Multi-output power management ICs in infotainment systems

Multi-output power management ICs in infotainment systems

Technology News |
By eeNews Europe

As product form factors are decreasing, demand for their functionality and features continue to increase. Furthermore, the industry trend for sophisticated digital ICs such as microprocessors (uP) and microcontrollers (uC) or field programmable gate arrays (FPGAs) that power these products continues to lower their operating voltage while simultaneously increasing their amperage. Microprocessors are among the most popular of these to design in, and there is a growing list of power efficient types from such suppliers as Freescale, Intel, NVIDIA, Samsung, ARM and others. They are designed to provide low power consumption and high performance processing for a wide range of wireless, embedded and networking applications.

The original intent of these processors was to enable OEMs to develop smaller and more cost-effective portable handheld devices with long battery life, while simultaneously offering enhanced computing performance to run feature-rich multimedia applications. Nevertheless, demand for this same combination of high power efficiency and processing performance has spread to non-portable applications. A couple of examples include automotive infotainment systems and other embedded applications, both of which demand similar levels of power efficiency and processing horsepower. In all cases, a highly specialized, high performance power management IC (PMIC) is necessary to properly control and monitor the microprocessor’s power so that all of the performance benefits of these processors can be attained. Further, as the electronic content of automobiles continues to dramatically increase, so too has the use of microprocessors as the workhorse of various control systems within the vehicle. Infotainment systems have captured a wide array of functions to enhance the driving experience. Touch screens, Bluetooth communication, digital and high-definition television (HDTV), satellite radio, CD/DVD/MP3 players, global positioning system (GPS) navigation and video game systems have created a full-fledged entertainment center inside the car!

Automotive PMIC Challenges

Electronic systems designed for automotive applications are challenging for many reasons, including the wide operating temperature range, strict EMC and transient requirements, as well as the high quality levels demanded by automotive OEMs. Starting with the wide operating temperature range, power management ICs are challenged on two fronts. First, power conversion – even when highly efficient – must dissipate some level of power as heat. When several DC-DCs and LDO regulators are packed into a single device, the combined power dissipation can be significant, easily approaching two Watts or more. Typical PMIC packages such as the 6mm x 6mm 40 pin, exposed pad QFN have a thermal resistance of 33°C/W resulting in a junction temperature rise in excess of 60°C. When this is combined with the additional challenge of a wide ambient operating temperature range, the maximum junction temperature of the PMIC can often exceed 125°C. Even in body electronics, not under the hood, the ambient temperature inside a sealed plastic electronic control module can reach 95°C. Due to these temperature challenges many PMICs rated for 85°C, or even 125°C, are not sufficient for sustained high temperature operation.

Another key to operating an integrated power management device in a high ambient temperature environment is for the device to self-monitor its own die temperature and report when its junction temperature is getting too high so that the system controller can make an intelligent decision on whether to reduce power to the load(s). Operating system software can do this by turning off less critical functions or by turning down the performance in processors and other high power functions such as displays and network communications.

The environment within a current vehicle’s dashboard is crowded with electronics. Adding to this crowding are radios from Bluetooth to cell phone based network connectivity. Therefore, it is imperative that any additional entries to this thermally constrained environment not contribute excessive heat or EMI. There are strict Electromagnetic Compatibility (EMC) requirements which cover radiated and conducted emissions, radiated and conducted immunity or susceptibility, and Electrostatic Discharge (ESD). Being able to conform to these requirements affects the performance aspects of a PMIC design. Some are straightforward, such as the DC-DC switching regulators must operate at a fixed frequency well outside of the AM radio band. However, another common radiated emission source found in DC-DC converters comes from the switching edge rates of its internal power MOSFETs. These edge rates should be controlled to reduce radiated emissions.

Many of today’s embedded systems and advanced processors require controlled and choreographed sequencing as power supplies are powered up and applied to various circuits. Allowing for system flexibility and a simple approach to sequencing not only makes the system design easier, but it also enhances system reliability and allows for a single PMIC to handle a broader range of the system than just a specific processor’s requirements.

