NSIC2020JBT3G

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6

Higher Current LED Strings

Two or more fixed current CCRs can be connected in

parallel. The current through them is additive (Figure 11).

Figure 11.

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 12).

Figure 12.

Dimming using PWM

The dimming of an LED string can be easily achieved by

placing a BJT in series with the CCR (Figure 13).

Figure 13.

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 14).

Figure 14.