UC2906 / UC3906 dual level float charger configurator

This page aims to simplify the design process for a UC2906 or UC3906-based lead-acid battery charger circuit. Only a handful of resistors need to be calculated to configure the IC, but the math can be somewhat tedious to complete by hand.

This configurator is intended to be used with the dual level float charger circuit, as seen on page 4 of the chip's datasheet:

The circuit uses a PNP transistor as the high-current pass device, and also includes a trickle charging stage to handle deeply discharged batteries.

Limited error checking is implemented in this tool, and the selection of components is ultimately left up to the user. A summary of resistor values is shown at the end of this page, along with some hints for choosing the PNP pass device. Multipliers (k, M, etc.) are not supported.

Fill in the supply voltage, maximum charging current, and the various battery voltage levels. Maximum supply voltage is 40V, and all other voltages must be lower than it. Maximum charging current is only limited by the properties of the pass device and the current-limiting resistor R_s. Unless the maximum charging current is very low, the pass device will most likely require a heatsink.

The design procedure on page 5 of the datasheet starts with the selection of the divider current I_d. Resistor R_c is then calculated based on I_d and a voltage reference of 2.3V. With this tool you can set either I_d or R_c. The other value is then automatically calculated. The datasheet recommends an I_d value between 50µA and 100µA.

The values for resistors R_a, R_b and R_d are then calculated using I_d. Because their values most likely end up not aligning with any standard resistor sizes, you can choose available resistors that closely match the calculated values by typing them in the selected column below.

Lastly, configure the trickle charging circuit. Input either the desired trickle charging current in mA or set the trickle charge resistor directly. The absolute maximum current is 40mA.