LM3414 datasheet(17/20 Pages) NSC

17 / 20 page

DESIGN EXAMPLE

Figure 7 shows an example circuit for analog dimming control

using simple external biasing circuitry with a variable resistor.

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FIGURE 7. Example Analog Dimming Control Circuit

In the figure, the variable resistor VR1 controls the base volt-

age of Q1 and eventually adjusts the bias voltage of current

to the IADJ pin (I

EXT). As the resistance of VR1 increases and

the voltage across VR1 exceeds 1.255V + 0.7V, the LED cur-

rent starts to decrease as I

EXT increases.

where

The analog dimming begins only when I

EXT > 0.

DESIGN CONSIDERATIONS

The overall performance of the LED driver is highly depends

on the PCB layout and component selection. To minimize

connection losses and parasitic inductance of the traces, the

best practice is to keep the copper traces connecting the in-

ductor, power switch and rectifier short and thick . Long traces

on critical power paths will introduce voltage and current

spikes to the LM3414/LM3414HV. If the voltage spike level

exceeds the absolute maximum pin voltage of the LM3414, it

could damage the device and LEDs. To avoid physical dam-

age of the circuit, a Transient Voltage Suppressor (TVS) can

be added across VIN and GND pins to suppress the spike

voltage. This also helps in absorbing the input voltage spike

when the circuit is powered through physical switch upon

power up.

LM3414 Datasheet(PDF) 17 Page - National Semiconductor (TI)