Li-ion battery charger adapts to different chemistries

Fran Hoffart, Linear Technology Corp, Milpitas, CA -- EDN, 9/2/1999

Rechargeable lithium-ion (Li-on) cells require a precise charging voltage for maximum performance. The battery chemistry, of which there are many, determines the optimum charge voltage. Two of the more common charging voltages are 4.1V±50 mV and 4.2V±50 mV per cell.

Consider the following situation: You have thousands of three-cell Li-ion battery chargers with an output voltage of 12.6V±1%. However, because of a battery-chemistry change, your chargers now need 12.3V±1%. The 12.6V output is well-regulated and has current limiting, but the voltage is 300 mV too high, and you cannot easily adjust it.

The circuit in Figure 1 can solve this problem by providing a constant 300-mV drop between V_IN and V_OUT at currents as high as 3A. The accuracy of the 300-mV drop is nearly as good as the accuracy of the input voltage, which in this case is approximately 1%. This circuit requires an input voltage that is fixed, regulated, and preferably current-limited. A precision voltage divider across the regulated input derives the 300 mV necessary for the circuit to operate, thus eliminating the need for an external voltage reference. The circuit also includes a logic-level low-quiescent-current shutdown. In shutdown, the series MOSFET, Q₁, is off, and the total circuit current drops to approximately 8 µA.

The circuit operates by developing a 300-mV reference voltage across R₁ and applying this voltage to the inverting input of the op amp. The noninverting input connects to the output side of the p-channel MOSFET. The resultant feedback loop forces the voltage across Q₁ to equal the voltage across R₁, which is 300 mV. In normal operation, the voltage drop across R₁ and Q₁ are equal in value but opposite in polarity, thus forcing the voltage between the two op-amp inputs to be 0V. C₁ provides stability, and C₂ and C₃ bypass the supply.

Although this circuit was intended to solve a Li-ion-charger problem, you can use the circuit for any application that needs to drop a constant voltage. Changing the appropriate resistor values allows the circuit to accommodate other input voltages or voltage drops. The voltage drop across R₁ determines the voltage drop between V_IN and V_OUT. For low input voltages of 5V, reduce R₂ to 470? to ensure adequate gate drive for Q₁. Q₁ has a low R_DS(ON) and comes in an SO-8 surface-mount package. Allow a minimum of 1 sq in. of pc-board copper around the eight leads for heat sinking. Higher voltage drops require additional copper area. For even higher power levels or higher currents, select a MOSFET in a TO-220 package and mount it to an appropriate heat sink. (DI #2408)