Efficiency is an important dc-dc converter characteristic, particularly for virtually every battery-based or embedded system. It impacts the thermal and electrical losses in the system, as well as the cooling required. Also, it affects the physical package sizes of both the power supply and the entire system. Plus, it has a direct effect on the system’s operating temperatures and reliability. These factors contribute to the total system cost, both in hardware and field support.
Battery-based dc-dc converter efficiency determines battery life and run time. These converters must be small and lightweight as well as efficient, so they usually employ analog techniques. This will probably continue because of size and cost limitations. Most handheld battery-based systems have a built-in dc-dc converter integrated with other circuits (see “DC-DC Converter Systems Glossary”).
The Texas Instruments TPS8267x is a complete dc-dc converter for low-power applications. Housed in a compact, low-profile ball-grid array (BGA) package, it suits automated assembly. Its IC-like package includes the switching regulator, inductor, and I/O capacitors, so it doesn’t require any additional components.
The TPS8267x is a synchronous, step-down converter that works with a 2.3- to 4.8-V input. Operating at 5.5 MHz, it can provide up to 600-mA output and has good load and line-transient response. Its ~17-μA quiescent current helps maintain high efficiency at light load while preserving a fast transient response for applications that require tight output regulation.
If the load current decreases, the converter automatically enters a power-save mode where it operates in discontinuous current (DCM), single-pulse pulse-frequency modulation (PFM) mode. The converter exits the PFM mode and enters pulse-width modulation (PWM) mode when the output current can no longer be supported in PFM mode. As a result, the dc output voltage is typically positioned approximately 0.5% above the nominal output voltage. There is a seamless transition between PFM and PWM.
Digitally Based Design
Digital control is now appearing in embedded systems to augment or supplant analog techniques. Point-of-load (POL) converters in embedded systems are among the first to incorporate digital technology. Because embedded systems usually use several POLs, they have been good candidates for efficiency optimization.
GE Energy’s DLynx Distributed-power Open Standards Alliance (DOSA) modules are digitally oriented POLs. The modules require only three external components and offer double the density of a discrete power design. Their standards-based DOSA footprints and analog/digital compatibility with existing circuit board designs shrinks the size, lowers the cost, and improves the performance of dc-dc converters.
Digital control provides the ability to monitor load power consumption. The DLynx modules deliver optimized current de-rating and 96% efficiency. They cost less than the previous generation of analog POLs as well. Also, adaptive voltage scaling (AVS) leverages silicon performance to reduce power consumption through tight digital control (±0.4%) of the output voltage and a ±1% controller set point reference.
These modules include Tunable Loop technology that improves the converter’s transient response by modifying the POL’s control bandwidth. As the control bandwidth increases, the transient response improves for a fixed external capacitance. Increasing the control bandwidth and external capacitance together would improve transient response.
However, these parameters are interdependent. Increasing the external capacitance degrades the system’s control bandwidth. With the Tunable Loop, the control loop can be retuned to compensate for the additional external capacitance, which yields the best possible transient response.
The Tunable Loop employs an external network consisting of a resistor and capacitor in series connected across the TRIM and VOUT (or SENSE) pins of the POL module (Fig. 1). These networks use resistors of up to a few kilohms and capacitors from a few hundred picofarads to a few hundred nanofarads. This allows a single POL module to be externally optimized across multiple applications of significantly varying demands and yields the optimum board area, cost, response, and reliability.
Transients and surges applied to system power supplies can affect associated loads and cause system and component failure. To combat this, Linear Technology’s LTC4366 “surge stopper” IC interfaces between a system power supply and its loads, protecting the loads from the supply’s input voltage surges.
The LTC4366 protects loads by controlling the gate of an external N-channel MOSFET to absorb the surge (Fig. 2). Normally, the IC and MOSFET allow the power supply to service its loads with minimal insertion loss. But if the power supply input receives a surge or transient, the LTC4366 and MOSFET clamp the voltage applied to the loads to protect them from damage or failure.
A floating topology allows the LTC4366 to operate from inputs of 9 V to more than 500 V. Its adjustable, regulated output clamp voltage does not affect system operation.
The Ericsson Power Modules 3E series BMR462 converter also uses digital control to offer a wide range of monitoring functions (Fig. 3). It operates with a 4.5- to 14-V input and produces an output from 0.6 to 5 V. At half-load, 5 VIN and 3.3 VOUT, typical efficiency is 97.1%.
A PMBus command can automatically control power dissipation under light-load conditions, minimizing current drain and switching losses. At low load currents, the BMR462 turns off its low-side MOSFET gate drive in its synchronous rectifier output, which boosts efficiency.