In summary, the main challenges facing the automotive infotainment system designer include the following:

  • Balancing power dissipation with the high level of integration of multiple switching regulators and linear regulators
  • Accurate output voltage regulation and load step response required by advanced nano-meter technology processors and FPGAs.
  • Monitoring junction temperatures
  • Immunity to radiated and conducted noise, with low emissions contributions
  • Large voltage transients and temperature extremes
  • Managing power sequencing during startup and shutdown
  • Minimizing solution size and footprint.

A Simple Solution

Historically, many PMICs have not possessed the necessary power to handle these modern systems and microprocessors. Any solution to satisfy the automotive power management IC design constraints as already outlined must combine a high level of integration, including high-current switching regulators and LDOs, wide temperature range of operation, power sequencing and dynamic I2C control of key parameters with hard-to-do functional blocks. Furthermore, a device with high switching frequency reduces the size of external components and ceramic capacitors reduce output ripple. This low ripple combines with accurate, fast response regulators to satisfy demanding voltage tolerances of 45nm type processors. Such power ICs must also be capable of meeting the rigorous automotive environment including radiated emission suppression, although the input voltage is typically a pre-regulated 5V or 3.3V rail off the system or battery voltage.

A High Power, Power Management Solution

The LTC3676/-1 are complete power management solutions for Freescale i.MX6 processors, ARM based processors and other advanced portable microprocessor systems. The LTC3676/-1 contain four synchronous step-down DC/DC converters at up to 2.5A each for core, memory, I/O and system on-chip (SoC) rails plus three 300mA linear regulators for low noise analog supplies. The LTC3676-1 configures a 1.5A buck regulator for source/sink and tracking operation to support DDR memory termination and also adds a VTTR reference output for DDR. These two pin features replace the LDO4 enable pin and feedback pins of the LTC3676. LDO4 is still programmable by I2C. Supporting the multiple regulators is a highly configurable power sequencing capability, dynamic output voltage scaling, a pushbutton interface controller, plus regulator control via an I2C interface with extensive status and fault reporting via an interrupt output. The LTC3676 supports i.MX6, PXA and OMAP processors with eight independent rails at appropriate power levels with dynamic control and sequencing. Other features include interface signals such as the VSTB pin that toggles between programmed run and standby output voltages on up to four rails simultaneously. The device is available in a low profile 40-pin 6mm x 6mm x 0.75mm exposed pad QFN package.

Figure 1. LTC3676-1 Simplified Typical Application Diagram

The LTC3676 Power Management Solution for Application Processors can solve the automotive infotainment system design challenges outlined above. The LTC3676HUJ PMIC is available in a high temperature (H-Grade) option with a junction temperature rating from -40°C to +150°C, easily satisfying the high temperature automotive operating requirement. The IC includes a thermal warning flag and interrupt specifically for junction temperature monitoring and also includes a hard thermal shutdown for reliable protection of the hardware, should power dissipation be mismanaged, or in the event of a severe fault condition.

The LTC3676 PWM switching frequency is specifically trimmed to 2.25MHz with a guaranteed range of 1.7MHz to 2.7MHz. Its internal regulators can also be set to a forced continuous PWM operating mode to prevent operation in pulse skip or burst-mode even at light loads. This not only keeps the frequency fixed but also further reduces voltage ripple on the DC-DC output capacitors.

Suppressing Radiated & Conducted Emissions

Since there are 4 switching regulators onboard the LTC3676, each has an associated reactive device (inductor) to be concerned about. One possible solution is to shield the LTC3676 area to prevent EMI from being emitted. Besides being expensive and heavy, this does not solve the problem of contamination by any wires that might be connected to the power supply area. It is better to use source suppression and antenna elimination.

Source suppression necessitates good layout/component selection (and internal IC design) to prevent the generation of radio frequency energy. It is often necessary to use shielded inductors and to place those inductors further away from the LTC3676 than the output capacitors. This is because the AC currents circulate from the LTC3676 through the inductor, through the output capacitor, to ground and back to the LTC3676. It is clear that wide traces, preferably area fill, should be used to connect the ground of the output capacitors to the ground of the LTC3676 and to the ground of the PVIN input decoupling capacitors also.

