LP3936 Datasheet, PDF (21/30 Pages) National Semiconductor (TI) – Lighting Management System for Six White LEDs and One RGB or FLASH LED

LP3936
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WLED Current Adjustment
WLED[7:0]
0000 0000
0000 0001
0000 0010
0000 0011
•
•
1111 1101
1111 1110
1111 1111
SNVS259D – NOVEMBER 2003 – REVISED MAY 2013
WLED Current (Typical)
0
0.1
0.2
0.3
•
•
25.3
25.4
25.5
30
25
-40°C
20
+85°C
15
10
Set value = FFh
Set value = 80h
5
Set value = 40h
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
WLED VOLTAGE (V)
Figure 26. WLED Output Current vs Voltage
Temperatures −40°C, +25°C, +85°C
Units
mA
mA
mA
mA
•
•
mA
mA
mA
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 ceramic 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 recommended. 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 voltage 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 current (1A). 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 regulation to suffer.
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