Maxim Integrated’s MPPT-Based Solar Harvesting IC Helps Designers Optimize System Efficiency and Output

The MAX20361, a new solar harvester integrated circuit (IC) from Maxim Integrated is an optimal solution for designers with stringent space constraints according to the press release. This IC is a fantastic fit especially for wearable and Internet of Things (IoT) applications where small form factors are a must. While meeting the reduced footprint, the IC does not sacrifice harvesting efficiency but actually improves it as compared to competitors, reaching efficiencies up to 86%.

 

Image courtesy of Maxim Integrated. 

 

As stated before, this new IC is the industry’s smallest such solution, with an area of 1.63mm x 1.23mm, which is 50% the size of its closest competitor, and the IC requires very few peripherals to operate. Shown below is the typical application circuit for the MAX20361 according to the datasheet.

 

Fig. 1: Image Courtesy of Maxim Integrated

 

As seen in Figure 1, the MAX20361 is a fully integrated solution that only requires approximately 10 external components to operate properly. It includes a boost converter that has an ultra-low quiescent-current and operates with input voltages as low as 225mV and efficiently harvests energy from input powers of only 15uW all the way up to 300mW utilizing a proprietary maximum power point tracking (MPPT) technique. This technique measures the open-circuit SRC voltage in order to compute the optimal value to transfer maximum power from the solar cell. 

The switching frequency for this converter is determined by the external inductor value shown in the block diagram. The converter also can be halted based on the monitoring of the voltage at the SYS pin to prevent overcharging or overvoltage. This monitoring is done via the comparators shown in the diagram with programmable levels to compare the SYS output to. 

There is a harvesting monitor which measures the current pulled from the SRC pin, and if it is lower than the programmed threshold value the IC enters a sleep mode in order to save power. Furthermore, the IC contains a thermal monitor that utilizes an external voltage divider consisting of a thermistor and resistor to halt the boost converter if the temperature is out of the range, which is when the THM pin has a voltage outside of the bounds of 18.7% to 57.5% of the REF pin. 

For example, if the user utilizes a 10kOhm NTC thermistor with Beta=3380 and a 22kOhm pull-up resistor, the temperature bounds will be 0oC to 45oC. Lastly, there is an ability to add a source clamp circuit that can discharge the SRC signal via an external load resistor, or when the boost converter is enabled, can divert excess current from the pin. In order to have the IC operate ideally, the following requirements must be met for some of the external components.

• Inductor: 
  • The inductor placed between the SRC and LX pins has a recommended inductance range of 4.7uH to 22uH where the switching behavior of the boost regulator is optimized when the inductor has a nominal value of 4.7uH. In order to minimize loss, the inductor must have a low series resistance (DCR).
• Capacitors:
  • All capacitors must have low leakage in order to maximize efficiency
  • SRC Capacitor: This capacitor is used to initially store energy from the input source being harvested. Minimum of 10uF is recommended and larger capacitances are needed if the inductance value is on the higher side of its range.
  • SYS and VCC Capacitor: Low equivalent series resistance (ESR) is required and a maximum effective capacitance of 1uF is preferred
• NMOS Transistor:
  • The NMOS transistor has its source connected to the SRC pin while the gate is controlled via the INT pin, and is used to clamp the input signal. The only requirements are that the gate to source threshold and drive voltages must be lower than 2V.

The MAX20361 IC has the opportunity to optimize any system it is placed within for both size and efficacy. Due to its integration of the functionality an energy harvester requires within a single package, a designer can meet these specs without adding complexity and cost to the design, as simple external components are only required for proper operation.