Datasheet

NCL30100
http://onsemi.com
14
Figure 26. R
ivc
Impact versus V
in
for I
LED
=
700 mA Nominal
100
200
300
400
500
600
700
800
900
1000
7 8 9 101112131415161718
INPUT VOLTAGE (V)
I
LED
(mA)
0
100
200
300
400
500
600
7 8 9 10 11 12 13 14 15 16 17 18
INPUT VOLTAGE (V)
Figure 27. Rivc Impact versus V
in
for I
LED
=
350 mA Nominal
I
LED
(mA)
Rivc = 300k
Rivc = 510k
Rivc = 1 Meg
Rivc = 1.5 Meg
Rivc = 2.2 Meg
Rivc = 3.7 Meg
Rivc = infinity
Rivc = 300k
Rivc = 510k
Rivc = 1 Meg
Rivc = 1.5 Meg
Rivc = 2.2 Meg
Rivc = 3.7 Meg
Rivc = infinity
A value 1.5 MW was used since the input voltage has a
sinusoidal component due to the low voltage AC input and
desires to have a small bulk capacitance, thus compensating
for part of this variation.
The dependence of V
CT
on IVC current is described by the
following equation:
V
CT
+
−0.097 @ X
2
) 24.5 @ X ) 1358.1
976.8
[
V
]
(eq. 13)
Using the result from Equation 12 and put it to Equation 13
the V
CT
threshold will be calculated:
V
CT
+
−0.097 @ 7.91
2
) 24.5 @ 7.91 ) 1358.1
976.8
(eq. 14)
^ 1.58 V
The CT capacitance can be calculated using the equations
above:
I +
dv @ C
dt
å C
CT
+
I
CT
@
ǒ
t
OFF
* CT
delay
Ǔ
V
CT
(eq. 15)
+
50 @ 10
−6
@
ǒ
1.654 @ 10
−6
* 220 @ 10
−9
Ǔ
1.58
^ 45.8 pF
The intrinsic pin capacitance CT pin (~8 pF) in
conjunction with the dimming transistor (~10 pF) in this
schematic approximately 18 pF so this value must be
subtracted from the calculated value in Equation 15. The
calculated value is not standard, so the nearest value 33 pF
has been selected.
Now we can calculate the I
PK
of LED. The average value
is set to 700 mA and the target ripple is set at 120 mA, the
I
PK
equals 760 mA. R
shift
has been chosen to be as small a
voltage drop as possible to minimize power dissipation so an
R
shift
of 100 mW has been selected.
Before calculation of R
shift
we need to know the I
CS
current, which affects the offset on R
shift
. The I
CS
value
dependents on the IVC current and for IVC currents between
0 − 50 mA, it can be described by this formula:
I
CS
+ −0.75 @ IVC ) 50 @ 10
−6
[
mA
]
(eq. 16)
Therefore:
I
CS
+ −0.75 @ IVC ) 50 @ 10
−6
(eq. 17)
+ −0.75 @ 7.91 @ 10
−6
) 50 @ 10
−6
^ 44.07 mA
To calculate R
shift
it is necessary to know the I
pk
current
through the inductor. From the time that the current sense
comparator detects that the peak current threshold has been
crossed to the time that the external MOSFET switch is
turned off there is a propagation delay. Depending on the
value of the inductor selected (which is based on the target
switching frequency), there is a current error between the
intended peak current and the actual peak current, this is
illustrated in Figure 28.