Integrated power ICs for mobile MPUs: Not just for portables

There is a long and growing list of power efficient microprocessors offered by Freescale, Intel, ARM and others 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 products was to enable OEMs to develop smaller, more cost-effective portable handheld devices with long battery life, while at the same time offering enhanced processing performance to run feature-rich multimedia applications. Recently, demand for this same combination of high power efficiency and processing performance has spread to non-portable applications as well. Automotive infotainment systems and other embedded applications demand similar levels of power efficiency and processing horsepower as the latest high-end portable devices. In all cases, however, a highly specialized, high performance power management companion IC is necessary to properly control and monitor the microprocessor’s power system and ensure that all of the efficiency benefits possible with these processors can be realized—regardless of the application.

Achieving higher processing power without increasing system power consumption requires lower voltage operation at ever increasing currents. Both portable and embedded systems include a variety of components optimized for operation at different voltages due to either the needs of the application or the line widths of the processing technology. The end result is that systems employing the latest “portable” processors require a large number of high current, low voltage rails—typically at or below 1.8V. In addition to numerous low voltage rails, many of these applications also require 3V or 3.3V rails for powering large portable hard disk drives, memory, I/O supplies for external logic circuitry, etc. In automotive or embedded applications, all supply voltages that directly interface to the processor can be generated by high efficiency buck DC/DCs or LDOs, depending on the current requirements.

In the case of portable applications, the main power source is typically a large single cell Li-ion/polymer battery which may have a voltage above or below the 3.3V system supply in the product. Applications such as these—handy terminals, bar code scanners, RFID readers, etc.—require a buck-boost supply to generate the 3.3V rail. Additional complexity of these “portable” processor systems, whether they are battery powered or not, include the need to sequence all supplies on and off in a specific order and the ability to be adjusted up and down dynamically depending on the processing needs of the system. For the system designer, a single integrated solution that addresses all of the microprocessor and associated application power supply needs is a huge advantage. Handling these needs across a wide range of applications requires a highly flexible, programmable and efficient multi-output power supply solution.

Design challenges
Buck-boost capability
Most of today’s modern feature-rich electronic systems still require voltage rails in the +3V range, for example to power I/O or a peripheral rail in an automotive infotainment system. Integrating synchronous buck-boost switching capability into the power management IC (PMIC) allows 3.3V regulation across the entire input voltage range, 2.7V to 5.5V, with high efficiency, resulting in increased operating margin. However, achieving high efficiency with a buck-boost design is much more challenging than a simple step-down DC-DC converter—particularly if low noise and good load step transient response are required.

The 12V car battery, the starting point for many automotive supplies, is far from the quiet, stable supply required by these systems. In addition to noise, this 12V “supply” can be subjected to reverse battery conditions or load dumps where the voltage can range or spike anywhere from -36V to 80V. The ideal supply for these systems must protect both itself and the applications circuits from these demanding electrical conditions while providing stable, low noise output voltages. Thermal conditions in automotive environments are equally challenging; PMIC junction temperatures may approach 125°C even for 85°C ambient, requiring supply stability over a -40°C to 125°C junction temperature range with robust overtemperature protection. Due to these harsh conditions, typically this system/battery voltage is pre-regulated to 3.3V or 5V prior to supplying the PMIC. In many instances, these intermediate supplies can be disrupted due to cold crank and severe noise transients. A buck-boost supply is advantageous here as well to ensure that the critical 3.3V rail associated with the processor system does not trigger a power-on reset.

Reducing heat, optimizing system efficiency
Many industry-standard PMICs come with a variety of linear regulators onboard. However, linear regulators, if not managed properly with sufficient copper trace routing, heat sinks, or well-designed input/output voltage and output current levels, can generate localized thermal “hot spots” on the PCB itself. Alternatively, a switching regulator provides a more efficient way to step down voltages when the difference between input and output voltage is high, and/or if the output current is large. PMIC usage is commonplace in today’s feature-rich devices with low-voltage microprocessors onboard. As a result, implementing switchmode based power supplies for the majority of voltage rails is increasingly necessary. LDOs, however, provide low noise outputs and great power supply rejection ratio (PSRR) performance, so tradeoffs must be assessed. In many cases, the correct IC partitioning includes both DC/DC and linear regulators.

Virtually all applications today are sensitive to heat in the system. As the processing performance and associated operating currents increase, it is increasingly important to use switching regulators in place of LDOs. This is particularly true in highly integrated power supplies since single ICs are limited in their ability to dissipate power. Furthermore, achieving optimal power dissipation requires many of the core processing rails to dynamically adjust, depending on the processing operations being performed. Higher supply voltages are necessary to achieve higher clock rate operation. Similarly, very low voltages are adequate for less processing-intensive modes of operation. Since the corresponding supply currents tend to track the input supply voltage, it is desirable to operate the processor at the lowest supply voltage possible. Dynamically adjusting the processor voltage supplies requires a serial port, such as I2C, to communicate the changes. Virtually all of today’s high end portable processors support this functionality—however, taking advantage of it requires an equally flexible and programmable power solution.

