Rainbow Electronics ATtiny43U - Manual (Page 44)

44
8048B–AVR–03/09
ATtiny43U
... where D is the duty cycle and T
S
the switching period of the boost converter. See “Boost Con-
verter Characteristics” on page 159 for limits. The steady-state duty cycle is calculated as
follows:
Overall efficiency of the boost converter is also affected by the resistance of the inductor.
8.6.2 Diode
It is recommended to use a Schottky diode with forward voltage, V
F
, and reverse leakage cur-
rent, I
R
, values as low as possible. This is because converter efficiency mainly depends on the
forward voltage of the diode when I
LOAD
is at maximum and V
BAT
is at minimum. It should be
noted that the reverse leakage current easily becomes a dominant factor, especially in Active
Low Current Mode. For reference, see converter current consumption during Low Current Mode
in
Table 20-7 on page 159 .
The diode is subject to peak currents the same magnitude as the inductor. See “Inductor” on
page 43 .
It should be noted that reverse leakage current is a highly temperature dependent variable.
8.6.3 Input Capacitors
A voltage drop occurs between the voltage source and inductor L
1
because voltage sources are
not ideal and tracks have a non-zero resistance. The voltage drop is application specific and
depends on the quality of the voltage source, inductor current, track size and track length.
Capacitor C
1
counteracts the voltage drop by providing energy to the inductor during high peak
currents.
The highest inductor peak currents are reached at highest load current and lowest V
BAT
of the
application. The input capacitor stabilises the input voltage and guarantees stable operation at
all load currents. The size of the capacitor can be decreased if load currents remain low, or if a
voltage supply with low internal resistance is available. Also, a good low-pass filter design (see
section
“RC Filter” below) reduces the size requirements of the capacitor.
8.6.4 RC Filter
A secondary input capacitor, C
2
, and a series input resistor, R
1
, are recommended. Together
they form a Low-Pass Filter (LPF), the purpose of which is to reduce the voltage ripple at the
V
BAT
pin. The corner frequency of the filter can be calculated as follows:
Component values are application specific and depend on the stability of the supply voltage. The
LPF reduces voltage ripple at the V
BAT
pin and helps to prevent the boost converter unintention-
ally entering Stop Mode.
D
V
CC
V
BAT
-------------
1
2
--- – ⎝ ⎠
⎛ ⎞
I
LOAD
×
=
f
LPF
1
2
π ×
R
1
×
C
2
Ч
--------------------------------------- =