AN843 Datasheet, PDF (1/4 Pages) Silicon Laboratories – Using Analog Components to Manage Power in Low-Power Solar Systems

AN843
Using Analog Components to Manage Power in Low-Power Solar Systems
1. Introduction
From large panels to harvested microwatts from a few photodiodes, solar power is increasingly prevalent in
autonomously powered systems. With the worldwide evolution toward lower power operation using more “green”
energy sources, emphasis on deploying solar power in a greater variety of environments has been on the rise.
In low-power solar systems, it is critical to assess whether there is sufficient sunlight at a given time to power the
system. In some cases, this involves determining whether there is sufficient power to enable the microcontroller. In
many ultra-low-power systems, the simple act of waking the microcontroller to make a voltage measurement might
collapse the solar source or waste precious power from a reservoir capacitor.
One solution is to incorporate a simple analog op amp into the system. An ultra-low-power analog op amp can
support “always-on” circuitry around the microcontroller and may be the simplest and best solution.
2. Ultra Low-Power Op Amps
Figures 1 to 3 illustrate a few simple circuits using an ultra-low-power op amp in a continuous “always-on”
measurement mode assessing the state of the solar cell. The technique hangs on using an op amp whose total
power is as low as practical driven primarily by ultra-low supply voltage operation.
The circuit in Fig. 1 shows a photodiode in photovoltaic zero-bias mode. The short-circuit current is measured and
converted to a voltage across resistor R1 while the feedback action of the op amp forces 0 V across D1. Zero-bias
photovoltaic current is a convenient parameter that is generally well characterized and can be directly referenced
to most photodiode manufacturer datasheets. Note that with the rail-to-rail input range of the analog op amp, in this
case the TS1001 op amp, the photodiode may be connected directly to the positive supply voltage rail.
Figure 1. An Ultra-Low-Power Op Amp can be in a Continuous “Always- On” Measurement Mode
The circuit in Figure 2 generates a simple positive-polarity output for a similar zero-bias-mode measurement. In
this case, the op amp servos its output to sink sufficient current to support the zero bias condition for D1. This
creates a voltage across resistor R1 at the negative supply voltage pin of the TS1001. Since this ultra-low-power
op amp contributes less than 1 µA to this current, the current measured from D1 has minimal error.
Rev. 1.0 1/15
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