Step-up/step-down current source to charge the battery

For battery charging, the efficient step-down (buck) approach is the usual choice. However, special cases require a different approach, such as when the supply voltage is less than the battery voltage or (worse) when the supply voltage varies between above and below the battery voltage. The charger may need to accommodate multiple voltage sources, depending on which one is working, and it may also need to charge batteries with different numbers of cells. The circuit in Figure 1 meets all of these requirements, charging from 1 to 15 cells from a 4V to 15V input.

Figure 1. This common battery charger is built around a controller IC that produces an average current whose amplitude is regulated by an op amp.

The topology shown is a single-ended, primary-inductor converter (SEPIC) with step-up/step-down capability. The controller (IC1) normally regulates the output voltage, but in this case, the resistor divider at pin 3 maintains insufficient feedback, causing the system to generate current pulses whose magnitude is determined by the current-limiting circuit. To regulate the charge current, the op amp adjusts the current limit of Q1 by comparing the voltage across R2 (proportional to the charge current) with a reference voltage from within IC1. S1 and S2 set the charge current magnitude.

The maximum Q1 current (4A) set by R1 is within the capacity of L1, but it allows for some saturation and heating. If the peak inductor current is insufficient, IOUT will drop slightly and fall short of the required maximum value (1A). If VIN is high and VOUT is low, a larger charging current can be obtained by changing the resistor at the inverting input of the op amp. Larger currents require a larger peak current to be set by reducing R1. In this case, L1, L2, C1, and C2 must be larger to handle the larger current.

To limit the voltage across Q1, C1, C2, and D1, the resistor connected to pin 3 of IC1 sets the maximum output voltage across the battery to 28V. This voltage can be extended by adjusting the resistors, but be aware that Q1 and D1 must withstand a voltage slightly greater than VIN + VOUT, and the coupling capacitor (C1) must withstand VIN. The entire charging current flows through C1, so be sure that any replacement components meet the required voltage and ripple current. C1 and C2 are nonpolarized ceramic capacitors. If replaced with polarized capacitors, maintain the polarity shown.

As shown, VIN has a maximum value of about 15V. This value can be higher if the supply voltage applied to IC1 (pin 2) is limited. To achieve this, a linear regulator can be added, or the MAX773 can be used instead of the MAX770, which derives its power from an internal shunt regulator. Note that any coupling between L1 and L2 assumes the polarity shown by the dots, but the operation of the circuit does not depend on this coupling.