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NCP5010
LED Current Selection
The feedback resistor (R FB ) determines the average
300
maximum current through the LED string. The control loop
regulated the current such that the average voltage at the FB
input is 500 mV (nom). For example, should one need a
20 mA output current in the primary branch, R FB should be
selected according to the following equation:
250
200
L = 10 m H
+ 500 mV + 25 W
RFB +
FBV
IOUT 20 mA
In white LED applications it is desirable to operate the
LEDs at a specific operating current as the color will shift
as the bias current is changed. As a result of this effect, it
is recommended to dim the LED string by a pulse width
modulation techniques. A low frequency PWM signal can
150
100
50
10
20
L = 15 m H
L = 22 m H
30 40 50
I OUT (mA)
60
V IN = 3.1 V
V IN = 4.2 V
70
80
be applied to the CTRL input and by varying the duty cycle
the brightness of the LED can be changed. To avoid any
optical flicker, the frequency must be higher than 100 Hz
Figure 24. Peak Inductor Currents vs. I OUT (mA)
@ 3 LEDs, 10.5 V
and preferably less than 1 kHz. Due to the soft?start
function set at 600 m s (nom) with higher frequency the
device remains active but the brightness can decrease.
Nevertheless in this case, a dimming control using a
filtered PWM signal (See Figure 33) can be used. Also for
DC voltage control the same technique is suitable and the
filter is takes away.
300
250
200
L = 10 m H
Inductor Selection
To choose the inductor there are three different electrical
parameters that need to be considered, the absolute value
of the inductor, the saturation current and the DCR. In
normal operation, this device is intended to operate in
Continuous Conduction Mode (CCM) so the following
equation below can be used to calculate the peak current:
150
100
50
10
L = 15 m H
L = 22 m H
20 30 40 50
I OUT (mA)
60
V IN = 3.1 V
V IN = 4.2 V
70
80
) IN
IPEAK +
IOUT
h (1 * D)
V D
2LF
Figure 25. Peak Inductor Currents vs. I OUT (mA)
@ 4 LEDs, 14 V
In the equation above, V IN is the battery voltage, I OUT is
the load current, L the inductor value, F the switching
frequency, and the duty cycle D is given by:
D + 1 * VIN
VOUT
300
250
h is the global converter efficiency which can vary with
load current (see Figure 3 thru Figure 8). A good
approximation is to use h = 0.8. Figure 24 ? Figure 26 are
a graphical representation of the above equations, as a
200
150
L = 10 m H
L = 15 m H
function of the desired I OUT , V IN , and number of LEDs in
series (V F = 3.5 V nominal). The curves are limited to an
I PEAK_MAX of 300 mA. It is important to analyze this at
worst case Vf conditions to ensure that the inductor current
100
50
10
20
L = 22 m H
30
40
50
60
V IN = 3.1 V
V IN = 4.2 V
70
80
rated is high enough such that it not saturate.
The recommended inductor value should range between
10 m H and 22 m H. As can be seen from the curves, as the
inductor size is reduced, the peak current for a given set of
I OUT (mA)
Figure 26. Peak Inductor Currents vs. I OUT (mA)
@ 5 LEDs, 17.5 V
conditions increases along with higher current ripple so it
is not possible to deliver maximum output power at lower
inductor values.
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