LP3950 Datasheet, PDF (28/38 Pages) National Semiconductor (TI) – Color LED Driver with Audio Synchronizer

LP3950
SNVS331C – NOVEMBER 2004 – REVISED APRIL 2013
www.ti.com
INDUCTOR, L₁
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 mΩ 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
C_VDD1
C_VDD2
C_OUT
C_IN
C_VDDIO
C_VDDA
C_1,2,3
R_T
R_SO
C_VREF
L₁
D₁
R_RX, R_GX, R_BX
Symbol Explanation
V_DD1 Bypass Capacitor
V_DD2 Bypass Capacitor
Output Capacitor from FB to GND
Input Capacitor from Battery Voltage to GND
V_{DD_IO} Bypass Capacitor
V_DDA Bypass Capacitor
Audio Input Capacitors
Oscillator Frequency Bias Resistor
SO Output Pull-up Resistor
Reference Voltage Capacitor, between V_REF and
GND
Boost Converter Inductor
Rectifying Diode, V_F @ Maxload
RGB LED
Red, Green, Blue or White LEDs
Current Limit Resistors
Value
100
100
10 ± 10%
10 ± 10%
100
100
10
82
100
100
4.7
0.3
Unit
Type
nF
Ceramic, X5R
nF
Ceramic, X5R
µF
Ceramic, X5R
µF
Ceramic, X5R
nF
Ceramic, X5R
nF
Ceramic, X5R
nF
Ceramic, X5R
kΩ
1% ⁽¹⁾
kΩ
nF
Ceramic, X5R
µH
Shielded, Low ESR,
I_SAT1.0A
V
Schottky Diode
User Defined
(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
capacitors should be close to the V_DD pins of the integrated circuit. Special attention must be given to the routing
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
Product Folder Links: LP3950
Copyright © 2004–2013, Texas Instruments Incorporated