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Photovoltaic Charger with Extremely Low Leakage When Not Charging

A photovoltaic (PV) application for the LTC4070 is illustrated in this example. In this application, transistor Q1 has been added to further reduce the already low quiescent current of the LTC4070 to achieve extremely low battery discharge when the PV cells are not charging the battery. In long battery life applications, Q1 isolates the battery from the LTC4070 when Q1’s base voltage falls. Under normal operation, the PV cells provide current through the VBE and VBC diodes of Q1. While the battery is charging, the majority of PV current flows to the battery. When VCC reaches the programmed float voltage, in this case 4.1V with ADJ floating, then the LTC4070 shunts base-collector junction current from Q1, effectively reducing the battery charging current to zero and saturating Q1. In the event that the thermistor temperature rises and the float voltage drops, the LTC4070 shunts more current, and Q1 is forced to operate in reverse active mode until the battery voltage falls. Once equilibrium is achieved, the difference between VBAT and VCC should be less than a few mV, depending on the magnitude of the shunt current.

The PV application schematic in this circuit also illustrates using the LTC4070 with a 10k, 5% curve 2 type NTC thermistor, NTHS0402N02N1002F. Here RNOM is 10k, and the rising temperature trip points are 40°C, 50°C, 60°C and 70°C.

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