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The goal of this work was to create most efficient constant
current boost driver operational from single cell Li−Ion battery
recharged from solar module. Savings and the environment quality are
a concern. This solar LED circuit diagram describes implementation of the
CAT4139, a dc−dc step−up converter that delivers an accurate
constant current for driving serial LED string in solar LED lamp.
Battery cell is charged during daylight, when average voltage on
solar module (SOLAR+ pin) is around 4.2 V. Charge current at solar
module output varies proportionally to incident light and charging
is controlled by incorporated protection battery circuit. SOLAR+
voltage switches on transistor T1 and its collector voltage disables
the CAT4139.
When ambient light is decreased and voltage on SOLAR+
pin is below threshold given by base−emitter voltage of T1, the
CAT4139 driver is enabled and supplies constant current to LED
string (T1, R5, R1 circuit is used to invert voltage coming from
solar module.).
During night, battery is being discharged. D2 is
used to prevent from reverse current flow from battery to solar
module. The constant LED current is given by value of R4 resistor
(in this case, 0.3 V/3.3 = 0.091 A.
Source: onsemi.com
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