ATtiny43U datasheet(38/182 Pages) ATMEL

38 / 182 page

8048B–AVR–03/09

ATtiny43U

8.2.1

Stop Mode

The boost converter enters Stop Mode (see Figure 8-3 on page 37 for modes of operation) when

input voltage ,V

BAT, is below the shutdown voltage, VSTOP (see Table 20-7 on page 159). Alter-

natively, the boost converter enters Stop Mode when instructed by firmware to do so.

In this mode of operation the boost converter is not active and current consumption is decreased

to a minimum. This is in order to prevent battery discharge and to avoid battery damage.

The voltage at the converter output, V

CC, has no effect in this mode. The converter monitors the

voltage of the input pin, V

BAT, and waits for it to rise above the start voltage, VSTART (see Table

20-7 on page 159). When there is sufficient voltage at the input the converter exits Stop Mode

and enters Start Mode.

8.2.2

Start Mode

The converter switches from Stop Mode to Start Mode (see Figure 8-3 on page 37) as soon as

the input voltage, V

BAT, goes above the start voltage, VSTART (see Table 20-7 on page 159).

Alternatively, the boost converter switches from Active Mode to Start Mode when the output volt-

age drops below V

BOOST.

In this mode of operation the boost converter pumps up the V

CC voltage by switcing at a 50%

duty cycle and high frequency, until it reaches V

BOOST.

All loads should be disconnected during this stage. The boost converter is designed to remain in

Start Mode for a short moment, but it is optimised to bring the microcontroller on line, only. If

there are additional loads connected to the V

CC pin the boost converter may not reach the VCC

voltage required to go into Active Mode.

8.2.3

Active Mode

The converter enters Active Mode of operation (see Figure 8-3 on page 37) when both input and

output voltages are sufficiently high. This means that V

BAT is above VSTART and VCC is above

BOOST. If input voltage drops below VSTOP or output voltage drops below VBOOST the converter

will exit Active Mode. Alternatively, firmware can force the boost converter to exit Active Mode

and enter Stop Mode.

In this mode of operation, the boost converter keeps V

CC within limits given in Table 20-7 on

page 159 by constantly adjusting the duty cycle between energy charge and discharge phases.

The duty cycle is affected mainly by input voltage, V

BAT, load current, ILOAD, and temperature.

By default, the boost converter operates in Active Regulated Mode but when load current drops

sufficiently low it will enter Active Low Current Mode, as explained in “Output Voltage versus

Load Current” . In Active Low Current Mode current consumption is minimised on the expense of

output voltage regulation.

8.2.4

Examples

Figure 8-4 illustrates operating modes and input and output voltages of the boost converter. As

input voltage, V

BAT, rises above VSTART (see “Boost Converter Characteristics” on page 159) the

converter enters Start Mode and output voltage, V

CC, begins to rise. At VBOOST output voltage

the converter exits Start Mode and goes into Active Mode. When output voltage exceeds the

power-on threshold V

POT (see “System and Reset Characteristics” on page 158) the microcon-

troller is released from reset.

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