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Powering mobile devices with ICs is becoming easier

Whilst low power precision components have enabled rapid growth of the mobile device market, those portable products targeted at industrial, medical and military applications typically have much higher standards for reliability, run time and robustness. Much of this burden falls on the power system and its components.

A common feature of such products is that they must operate properly and switch seamlessly between a variety of power sources. As a result, care must be taken to protect against and tolerate faults, maximise operating time when powered from batteries and ensure that operation is reliable whenever a valid power source is present.

Clearly, the power management ICs (PMICs) required to address these needs must allow an application to receive power from multiple power sources, which could include: mains power; a USB port; or a Li-ion battery. Amongst PMIC requirements are that system power remains uninterrupted and hot plugging between external power and battery power is tolerated.
In some instances, the PMIC may also feature a battery charger. If so, this needs to ensure the battery is not charged using power required by the application.

Industry trends
While product form factors are decreasing, demand for functionality and features are increasing continuously. Furthermore, the industry trend is for digital ICs, such as microprocessors, microcontrollers and FPGAs, to have lower operating voltages, but higher currents.



Microprocessors are among the most popular of these, originally intended to enable the development of smaller, more cost effective portable handheld devices with long battery life and feature rich multimedia applications. Demand for high power efficiency and processing performance has spread to non portable applications; examples include automotive infotainment systems and embedded applications, both of which require similar levels of power efficiency and processing horsepower.

In all cases, a high performance PMIC is needed to control and monitor the microprocessor's power consumption so its performance benefits can be attained.

Many industrial and medical mobile devices require controlled and choreographed sequencing as supplies are powered up and applied to various circuits. A simple approach to sequencing not only makes system design easier, but it also enhances system reliability and allows for one PMIC to handle a broader range of the system than just a specific processor's requirements. But not all PMICs can handle modern systems and microprocessors.

Any solution that satisfies industrial or medical PMIC design constraints must combine high current switching regulators and LDOs, a wide operating temperature range, 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, whilst ceramic capacitors reduce output ripple. This low ripple combines with accurate, fast response regulators to satisfy demanding voltage tolerances of 45nm type processors. Such PMICs must also meet rigorous environmental constraints, such as radiated emission suppression, even if the input voltage is directly from the battery.

Design challenges
Designers of smart phones and tablets face unprecedented challenges, including the demand for high performance power management systems to accommodate growing system complexity and higher power budgets. These systems strive for an optimum balance among competing objectives, including long battery runtime, compatibility with multiple power sources, high power density, small size and effective thermal management.

One common goal is to reduce the amount of power these devices consume. Power consumption in any system can be addressed in two ways: firstly, by maximising the conversion efficiency across the entire range of load current; and, secondly, by reducing the quiescent current drawn from the DC/DC converters in all modes of operation. In order to play an active role in the reduction of system power consumption, power conversion and management ICs must be more efficient, with lower levels of power consumption under all operating conditions.

To address these specific requirements, Linear Technology has integrated its Burst Mode technology into many of its power management and conversion ICs. This technique minimises the current needed by the IC during standby mode; in many cases, this standby quiescent current is less than 20µA.

Until recently, designers of Li-ion battery powered products used two basic approaches: architect the system using discrete components, each optimised for a single function; or choose from a variety of highly integrated PMICs. These typically support a superset of the functionality needed for most applications, including unwieldy combinations of switching DC/DC controllers, monolithic switchers and numerous LDOs integrated with unrelated mixed signal functions like touch screen controllers, audio CODECs and more.

As a result, they can be cumbersome to use and most require a substantial investment in firmware just to turn them on. These products tend to favour integration over performance and often complicate thermal management by creating a 'hot spot'. Ironically, these solutions may also require relatively more board space due to their large, high pin count packages. Finally, they create board layout challenges in order to accommodate the related external components – MOSFETs, inductors, diodes and assorted passive components – and associated routing.

A new approach fits between these two approaches. The LTC3676 and 3676-1 offer a complete power management solution for advanced microprocessor based portable equipment.
They contain four synchronous step down DC/DC converters, each rated at up to 2.5A, plus three 300mA linear regulators for low noise analogue supplies.

The LTC3673-1 configures a 1.5A buck regulator for source/sink and tracking operation to support DDR memory termination and adds a VTTR reference output for DDR. These two pin features replace the LDO4 enable pin and feedback pins of the LTC3676.

Alongside the multiple regulators the parts feature: configurable power sequencing; dynamic output voltage scaling; a pushbutton interface controller; and regulator control via an I2C interface with status and fault reporting.

The LTC3676 supports i.MX6, PXA and OMAP processors with eight independent rails at appropriate power levels and with dynamic control and sequencing.

The LTC3676 can solve the industrial and military system design challenges outlined above. The LTC3676IUJ is available in a high temperature (I-Grade) option with a junction temperature rating from -40 to 125°C, satisfying the high temperature operating requirement. The device includes a thermal warning flag and interrupt specifically for junction temperature monitoring. It also includes a hard thermal shutdown for reliable protection of the hardware, should power dissipation be mismanaged, or in the case of a severe fault.

The LTC3676's PWM switching frequency is trimmed to 2.25MHz, with a guaranteed range of 1.7MHz to 2.7MHz. 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.

Conclusion
Designing a modern mobile device for the industrial, medical or military markets is a challenging task. However, systems designers now have the option to take a 'middle ground' approach to meeting their power needs with a modestly integrated PMIC. This is a more practical approach when compared to using either discrete ICs or highly integrated PMICs. Either way, the choice is theirs to make.

Tony Armstrong is director of power product marketing for Linear Technology.

Author
Tony Armstrong

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