Summarizing, the main challenges for the system designer include:
• Integration of buck-boost regulators
• Balancing power dissipation with the high level of integration of multiple switching regulators and LDOs
• Voltage transients and temperature extremes of automotive systems
• Integrating dynamic I2C control
• Solution size and footprint

A simple solution
In the past, PMICs have not had sufficient power to handle these modern systems and microprocessors. Any solution to satisfy the PMIC design constraints outlined above must combine a high level of integration, including high-current switching regulators and LDOs, dynamic I2C control of key parameters with hard-to-do functional blocks such as buck-boost regulators. Further, a device with high switching frequency reduces the size of external components, and ceramic capacitors reduce output ripple. Such an IC must also be suitable for the rigorous automotive environment, although the input voltage is typically pre-regulated from the system or battery voltage.

The LTC3589 is a complete power management solution for ARM-based processors and advanced portable microprocessor systems. The device contains three synchronous step-down DC/DC converters for core, memory and system on chip (SoC) rails, a synchronous buck-boost regulator for I/O at 2.5V to 5V, and three 250mA LDO regulators for low noise analog supplies. An I2C serial port is used to control regulator enables, output voltage levels, dynamic voltage scaling and slew rate, operating modes and status reporting. Regulator start-up is sequenced by connecting regulator outputs to enable pins in the desired order or via the I2C port. System power-on, power-off and reset functions are controlled by a pushbutton interface, pin inputs, or I2C interface. Voltage monitors and active discharge circuits guarantee a clean power-down before the next enable sequence plus selected regulators can be exempt from pushbutton control for supplies, such as memory, when the system must be kept alive during a standby mode. The LTC3589 supports i.MX, PXA and OMAP processors with eight independent rails at appropriate power levels, with dynamic control and sequencing.


Figure 1: LTC3589 simplified block diagram (3589 TA01a).

The LTC3589 is a complete power management solution for portable microprocessors and peripheral devices. It generates a total of eight voltage rails for supplying power to the processor core, SDRAM, system memory, PC cards, always-on real time clock and hard disk drive (HDD) functions. Supplying the voltage rails are an always-on low quiescent current 25mA LDO, one 1.6A and two 1A step-down regulators, a 1.2A buck-boost regulator, and three 250mA low dropout linear regulators. Supporting the multiple regulators is a highly configurable power sequencing capability, dynamic voltage slewing DAC output voltage control, a pushbutton interface controller, regulator control via an I2C interface, with extensive status reporting and interrupt output.

Dynamic rail control
The LTC3589 has the I2C control features required by high-end portable applications processors, Dynamic Voltage Scaling and selectable voltage slew settings. To enable the IC’s slewing DAC reference operation, the three LTC3589 step-down switching regulators and linear regulator LDO2 have programmable DAC reference inputs. Each DAC is programmable from 0.3625V to 0.75V in 12.5mV steps:




R1 and R2 make up the feedback resistor divider for setting the output voltage of the regulators, see Figures 2 and 3 for details. 0.3625 is the minimum value of the 5-bit DAC reference into the error amplifier. 0.0125V is the DAC LSB step-size. BxDTVx is the binary code (0 to 31 decimal) stored in the I2C register.


Figure 2: LTC3589 LDO regulator application circuit.



FIGURE 3

The DAC references can be commanded to independently slew between two voltages at one of four selectable slew rates. Each DAC has two independent output voltage registers, voltage register select, slew rate and start controls. The regulators do not need to be enabled to change the DAC outputs.

The versatile I2C serial port is used to control regulator enables, output voltage levels, operating modes and status reporting. The I2C serial port on the LTC3589 contains 13 command registers for controlling each of the regulators, one read-only register for monitoring each regulator’s power good status, one read-only register for reading the cause of an IRQ event and one clear IRQ command register.

Conclusion
By replacing discrete power IC components or traditional large overly integrated PMICs (i.e. with audio, codecs, touch screen interfaces, etc.), a system designer can use a new generation of compact PMICs that integrates key power management functions for improved performance with smaller and simpler solutions. High performance mobile processors typically have a unique set of power supply requirements, including multiple high current and low noise rails, programmable sequencing and dynamic I2C adjustment. These high-end processors were originally developed for handheld applications but are now being implemented in non-portable and embedded systems such as automotive infotainment. New products like the LTC3589 PMIC from Linear Technology enable system designers to exploit the full power-saving and performance benefits of new processors from Freescale, Marvell, Samsung and others across a broad range of applications.


Author Information
Sam Nork is the Director, Boston Design Center; Jeff Marvin is the Manager, Burlington Design Center; and Steve Knoth is the Senior Product Marketing Engineer, Power Products Group, of Linear Technology Corp.


Caption
Figure 1: LTC3589 simplified block diagram (3589 TA01a).
Figure 2: LTC3589 LDO regulator application circuit.
Figure 3: LTC3589 step down switching regulator application circuits.