NSIC2030BT3G Datasheet, PDF (8/9 Pages) ON Semiconductor – Constant Current Regulator & LED Driver for A/C off-line Applications

NSIC2030BT3G
The current through the LEDs is constant during the period
they are turned on resulting in the light being consistent with
no shift in chromaticity (color). The brightness is in proportion
to the percentage of time that the LEDs are turned on.
Figure 15 is a typical response of Luminance vs Duty Cycle.
6000
5000
4000
3000
2000
1000
Lux
Linear
0
0 10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
Figure 15. Luminous Emmitance vs. Duty Cycle
Reducing EMI
Designers creating circuits switching medium to high
currents need to be concerned about Electromagnetic
Interference (EMI). The LEDs and the CCR switch
extremely fast, less than 100 nanoseconds. To help eliminate
EMI, a capacitor can be added to the circuit across R2.
(Figure 13) This will cause the slope on the rising and falling
edge on the current through the circuit to be extended. The
slope of the CCR on/off current can be controlled by the
values of R1 and C1.
The selected delay / slope will impact the frequency that
is selected to operate the dimming circuit. The longer the
delay, the lower the frequency will be. The delay time should
not be less than a 10:1 ratio of the minimum on time. The
frequency is also impacted by the resolution and dimming
steps that are required. With a delay of 1.5 microseconds on
the rise and the fall edges, the minimum on time would be
30 microseconds. If the design called for a resolution of 100
dimming steps, then a total duty cycle time (Ts) of 3
milliseconds or a frequency of 333 Hz will be required.
Thermal Considerations
As power in the CCR increases, it might become
necessary to provide some thermal relief. The maximum
power dissipation supported by the device is dependent
upon board design and layout. Mounting pad configuration
on the PCB, the board material, and the ambient temperature
affect the rate of junction temperature rise for the part. When
the device has good thermal conductivity through the PCB,
the junction temperature will be relatively low with high
power applications. The maximum dissipation the device
can handle is given by:
P_D(MAX)
+
T_J(MAX) *
R_qJA
T_A
Referring to the thermal table on page 2 the appropriate
R_qJA for the circuit board can be selected.
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