Ultra-Low-Power energy harvesting applications often requires some “black-boxes” for finding the maximum power point between current and voltage consumption to harvest energy, a DC-DC converter to charge a storage element, some LDO for providing the target MCUs operation voltage and some kind of timer to wakeup the application periodically or wakeup by request via external interrupt (for example by push-button).
For these requirements a combination of e-Peas AEM10941 for solar harvesting (chips for thermal or RF harvesting are available as well) and Ambiq Micros RTC AM1805AQ with powermanagement is a perfect fit.
The e-Peas PMIC offers the capability of harvesting with the best MPP. The MPP is periodically adjusted depending on the available energy. This is generating the best result in energy-harvesting and in terms of solar harvesting it is working for viarious light environments. In addition the e-Peas PMIC includes also a DC-DC buck converter and several settings to charge Li-Ion, NiMH, LiFePo4 and Supercaps. To provide a stable voltage to the target system two LDOs can provide 1.8V and an adjustable 2.5-3.3V output voltage. Other voltage options are possible with own designed restistor networks as well.
For the combination of PMIC + RTC some options were removed and only options with 1.8V/2.5V and 1.8V/3.3V can be set via solder jumper. The following schematic shows the PMIC part only:
The RTC of Ambiq Micro is communicating with I2C or SPI with additional interrupt pin and has several possibilities: Time-Keeping, Power-Management, External-Wakeup-Pin, 256 byte data storage, etc. The most interesting part is the possibility to totally remove power from the target MCU, so the RTC can run with 22nA (15nA) only and can periodically by timer, alarm, external-pin, etc. wakeup the target-system. In addition it is also possible to track the status1-pin of the e-Peas PMIC to react on power-early-warning events via RTC. So the RTC can in addition to its own interrupt sources also react on the power-early-warning with an interrupt and extra pins can be saved.
The only tricky part is the power supply of 1.5V-3.6V only while the e-Peas chip can charge a storage-element from 0V – 4.5V and has only at power-good some voltage between 1.8V and 3.3V available. As soon power is good (in most configurations at around 3.67V), high-voltage-out LDO (HVOUT) and low-voltage-out LDO (LVOUT) are enabled. So during 0-3.67V the RTCs backup battery can be charged togeter with the storage element (BATT). As soon the HVOUT is enabled it is an good idea to switch from BATT to HVOUT which is 3.3V or 2.5V depending on the configuration.
The following schematic-snipplet shows the RTCs VBAT is buffered via C2, 22uF. Via R2 1k the current is limited. If HVOUT is disabled, T2 is charging C2 via VBATT. As soon HVOUT is enabled, C2 is charged via HVOUT and BATT is disabled.
The power-early-warning status1-pin of the PMIC can have a higher voltage than 3.6V, so it makes sense to add an inverter realized with a mosfet and resistor, so it is ensured maximum voltage will be VCC of the RTC:
The schematic of the RTC looks like this:
Ambiq Micro RTCs and e-Peas PMICs are available at the FEEU webshop: http://shop.feeu.com
Further information for e-Peas can be found here: http://www.feeu.com/e-peas
Further information for Ambiq Micro RTCs can be found here: http://www.feeu.com/rtc
Created by Manuel Schreiner, Fujitsu Electronics Europe GmbH Copyright © 2019 Fujitsu Electronics Europe GmbH. All rights reserved.
This schematic is provided by the copyright holder and contributors “AS IS” and any warranties related to this schematic are DISCLAIMED. The copyright owner or contributors be NOT LIABLE for any damages caused by use of this schematic.