NCL30100 Datasheet, PDF (15/27 Pages) ON Semiconductor – Fixed Off Time Switched Mode LED Driver Controller

I_LED
I_peak
I_cs
t_d
(V_in-V_LED)
L
NCL30100
-V_LED- V_diode
L
Δi_L
t_on
t_off
Figure 28. A Current Error Between the Intended Peak Current and the Actual Peak Current
V
=
di ⋅
dt
L
⇒
I_delay
=
V⋅
L
t
=
8.8 ⋅ 215 ⋅ 10^--9
47 ⋅ 10^--6
≅ 0.0402 A
(eq. 18)
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:
  _{Rsense ⋅ Ipk − Idelay + Vth}
R_shift =
I_CS
(eq. 19)
= ^{0.1 ⋅ (0.76 − 0.0402) + 0.038} ≅ 2496 Ω
44.07 ⋅ 10^--6
This value of resistance can be a parallel combination of
2.7 kΩ and 30 kΩ.
To understand the operating junction temperature, we
calculate the die power dissipation:
  _{PDIE = VCC ⋅ 300 ⋅ 10}^--6 _{+ CMOSFET ⋅ VCC ⋅ fswitching}
= 12 ⋅ ^300 ⋅ 10^--6 + 560 ⋅ 10^--12 ⋅ 12 ⋅ 450 ⋅ 10^3
= 39.8 mW
(eq. 20)
Using the P_DIE, we can calculate junction temperature:
Temp_IC = T_A + P_DIE ⋅ R_θJA = T_A + 0.0399 ⋅ 178
= T_A + 7.1° C
(eq. 21)
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.
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