Application Note
Using the EnerChip™ CC in Energy Harvesting Designs
EnerChip CC Overview
Cymbet EnerChip
TM
solid state batteries are highly rechargeable energy storage devices packaged in low
profile, surface mount, solder reflowable packages. Their low self-discharge, high charge-discharge cycle life,
and ability to accept very low input charging current combine to make them ideally suited as energy storage
devices in low power applications. These include backup power sources for real-time clocks, microcontrollers,
and memory chips to maintain operation during periods when main power is interrupted. EnerChips are also
well suited as energy storage devices for wireless sensor nodes where energy is harvested from ambient
power sources. The integrated battery and power management circuitry included in the EnerChip CC products
provides supply voltage threshold detection, an automatic energy storage switchover function, and EnerChip
charge-discharge control circuitry - all in a compact surface mount package that includes the EnerChip storage
device. Figure 1 depicts the EnerChip CC surface mount package and the CBC-EVAL-10 development module
designed for evaluating the EnerChip CC devices in energy harvesting applications.
AN-1036
Figure 1. EnerChip CC CBC3150 (left) and the CBC-EVAL-10 Energy Harvesting Evaluation Kit (right)
EnerChip CC Energy Harvesting
The EnerChip CC can be configured to collect energy from transducers such as low power photovoltaic (PV)
cells and use that harvested energy to charge the integrated EnerChip and deliver self-sustaining power to
components such as microcontrollers, sensors, and radios in wireless systems. Figure 2 illustrates a typical
implementation of the EnerChip CC in energy harvesting applications. In order to make most efficient use
of the power available from the transducer (for example, a PV cell), it is necessary to know the electrical
characteristics including voltage and peak power point of the transducer being used. This Application Note
describes a low cost energy harvesting circuit using the EnerChip CC to collect, convert, store, and deliver to the
Figure 2. Implementing Energy Harvesting with the EnerChip CC CBC3150
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AN-1036: EnerChip™ CC in EH Designs
system components the energy available from power transducers such as small photovoltaic cells.
EnerChip CC Description and Operation
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The EnerChip CC is a multi-functional circuit that serves two primary application purposes:
Backup power source with integrated power management for real-time clocks, SRAM, and microcontrollers
Energy harvesting, power management, and energy storage in wireless network systems
The EnerChip CC delivers power to the system load from either the main supply or the integrated EnerChip
rechargeable storage cell.
Functional elements of the EnerChip CC are:
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Internal charge control circuit that converts a wide range of input voltages to a tightly regulated battery
charging voltage;
Battery cutoff circuit that disconnects the load from the embedded EnerChip when the discharge voltage
reaches a preset limit;
Output pin that can be used to drive an interrupt line on an MCU to indicate that the system is operating in
battery-backed mode;
A voltage detection circuit allowing a user-selectable threshold trip voltage;
A control line that enables and disables the internal charge pump that can be controlled externally to
reduce power consumption when the internal EnerChip does not need to be charged.
Figure 3 illustrates the functional blocks of the EnerChip CC. The only required external component is the
flying capacitor - typically 0.1uF to 0.47uF. In normal operation, VDD passes directly through to VOUT by way
of a 2.5 Ohm (typical) pass transistor. When VDD drops below the switchover threshold (as determined by
the voltage on VMODE), the internal energy storage device - the EnerChip - delivers power to the load through
VOUT at a nominal voltage of 3.3V (unregulated). An internal charge pump regulates the EnerChip charging
voltage at 4.1V. To reduce power consumption, the charge pump can be disabled by forcing ENABLE to a logic
low. Additional EnerChips may be appended to the VCHG pin for incremental energy storage. The EnerChip CC
operates over a supply range of 2.5V to 5.5V. See EnerChip CC data sheets for a more complete description of
the circuit functions, operating modes, and specifications, including EnerChip device characteristics.
Figure 3. Block Diagram of the EnerChip CC Energy Storage Device with Integrated Power Management
©2011 Cymbet Corporation • Tel: +1-763-633-1780 • www.cymbet.com
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AN-1036: EnerChip™ CC in EH Designs
In addition to the intrinsic features of the EnerChip CC, the CBC-EVAL-10 makes provision for system-level
requirements in energy harvesting applications, including:
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Energy harvesting circuitry that matches the impedance of photovoltaic cells to ensure maximum power
transfer to system load and on-board energy storage devices
Integrated battery management that controls battery charging and discharge cutoff, ensuring maximum
service life of on-board storage cells
Provision for additional energy storage (primary or rechargeable batteries) with switchover control circuit to
meet application requirements
Regulated output voltage with user-configurable voltage settings
Input/output headers for connection to system components such as radios and microcontrollers
The following sections describe energy harvesting concepts and methods of configuring the EnerChip CC as a
low cost energy harvesting, storage, and delivery device in low power wireless systems.
Energy Harvesting Design Considerations
Energy harvesting refers to the collection of ambient energy in one form and conversion to another form - for
example, the conversion of ambient light to electrons using PV cells. The energy is typically then stored in
a highly rechargeable storage reservoir - such as the EnerChip - and delivered to other system components
such as MCUs, sensors, radios, and so forth. A typical application is a wireless sensor node that: measures
any number of environmental variables (e.g., temperature, proximity, pressure, humidity), processes the
information, and sends it via a wireless radio link to a central location or another wireless node.
