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Power supply for the battery charger is shown in Appendix 2.

## Atmel AVR Controller

AT90S with 14 kHz Buck Converter The 90S battery charger design uses an external op-amps to amplify the voltage for the current measurement.

This ensures the highest accuracy for the battery mea- surement. The charger is capable of communicating with a PC, which can be used to monitor charging parameters and to debug the charging algorithm. These charge algorithms include fast-charge mode and a top-off trickle-charge to gain minimum charge time with maximum battery capacity.

The switching transistor is connected to an inductor, a diode and a capacitor see Figure 3. An additional diode prevents the battery from supplying voltage into the microcontroller when the power is disconnected. When the switching transistor is on illustrated by a switch on the figures below the current will flow like Figure 3A illus- trates. The capacitor is charged from the input via the inductor the inductor is also charged up.

When the switch is opened Figure 3Bthe inductor will try to maintain its current-flow by inducing a voltage. The current flows through the diode and the inductor will charge the capacitor. Then dataasheet cycle repeats itself. If the duty cycle is decreased, by shorter on time, longer off time, the voltage will decrease.

If the satasheet cycle is increased longer on timer, shorter off timethe voltage will increase. The voltage reference is common for both battery charger designs Battery Temperature Temperature is measured by a at90s443 temperature coefficient NTC resistor.

The NTC is part of a voltage divider, which is powered by the reference voltage. The resolution in respect to the voltage measured across the NTC is the same as for the voltage measurement circuit.

The steps can be calculated by the following equation: The NTC resistance does not follow a linear curve, which makes it difficult to calculate the temperature from the ADC value. Using a table to look up the temperature solves this see Table 2.

### AT90SAI IC MCU 4K 8MHZ A/D IT 32TQFP Atmel datasheet pdf data sheet FREE from

The table indicates the steps equal to 0. H will make it easy to implement any NTC resistor. The ATtiny15 battery charger design assumes that the linearity of the ther- mistor is sufficient to detect a temperature increase. Therefore, it uses a constant compare value to monitor the temperature. Parameters for Layout Oscillator frequency: In order to select a suitable measurement range for the charger, decide how many battery cells and what type of batteries to charge, select a suitable input voltage V1 – V2 and scale resistors for the voltage measurement.

The op-amp circuit for measuring the battery voltage is an ordi- nary differential op-amp circuit. The equation for the output voltage from the op-amp circuit is shown below.

The output voltage VBAT2 from the op-amp has to be within this range:. The maximum charge voltage will be: Charge Current The charge current is measured by sensing the voltage over a 0. This voltage is amplified by the factor: The op-amp output voltage is therefore: Ar90s4433 maximum current that can be measured is: This gives a resolution of: The step number for a given current can now be calculated from: The current from a certain step number is: The reference design is shipped without resistors for dividing down the voltage of the battery.

This limits the maximum voltage to 3. To use higher voltages, simply add the required resistors to divide down the voltage datashet the Calculation at09s4433 the resistors are described at the end of this section.