AN-1036: EnerChip™ CC in EH Designs

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AN-72-1036 Rev A

Page 6 of 7

Figure 6. EnerChip CC EH Energy Harvesting Circuit as Implemented in the CBC-EVAL-09

Case Study

When designing a ‘batteryless’ power system using energy harvesting, it is essential to know the system power

requirements, including cumulative standby power of all system components, transmit/receive power and duty

cycle where applicable, and run-time requirements during periods when ambient power might not be available

for harvesting.

A common scenario is a wireless sensor node that measures temperature in a room every several minutes and

reports the temperature to a centralized receiver via a short range radio link. Total power consumption can be

broken down as follows:

1) Microcontroller in standby mode

1uA

2) Temperature sensor

500nA

3) A/D conversion for temperature measurement

50uA x 20ms

4) Radio standby current

2uA

5) Radio transmit pulse

20mA x 20ms

6) Temperature measurement duty cycle

once per 5 minutes

7) Transmit duty cycle

once per 5 minutes

8) Average current required by energy harvesting

15uA

and power management circuit, including EnerChip at

>90% state-of-charge and assuming the charge pump is

active 50% of the time

Total average power is the sum of the above, calculated based on a system voltage of 3.3V.

Pavg.

= 3.3uW + 1.65uW + (165uW x 20ms/300 sec.) + 6.6uW + (66mW x 20ms/300 sec.) + 49.5uW

= 65.5uW.

The total energy consumed in any 24-hour period, using the foregoing assumptions, is:

65.5uW x 24 hours x 3600 sec./hour = 5.7 Joules.

Depending on the output of the transducer and the voltage requirement for the system load, the input shunt

diode and the output regulator might or might not be necessary. The circuit of Figure 6 includes the shunt

regulator, power-on reset circuit, and input shunt diode typical of an energy harvesting application circuit using

PV cells, but excludes the external battery.