Charger for a current of 300 milliamps. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Chargers, batteries, galvanic cells
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The low-current charger circuit is required, in particular, for miniature computers of various types and systems. In this case, the charger should be assembled only on elements intended for surface mounting, and occupy a minimum area on the printed circuit board.
(click to enlarge)
The circuit shown in the figure, using the LTC1174, satisfies both requirements. The LTC1174 is a stabilized buck switching voltage converter operating in current mode. The power switch is a built-in low-loss p-channel high power MOSFET.
In this circuit, the charge current is set to 600 mA. The average charging current depends on the inductance of the inductor and the operating voltage of the battery, according to the formula shown in the figure.
The divider resistor values in the voltage feedback circuit correspond to an output level of 7 V. For nickel-cadmium cells, the value is 1,2 V, when discharged - 0,9 V, and when fully charged - 1,5 V. Therefore, on the battery of four nickel-cadmium battery cells can be from 3,6 to 6 V, depending on the degree of charge or discharge. When such a battery is connected to the charger, its output voltage drops below 7V, and the LTC1174 goes into current limiting mode at about 300mA if the inductor inductance is 50µH, according to the above formula.
Diode D2 prevents the battery from discharging through the divider circuit when the charger turns off when the SHUTDOWN pin is driven low. In the off state, the charger draws less than 10µA of its own current from the power supply.
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