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A BETTER BATTERY CHARGER

Part 2: Hardware and Software Implementation
by Thomas Richter

Start ý The Buck Converter ý Voltage Reference and Battery Temperature ý AT90S2333 Battery Charger ý ATtiny15 Battery Charger ý Charge Current ý Software Implementation ý User Settings ý Source Code ý INT Battery Function ý The Stable_ADC Function ý BC.H ý B_DEF.H ý Charge MethodýSLA.C ý Charge MethodýNiCd.C and NiMH.C ý Charge MethodýLilon.C ý Suggested Improvement ý Sources and PDF

THE BUCK CONVERTER

The ýtiny15 includes special features to make it specially suited for battery-charger applications. First, the internal 100-kHz PWM is connected to a buck converter. The high switching frequency and accuracy reduce the size of the external coil and capacitors, and test points were added to easily monitor the PWM output, voltage input, and current input.

The ATtiny15 also includes an internal gain stage that can amplify the differential voltage between two A/D channels. This eliminates the need for external op-amps. The charge current is measured as the differential between two A/D channels over a 0.25-ohm resistor.

To ensure the highest accuracy, the AT90S2333 battery charger design uses an external op-amp to amplify the voltage for the current measurement. The charger is capable of communicating with a PC, which then can be used to monitor charging parameters and debug the charging algorithm.

The battery charger circuit was designed to charge any of the four battery types (SLA, NiCd, NiMH, and Li-Ion) with the appropriate charge algorithm. These charge algorithms include fast-charge mode and a top-off trickle-charge to gain minimum charge time with maximum battery capacity.

The buck converter is similar for both the AT90S2333 (see schematic 1) and the ATtiny15 (see schematic 2). It is made up of one P-channel MOSFET switching transistor driven by the AVR via one bipolar NPN transistor. The switching transistor is connected to an inductor, a diode, and a capacitor (see Figure 1).

(a)

(b)

Figure 1ýHere you can see the buck converter switching schemeý(a) charging and (b) inducing voltage.

An additional diode prevents the battery from supplying voltage to the microcontroller when the power is disconnected. When the switching transistor is on (illustrated by a switch in Figure 1) the current will flow. The capacitor is charged from the input via the inductor (the inductor is also charged).

When the switch is opened (see Figure 1b), the inductor will try to maintain its current flow by inducing a voltage. The current flows through the diode and the inductor charges the capacitor. Then the cycle repeats itself. If the duty cycle is decreased, by shorter on time, longer off time, the voltage will decrease. If the duty cycle is increased (longer on time, shorter off time), the voltage will increase. The buck converter is most efficient when running on a duty cycle of 50%.

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