nsi50350ad ON Semiconductor, nsi50350ad Datasheet - Page 5

• Manufacturer Part Number: nsi50350ad
• Description: Nsi50350adt4g Constant Current Regulator & Led Driver
• Manufacturer: ON Semiconductor

Other Currents
other CCR to obtain a desired current. The adjustable CCR
provides the ability to adjust the current as LED efficiency
increases to obtain the same light output (Figure 10).
Dimming using PWM
placing a BJT in series with the CCR (Figure 11).
known as Pulse Width Modulation (PWM) and has become
the preferred method of changing the light level. LEDs being
a silicon device, turn on and off rapidly in response to the
current through them being turned on and off. The switching
time is in the order of 100 nanoseconds, this equates to a
maximum frequency of 10 Mhz, and applications will
typically operate from a 100 Hz to 100 kHz. Below 100 Hz
the human eye will detect a flicker from the light emitted
from the LEDs. Between 500 Hz and 20 kHz the circuit may
The adjustable CCR can be placed in parallel with any
The dimming of an LED string can be easily achieved by
The method of pulsing the current through the LEDs is
Figure 10.
Figure 11.
http://onsemi.com
5
generate audible sound. Dimming is achieved by turning the
LEDs on and off for a portion of a single cycle. This on/off
cycle is called the Duty cycle (D) and is expressed by the
amount of time the LEDs are on (Ton) divided by the total
time of an on/off cycle (Ts) (Figure 12).
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.
Reducing EMI
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 11) 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.
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.
6000
5000
4000
3000
2000
1000
The current through the LEDs is constant during the period
Figure 13 is a typical response of Luminance vs Duty Cycle.
Designers creating circuits switching medium to high
The selected delay / slope will impact the frequency that
Figure 13. Luminous Emmitance vs. Duty Cycle
0
0
10
20
30
DUTY CYCLE (%)
Figure 12.
40
50
60
70
80
Lux
Linear
90
100