Reliable in-vehicle power management design solution

The requirements for automotive power management 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 the 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 emergence of automotive subsystems has brought higher load requirements that make these data designs more complex. This article will provide designers with a brief introduction to automotive power requirements and introduce a new DC-DC converter recently released by TI, the TPIC74100.

Transient Protection

Load Dump

Almost all electronic components and circuits directly connected to the car 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 survive a certain level of overvoltage on the power supply line. This is especially true for systems that require the main power input of any particular automotive electronic system to be able to operate under a variety of transient voltage conditions, including alternator load dump.

Because the alternator control loop does not close quickly 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, automotive manufacturers have specified to their suppliers the residual overvoltage that may appear at their power supply input terminals. This varies from car manufacturer to car manufacturer, but the standard peak value for cars is about 40V, while 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, and the figure below (Figure 1) shows a typical pulse for this load dump condition.



Cold Crank in an Instrument Cluster Application

The automotive environment is placing increasing demands on power management chips. One of these requirements 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 this transient can be described by looking at the cold crank pulse. This condition can occur when a vehicle is first started in cold conditions. 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 condition occurs, the power management chip must boost the input voltage in order to maintain a properly regulated output voltage so that these electronic systems can function correctly.

There are several topologies that can be used for boost/buck 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 equals or drops below the output voltage level. It will then provide step-up conversion until the battery voltage drops to the minimum allowable input voltage level. One of the main drawbacks 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. This is particularly undesirable in applications where size and board space must be maintained.

Starting 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" during voltage transients (such as cold crank).

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

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 special chip mode of operation is a function of the input-to-output voltage ratio, which is also the control topology of the chip.

The TPIC74100-Q1 is a buck-boost switch-mode regulator that works under the power supply concept to ensure a stable output voltage with input voltage offset and a 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 applied to many high input voltages. The device also provides a reset function for detection and indication when the 5V output rail is out of the specified tolerance.

The TPIC741 00-Q1 has a frequency modulation scheme that allows system designs to meet EMC requirements by spreading the spectral noise over a frequency band rather than peaking at a specific frequency…


The 5Vg output is a switching 5V regulated output with an internal current limit function to prevent a “reset” assertion when driving a power line capacitive load. This function 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 during this fault condition.



Buck-Boost Conversion

The operating 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 are load dependent, with higher performance at higher load conditions.



Low Power Operation

In some applications, such as power train and instrument cluster, 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 power 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 amount of 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 function is implemented to achieve buck mode operation. In boost mode conditions, the device will automatically enter PWM mode. By turning on the low power mode, the transition between buck and boost as well as the transition between PWM mode and PFM mode will be performed simultaneously.

Conclusion

In many automotive applications, vehicle transient voltages are an issue that will continue to challenge designers. Buck-boost converters play a key 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 the design in the automotive environment and allow designers to save 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 for operation over the -40°C to +125°C temperature range.