Takashi Kimura ROHM
As the use of electronics in automobiles increases steadily in recent years, there is a higher demand to improve fuel efficiency. Wide-ranging development efforts, such as reducing car body weight, improving tire performance, and raising the efficiency of electronic circuits are being made by automobile manufacturers as a main driving force.
On the other hand, in the car infotainment equipment sector new functions like Universal Serial Bus (USB) and Bluetooth has led to an increase in the number of power supply channels and current consumption. Such high power consumption directly impacts the fuel efficiency of electric vehicles (EV) and hybrid vehicles (HV) as well as the fuel efficiency of gasoline engine automobiles because the power generator is used and the engine load increases.
To address these issues, ROHM introduced a next-generation car audio system power supply IC called the BD49101AEFS-M, with a high-efficiency switching regulator to reduce the operating power by 65% over conventional operating power the company’s previous products (see Figure 1).
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The BD49101AEFS-M has a maximum rated voltage of 42V, an operating voltage from 5.5 to 25V, a 2-channel high-withstand voltage switching regulator, a 2-channel high-withstand voltage series regulator a 3-channel low-withstand voltage series regulator and a high-withstand voltage Hi Side switch one channel. The BD49101AEFS-M is a system power supply IC that is equipped with a battery power supply detection circuit, serial interface, and various protection functions, and it integrates all the power supplies required for car audio on a single chip by adopting a HTSSOP-A44 surface-mount package. In addition to reducing the operating power of car audio systems, it also has been possible to reduce the weight of a conventionally required heat sink, which in turn contributes to improving fuel consumption in vehicles.
Configurations of car audio power IC
There are two primary types of power supply configurations depending on the application. One is a configuration that combines discrete ICs, and the other is a system power supply that integrates multiple power supplies in a single chip. The former type of configuration offers users a wider range in the choice of components and design flexibility, but the number of components to be used increases, and the On/Off control of each IC must be performed independently.
On the other hand, the latter type of configuration offers advantages such as a significant reduction in the number of components to be used as the required power supplies as well as functions including the battery voltage detection function are assembled on a single IC. This reduces the design workload of users and provides simultaneous On/Off control of all power supplies.
Power supply circuit methods
This section describes power circuit methods. Power circuit methods can be roughly classified into two types. One is the linear type called series regulators, droppers, or low dropout regulators (LDO). The other is the switching type called switching regulators or DC/DC converters.
Series regulators have advantages such as requiring less external components, low cost, and being comparatively simple to use. The downsides of series regulators are poor power efficiency and high heat generation. Car audio power supply ICs use the 12V car battery for their power supply and from this battery the ICs supply power to internal devices.
With the advances made in miniaturization of devices in recent years, there is an increase in the use of car batteries with an operating voltage of 5V or less and an increase in current consumption due to implementation of higher functionality. For example, when the input voltage, output voltage, and current consumption of a series regulator are 12V, 3.3V, and 0.3A, respectively, its power consumption will be: (12 – 3.3) × 0.3 = 2.61W. The power efficiency of this example is 27.5 percent and more than 70 percent is consumed as heat.
On the other hand, there are downsides to switching regulators such as an increase in the number of external components, larger circuit scale, higher cost, and more complicated design. Under the same conditions as mentioned above, however, power efficiency of switching regulators can be improved by more than 85 percent and heat generation can be suppressed to a considerable extent.
Conventional vs. new
As conventional car audio system power supply ICs are configured with series regulators, they have high heat generation and most of them require heat sinks. For that reason, insertion type large-size packages such as Single Inline Package (SIP) or Zigzag Inline Package (ZIP) are used.
The number of power supply channels that can be assembled in conventional system power supply ICs is also limited, and therefore it is often necessary to add an additional power supply IC in order to cope with the multi-functionality of car audio systems. When the series regulator is replaced with a high-efficiency switching regulator, heat generation can be significantly reduced, but this results in disadvantages such as an increase in the number of external components and higher cost of the IC.
To resolve the above disadvantages, BD49101AEFS-M has adopted a hybrid configuration in which the battery voltage is temporarily stepped down by the switching regulator and some of the series regulators are placed under the above switching regulator (see Figure 2).
