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SINGLE_CELL_LITHIUM_BATTERY_CHARGER

2016-05-10 12:35  
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SINGLE_CELL_LITHIUM_BATTERY_CHARGER
An LP2951 regulator was chosen for this single lithium cell-charging circuit for its built-in cur-rent-limiting capability. In addition, the regulator's output voltage is extremely stable, which is a pre-requisite for lithium battery charging. This figure details an example circuit designed to recharge a single cell. The required output set voltage was specified as 4.200 V (±0.025 V) with a maximum charging current of about 150 mA.An LP2951 regulator was selected for two reasons. One is that its built-in current limiter holds the maximum current to 160 mA (typical). The other is because the output voltage can be very ac-curately set to 4.200 V, thanks to the regulator's stable internal bandgap reference.The 1.23-V reference appears between the feedback pin and ground, which causes a precise current to flow in the output resistive-divider string. The amount of current flowing in these resis-tors determines (sets) the charger output voltage that appears across the battery terminals. Large-value resistors keep the battery drain below 2μA when the dc input is removed (a customer requirement). A trimming potentiometer sets the output to 4.200 V. It must be adjusted when the battery isn't connected to the charger output. A blocking diode is required at the LP295l's output to prevent current from flowing out of the battery and back into the output when the dc-input source is removed. Because the diode is in series with the output, the minimum input-output voltage dif-ferential required for this circuit to operate is about 1.5 V.