LP3933 Datasheet, PDF (15/18 Pages) National Semiconductor (TI) – Lighting Management System for Six White LEDs and Two RGB or FLASH LEDs

WLED and CLED Driver Electrical Characteristics
Symbol
I_RANGE
I_MAX
I_LEAKAGE
I_MATCH 1–4
Parameter
Sink Current Range
Maximum Sink Current
Leakage Current
Sink Current Matching (Note
17)
Conditions
V_FB = 5V, Control 00h–FFh
(Note 16)
V_FB = 5V
I_SINK = 13 mA, between
WLED1...4 or CLED1...2
Min
Typ
Max Units
0–25.5
mA
25.5
30
mA
0.03
1
µA
0.5
2.7
%
Note 16: A minimum voltage, Dropout Voltage, is required on the WLED and CLED outputs for maintaining the LED current. The current reduction at lower voltages
is shown in the graph WLED Output Current vs Voltage
Note 17: Match % = 100% * (Max − Min)/Min
Adjustment
WLED[7:0] or
CLED[7:0]
0000 0000
0000 0001
0000 0010
0000 0011
•
•
1111 1101
1111 1110
1111 1111
WLED or CLED
Current (Typical)
0
0.1
0.2
0.3
•
•
25.3
25.4
25.5
Units
mA
mA
mA
mA
•
•
mA
mA
mA
WLED or CLED Output Current vs Voltage
Temperatures −40˚C, 25˚C, 85˚C, 100˚C
ramic capacitors with low ESR 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 than the tantalums under all load conditions.
Capacitor voltage rating must be sufficient, 10V is recom-
mended.
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.
OUTPUT DIODE, D_OUT
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 (1A). Schottky diodes with a low forward drop and fast
switching speeds are ideal for increasing efficiency in por-
table 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.
20080523
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-
INDUCTOR, L
The LP3933’s high switching frequency enables the use of
the small surface mount inductor. A 10 µH shielded inductor
is suggested. Values below 4.7 µH should not be used. The
inductor should have a saturation current rating higher than
the peak current it will experience during circuit operation
(A1A). Less than 300 mΩ ESR is suggested for high effi-
ciency. 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 SW pin as close to the IC as
possible. Examples of suitable inductors are TDK types
LLF4017T-100MR90C and VLF4012AT-100MR79 and Coil-
craft type DO3314T-103.
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