High-performance, highly integrated power ICs for new uPs: Entering the industrial and medical markets

background

Freescale, Marvell, ARM and other semiconductor manufacturers design power-efficient microprocessors (uP/CPU) in an ever-expanding family to provide low power and high-performance processing for a variety of wireless, embedded and networking applications. These microprocessor products were originally designed to help consumer electronics OEMs develop portable handheld devices with longer battery life, smaller size and more affordable, while providing enhanced processing performance to run feature-rich multimedia consumer applications.

Achieving greater processing power without increasing system power consumption requires operating at lower voltages at increasingly higher currents. Portable and embedded systems contain a variety of components optimized to operate at different voltages, either due to the application or due to the line width of the process technology. The end result is that systems using the latest "portable" processors require a large number of high-current, low-voltage rails, typically 1.8V or lower. In addition to the countless low-voltage rails, many of these applications also require 3V or 3.3V rails to power large portable hard drives, memory, I/O supplies for external logic circuits, etc. In embedded applications, all supply voltages directly connected to the processor can be generated by high-efficiency step-down DC/DCs or LDOs, depending on the current requirements.

Recently, this need for both high power efficiency and high processing performance has also extended to industrial and medical portable applications. As the newest and most feature-rich high-end consumer portable devices, handheld data collection devices, rugged inventory control and tracking devices, portable gas detectors, blood analyzers, portable EKG devices, and other portable medical devices all require similar or even higher power efficiency and processing capabilities. In addition, these devices must be rugged, reliable, and light enough to be considered "portable." In all cases, however, no matter what the application, 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 the efficiency advantages possible with these processors are achieved.

For portable applications, the main power source is typically a large single-cell Li-Ion/Polymer battery, which may provide a voltage higher or lower than the 3.3V system supply in the product. Applications such as handheld terminals, barcode scanners, RFID readers, etc. require a buck-boost supply to generate the 3.3V rail. Whether or not these "portable" processor systems are battery powered, they come with additional complexities including the need to sequence all power supplies on and off in a specific order, and the ability to dynamically adjust supply voltages up and down depending on the system's processing needs. For system designers, a single integrated solution that meets the power needs of all microprocessors and related applications is extremely beneficial. Meeting these needs in a variety of applications requires a highly flexible, programmable, and efficient multi-output power solution.

Buck-Boost Function Design Challenges

Most of today's new feature-rich electronic systems still require voltage rails in the +3V range, for example, to power I/O or external device rails in automotive infotainment systems. Integrating the synchronous buck-boost switching function in a power management IC (PMIC) allows 3.3V regulation with high efficiency across the entire input voltage range of 2.7V to 5.5V, resulting in higher operating margin. However, achieving high efficiency with a buck-boost design is much more challenging than a simple step-down DC-DC converter, especially if low noise and good load step transient response are required.

Reduce heat and optimize system efficiency

Many industry-standard PMICs come with a variety of built-in linear regulators. However, linear regulators can create localized thermal “hot spots” on the PC board if they are not properly managed with adequate copper trace routing, heat sinks, or well-designed input/output voltage and output current values. Alternatively, switching regulators can provide a more efficient way to step down the voltage when the difference between the input and output voltages is large, and/or if the output current is high. The use of switching regulators is common in today's feature-rich devices with built-in low-voltage uPs. Therefore, it is increasingly important to deploy switch-mode based power supplies for most voltage rails. However, LDOs offer low noise output and high PSRR performance, so the two trade-offs must be evaluated. In many cases, the proper IC partitioning includes both types of regulators.

Today, almost all applications are sensitive to heat in the system. As processing performance and associated operating currents rise, replacing LDOs with switching regulators becomes increasingly important. This is especially true in highly integrated power supplies, where the ability of a single IC to dissipate heat is limited. In addition, achieving optimal power consumption requires dynamic adjustment of many core processing rails, depending on the processing operations performed. To operate at higher clock rates, higher supply voltages are necessary. Similarly, for operating modes that are less processing intensive, very low voltages are sufficient. Since the corresponding supply current tends to track the input supply voltage, it is desirable to have the processor operate at the lowest supply voltage. Dynamically adjusting the processor voltage supply requires a serial port such as I2C to communicate the changes. Today's high-end portable processors almost all support this capability, but taking advantage of it requires an equally flexible and programmable power solution.

Power Management Issues in Portable Medical and Industrial Instruments

As is the case with many other applications, low-power precision components have enabled a rapid growth in portable medical instruments. However, unlike many other applications, portable medical products, in addition to being lightweight for portability, generally have much higher standards for reliability, operating time, and ruggedness. Much of this burden falls on the power system and its components. Medical products must operate correctly and, depending on the design and input power requirements, must often switch seamlessly between various power sources. Every effort must be made to protect the device from and withstand faults, maximize operating time when powered by batteries, and ensure reliable operation whenever a valid power source exists. In addition, power values ​​increase with the number of functions and related voltage rails. Industrial portable equipment has many of the same requirements as medical equipment, except that remote applications require less maintenance, can withstand extreme temperature changes, and will not be damaged when subjected to mechanical vibration or shock.

In summary, the key challenges facing system designers include:
• Integrating buck-boost regulators
• Balancing power consumption with high integration of multiple switching regulators and LDOs
• Integrating dynamic I2C control
• Reliability and ruggedness requirements for industrial and medical equipment systems
• Solution size and footprint

A simple solution

Industrial PMICs of the past did not have enough power to handle these modern systems and microprocessors. Any solution that meets the above power management IC design constraints must combine: high integration, including integration of high current switching regulators and LDOs; dynamic I2C control of key parameters with hard-to-use functional building blocks such as buck-boost regulators. In addition, a device with high switching frequency can reduce external component size, and ceramic capacitors can reduce output ripple.

