Rainbow Electronics ATtiny43U - Manual (Page 45)

45
8048B–AVR–03/09
Too high resistor values may lead to Start Mode failures. See “Boost Converter Component Val-
ues” on page 45 for component recommendations and limits.
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 frequency of the boost converter, V
PP
is the allowed voltage ripple
and D is the duty cycle, calculated as shown in “Inductor” on page 43 .
The recommended ESR is calculated as follows:
A secondary output capacitor, C
4
, is recommended and should be placed close to the device.
8.6.6 Summary
The table below summarises recommended component values for a typical application.
Note: 1. With these values the LPF provides a 32dB attenuation at the switching frequency of the boost
converter while permitting a supply voltage ripple of about ±200mV
2. Application specific limits may be tighter
C
OUT
I
LOAD
T
S
D
Ч Ч
V
PP
-------------------------------------- =
ESR
V
PP
I
PEAK
---------------
≤
Table 8-1. Boost Converter Component Values
Component Recommended Value Min (2) Max (2)
C
1
C = 4.7µF 1µF
C
2
C = 100nF (1)
C
3
C = 22µF, ESR < 100m
Ω
10µF
C
4
C = 100nF 100nF 100nF
D
1
I
R
= 1µA @ 25
°
C, V
F
= 0.5V @ 1A
L
1
L = 15µH ±20%, I
MAX
= 700mA, R < 150m
Ω
15µH 15µH
R
1
R = 680
Ω (1)
1k
Ω