ATtiny43U datasheet(41/182 Pages) ATMEL

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8048B–AVR–03/09

Figure 8-6.

Typical Output Voltage of Boost Converter in Active Regulated Mode.

8.3.2

Active Low Current Mode

The boost converter enters Active Low Current Mode from Active Regulated Mode when output

voltage reaches its maximum and duty cycle is at its minimum. In practice, this means that the

load current drops below a threshold. The threshold varies with converter input voltage and tem-

perature but a typical plot is shown in Figure 21-1 on page 165.

From Figure 21-1 on page 165 can be seen that at low input voltages (V

BAT typically below 1.0V)

and high load currents (I

LOAD typically above 0.6mA) the boost converter will never enter Low

Current Mode. Using Full Duty Cycle mode the boost converter can be forced to enter Active

Low Current Mode at input voltages lower than those shown in Figure 21-1 on page 165. See

“Full Duty Cycle” on page 41.

In Low Current Mode the boost converter stops switching and reduces current consumption to a

minimum, while still remaining active. Provided there are no external loads active the boost con-

verter enters Low Current Mode automatically when the microcontroller goes into Power Down

Mode (see “Sleep Modes” on page 31).

In this mode of operation the converter periodically reaches its duty cycle low limit. When this

happens the converter stops switching and the output voltage starts dropping. The converter

starts switching again when the output voltage has decreased to the low limit of Active Low Cur-

rent Mode. This results in a periodical pattern as illustrated in Figure 8-5 on page 39.

If the output voltage, V

CC, drops below VBOOST (due to an overload or a short circuit) the con-

verter goes back to Start Mode. In addition, the firmware can instruct the converter to leave this

mode and enter Stop Mode. See “Software Control of Boost Converter” on page 42.

8.3.3

Full Duty Cycle

By default, the boost converter keeps V

CC within limits by controlling the duty cycle of the switch-

ing waveform. It is possible to bypass the duty cycle regulation and lock the duty cycle at its

maximum, resulting in a V

CC voltage that quickly ramps up to the maximum limit and then starts

dropping when the boost converter enters Low Current Mode. See Figure 8-7, below.

VCC

ACTIVE REGULATED MODE

fSW

VRPP

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