By taking advantage of increasingly efficient energy harvesting transducers and ultra-lower power ICs, engineers can build more sophisticated designs powered by ambient energy sources. However, as these designs combine more complex application circuits, engineers are faced with the challenge of ensuring reliability and extended lifetime of devices operating with uncertain ambient energy sources. To make sure their circuits operate within required power limits engineers can take advantage of available power supervisor capabilities in ICs from semiconductor manufacturers including Analog Devices, Cymbet, Linear Technology, Maxim Integrated, Microchip Technology, ON Semiconductor, and Texas Instruments, among others.
At the heart of a typical energy harvesting design, power management circuitry provides critical functionality for 1) maximizing energy extraction from ambient sources, 2) charging energy storage devices and 3) supervising power for proper operation of the system as a whole despite changes in voltage and power (Fig. 1). In energy harvesting circuits operating near the limits of available ambient power, power supervision also is critical to ensure safe and reliable operation. When voltage and current fall outside operating specifications, system operation, and lifetime can be compromised with loss of program state in MCUs and damage to Li-ion storage cells, for example.
Figure 1: In a typical energy harvesting design such as a wireless sensor, power management circuitry plays a critical role in maximizing energy conversion and power efficiency while ensuring system reliability (Courtesy of Texas Instruments).
Basic power monitoring circuitry is typically built into complex devices such as MCUs, which typically include a brownout reset (BOR) capability to ensure that the device will not try to run at a voltage that is too low for correct operation. Similarly, built-in power-on reset (POR) ensures that the MCU powers up with proper initialization.
For example, the TI MSP430xG461x can operate within a relatively wide range of supply voltage in normal operating mode and flash programming mode, and includes an on-chip supply voltage supervisor (SVS) circuit. The MCU's SVS circuit detects if the supply voltage drops below a user selectable level. The designer can select if low supply voltage levels cause the device to automatically reset or simply serve to monitor the low levels.
On some devices, power-on reset can start a power-on reset timer designed to hold the device in reset for an extra delay to allow more time for power to stabilize. For example, the Microchip PIC16F182 XLP MCU features a power-up timer that adds a delay on power up resulting from a power-on reset or brown-out reset signal. This delay allows additional time for the MCU supply voltage to settle at the required level.
Dedicated power supervisor ICs provide improved threshold accuracy with tighter tolerances typically programmed simply with external resistors. These specialized devices are also typically designed to provide greater immunity to spurious resets if glitches in power sources from load transients or noise are seen on device reset lines.
More advanced power supervisor ICs provide an extended set of features beyond on-chip power-on reset and brownout reset functionality built into MCUs (or the corresponding functionality for MCUs without built-in BOR features). Sophisticated devices designed for complex multi-rail designs provide a broad range of features including power sequencing, fault detection, monitoring, trimming, and even margining support. Although many of these features are typically beyond the scope or power budget of power-limited energy harvesting designs, key features such as power sequencing and fault detection are critical for system reliability.
Applying power in the proper sequence is important for semiconductor devices, which can be stressed or damaged from excessive current flows or voltage differentials resulting from improper sequencing. While engineers can find advanced multi-voltage supervisor ICs such as the ON Semiconductor MC34160 or Maxim MAX6877 typically used in complex systems, other dedicated devices offer simpler supervisory function useful in energy harvesting designs.
Simple sequences like the Maxim Integrated MAX6819/MAX6820 or the Analog Devices ADM1085 provide means to enable a secondary voltage source with either a fixed or programmable time delay. With the Maxim MAX6819 and MAX6820, when the primary supply exceeds an adjustable threshold, these devices enable an external MOSFET switch to connect the secondary supply to the load using a fixed (MAX6819) or variable delay (MAX6820) adjustable using an external capacitor (Fig. 2).
Figure 2: Simple power sequencers such as the Maxim Integrated MAX6819 and MAX6820 enable an external MOSFET switch connecting a secondary supply to the load when the primary supply reaches a preset threshold (Courtesy of Maxim Integrated).
With the Analog Devices ADM1085, when the output voltage of the primary power source reaches a preset threshold, a time delay is initiated before other power sources are enabled. Engineers can cascade several ADM1085 devices with voltage converters to allow sequencing of multiple power sources, setting separate threshold levels external resistors. With appropriate resistor values, the threshold can be adjusted to monitor voltages as low as 0.6 V.
Some DC/DC converters also include basic power sequencing capability. For example, the Analog Devices ADP2140 DC/DC converter with LDO provides programmable power sequencing modes allowing engineers to enable the primary output and LDO output through a pair of enable pins (Fig. 3), or put the device in an auto sequencing mode that enables primary output and LDO in either order with a fixed delay between them.
Figure 3. The Analog Devices ADP2140 DC/DC converter provides a built-in sequencing feature that can automatically enable separate activation of its primary output and LDO output or allow engineers to control activation programmatically using two enable pins, EN1 and EN2 (a) with specified delays between activation (b) (Courtesy of Analog Devices).
Besides dedicated supervisory ICs, devices developed with energy harvesting applications in mind offer many required power supervisory function. For example, the Linear Technology LTC3108/9 ultra-low-voltage DC/DC converters sequence the output of their multiple voltage source outputs based on thresholds and timing set for each output source.
Energy-harvesting ICs designed to support energy storage management include built-in power supervisory functionality. Such devices, including the Cymbet CBC915, Maxim Integrated MAX17710, and Texas Instruments bq25504, feature capabilities such as undervoltage lockout and overvoltage protection, typically designed to protect storage devices and the load when voltage levels fall outside of specified operating ranges.
In summary, ensuring proper supply levels is critical not only for overall system performance but also for reliability and longevity. Dedicated power supervisor ICs provide engineers with flexible control of power sequencing and circuit protection, offering enhanced capabilities beyond those built into complex devices such as MCUs. What's more, devices specifically intended for energy harvesting applications typically provide the basic power supervisory functionality needed in many of these designs. Using built-in or dedicated power supervisory functionality, engineers can build energy harvesting systems designed for reliable, long-term operation across the widest possible range of supply voltages.