The LTC3676 also provides some tools for source suppression. Its DC-DC converters specifically include a dv/dt control feature which slows down the switching edge rates to reduce radiated emissions. Since the buck regulators are synchronous, both the rise and the fall time are both controlled. A slow edge rate of about 3ns rise/fall, was selected to both pass emission requirements and still limit switching losses, which helps to optimize power converter efficiency. Each of the 4 buck switching regulators in the LTC3676 default to this 3ns edge rate control mode, but can also be individually set via I2C, to a faster 1ns rate, to improve efficiency, if limited edge rate and emissions control is not required.

In addition to switching time control, the LTC3676 offers some other EMI suppression tools. The frequency of the buck regulators can be changed from 2.25MHz to 1.12MHz. Also, to minimize the input ripple, which can end up radiating through the power input wiring, the buck regulators can be staggered between two different clock phases.

The LTC3676 is also capable of providing considerable power, in excess of 10W. This can result in substantial circulating currents and so it is imperative to provide an uninterrupted path for this current to circulate. In particular, slots in the ground plane, which force the large circulating currents to flow around them, create slot antennas. But other obstacles, such as changing layers, contribute some energy in the EMI signature and should be minimized. Ideally the top and bottom layers should be all, or mostly, ground plane, with the signal layers flowing internally. Since this is usually not practical, some thought needs to go into how the ground plane will be connected prior to commencing layout. For example, it is not a good idea to put the LTC3676 into a corner or tab on the PCB. This will make it very difficult to properly route the ground plane. However, it is a good idea to route the high circulating current areas of the LTC3676 first, to ensure the most optimum layout possible.

If EMI control is planned and executed with the concepts of source suppression and antenna elimination in mind, it is possible to create a full power system that is a good EMI neighbor without increasing product cost or weight.

More Key Features

The LTC3676 fully satisfies the automotive ESD requirements of 2kV HBM and 1000V CDM which is another key requirement for approaching zero defects in the automotive assembly process. Further, the IC has very low standby current consumption, typically 12uA, which is desired in “always on” automotive navigation, security and safety systems which must maintain continuous power to real time clock circuits for temporal awareness, even when the engine is not running.

Finally, the LTC3676 supports simple and effective power sequencing which can be handled through serial communications or via pin strapping where power supply output voltages are tied to enable pins in the desired turn-on sequence. Internally each enable is delayed micro seconds to further time stagger the startup sequence. This feature is supported with precision low voltage enable thresholds so the sequencing is possible even with output voltages as low as 0.43V. Each supply voltage output is also soft-started to limit inrush current and produce clean voltage transitions. See Figure 2.

The LTC3676 also includes easily programmable power-down sequence control. The IC includes two registers for initializing a power down sequence configuration which will be followed at the next turn-off event or overpower fault condition. Each regulator (DC-DC and LDO) can be pre-set to any one of four time slots for shutdown. Each regulator output includes an internal pull-down resistor that is engaged when disabled to guarantee the controlled discharges as shown in Figure 3 and a low starting point for the next turn-on sequence

Figure 2. The LTC3676 Startup Sequence

Figure 3. The LTC3676 Power-Down Sequence


Modern automobiles have advanced significantly from those of yesteryear. Simple AM/FM radios have given way to modern technological advancements such as satellite radio, touch screens, navigation systems, Bluetooth, HDTV, integrated cell phones, media players, and video game systems. Further, by replacing discrete power IC components or traditional large overly-integrated PMICs (i.e. with audio, codecs, etc.), a system designer can use a new generation of compact Power Management Solutions ICs that integrate key power management functions for a new level of performance with smaller and simpler solutions. High performance processors typically have a unique set of power supply requirements, including multiple high current and low noise rails, programmable sequencing and dynamic I2C adjustment. Therefore it is important to select the right PMIC to control and power them.

New products, such as the octal-rail LTC3676/-1 Power Management Solution for Application Processors from Linear Technology, enable system designers to exploit the full power savings and performance benefits of these new processors from Freescale, Marvell, Samsung and others across an ever-growing range of applications. The LTC3676/-1 solves many of the traditional problems associated with automotive infotainment system design, thus enhancing the modern automotive experience.

About the authors:

Steve Knoth is Senior Product Marketing Engineer, Power Products Group, Linear Technology Corporation.

Jeff Marvin is Design Center Manager, Power Products Group, Linear Technology Corporation

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