Rainbow Electronics ATtiny43U - Manual (Page 42)

42
8048B–AVR–03/09
ATtiny43U
Figure 8-7. Typical Output Voltage of Boost Converter at Constant Full Duty Cycle.
See section “Software Control of Boost Converter” on page 42 for instructions on how to turn on
and off duty cycle control.
The use of Full Duty Cycle mode is recommended only at low load currents.
8.4 Overload Behaviour
The output is considered overloaded when the load current, I
LOAD
, exceeds the maximum given
in Table 20-7 on page 159 . During an overload condition the boost converter operates at maxi-
mum duty cycle and can no longer regulate V
CC
. If the overload condition prevails the output
voltage will drop as load current increases. If V
CC
drops below its minimum level the converter
will switch to Start Mode.
In Start Mode the converter has a low load current capability, which means nearly all overload
current will be drained straight from the battery (or other power source) via the inductor and the
diode. The resistance of the inductor is typically very low and, provided the voltage of the power
source remains constant, the output voltage during overload will stabilise to battery voltage,
V
BAT
, minus the forward voltage drop, V
F
, of the diode used.
8.5 Software Control of Boost Converter
The boost converter is an independent hardware module that requires no interaction by the
microcontroller, although some features can be controlled by firmware. Features that can be
controlled by firmware are described in the following sections.
8.5.1 Stopping the Boost Converter
The device firmware can stop the boost converter on demand. When issued a stop signal, the
boost converter will exit Active Mode and enter Stop Mode, as illustrated in
Figure 8-3 on page
37 . This procedure allows the device to read true battery voltage using the on-board ADC,
assess if the voltage is sufficient for the selected battery chemistry and then control the boost
converter accordingly.
V
CC
FDC = ON
FDC = OFF
FULL VAR
IAB
LE
DUTY
CYCLE