Rainbow Electronics ATtiny43U - Manual (Page 38)

38
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, V
STOP
(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, V
START
(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, V
START
(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 V
CC
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 V
START
and V
CC
is above
V
BOOST
. If input voltage drops below V
STOP
or output voltage drops below V
BOOST
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, I
LOAD
, 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 V
START
(see “Boost Converter Characteristics” on page 159 ) the
converter enters Start Mode and output voltage, V
CC
, begins to rise. At V
BOOST
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.