L DESIGN FEATURES Designing a Solar Cell Battery Charger
by Jim Drew LT3652 Input Voltage
Regulation Loop The input voltage regulation loop of the
LT3652 acts over a specific input voltage range. When VIN, as measured via a
resistor divider at the VIN_REG pin, falls
12 2.4 24 2.2 IPANEL (A) 22 1000W/m2 2 20 1.8 18 1.6 16 1.4 14 1.2 12 1 10 0.8 8 0.6 6 0.4 4 0.2
0 100W/m
0 2 4 2 2 6
8
10
VPANEL (V) 12 14 16 0 Figure 1. A solar cell produces current in proportion to the amount of sunlight falling on it, while
the cell’s open-circuit voltage remains relatively constant. Maximum power output occurs at
the knee of each curve, where the cell transitions from a constant voltage device to a constant
current device, as shown by the power curves. below a certain set point, the charge
current is reduced. The charging current is adjusted via a control voltage
across a current sensing resistor in
series with the inductor of the buck
regulator charging circuit. Decreased
illumination (and/or increased charge
current demands) can both cause the
input voltage (panel voltage) to fall,
pushing the panel away from its point
of maximum power output. With the …