EDN -- 06.06.96 Step-up/step-down current source charges batteries

Design Ideas: June 6, 1996

Step-up/step-down current source charges batteries

Michael Keagy
Maxim Integrated Products, Sunnyvale, CA

For battery charging, the highly efficient step-down (buck) configuration is usually the topology of choice. However, a different approach is necessary if the following conditions prevail: The supply voltage is less than the battery voltage, or, even worse, the supply voltage ranges above and below the battery voltage. The charger may need to accommodate one of several voltage sources, according to which is active. The charger may need to charge batteries with different cell counts. The circuit in Figure 1, which can charge one to 15 cells from an input of 4 to 15V, meets all of these requirements.

The topology in Figure 1 is the single-ended, primary-inductance converter, which is notable for its step-up/step-down capability. The controller (IC₁) usually regulates an output voltage. However, in this case, the resistive dividers at pin 3 keep the feedback unsatisfied, which causes the system to produce current pulses at a level determined by the IC's current-limit circuitry. To regulate charging current, op amp IC₂ adjusts Q₁'s current limit. IC₂ does this by comparing R₂'s voltage, which is proportional to charging current, with a voltage derived from the reference in IC₁. S₁ and S₂ let you set the level of charging current.

The maximum Q₁ current of 4A that R₁ sets is within the capability of L₁ but allows some saturation and heating. If this peak inductor current is insufficient, I_OUT falls gracefully short of the desired maximum value of 1A. If V_IN is high and V_OUT is low, you can obtain more charging current by changing resistor values at the op amp's inverting input. Otherwise, higher current requires that you set a higher peak current by lowering R₁. In that case, L₁, L₂, C₁, and C₂ must be larger to withstand the higher currents.

The resistor values connected at pin 3 of IC₁ set a maximum output voltage of 28V across the battery to limit the voltage stresses on Q₁, C₁, C₂ and D₁. You can extend this voltage by adjusting the resistor values. But, note that Q₁ and D₁ must withstand slightly more voltage than V_IN+V_OUT, and the coupling capacitor C₁ must withstand V_IN. The full charging current flows through C₁, so be sure that any substitutes can handle the required voltage and the ripple current. C₁ and C₂ are nonpolarized ceramic capacitors, but you should heed the polarities the figure shows if you substitute polarized capacitors.

The maximum V_IN is about 15V. This value can be higher if you limit the supply voltage applied to IC₁'s pin 2. Either add a linear regulator for this purpose, or replace the MAX770 with a MAX773, which takes its power from a built-in shunt regulator. Note that any coupling between L₁ and L₂ assumes the polarities shown by the dots, but circuit operation does not depend on such coupling. (DI #1874)