Datasheet

MIN

MAX

T T

DIM

t+t

SUD

MIN

PWM

tT -

MAX

Low Power Shutdown

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Figure 4. PWM Dimming Limits

The logic of DIM1 is active low, hence the LM3406 will deliver regulated output current when the voltage

at DIM1 is high, and the current output is disabled when the voltage at DIM1 is low. Connecting a constant

logic low will disable the output. Note that an internal pullup esnures that the LM3406 is enabled if the DIM

pin is open-circuited. The DIM1 function disables only the power MOSFET, leaving all other circuit blocks

functioning to minimize the converter response time, t

The DIM2 terminal provides a second method for PWM dimming by connecting to the gate of MOSFET

Q1. Q1 provides a parallel path for the LED current. Shunting the output current through a parallel

MOSFET reduces the PWM dimming delays because the inductor current remains continuous, providing

faster response time for higher frequency and/or greater resolution in the PWM dimming signal. The trade-

off in this method is that the full current flows through Q1 while the LED is off, resulting in lower efficiency.

The standard BOM of the LM3406 evaluation board includes an output capacitor to reduce output current

ripple, but the drawback of this output capacitor is that it causes significant delays when using parallel

MOSFET dimming. The output capacitor should be removed to take full advantage of parallel MOSFET

dimming.

The logic of DIM2 is active high, hence the regulated output current flows through the LED array when the

voltage at DIM2 is low, and the current flows through the shunt FET when the voltage at DIM2 is high.

Connecting a constant logic high to the DIM2 will turn off the LED but will not shut down the LM3406.

6 Low Power Shutdown

The LM3406 can be placed into a low power shutdown (typically 240 µA) by grounding the OFF* terminal.

During normal operation this terminal should be left open-circuit.

7 Output Open Circuit

The LM3406 will begin to operate as soon as VIN is greater than 6V and the DIM and RON pins are not

grounded. If the regulator is powered and enabled but no LED array is connected, the output voltage will

rise to V

. The output of the circuit is rated to 50V (beyond the maximum input voltage) and will not suffer

damage, however care should be taken not to connect an LED array if the output voltage is higher than

the target forward voltage of the LED array in steady state.

AN-1750 LM3406 Evaluation Board SNVA300A–September 2008–Revised May 2013

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