NSI50350AST3G

http://onsemi.com

5

Other Currents

The adjustable CCR can be placed in parallel with any

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).

Figure 10.

Dimming using PWM

The dimming of an LED string can be easily achieved by

placing a BJT in series with the CCR (Figure 11).

Figure 11.

The method of pulsing the current through the LEDs is

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

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).

Figure 12.

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 13 is a typical response of Luminance vs Duty Cycle.

Figure 13. Luminous Emmitance vs. Duty Cycle

DUTY CYCLE (%)

100

90

80

70

60

50

40

0

1000

3000

2000

30

4000

6000

20

10

0

5000

Lux

Linear

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 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.

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.