Question 22.17: What does the circuit of Fig. 22-35 do?
What does the circuit of Fig. 22-35 do?
The circuit of Fig. 22-35 is a solar LED lamp application. It uses a solar panel to charge up a battery. The battery supplies the input voltage to the CAT4139 constant current driver, which drives the LED string. So how does this circuit work?
The solar panel consists of a solar module constructed with 10 cells connected in series. Each cell produces approximately 0.5 V to 1.0 V depending on ambient solar conditions. This produces an output voltage of 5–10 V, under no load, between the SOLAR+ output and ground GND.
When the output of the solar module is high enough, it charges up a 3.7-V lithium-ion (Li-Ion) cell through diode D_2. The Li-Ion cell (or parallel cells if needed) has built-in protection for overcharge current/voltage or discharge current.
The voltage at SOLAR+ also provides the input base-bias for transistor Q_1. When Q_1 conducts, the voltage drop across R_1 pulls the collector towards ground and the CAT4139 converter goes into a shutdown mode. When the ambient light is low, the output of the solar module drops significantly. The low output voltage at SOLAR+ no longer forward-biases Q_1. This transistor turns off and its collector voltage rises to the BAT+ level. If the Li-Ion cell is charged to a sufficient level, the converter goes into switching mode and the voltage at BAT+ supplies the input voltage to the converter’s input pin V_{IN}. During this time, D_2 is used to prevent reverse current flow from the battery to the solar module. The dc-to-dc boost converter now supplies the necessary output voltage and current for the LED string. The constant current through the LEDs is controlled by the resistor R_4 where I_{LED} = 0.3 V/ 3.3 Ω = 91 mA.