especially true for automotive systems where the main power input to any particular automotive electronic system is required to operate under a variety of transient voltage conditions, including alternator load dump.

Because the alternator control loop does not close fast enough, it generates a high output voltage pulse when the battery voltage is removed. Normally, this high energy pulse is controlled (or suppressed) to a lower voltage range at a central location in the vehicle. However, car manufacturers still specify to their suppliers the residual overvoltage that may appear at their power input terminals. This varies from car manufacturer to car manufacturer, but the standard peak value for cars is about 40V and the standard peak value for commercial vehicles is about 60V. The duration of a typical load dump pulse is a few tenths of a second. The figure below (Figure 1) shows a typical pulse in this load dump condition.

Cold Start in Dashboard App

The automotive environment is placing increasing demands on power management ICs . One of these demands is the need for power management ICs to operate over a wide voltage excursion range, which is common in electronic systems directly connected to the battery. An example of such a transient can be described by looking at this cold crank pulse. This condition can occur when a vehicle is first started in a cold environment. If the temperature is low enough (cooled to zero degrees Celsius), the engine oil becomes viscous, which places a heavy load demand on the motor by requiring more power (torque). This requires a battery that can provide higher current. The heavy load demand can immediately pull the battery voltage down to 3V during this ignition cycle.

The challenge is that some applications must remain operational during this process. These applications are not limited to powertrain ECUs or safety-critical applications, but can also be found in some cluster and infotainment subsystems. When this state occurs, the power management chip must boost the input voltage in order to maintain the correct regulated output voltage so that these electronic systems can function correctly.

There are several topologies available for step-up/ step-down conversion: SEPIC (Single-Ended Primary Inductor Converter), or a pure buck/boost converter.

SEPIC Converter

The SEPIC converter provides a step-down conversion until the input voltage is equal to or drops below the output voltage level. It will then provide a step-up conversion until the battery voltage drops to the minimum allowable input voltage level. A major drawback of using a SEPIC is that it requires a single coupled inductor (transformer) or two separate inductors and a coupling capacitor , as shown in Figure 3.

These inductors and coils are physically larger and require more PCB space, which is particularly undesirable in applications where size and board space must be maintained.

Startup Buck - Boost Converter

The demand for buck-boost converters in automotive applications has grown dramatically over the past few years. This is particularly beneficial for applications that need to survive voltage transients, such as cold crank.

The buck-boost converter is a typical DC-DC converter with an output voltage amplitude that is greater or less than the input voltage amplitude. It is a switch-mode power supply with a similar circuit topology to the boost and buck converters. The output voltage can be regulated depending on the duty cycle of the switching transistor.

This topology consists of a buck power stage and its two power switches, which are connected to a boost power stage and its two power switches through a power inductor. These switches can be controlled in three different operating modes: buck-boost mode, buck mode, and boost mode. The specific mode of operation is a function of the input-to-output voltage ratio and the control topology of the chip.

The TPIC74100-Q1 is a buck-boost switch-mode regulator that operates under the power concept to ensure a stable output voltage with input voltage offset and specified load range.

The TPIC74100-Q1 has a complete voltage mode control switch and is also designed in a synchronous configuration to obtain overall enhanced efficiency. With the help of some external components (LC combination), the device can regulate the output to 5V±3% to achieve a wide input voltage range, allowing it to be used in many high input voltage applications. The device also provides a reset function for detection and indication when the 5V output rail is out of the specified tolerance.

The TPIC74100-Q1 has a frequency modulation scheme that allows system designs to meet EMC requirements by spreading the spectral noise across a frequency band rather than peaking at a specific frequency…

The 5Vg output is a switched 5V regulated output with internal current limiting to prevent a "reset" assertion when driving a power line capacitive load. This functionality is controlled by the 5Vg_ENABLE terminal. If there is a short to ground on this output (5Vg output), the output will protect itself by operating in chopping mode. However, this will increase the output ripple voltage of VOUT in this fault condition.

Buck-Boost Conversion

This operation mode automatically switches between buck and boost modes depending on the input voltage (Vdriver) and output load conditions.

In normal operating mode, the system will be configured as a buck converter. However, during low input voltage pulses, the device automatically switches to boost mode operation to maintain 5V voltage regulation. When the device is operating in boost mode and in the 5.8V to 5V crossover window, the output regulation may contain a higher ripple than normal and only maintain a 3% tolerance. This ripple and tolerance depends on the load conditions, and the higher the load conditions, the higher the performance.

Low power consumption Operation

In some applications, such as powertrain and instrument clusters, low power mode operation is required to minimize power consumption when the vehicle ignition is "off". The TPIC74100-Q1 has an input LPM and will operate in PFM (Pulse Frequency Modulation) when it is turned on during light loads (typically less than 30mA). In most systems, many memory devices still require some power to retain data when the ignition is "off", typically less than 100uA. To support this mode of operation, the total mode consumption should be less than 300uA. The TPIC74100-Q1 has a low power mode with a quiescent current of 150uA (typical). Regulation is achieved by changing the switching frequency.

In PFM mode, the reduced load current for the output load is non-existent. In this mode, the converter efficiency is lower and the output voltage ripple will be slightly larger than in PWM mode due to the higher load current. The low power mode feature is implemented to achieve buck mode operation. In boost mode conditions, the device will automatically enter PWM mode. By turning on low power mode, the transition between buck and boost as well as the transition between PWM mode and PFM mode will be performed simultaneously.

in conclusion

In-vehicle transient voltages are an issue that will continue to challenge designers in many automotive applications. Buck-boost converters play a critical role in many automotive power management system applications that need to continue to operate under these conditions, or when the battery voltage unexpectedly drops below the required output voltage level. The TPIC74100-Q1 automotive buck/boost converter will simplify designs in automotive environments and allow designers to save on external component count and PCB space (it features integrated power switches and synchronous operation). The TPIC74100-Q1 is available in a 20-pin PWP package with a thermal pad and is specified over a -40°C to +125°C temperature range.