MIC79050 Linear Li-Ion Battery Charger - Micrel - #9

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August 2005 9 MIC79050 MIC79050 Micrel, Inc. Protected Constant-Current Charger Another form of charging is using a simple wall adapter that offers a fixed voltage at a controlled, maximum current rating. The output of a typical charger will source a fixed voltage at a maximum current unless that maximum current is exceeded. In the event that the maximum current is exceeded, the voltage will drop while maintaining that maximum current. Using an MIC79050 after this type of charger is ideal for lithium-ion battery charging. The only obstacle is end of charger termination. Using a simple differential amplifier and a similar comparator and reference circuit, similar to Figure 1, completes a single cell lithium-ion battery charger solution. Figure 2 shows this solution in completion. The source is a fixed 5V source capable of a maximum of 400mA of current. When the battery demands full current (fast charge), the source will provide only 400mA and the input will be pulled down. The output of the MIC79050 will follow the input minus a small voltage drop. When the battery approaches full charge, the current will taper off. As the current across RS approaches 50mA, the output of the differential amplifier (MIC7300) will approach 1.225V, the reference voltage set by the LM4041. When it drops below the reference voltage, the output of the comparator (MIC6270) will allow the base of Q1 to be pulled high through R2. Zero-Output Impedance Source Charging Input voltage sources that have very low output impedances can be a challenge due to the nature of the source. Using the circuit in Figure 3 will provide a constant-current and constant voltage charging algorithm with the appropriate end-of-charge termination. The main loop consists of an op-amp controlling the feedback pin through the schottky diode, D1. The charge current through RS is held constant by the op-amp circuit until the output draws less than the set charge-current. At this point, the output goes constant-voltage. When the current through RS gets to less than 50mA, the difference amp output becomes less than the reference voltage of the MIC834 and the output pulls low. This sets the output of the MIC79050 less than nominal, stopping current flow and terminating charge. Lithium-Ion Battery Charging Single lithium-ion cells are typically charged by providing a constant current and terminating the charge with constant voltage. The charge cycle must be initiated by ensuring that the battery is not in deep discharge. If the battery voltage is below 2.5V, it is commonly recommended to trickle charge the battery with 5mA to 10mA of current until the output is above 2.5V. At this point the battery can be charged with constant current until it reaches its top off voltage (4.2V for a typical single lithium-ion cell) or a time out occurs. For the constant-voltage portion of the charging circuit, an extremely accurate termination voltage is highly recommended. The higher the accuracy of the termination circuit, the more energy the battery will store. Since lithium-ion cells do not exhibit a memory effect, less accurate termination does not harm the cell but simply stores less usable energy in the battery. The charge cycle is completed by disabling the charge circuit after the termination current drops below a minimum recommended level, typically 50mA or less, depending on the manufacturer’s recommendation, or if the circuit times out. Time Out The time-out aspect of lithium-ion battery charging can be added as a safety feature of the circuit. Often times this function is incorporated in the software portion of an application using a real-time clock to count out the maximum amount of time allowed in the charging cycle. When the maximum recommended charge time for the specific cell has been exceeded, the enable pin of the MIC79050 can be pulled low, and the output will float to the battery voltage, no longer providing current to the output. As a second option, the feedback pin of the MIC79050 can be modulated as in Figure 4. Figure 4. shows a simple circuit where the MIC834, an integrated comparator and reference, monitors the battery voltage and disables the MIC79050 output after the voltage on the battery exceeds a set vaue. When the voltage decays below this set threshold, the MIC834 drives Q1 low allowing the MIC79050 to turn on again and provide current to the battery until it is fully charged. This form of pulse charging is an acceptable way of maintaining the full charge on a cell until it is ready to be used. MIC79050-4.2BM MIC834 SD101 1/2 MIC7122 1/2 MIC7122 IN BAT FB GND EN 8.06M 4.7ìF R2=124k R3=1k R4=124k 0.01ìF VDD OUT INP GND R1=1k D1 221k 16.2k 16k 10k 5V RS=0.200Ù ICC= 80mV RS IEOC= 1.24V × R1 R2 × RS Figure 3.