44
8048C–AVR–02/12
ATtiny43U
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, R1, are optional but 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 stabilize ADC measurement.
Capacitor C
2 should be located close to the device.
8.6.5
Output Capacitors
An output capacitor, C
3, is required to keep the output voltage stable at times when energy is
transferred to the inductor. It is recommended to use a capacitor with high capacitance and low
Equivalent Series Resistance, ESR. A large capacitance helps to reduce the voltage ripple at
the output and a low ESR reduces voltage ripple and helps to keep the temperature of the
capacitor within limits.
The recommended capacitance at a given, steady-state load is calculated as follows:
... where T
S is the switching period of the boost converter, VPP is the allowed voltage ripple and
The recommended ESR is calculated as follows:
A secondary output capacitor, C
4, is recommended and should be placed close to the device.
f
LPF
1
2
π
×
R
1
×
C
2
×
---------------------------------------
=
C
OUT
I
LOAD
T
S
D
×
V
PP
--------------------------------------
=
ESR
V
PP
I
PEAK
---------------
≤