LTC3589 – High Power PMIC for Modern Processors

The LTC®3589 is a complete power management solution for ARM-based processors and advanced portable microprocessor systems. The device contains: 3 synchronous step-down DC/DC converters for core, memory and SoC rails; a synchronous buck-boost regulator for 2.5V to 5V I/O; and 3 250mA LDO regulators for low noise analog supplies. An I2C serial port is used to control regulator startup, output voltage levels, dynamic voltage regulation and slew rate, operating modes and status reporting. Regulator startup can be sequenced by connecting the regulator outputs to the enable pins or through the I2C port in the desired order. The power-up, power-down and reset functions of the system can be controlled through a push button interface, pin input or I2C interface. Voltage monitors and active discharge circuits ensure a clean power-down before the next enable sequence, and selected regulators can eliminate push button control for power supplies (such as memory, when it must remain operational in shutdown mode). The LTC3589 supports i.MX, PXA and OMAP processors with 8 independent rails, appropriate power levels, dynamic control and sequencing. Other features include interface signals such as the VSTB pin, which switches between set operating and standby output voltages on up to 4 rails simultaneously. The device is available in a low-profile 40-lead 6mm x 6mm exposed pad QFN package.

Figure 1: Simplified block diagram of the LTC3589

High integration - supports multiple high power rails

The LTC3589 is a complete power management solution for portable microprocessors and external devices. It provides a total of eight voltage rails to power the processor core, SDRAM, system memory, PC Card, always-on real-time clock, and HDD functional components. Providing these rails is an always-on low quiescent current 25mA LDO, a 1.6A and two 1A buck regulators, a 1.2A buck-boost regulator, and three 250mA low dropout linear regulators. Supporting multiple regulators is highly configurable power sequencing, dynamic voltage conversion DAC output voltage control, a push button interface controller, regulator control through an I2C interface, and a large number of status reporting and interrupt outputs.

The LTC3589's internally compensated, constant frequency current mode step-down switching regulators provide currents up to 1A, 1A and 1.6A. The step-down regulator switching frequency (including phase) of 2.25MHz or 1.125MHz is independently selectable for each step-down regulator using an I2C command register. The power-on default frequency is 2.25MHz and includes edge rate adjustment to reduce EMI. Each step-down converter has a dynamically switching DAC input reference and external feedback pin to set the output voltage range. The three operating modes of these step-down regulators - pulse skipping mode, burst mode operation or forced continuous mode - are set using the I2C interface. In pulse skipping mode, the regulator supports 100% duty cycle. Burst Mode operation is beneficial for achieving maximum efficiency at light output loads. In addition to the best dynamic transition control between voltage output set points, forced continuous mode also minimizes output voltage ripple at light loads.

The single inductor, 4-switch buck-boost DC/DC voltage mode converter generates a user-programmable output voltage rail from 2.5V to 5V. The buck-boost converter utilizes a proprietary switching algorithm to maintain high efficiency and low noise operation with input voltages above, below or equal to the desired output rail. The buck-boost error amplifier uses a fixed 0.8V reference and the output voltage is set by an external resistor divider. Burst Mode operation is enabled via an I2C control register. No external compensation components are required for the buck-boost converter.

Dynamic voltage rail control and other I2C controlled functions

The LTC3589 has the I2C control function, dynamic voltage scaling and selectable voltage conversion settings required by high-end portable application processors. To enable the conversion DAC reference of the IC to work, the three LTC3589 step-down switching regulators and the linear regulator LDO2 have programmable DAC reference inputs. Each DAC is programmable in 12.5mV steps from 0.3625V to 0.75V:

R1 and R2 form the feedback resistor divider to set the output voltage of the regulator, see Figure 2 and Figure 3 for details. 0.3625 is the minimum value of the 5-bit DAC reference going 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: LTC3589 step-down switching regulator application circuit

The DAC reference can also be commanded to switch between two voltages independently at one of four selectable conversion rates. Each DAC has two independent output voltage registers as well as voltage register selection, conversion rate and enable control. It is not necessary to enable these regulators to change the DAC output.

Figure 4 shows buck regulators 1, 2, 3, and LDO2 switching between 0.8V and 1.2V at four possible slew rates, initiated by the VSTB pin (gray). These values ​​are eight individual DAC codes.

Figure 4: LTC3589 dynamic voltage regulation conversion

The general purpose I2C serial port is used to control regulator startup, output voltage levels, operating modes and status reporting. The I2C serial port on the LTC3589 contains 13 command registers for controlling each regulator, a read-only register for monitoring the power good status of each regulator, a read-only register for reading the cause of an IRQ event and a clear IRQ command register. The LTC3589 I2C port supports random addressing of any register and can write registers in any order using a variety of START sequences. All registers can be read back to verify software and hardware integrity.

in conclusion

By replacing discrete power IC components or traditional, over-integrated large PMICs (i.e., with audio, codec, touch screen interface, etc.), system designers can achieve new levels of performance in smaller and simpler solutions with a new generation of compact PMICs that integrate key power management functions. Today's high-performance processors typically have a unique set of power requirements, including multiple high current and low noise rails, programmable sequencing, and dynamic I2C regulation. These high-end processors were originally developed for handheld applications, but are now being used in embedded systems in the industrial and medical markets. New processors from Freescale, Marvell, Samsung and other semiconductor manufacturers offer power savings and high performance, and new products such as the LTC3589 PMIC from Linear Technology enable system designers to take full advantage of the full benefits of these new processors in an ever-expanding range of applications.