Rainbow Electronics ATtiny43U - Manual (Page 41)

41
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 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 .
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.
V
CC
ST
AR
T MODE
ACTIVE REGULATED MODE
t
f
SW
V
RPP