Recommended External

Components

OUTPUT CAPACITOR, C

The output capacitors C

nitude of the output ripple voltage. In general, the higher the

value of C

layer ceramic capacitors with low ESR are the best choice.

At the lighter loads, the low ESR ceramics offer a much

lower V

value. At the higher loads, the ceramics offer a slightly lower

V

However, the dv/dt of the V

much lower that the tantalums under all load conditions.

Capacitor voltage rating must be sufficient, 10V is recom-

mended

Some ceramic capacitors, especially those in small

packages, exhibit a strong capacitance reduction with

the increased applied voltage. The capacitance value

can fall to below half of the nominal capacitance. Too

low output capacitance can make the boost converter

unstable.

INPUT CAPACITOR, C

IN

The input capacitor C

input ripple voltage and to a lesser degree the V

higher value C

age rating must be sufficient, 10V or greater is recom-

mended.

OUTPUT DIODE, D

1

The output diode for a boost converter must be chosen

correctly depending on the output voltage and the output

current. The diode must be rated for a reverse voltage

greater than the output voltage used. The average current

rating must be greater than the maximum load current ex-

pected, and the peak current rating must be greater than the

peak inductor current (~1.6A at maximum load). A Schottky

diode should be used for the output diode. Schottky diodes

with a low forward voltage drop (V

speeds are ideal for increasing efficiency in portable appli-

cations. Do not use ordinary rectifier diodes, since slow

switching speeds and long recovery times cause the effi-

ciency and the load regulation to suffer. In Schottky barrier

diodes reverse leakage current increases quickly with the

junction temperature. Therefore, reverse power dissipation

and the possibility of thermal runaway has to be considered

when operating under high temperature conditions. Ex-

amples of suitable diodes are Diodes Incorporated type

DFLS220L, ON Semiconductor type MBRA210LT3 and Phil-

ips type PMEG1020.

INDUCTOR, L

1

The LP5527 high switching frequency enables the use of the

small surface mount inductor. A 4.7 µH shielded inductor is

suggested for 2 MHz switching frequency. The inductor

should have a saturation current rating higher than the peak

current it will experience during circuit operation (~1.7A at

maximum load). Less than 300 m

Ω ESR is suggested for

high efficiency. Open core inductors cause flux linkage with

circuit components and interfere with the normal operation of

the circuit. This should be avoided. For high efficiency,

choose an inductor with a high frequency core material such

as ferrite to reduce the core losses. To minimize radiated

noise, use a toroid, pot core or shielded core inductor. The

inductor should be connected to the SW1 and SW2 pins as

close to the I

TDK type VLCF5020T-4R7N1R7-1.

Table List of Recommended External Components

Symbol

Symbol Explanation

Value

Unit

Type

C

VDD1

V

100

nF

Ceramic, X5R

C

VDD2

V

100

nF

Ceramic, X5R

C

OUT1,2

Output Capacitors from FB to GND

µF

Ceramic, X5R, 10V

C

IN

Input Capacitor from Battery Voltage to GND

µF

Ceramic, X5R, 10V

C

VDDIO

V

100

nF

Ceramic, X5R

C

VDDA

V

4.7

µF

Ceramic, X5R, 6.3V

C

1,2

Audio Input Capacitors

47

nF

Ceramic, X5R

RT

Oscillator Frequency Bias Resistor

82

k

Ω

1%

RF

Flash Current Set Resistor for 400 mA Sink

Current

1200

Ω

1%

C

VREF

Reference Voltage Capacitor, between V

REF

and GND

100

nF

Ceramic, X5R

L

1

Boost Converter Inductor

4.7

µH

Shielded, low ESR,

I

SAT

~1.7A

D

1

Rectifying Diode, V

0.35

V

Schottky diode

Flash LED

User defined

LED1 to LED4

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