LP3936 Datasheet, PDF (18/20 Pages) National Semiconductor (TI) – Lighting Management System for Six White LEDs and One RGB or FLASH LED

WLED and CLED Driver Electrical Characteristics
Symbol
I_RANGE
I_MAX
I_LEAKAGE
I_MATCH
Parameter
Sink Current Range
Maximum Sink Current
Leakage Current
Sink Current Matching
(Note 15)
Conditions
Min
V_FB = 4.55V, Control 00h–FFh
(Note 14)
24
22
V_FB = 5V
I_SINK = 13 mA, between
WLED1 … 4 or WLED5 … 6
Typ
0–25.5
25.5
0.04
1.0
Max Units
mA
27
mA
28
mA
1
µA
4
%
Note 14: A minimum voltage, Dropout Voltage, is required on the WLED outputs for maintaining the LED current. The current reduction at lower voltages is shown
in the graph WLED Output Current vs. Voltage.
Note 15: Match % = 100% * (Max – Min)/Min
WLED Current Adjustment
WLED[7:0]
0000 0000
0000 0001
0000 0010
0000 0011
•
•
1111 1101
1111 1110
1111 1111
WLED Current
(Typical)
0
0.1
0.2
0.3
•
•
25.3
25.4
25.5
Units
mA
mA
mA
mA
•
•
mA
mA
mA
WLED Output Current vs Voltage
Temperatures −40˚C, +25˚C, +85˚C
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. It should be noted that with some capacitor types
the actual capacitance depends heavily on the capacitor DC
voltage bias.
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.
20081431
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 are the best choice. At the
INDUCTOR, L
The high switching frequency enables the use of the small
surface mount inductor. A 10 µH shielded inductor is sug-
gested. Values below 4.7 µH should not be used. The induc-
tor 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 efficiency.
Open core inductors cause flux linkage with circuit compo-
nents and interfere with the normal operation of the circuit.
This should be avoided. For high efficiency, choose an in-
ductor 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 OUT 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 (unshielded).
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