Datasheet
NCL30100
http://onsemi.com
13
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%
Figure 25. Example Design Schematic
C1
R3
R4
C2
C3
R2
R1
Q1
D1
L3
D2
12 V
NCL30100
IC1
Q2
D3
C5
R5
D4
D5
D7
D6
R8
R9
DIMM
0/5 V
A
K
Vac
Vac
CS
GND
CT
DRV
VCC
IVC
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
t
off
+
V
LED
) V
f
V
in
* V
LED
+
3.2 ) 0.5
12 * 3.2
+
3.7
8.8
(eq. 7)
Next the typical duty cycle (DC) will be calculated:
DC +
t
ON
t
ON
) t
OFF
+
3.7
3.7 ) 8.8
+ 0.296
(eq. 8)
Target switching frequency is set at 450 kHz, now we need
to determine the period:
T +
1
f
op
+
1
450 @ 10
3
+ 2.222 ms
(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 ms
Now all the parameters are defined to calculate inductor
value:
V +
di @ L
dt
å L +
ǒ
V
IN
* V
LED
Ǔ
@ t
ON
I
ripple
(eq. 11)
+
(
12 * 3.2
)
@ 658 @ 10
−9
0.12
^ 48.3 mH
A standard value 47 mH is chosen.
Next the CT capacitor can be calculated, but we need to
first determine the IVC current which can be simply
calculated.
I
IVC
+
V
R ) R
IVC
+
12
1.5 @ 10
6
) 17 @ 10
3
^ 7.91 mA
(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).