NUD4011
http://onsemi.com
5
APPLICATION INFORMATION (continued)
Design Guide for AC Applications
1. Define LED’s current:
a. I
LED
= 30 mA
2. Define V
in
:
a. Per example in Figure 5, V
in
= 120 Vac
3. Define V
LED
@ I
LED
per LED supplier’s data
sheet:
a. Per example in Figure 6,
V
LED
= 3.0 V (30 LEDs in series)
V
LEDs
= 90 V
4. Calculate Resistor Value for R
ext
:
The calculation of the R
ext
for AC applications is
totally different than for DC. This is because
current conduction only occurs during the time
that the ac cycles’ amplitude is higher than V
LEDs
.
Therefore R
ext
calculation is now dependent on the
peak current value and the conduction time.
a. Calculate for V
LEDs
= 90 V:
V = V
peak
Sin
90 V = (120 2
Ǹ
) Sin
= 32.027°
b. Calculate conduction time for = 32.027°. For
a sinuousoidal waveform Vpeak happens at
= 90°. This translates to 4.165 ms in time for
a 60 Hz frequency, therefore 32.027° is 1.48 ms
and finally:
Conduction time = (4.165 ms – 1.48 ms) 2
= 5.37 ms
c. Calculate the I
peak
needed for I
(avg)
= 30 mA
Since a full bridge rectifier is being used (per
Figure 6), the frequency of the voltage signal
applied to the NUD4011 device is now 120 Hz.
To simplify the calculation, it is assumed that
the 120 Hz waveform is square shaped so that
the following formula can be used:
I
(avg)
= I
peak
duty cycle;
If 8.33 ms is 100% duty cycle, then 5.37 ms is
64.46%, then:
I
peak
= I
(avg)
/ duty cycle
I
peak
= 30 mA / 0.645 = 46 mA
d. Calculate R
ext
R
ext
= 0.7 V / I
peak
R
ext
= 15.21
5. Calculate V
drop
across the NUD4011 device:
a. V
drop
= V
in
– V
sense
– V
LEDs
b. V
drop
= 120 V – 0.7 V – 90 V
c. V
drop
= 29.3 V
Figure 6. 120 Vac Application
(Series LED’s array)
R
ext
PWM
V
in
I
out
Current
Set Point
NUD4011
Boost
I
out
I
out
I
out
1
2
3
4
8
7
6
5
L
ED1
L
ED2
L
ED30
Full
Bridge
Rectifier
120 Vac
60 Hz
−
+
1
23
4
6. Calculate Power Dissipation on the NUD4011
device’s driver:
a. P
D_driver
= V
drop
* I
(avg)
b. P
D_driver
= 29.3 V 0.030 A
c. P
D_driver
= 0.879 W
7. Establish Power Dissipation on the
NUD4011device’s control circuit per below
formula:
a. P
D_control
= (V
in
– 1.4 – V
LEDs
)@ / 20,000
b. P
D_control
= 0.040 W
8. Calculate Total Power Dissipation on the device:
a. P
D_total
= P
D_driver
+ P
D_control
b. P
D_total
= 0.879 W + 0.040 W = 0.919 W
9. If P
D_total
> 1.13 W (or derated value per
Figure 3), then select the most appropriate
recourse and repeat steps 1−8:
a. Reduce V
in
b. Reconfigure LED array to reduce V
drop
c. Reduce I
out
by increasing R
ext
d. Use external resistors or parallel device’s
configuration
10. Calculate the junction temperature using the
thermal information on Page 8 and refer to
Figure 4 to check the output current drop due to
the calculated junction temperature. If desired,
compensate it by adjusting the value of R
ext
.