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The BD49101AEFS-M is optimized with a combination of two switching regulators and five series regulators and as a result it achieves both a significant improvement of power efficiency and reduction of cost. The improved power efficiency drastically suppresses heat generation. BD49101AEFS-M enables the use of a surface-mount package instead of a conventional insertion type package and allows an approximately 1 to 14th size reduction compared to previous products (see Figure 3).
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Low power consumption
The power efficiency of switching regulators is high, but the efficiency drops when the load current is small. Some of the circuits such as microcontrollers (MCU) of car audio systems operate in standby state and therefore require power to be supplied even when the car audio power is turned Off while the car is parked, for example. In such a state, the current consumption of MCUs drops to less than 1mA. This drop in current consumption occurs when the operating current of the circuit itself is higher than the load current of the switching regulator, and power efficiency drops to less than 7 percent, which is even lower than the efficiency of series regulators (see Figure 4).
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There is an efficiency-enhancing technique for the light load state where the operation is performed using pulse-width modulation (PWM) control during a high load state and switched to pulse-frequency modulation (PFM) control during a light load state. This technique decreases the switching frequency during a light load state and reduces the switching loss.
When this technique is used, however, noise suppression becomes difficult because the switching frequency changes and it is also difficult to provide higher power efficiency than series regulators when the load current is very small, such as less than 1mA. For that reason, BD49101AEFS-M has adopted the method of switching between switching regulators and standby series regulators for the MCU power supply, which is required to operate at all times. In this way, BD49101AEFS-M achieves both high power efficiency and low current consumption during standby state.
There is, however, the issue of voltage fluctuation when the regulators are switched in this method. When BD49101AEFS-M enters normal operation from standby state, the MCUs start operating and the load current increases significantly. At this time, if the power supply is simply switched from the series regulators to switching regulators, the output voltage may drop sharply. If the voltage drop is large, the MCUs are reset in some cases. Although the condition can be improved by mounting a large-capacitance capacitor, it will lead to other issues such as additional component costs and a bigger footprint.
The operation principle of switching regulators.
Switching regulators are configured with an error amplifier for comparing the output voltage with the reference voltage, a PWM comparator for comparing the above output with ramp waves, an output transistor that is operated using pulses generated by the comparator, and an LC filter for smoothing the above current output. Switching regulators maintain the voltage by performing feedback operations that keep the pulse width constant (see Figure 5).
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When the regulators are switched, the voltage drops because the abovementioned series of feedback operations cannot be performed immediately after the switching occurs. To improve this condition, BD49101AEFS-M applies the following three controls: (1) Operates both regulators for a specific period of time while the regulators are switched: (2) The error amplifier output is pre-charged to a specific voltage so that the PWM comparator can immediately output the specified pulse width during the period of item (1); (3) The series regulators are turned Off and operation is performed using switching regulators only.
Because of the above controls, the feedback loop enters into a stable operation within a short period of time, and this makes it possible to suppress voltage drop. Thus, it has been possible to suppress the output voltage drop of BD49101AEFS-M to less than 40mV compared with a more than 300mV drop when the power supply is simply switched from series regulators to switching regulators (see Figure 6).
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As a result of adopting this method, the BD49101AEFS-M IC achieves a current consumption of 100A (typ.) during standby state and enables reduction of the dark current produced by the car audio while the automobile engine is turned Off.
BD49101AEFS-M is equipped with a cable impedance voltage compensation function for the USB power supply in order to cope with the incorporation of higher functionalities for car audio systems (Fig. 5). This function has been provided in order to meet the requirements of USB standards by increasing the output voltage for the amount of voltage dropped by the power supply IC side and maintaining a stable voltage at the end of the cable. This compensation amount can be adjusted in accordance with the cable to be used by setting the amount using an external resistor. This IC is provided with the dedicated terminal for stopping the output and sending an overcurrent notification to MCUs when overcurrent is detected.
Furthermore, the battery power supply detection circuit of this IC can be set in 0.1V steps and provides a detection voltage range between 5.7V and 6.4V in addition to one between 7.7V and 8.4V so that this IC can detect the battery power supply for low-voltage operation of the unit.