ATTINY43U_1 Datasheet, PDF (41/182 Pages) ATMEL Corporation – 8-bit Microcontroller with 4K Bytes In-System Programmable Flash and Boost Converter

Figure 8-6. Typical Output Voltage of Boost Converter in Active Regulated Mode.
V_CC
ACTIVE REGULATED MODE
V_RPP
f_SW
8.3.2
8.3.3
t
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 V_BOOST (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.
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.
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