ATtiny43U datasheet(42/182 Pages) ATMEL

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8048B–AVR–03/09

ATtiny43U

Figure 8-7.

Typical Output Voltage of Boost Converter at Constant Full Duty Cycle.

See section “Software Control of Boost Converter” on page 42 for instructions on how to turn on

and off duty cycle control.

The use of Full Duty Cycle mode is recommended only at low load currents.

8.4

Overload Behaviour

The output is considered overloaded when the load current, I

LOAD, exceeds the maximum given

in Table 20-7 on page 159. During an overload condition the boost converter operates at maxi-

mum duty cycle and can no longer regulate V

CC. If the overload condition prevails the output

voltage will drop as load current increases. If V

CC drops below its minimum level the converter

will switch to Start Mode.

In Start Mode the converter has a low load current capability, which means nearly all overload

current will be drained straight from the battery (or other power source) via the inductor and the

diode. The resistance of the inductor is typically very low and, provided the voltage of the power

source remains constant, the output voltage during overload will stabilise to battery voltage,

BAT, minus the forward voltage drop, VF, of the diode used.

8.5

Software Control of Boost Converter

The boost converter is an independent hardware module that requires no interaction by the

microcontroller, although some features can be controlled by firmware. Features that can be

controlled by firmware are described in the following sections.

8.5.1

Stopping the Boost Converter

The device firmware can stop the boost converter on demand. When issued a stop signal, the

boost converter will exit Active Mode and enter Stop Mode, as illustrated in Figure 8-3 on page

37. This procedure allows the device to read true battery voltage using the on-board ADC,

assess if the voltage is sufficient for the selected battery chemistry and then control the boost

converter accordingly.

VCC

FDC = ON

FDC = OFF

DUTY

CYCLE

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