ATtiny43U datasheet(44/182 Pages) ATMEL

44 / 182 page

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 VBAT 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, R1, are recommended. Together

they form a Low-Pass Filter (LPF), the purpose of which is to reduce the voltage ripple at the

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.

BAT

-------------

---

–

⎝⎠

⎛⎞

LOAD

LPF

---------------------------------------

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