ATtiny43U datasheet(45/182 Pages) ATMEL

45 / 182 page

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, VPP 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

OUT

LOAD

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

ESR

PEAK

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

≤

Table 8-1.

Boost Converter Component Values

Component

Recommended Value

Min (2)

Max (2)

C = 4.7µF

1µF

C = 100nF

(1)

C = 22µF, ESR < 100m

10µF

C = 100nF

100nF

R = 1µA @ 25°C, VF = 0.5V @ 1A

L = 15µH ±20%, I

MAX = 700mA, R < 150mΩ

15µH

R = 680

Ω (1)

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