Micropower LDOs Simplify Energy-Harvesting Designs



Low-dropout (LDO) regulators offer a simple solution for managing supply voltage in energy harvesting designs. Along with their own low-power characteristics, LDOs feature power protection and management capabilities once found only in more advanced power management ICs. Today, designers can implement sophisticated power supply capabilities using available devices from Analog Devices, Linear Technology, Maxim Integrated, Micrel and Texas Instruments, among others.

For designs powered by energy-harvesting techniques, engineers can take advantage of multiple-regulator topologies to provide the load with a clean, well-regulated voltage source. Sophisticated switching regulators offer high-power efficiency while bucking or boosting harvested voltage to required levels. For example, in drawing on very-low-energy sources, such as temperature differentials and RF, designers apply boost converters to bring the millivolt-range voltages generated by thermoelectric generators or RF harvesters to useful levels. For very high-energy solar sources, buck converters reduce the high-voltage output from multistring solar panels to levels required to deliver power to the grid or charge local batteries. Between these extremes, low-dropout (LDO) regulators offer a combination of simplicity, low power and low noise needed in designs such as sensors or Internet of Things (IoT) nodes powered by ambient energy sources.

An LDO uses a simple topology comprising an error amplifier, a voltage reference, a feedback-voltage divider and a pass transistor (Figure 1). While the pass device delivers output current, an error amplifier compares the feedback voltage to the reference and controls the gate of the pass transistor to bring output voltage back to the regulated level. This simple topology provides suitable regulation at a modest cost in efficiency compared to switching regulators.


Figure 1: Low-dropout (LDO) regulators use a simple-feedback loop that controls current through a pass transistor to regulate output voltage (Courtesy of Analog Devices).

Available in fixed and adjustable versions, LDOs are ideal for maintaining regulation when the input supply and output-load voltage differ by only a relatively small margin. Unlike standard linear regulators, LDOs can continue to operate even as the input supply voltage level drops very close to the output load voltage. This difference between input and output levels — the dropout voltage — is little more than 0.1 V and substantially less at low current levels for advanced LDOs. For example, the Micrel MIC5387 LDO features dropout voltage in the low tens of millivolts at low current with little change across a wide-temperature range (Figure 2).


Figure 2: LDOs such as the Micrel MIC5387 exhibit very-low-dropout voltage that falls even further with reduced output current (Courtesy of Micrel).

Perhaps more critical in designs with limited-power budgets, the quiescent or ground current dissipated by these devices is negligible. For example, the Texas Instruments TPS79733 exhibits a typical ground current of only 1.2 μA even at full load with only modest variation with temperature (Figure 3).


Figure 3: LDOs such as the Texas Instruments TPS79733 feature ultra-low quiescent or ground current to help reduce power consumption in designs powered by low-energy ambient sources (Courtesy of Texas Instruments).

Beyond their own power characteristics, advanced LDOs provide engineers with additional features needed for managing power in designs with tight power budgets. For example, as with other devices in this class, the Analog Devices ADP222 LDO provides two independent LDOs that include thermal shutdown and current-limiting capabilities (Figure 4). Its internal-thermal-shutdown circuitry turns off the LDO if its temperature exceeds a specified value, while its current-limiting features constrain its output current and power-dissipation levels.


Figure 4: Along with protection features such as thermal shutdown and current limiting, the Analog Devices ADP222 dual LDO allows designers to enable/disable each LDO separately for finer control of power sequencing in designs (Courtesy of Analog Devices).

Along with internal protection features, many LDOs also provide mechanisms for power management and sequencing in designs. For example, the ADP222’s “enable” inputs (see Figure 4) allow designers to externally control LDO output. Engineers can use this capability to ensure proper power-startup sequencing in complex designs where an unmanaged power initialization might drain available energy and cause the circuit to collapse for lack of power.

If unmanaged, an attempt to power up an energy-limited design could result in a continued cycle of partial startup and collapse as the design fails to find sufficient power to complete power initialization. Instead, engineers can power components in a sequence designed to meet overall power-initialization requirements with limited available power. Similarly, engineers can find LDOs, such as TI’s TPS797xx family, includes a power-good (PG) output designed to help sequence power correctly for parts such as MCUs (Figure 5).


Figure 5: LDOs such as the Texas Instruments TPS79718 and other members of the TI TPS797xx LDO series feature a power-good (PG) output designed to interface directly with a microprocessor, such as the TI MSP430 series device (Courtesy of Texas Instruments).

Unlike most buck and boost regulators, LDOs' switch-free topology results in low- power supply and excellent power supply rejection-ratio (PSRR) characteristics. For example, as with many devices in this class, the Analog Device ADP222 dual LDO offers a high PSRR particularly at the lower frequencies (Figure 6).


Figure 6: With their switcher-free topology, LDOs feature highly effective power supply rejection ratios (PSRR) across a wide range of frequencies (Courtesy of Analog Devices).

A high PSRR is particularly critical for delivering clean power to sensitive components such as sensors or wireless devices where high-frequency noise in the supply would otherwise introduce errors. This characteristic is particularly useful in reducing output ripple produced by specialized energy-harvesting ICs based on switching topologies. For example, designers can pass the output of a piezoelectric energy harvester such as the Linear Technology LTC3588 through a Linear LT3009 LDO to provide clean power to the load.

The advantages of LDO regulation in energy-harvesting designs are sufficiently powerful that manufacturers often integrate LDO functionality in specialized devices and provide dedicated-LDO power-out pins. For example, the Maxim Integrated MAX17710 energy-harvesting IC provides LDO regulated output at selectable voltage levels. Similarly, along with separate outputs with selectable voltage levels, the Linear Technology LTC3108 energy harvesting IC offers a dedicated 2.2 V LDO output for powering microprocessors.

Conclusions

LDO regulators provide a simple, low-cost option for managing power in energy-harvesting designs. Along with low-power consumption, these devices feature power-management capabilities needed in power-limited applications. Furthermore, LDOs can reduce ripple in supply voltage to provide a clean power source for sensitive devices. For designers, available LDO devices and specialized energy-harvesting ICs with integrated LDO functionality offer a simple yet powerful alternative to complex devices for voltage regulation and power management.

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