Datasheet
NCL30100
http://onsemi.com
15
Figure 28. A Current Error Between the Intended Peak Current and the Actual Peak Current
I
peak
I
cs
t
d
t
on
t
off
I
LED
Δi
L
(V
in
-V
LED
)
L
-V
LED
- V
diode
L
I
peak
I
cs
t
d
t
on
t
off
I
LED
(V
in
-V
LED
)
L
-V
LED
- V
diode
L
-V
LED
- V
diode
L
V +
di @ L
dt
å I
delay
+
V @ t
L
+
8.8 @ 215 @ 10
−9
47 @ 10
−6
(eq. 18)
^ 0.0402 A
V/L is simply the slew rate through the inductor and t
d
is
the internal propagation delay so the current overshoot from
target is approximately 40 mA as calculated in Equation 18.
All values necessary for R
shift
calculation are known, the
R
shift
value is described by this formula:
R
shift
+
R
sense
@
ǒ
I
pk
* I
delay
Ǔ
) V
th
I
CS
(eq. 19)
+
0.1 @
(
0.76 * 0.0402
)
) 0.038
44.07 @ 10
−6
^ 2496 W
This value of resistance can be a parallel combination of
2.7 kW and 30 kW.
To understand the operating junction temperature, we
calculate the die power dissipation:
P
DIE
+ V
CC
@
ǒ
300 @ 10
−6
) C
MOSFET
@ V
CC
@ f
switching
Ǔ
(eq. 20)
+ 12 @
ǒ
300 @ 10
−6
) 560 @ 10
−12
@ 12 @ 450 @ 10
3
Ǔ
+ 39.8 mW
Using the P
DIE
, we can calculate junction temperature:
Temp
IC
+ T
A
) P
DIE
@ R
qJA
+ T
A
) 0.0399 @ 178
(eq. 21)
+ T
A
) 7.1° C
A design spreadsheet to aid in calculating the external
components necessary for a specific set of operating
conditions is available for download at the
ON Semiconductor website.
For a low voltage AC input diode D3 is placed into the Vcc
line. Since Capacitor C3 is charged from a sinusoidal
voltage. If the input voltage approaches zero, the IC is still
supplied from C3. Due to this diode, the IC keeps the LED
driver operating even if the sinusoidal voltage is lower than
V
CC(min)
until V
in
is lower than V
LED
. The use of this diode
make sense only if a single LED is used and the converter is
supplied by sinusoidal voltage 12 Vac. For two LEDs in
series their forward voltage is almost as high as V
CC(min)
of
the IC.
Parasitic capacitance and inductance are presented in real
applications which will have an influence on the circuit
operation. They are depending on the PCB design which is
user dependent. The BOM, PCB and some plots are enclosed
for better understanding of the system behavior.