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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

ATtiny15 BATTERY CHARGER

Now, letýs discuss the theory specific for the battery charger design based on ATtiny15. The 25.6-MHz oscillator frequency is generated with an on-chip PLL from a 1.6-MHz internal RC oscillator. The reference design is shipped without resistors for dividing down the voltage of the battery. This limits the maximum voltage to 3.67 V, making it suitable for one or two NiCd or NiMh batteries. To use higher voltages, simply add the required resistors to divide the voltage into the 0 to 3.67-V range. Figure 3 gives the parameters for the layout.

Figure 3ýThe ýtiny15 layout parameters can be seen here.

The charge voltage is measured directly on the positive battery pole. When a voltage higher than the reference voltage (3.67 V) is used to charge the battery, the charging voltage can be divided with two resistors to fit into the 0 to 3.67-V area. This input is also the negative input for the differential measurement of the battery charge current, as shown in Figure 4. The current is measured as the difference between the negative and positive input to the internal 20x gain stage. This voltage is measured over a 0.25-ohm shunt resistor. All measurements are done with 10-bit (1024-step) resolution.

The voltage resolution is decided by A_REF, which is determined by:

[13]

In order to select a suitable measurement range for the charger, decide how many battery cells and what type of batteries you want to charge. The ADC is capable of measuring the voltage range from A_GND to A_REF (3.67 V). The output voltage (V_ADC) from the voltage divider has to be within this range:

[14]

where V_ADC is the output voltage from the voltage divider to the AVR A/D, Vb is the battery voltage, Ra and Rb are the resistors used to scale down the battery voltage, Ra is equal to R8 in the reference design, and Rb is equal to R16 in the reference design.

Note that resistors R9 and R17 (for scaling down the voltage of the shunt resistors) must be equal to R8 and R16 (for scaling down the voltage measurement). The reference design uses R8 = R9 = 3.7 kilohms and R16 = R17 = 2.2 kilohms.

This gives a maximum charge voltage of:

[15]

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