LP3950

SNVS331C – NOVEMBER 2004 – REVISED APRIL 2013

www.ti.com

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 (1.0A). Less than 300 m

Ω ESR is

suggested for high efficiency. Open core inductors cause flux linkage with circuit components and, thus, may

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 SW pin as close to the IC as

possible. Examples of suitable inductors are TDK type VLF4012AT- 4R7M1R1 and Coilcraft type MSS4020-

472MLD.

Table 7. List of Recommended External Components

Symbol

Symbol Explanation

Value

Unit

Type

100

nF

Ceramic, X5R

100

nF

Ceramic, X5R

Output Capacitor from FB to GND

10 ± 10%

µF

Ceramic, X5R

Input Capacitor from Battery Voltage to GND

10 ± 10%

µF

Ceramic, X5R

100

nF

Ceramic, X5R

100

nF

Ceramic, X5R

Audio Input Capacitors

10

nF

Ceramic, X5R

Oscillator Frequency Bias Resistor

82

k

Ω

SO Output Pull-up Resistor

100

k

Ω

100

nF

Ceramic, X5R

GND

Boost Converter Inductor

4.7

µH

Shielded, Low ESR,

0.3

V

Schottky Diode

RGB LED

Red, Green, Blue or White LEDs

User Defined

Current Limit Resistors

(1)

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

of the switching 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 integrated circuit and use wide

routings for those components. Sensitive components should be placed far from those components with high

pulsating current. A ground plane is recommended.

The power switch loop (the switch is on) has the greatest affect on noise generation. The loop is formed by the

input capacitor, the inductor, the SW pin, the GND_BOOST pin and the ground plane, as shown by the dashed

line in Figure 31. The other switching loop, the rectifier loop, is formed by the input capacitor, the inductor, the

diode, the output capacitor and the ground plane, as shown by the dotted line. Arrange the components so that

the switching current loops curl in the same direction (see arrows in Figure 31). See also Application Note AN

1149, Layout Guidelines for Switching Mode Power Supplies.

28

Submit Documentation Feedback

Copyright © 2004–2013, Texas Instruments Incorporated

Product Folder Links: LP3950