Recommended External

Components

OUTPUT CAPACITOR, C

OUT

The output capacitor C

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

C

ramic capacitors with low ESR (Equivalent Series Resis-

tance) are the best choice. At the lighter loads, the low ESR

ceramics offer a much lower V

ESR tantalums of the same value. At the higher loads, the

ceramics offer a slightly lower V

the tantalums of the same value. However, the dv/dt of the

V

ums under all load conditions. Capacitor voltage rating must

be sufficient, 10V is recommended.

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 is recommended.

OUTPUT DIODE, D

1

A Schottky diode should be used for the output diode. To

maintain high efficiency the average current rating of the

schottky diode should be larger than the peak inductor cur-

rent (

fast switching speeds are ideal for increasing efficiency in

portable applications. Choose a reverse breakdown of the

schottky diode larger than the output voltage. Do not use

ordinary rectifier diodes, since slow switching speeds and

long recovery times cause the efficiency and the load regu-

lation to suffer.

INDUCTOR, L

1

LP3950’s high switching frequency enables the use of a

small surface mount inductor. A 4.7 µH shielded inductor is

suggested for 2.0 MHz switching frequency. Values below

2.2 µH should not be used at 2.0 MHz. At lower switching

frequencies 4.7 µH inductors should always be used. The

inductor should have a saturation current rating higher than

the peak current it will experience during circuit operation

(

Ω ESR is suggested for high

efficiency. Open core inductors cause flux linkage with circuit

components and, thus, may interfere with the normal opera-

tion 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 SW pin as close to the

IC as possible. Examples of suitable inductors are TDK type

VLF4012AT- 4R7M1R1 and Coilcraft type MSS4020-

472MLD.

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

OUT

Output Capacitor from FB to GND

µF

Ceramic, X5R

C

IN

Input Capacitor from Battery Voltage to GND

µF

Ceramic, X5R

C

VDDIO

V

100

nF

Ceramic, X5R

C

VDDA

V

100

nF

Ceramic, X5R

C

1,2,3

Audio Input Capacitors

10

nF

Ceramic, X5R

R

T

Oscillator Frequency Bias Resistor

82

k

Ω

1% (Note 19)

R

SO

SO Output Pull-up Resistor

100

k

Ω

C

VREF

Reference Voltage Capacitor, between V

100

nF

Ceramic, X5R

L

1

Boost Converter Inductor

4.7

µH

Shielded, Low ESR, I

D

1

Rectifying Diode, V

0.3

V

Schottky Diode

RGB LED

User Defined

Red, Green, Blue or White LEDs

R

Current Limit Resistors

Note 19: Resistor RT tolerance change will change the timing accuracy of RGB block. Also the boost converter switching frequency will be affected.

PCB Design Guidelines

Printed circuit board layout is critical to low noise operation

and good performance of the LP3950. Bypass capacitors

should be close to the V

Special attention must be given to the routing of the switch-

ing loops. Lengths of these loops should be minimized. It is

essential to place the input capacitor, the output capacitor,

the inductor and the schottky diode very close to the inte-

grated circuit and use wide routings for those components.

Sensitive components should be placed far from those com-

ponents with high pulsating current. A ground plane is rec-

ommended.

The power switch loop (the switch is on) has the greatest

affect on noise generation. The loop is formed by the input

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