LP3950 Datasheet, PDF (25/31 Pages) National Semiconductor (TI) – Color LED Driver with Audio Synchronizer

Recommended External
Components
OUTPUT CAPACITOR, C_OUT
The output capacitor C_OUT directly affects the magnitude of
the output ripple voltage. In general, the higher the value of
C_OUT, the lower the output ripple magnitude. Multilayer ce-
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_OUT ripple than the higher
ESR tantalums of the same value. At the higher loads, the
ceramics offer a slightly lower V_OUT ripple magnitude than
the tantalums of the same value. However, the dv/dt of the
V_OUT ripple with the ceramics is much lower that the tantal-
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_IN directly affects the magnitude of the
input ripple voltage and to a lesser degree the V_OUT ripple. A
higher value C_IN will give a lower V_IN ripple. Capacitor volt-
age rating must be sufficient, 10V is recommended.
rent (A1.0A). Schottky diodes with a low forward drop and
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, 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
(A1.0A). 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 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.
OUTPUT DIODE, D₁
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-
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% (Note 19)
kΩ
nF Ceramic, X5R
µH Shielded, Low ESR, I_SAT A1.0A
V Schottky Diode
User Defined
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_DD pins of the integrated circuit.
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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