30022903

FIGURE 2. PWM Dimming of LEDs by Applying a PWM Signal to the EN/DIM Pin

A logic high at V_PWM enables the IC, and a logic low dis-

ables the IC. In this manner, the LED current is turned on and

off. In order to eliminate flicker, the lowest PWM-dimming fre-

quency is normally chosen to be above 100 Hz. The upper

end of the PWM frequency is determined by the turn-on delay

of the LM3405. If the V_PWM signal has an ON-time of T

ON

and a period of T

by the equation below. Note that the delay in enabling and

disabling the IC is not included in the following equation. Typ-

ically the time delay from the instant where EN/DIM=1 to the

instant where I

(see Figure 4).

The average LED current is therefore controlled by T

T

change. Figure 3 shows the measured average LED current

for varying duty cycles and frequencies.

30022905

FIGURE 3. Average LED Current versus Duty Cycle and

Frequency of V_PWM at the EN/DIM Pin

The startup delay of the LM3405 device is shown in Figure

4 for an LED-current setting of 1A. This is the delay from the

instant when EN/DIM=1 to the instant when I

900 mA (90% of the set LED current).

30022906

FIGURE 4. Startup Response of the LM3405 to an EN/DIM

Signal with I

Deriving a Self-Biased Boost

Voltage When Driving Two or More

LEDs in Series

In a typical application where two or more LEDs are driven in

a single string, the boost voltage is derived from a separate,

external low-voltage source so as to meet the (VBOOST-

VSW) voltage requirement. This is because when powering

two or more LEDs in series, V

therefore V

case, V

VBOOST.

Figure 5 shows another approach for deriving a self-biased

boost voltage from the LED string itself without the need for

generating a separate low-voltage supply.

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