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NCL30100
Application Design Example:
A typical step down application will be used to illustrate the basic design process based on nominal design parameters:
? Input voltage: V in ? 12 Vac (12 V dc after the bridge)
? Nominal LED current: 700 mA (rms)
? LED ripple : 120 mA (peak ? to ? peak)
? V LED : 3.2 V
? Freewheel diode V f : 0.5 V
? Target Switching Frequency: 450 kHz
Dimming using PWM signal 1 kHz with duty cycle 0 – 99%
Vac
D4
12 V
R2
A
D5
D6
C1
NCL30100
R1
D3
D1
Vac
D7
R3
R5
C2
IC1
CS DRV
GND VCC
CT
IVC
D2
Q1
Q2
L3
K
R4
C5
C3
DIMM
0/5 V
R9
R8
Figure 25. Example Design Schematic
t off
t ON ) t OFF
T +
+ 2.222 m s
+
f op
450 @ 10 3
dt I ripple
+ ^ 48.3 m H
I IVC +
^ 7.91 m A
+
Note this simplified step ? by ? step design process neglects
any parasitic contribution of the PCB.
First, we need to determine the nominal t ON /t OFF ratio:
t on V LED ) V f 3.2 ) 0.5   3.7
+ + + (eq. 7)
V in * V LED 12 * 3.2 8.8
Next the typical duty cycle (DC) will be calculated:
t ON 3.7
DC + + + 0.296 (eq. 8)
3.7 ) 8.8
Target switching frequency is set at 450 kHz, now we need
to determine the period:
1 1
(eq. 9)
Combination the previous equation we can calculate the t ON
and t OFF durations:
t ON + DC @ T + 0.296 @ 2.222 @ 10 ? 6 + 658 ns (eq. 10)
t OFF + (1 * DC) @ T + (1 * 0.296) @ 2.222 @ 10 ? 6
+ 1.564 m s
Now all the parameters are defined to calculate inductor
value:
di @ L V IN * V LED @ t ON
V + ? L +
(eq. 11)
(12 * 3.2) @ 658 @ 10 ? 9
0.12
A standard value 47 m H is chosen.
Next the CT capacitor can be calculated, but we need to
first determine the IVC current which can be simply
calculated.
V 12
R ) R IVC 1.5 @ 10 6 ) 17 @ 10 3 (eq. 12)
The IVC current controls the dependence of the peak
current to the input voltage. If the input voltage is well
regulated, the IVC pin should be grounded. The value for
IVC resistor should be chosen based on graphs below.
Note as well that the IVC can be used to implement analog
dimming since increasing the current into IVC pin will
decrease the I peak of the LED).
http://onsemi.com
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