Design Ideas: June 22, 1995
The circuit in Fig 1 is a digitally programmable current source capable of sourcing currents as high as 2.55A. The circuit is intended for use in "smart chargers" (for NiCd or NiMH batteries containing as many as 10 cells) in applications that require fast recharge cycles. A smart charger contains a µP and sufficient memory to store exact charge/discharge profiles for different battery types and sizes. Many smart chargers "interrogate" a battery prior to charging and learn its specific needs from a ROM stored within the battery pack. The charger can thus safely charge different types of batteries from multiple manufacturers.
During the charge cycle, the charger constantly monitors the battery, compares its state with the optimum profile, and adjusts the charging current to hold it on target. This technique allows very fast charge cycles without harm to the battery-the battery never suffers from life-reducing overcharge. The fundamental building block for any smart charger is the current source used to charge the battery. The wish-list of features for this current source includes
In Fig 1, the LM2576 step-down (buck) dc-dc converter serves as a current source to provide battery-charging current. The buck-regulator configuration provides good power-conversion efficiency. The charger operates as a constant-current source by sensing the voltage drop across R9. The feedback loop holds this sense voltage constant. The DAC0832 D/A converter provides the dc voltage to the input of the error amplifier (IC4A), which sets the voltage the feedback loop locks in, thus setting the charging current.
R7 and R8 set the gain of the sensing amplifier IC4B such that a voltage of 0 to 1.22V from the output of the D/A converter produces a charging current of 0 to 2.55A in the battery. Because 255 programmed levels are available, the charging current can have any value from 0 to 2.55A in steps of 10 mA. (To obtain zero charging current, pull the ON pin of IC3 high, possibly with an open-collector output.)
Buck-regulator IC3 chops the input voltage at a switching frequency of 52 kHz and varies the pulse width as required by the load through the action of either the voltage- or current-control feedback loop. L1, C7, C9, and C10 make up the output filter, which converts the square wave at the SW pin of IC3 into a smooth dc voltage. Diode D5 is a blocking diode that prevents the battery from discharging upon the removal of input power. Diode D3 is the switching "catch" diode. Both diodes must be Schottky devices with at least 3A current rating.
R9, IC4, and associated components form the feedback loop that enables IC3 to function as a constant current source. The role of IC4B is to provide gain for the voltage developed across current-sense-resistor R9 to correspond with the output of D/A-converter IC2. Full-scale charging current (2.55A) must match the full-scale output of the D/A converter (1.22V) at the inputs of the current-error-amplifier IC4A. In typical operation, the D/A converter presents a voltage through R4 to the input of IC4A, and the feedback loop fixes the charging current at the set value. C8, C5, C4, R3, and C3 provide compensation for the current-control loop.
Although the charger operates as a constant current source in most cases, a voltage-control loop (comprising R5, R6, and D4) is present to prevent the output of the charger from flying high if the battery disconnects from the charger. With the values shown, the maximum output voltage under no-load conditions is about 18.5V. The voltage-control (from D4) and current-control (from D2) signals are in a wired-OR connection at the feedback pin of IC3. Either signal can thus control the pulse-width modulation of IC3 as determined by the amount of load current.
With a battery connected to the charger output, the current-control loop supplies the feedback signal to IC3 through diode D2. However, upon battery disconnection, no charge current flows, and IC4A holds its output low. This condition would cause IC3 to go to maximum duty cycle (and hence maximum output voltage) if D4 were not present.
The D/A-converter is configured in its "backwards" configuration, in which the R-2R resistor ladder operates in a voltage-dividing, rather than current-switching, mode. The output voltage at VREF varies between 0 and 1.225V in 255 steps of 4.8 mV, corresponding to charging currents of 0 to 2.55A. IC1 provides a constant 12V for IC2. As IC2 requires only a few milliamps to operate, the power dissipation in IC1 and IC2 is negligible.
Keep all leads and traces connected to the power ground (preferably a ground plane) as short as possible. Also, connect the ground lead of D3 as close as possible to the ground connection of C9. Tie the low-current signal-ground leads together, then connect them to power ground at a single point very near the ground connection of IC3. Keep the leads of R2 and C3 as short as possible, and connect them very near the pins of IC3. Connect the sense leads that drive IC4B directly to the ends of R9 to prevent errors from voltage drops in copper traces carrying the battery's return current.
Although the power-conversion efficiency of the circuit is high (84% with 25V input, 13V battery voltage, and 2.55A charging current), you'll need a small heat sink for IC3. Any heat sink whose thermal resistance is ó13øC/W is adequate, assuming a maximum ambient temperature of 60øC. Some examples include Thermalloy's 6032B, 7019B, 7020B, or 7021B; Aavid's 5903B or 5510B; or IERC's LATO127B4CB. (DI #1715)