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LM3405 Datasheet(PDF) 2 Page - National Semiconductor (TI) |
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LM3405 Datasheet(HTML) 2 Page - National Semiconductor (TI) |
2 / 6 page 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 PER, the average IF is given approximately 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 F is fully established is approximately 100 μs (see Figure 4). The average LED current is therefore controlled by T ON, T PER, or both and is perceived by the eye as a brightness 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 F settles to 900 mA (90% of the set LED current). 30022906 FIGURE 4. Startup Response of the LM3405 to an EN/DIM Signal with I F Set at 1A 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 IN will be greater than 5V and therefore V IN cannot be used to provide VBOOST. In this case, V OUT will also be high and cannot be used to derive 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. www.national.com 2 |
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