The equivalent circuit of the solar cell is shown as Figure 3-3. The solar cell is actually a power source which includes a somewhat larger internal resistance and has primary features of diode. The solar cell P-V and I-V curve is shown as Figure 3-4

Figure 3-3 Solar Cell Equivalent Circuit

Under certain light and temperature conditions, solar cell I-V and P-V curve is shown as Figure 3-4. There is a max power point which is changing along with voltage rising.

Figure 3-4 Solar Cell PV/IV Curve

Solar cell battery is not the same as normal battery such as Li-ion battery, the solar cell battery is most similar to a constant current source with a somewhat larger internal resistance. The output current and voltage is small on a cloudy day and night. Temperature is also a factor affecting current and max voltage of solar cell battery. so MPPT (Max Power Point Tracking) is an important feature in solar energy system, especially for power grid or high-power energy harvest system. But as a power supply for battery powered camera, instead of care about Max power point of solar cell battery, several designers mostly care about how to continuously and high-efficiency achieve energy both in day and night.

TLV61070A is 2.5-A boost component with pass-through feature. Quiescent current is 20 uA. If output voltage of connected USB adapter or solar panel is larger than configured voltage of boost, power can pass through TLV6170 and directly reach rear charger. And when solar panel output voltage is low in cloudy day or night light, TLV6170 start to convert input voltage to configured voltage such as 4.3 V. This process makes the charger continuously charge the battery even when voltage or current of solar panel becomes low.

Figure 3-5 Solar Charger Design Function Block

BQ21040/5 is very low cost and max 1-A 1-cell linear Li battery charger without power path management and ship mode feature. Mostly, video processing circuit stays in sleep mode, and total average current is less than 1 mA. When the battery is charged to full status, the charger automatically terminates charging and damage battery not due to continuously charging.

Figure 3-6 BQ21040/5 Charger Function Block

BQ21040/5 is a standalone charger with only a few external resistor configurations. Fast charging current and termination current can be separately configured, this makes it possible to input relatively big charging current in sunny day as well as input small charging current in darker days.

If using BQ21080 charger, the BQ21080 power path management function can provides shutdown charging when battery is full and video recording is continuously working. Ship mode can maintain that the battery has energy after finishing charging is delivered to end customer.

Figure 3-7 Solar Charger Design Function Block

BQ21080 is 800-mA, 1-cell linear Li battery charger with power path management and ship mode feature. With ship mode feature, the battery in camera can be designed nondetachable to save cost.

Figure 3-8 BQ21080 Charger Function Block

BQ21080 can also be easily configured through I2C such as charging current, termination current, enable or disable VINDPM, charging time, and so on. BQ21080 can configure battery regulation voltage with 0.5% accuracy from 3.6 V to 4.65 V in 10-mV steps, to support 4.35 V, 4.4 V , 4.45 V, high voltage Li battery charging. BQ21080 charging current range is from 5 mA to 800 mA.

Figure 3-9 BQ27427 Gauge Function Block

BQ27427 is system side, 1-cell and low cost Li battery gauge, integrated sensing resistor, and supports 4.2 V, 4.35 V and 4.4 V charging strategy battery, and automatically adjust battery aging, self-discharge, temperature, and rate changes.