Microcontroller controls multichemistry battery charger

Kelly Flaherty, National Semiconductor, Santa Clara, CA -- EDN, 9/28/2000

Figure 1 is a generalized block diagram of a multichemistry battery charger. A COP8ACC5 µC handles its key charging features. The µC is available in a 20-pin (15 I/O pins) SOIC or a 28-pin (23 I/O pins) SOIC/DIP. It contains 4 kbits of internal ROM. The controller's A/D inputs monitor the battery-voltage pin, ID pin, and thermistor pin. For more complex charging systems, you can add external EEPROM via the Microwire serial interface. Such an external EEPROM might be useful for storing battery-specific charge history, a battery-specific look-up table for more accurate charging, or both. The LP2950 doubles as a low-dropout-voltage regulator for the µC and as a ±0.5% reference for the charge-control block. The charge-control block is basically a constant-voltage, constant-current regulator, as the voltage-versus-current curve in Figure 1 shows. The µC reads the battery pack's ID pin and adjusts the circuit accordingly. If the battery is a lithium-ion type, the charge-control block adjusts the fast-charge rate according to the battery's capacity rating and adjusts the constant voltage to the critical maximum voltage for lithium-ion. If the battery is nickel-based, similar adjustments take place. However, the voltage adjustment is to a level greater than the maximum battery voltage that is still low enough to accommodate the power dissipation of the pass transistor.

Figure 2 shows a possible implementation of the charge-control block. It is an adjustable constant-current, constant-voltage regulator under control of a µC. The switches can be analog switches, such as the CD4066; discrete transistors, such as the 2N3904; or FETs, such as the 2N7002. The default setting (switches open) is 4.2V and 0.5A. When S₁ closes, current regulation increases by the change in equivalent resistance (R_EQUIV): I_CHRG=(V_REF×R₁)/(R_EQUIV×R_SENSE). S₁ switches in R₂, resulting in the doubling (to 1A) of the default regulated current. Closing S₂ similarly increases the level of the regulated voltage from the LM3420 lithium-ion charge controller. The LM3420 contains an error amplifier, a precision voltage reference, and a trimmed voltage divider that sets the regulated voltage to within ±0.5%. The IC is available in five fixed voltage levels that correspond to 4.2V per cell for one, two, three, and four cells. The Comp pin of the LM3420 switches an external resistor, R₃, in parallel with one of the internal divider resistors, and results in a regulated voltage of 7.2V. Q₁ provides a disconnect between the battery and the LM3420 upon removal of the input voltage. D₁ and D₂ act as an exclusive-OR gate for current regulation of voltage regulation. When V_REG is reached, D₂ overrides D₁.