The reliability of traditional batteries over years of operational life can be poor. Consequently, designers have
long sought alternative means of providing power to wireless systems. Energy harvesting, coupled with solid
state energy storage, is now a viable alternative in many operating environments. The average power consumed
by a wireless sensor node is often on the order of a few microwatts to a few tens of microwatts. The average
power is often dominated by standby currents of the MCU, radio (where applicable), and sensor itself. The radio
transmit and/or receive duty cycle, if kept to a low percentage, will not generally have an overwhelming effect
on the average power consumption of the sensor node. With proper system design, most or all elements of
the system can be put into low power standby modes to reduce the average power consumption and therefore
permit cost-effective power transducers like small PV cells to be used for collecting power in low ambient light.
Other design considerations include managing start-up power associated with linear devices when being
powered by a ‘soft’ supply such as a small PV cell in low light. For example, MCUs and radios, if not powered up
with a robust power supply, can enter high current states that prevent the system from ever reaching a stable
operating condition due to the limited power available from the supply.
Pulse currents demanded by radios can also render the system inoperable if not properly managed with output
capacitance and proper system configuration. Additionally, it is often advantageous to disconnect sensors
themselves from the power supply when not actively acquiring signals. This can result in a significant reduction
in average power consumption and lower the requirement for bulky power transducers and energy storage
devices. In some cases, it is necessary to utilize power-on reset circuits to ensure the load is not connected
to the supply until sufficient charge is injected into an output capacitor to overcome the start-up problems
described above. Linear dropout regulators (LDOs) must be carefully selected to ensure they do not place a
burdensome load on the limited power available from the energy harvesting source.
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AN-1036: EnerChip™ CC in EH Designs
When converting power from the source to the load, such as from a PV cell to an active electronic load,
impedance matching is essential to efficient power conversion. By matching the impedance between the
source and load, maximum power transfer is achieved. Conversely, any mismatch in impedance from the power
source to the load presented it will result in wasted power. As shown in Figure 4, the peak power output of
a PV cell occurs at a point on a nonlinear voltage-current (V-I) curve. Extracting power from the PV cell at an
operating point well away from the peak power point results in greatly diminished power output. Consequently,
proper impedance matching is critical to efficient circuit operation. Various impedance matching circuits are
available to the designer.
The power curve of Figure 4 is typical of a low power PV cell used in energy harvesting applications. Electrical
impedance of the cell varies strongly as a function of ambient light. As illustrated, the power curve is highly non-
linear; connecting an electrical load to the PV cell that is not matched to its impedance results in inefficient power
transfer to that load.
Figure 4. Typical Power Profile of a Photovoltaic Cell
Choice of a particular design depends on many factors, including:
• the nature of the system elements (e.g., capacitive, inductive, resistive) and the rate at which the
elements change over time with variations in ambient conditions;
• the amount of power being coupled from source to load;
• conversion efficiency needed for proper circuit operation;
• and the relative cost of implementing a given circuit.
The designer often must consider all of these factors when selecting a particular energy harvesting design
topology. The following section describes a circuit that relieves the designer of the complexity associated with
making those calculations.
EnerChip CC Energy Harvesting Circuit
As illustrated in Figure 5, the EnerChip CC can be configured to operate in an energy harvesting mode that
tracks the peak power point of the transducer (e.g., PV cell) by dynamically adjusting the duty cycle of the
internal charge pump - increasing the duty cycle with higher input power, decreasing it with lower input power.
This is accomplished by connecting the RESET/ output to the ENABLE input In so doing, the load presented
to the transducer varies in response to changes in transducer output characteristics. As the transducer drive
current diminishes at a given voltage, its impedance is raised.
©2011 Cymbet Corporation • Tel: +1-763-633-1780 • www.cymbet.com
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AN-1036: EnerChip™ CC in EH Designs
Figure 5. EnerChip CC EH Energy Harvesting Schematic
As the EnerChip CC loads the transducer output, the voltage at VDD decreases, until the point that the
supply threshold trip point 3.0V (as determined by GND level of the VMODE pin) is reached. At that point,
the RESET/ line is forced low, which in turn disables the charge pump, thereby increasing the effective
impedance presented to the transducer. A higher impedance attached to the transducer allows the transducer
voltage to rise, repeating the cycle. Thus, the duty cycle of the charge pump is dictated by the transducer I-V
characteristics. Ideally the threshold detect trip point will be set to the voltage at which the transducer operates
at its peak power point. VMODE has two discrete threshold switchover settings VDD and GND (GND in this
case) as well as the option for setting it to an arbitrary level through use of an external resistor divider. In the
latter mode, it is important to select resistor values that do not place a burdensome load on the supply. See the
EnerChip CC CBC3150 DS-72-03 or CBC3112 DS-72-04 data sheets for recommendations on resistor values
to use on the VMODE pin.
Figure 5 depicts an energy harvesting implementation using the CBC3150 power management IC. The
circuit in Figure 5 is useful for some energy harvesting applications where start-up currents of radios
and microcontrollers will place a load on a relatively underpowered transducer that can not be overcome
without additional circuitry incorporated to manage the transition of power from the transducer to the load.
This schematic uses a power-on reset circuit that enables operation of the output regulator only after the
capacitance is fully charged. This scheme allows for a sufficient burst of charge into the load to overcome the
start-up power barrier.
Another scenario shown in Figure 5, includes a larger, external battery (shown as BT1 either a rechargeable or
primary cell) that is used when the CBC3150 energy storage is insufficient to operate the circuit for prolonged
periods when ambient power from the transducer is not available. In this case, the service life of external
battery that would often have to be used as the main energy storage device can be extended by months or
years through the use of energy harvesting. The constant background current drain otherwise eats away at the
battery capacity. With energy harvesting, the external battery can be used only intermittently as ambient power
is not present, or as the source of higher pulse currents.
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