Rainbow Electronics ATtiny43U - Manual (Page 40)

40
8048B–AVR–03/09
ATtiny43U
As current consumption goes below I
MS
(see Figure 8-5 ) the converter goes from Active Regu-
lated Mode to Active Low Current Mode. After this, the more the load current is decreased the
longer the discharge time of the output capacitor, i.e. the time when the converter is not switch-
ing, will be. Similarly, the charge time, i.e. the time when the converter is switching, will be
shorter. Note that in Active Low Current Mode only the last part of the rising/switching slope is
changed.
Charge time can be minimised by forcing the device into Full Duty Cycle mode of operation (see
“Full Duty Cycle” on page 41 ).
When load current increases above I
MS
the converter goes back to Active Regulated Mode.
8.3 Output Voltage versus Load Current
The output voltage of the boost converter depends on the amount of load and method of duty
cycle control.
In Active Mode the boost converter operates in one of two sub-modes called (Active) Regulated
Mode and (Active) Low Current Mode. In Regulated Mode the emphasis is on maintaining a sta-
ble output voltage, while in Low Current Mode the emphasis is on minimising current
consumption. The converter always enters Active Regulated Mode at first and switches to Low
Current Mode automatically when required but it is possible to design the application such that
the converter always stays in Active Regulated Mode.
The boost converter goes from Active Regulated Mode to Active Low Current Mode when the
duty cycle reaches its minimum and the output voltage reaches its maximum. At this point the
converter stops switching and the output voltage starts to fall. The converter starts switching
again when the output voltage has fallen to the low limit defined for Low Current Mode. If load
current increases sufficiently the converter will go back from Active Low Current Mode to Active
Regulated Mode. See
Figure 21-1 on page 165 .
The boost converter goes back to Start Mode if output voltage drops below V
BOOST
, and starts
over from Stop Mode if input voltage drops below V
STOP
, or when instructed by firmware to do
so.
8.3.1 Active Regulated Mode
This is the default method of operation in Active Mode. The converter will remain in this mode
provided that load current is sufficiently high. See
“Active Low Current Mode” on page 41 .
In this mode of operation the output voltage is constantly regulated. This means a stable output
voltage with a low amplitude, high frequency ripple superimposed. See
Figure 8-6 and Table 20-
7 on page 159 .
The firmware can instruct the converter to leave this mode and enter Stop Mode. See “Software
Control of Boost Converter” on page 42 .