GND

DIM

BOOT

SW

VOUT

RON

LM3406/06HV

VIN,VINS

D1

L1

VCC

CS

COMP

Q1

GND

BOOT

SW

VOUT

RON

LM3406/06HV

VIN

D2

L1

VCC

CS

COMP

VINS

D1

DIM

LM3406, LM3406HV

www.ti.com

SNVS512C – SEPTEMBER 2008 – REVISED FEBRUARY 2010

Figure 13. Typical Application using Two-Wire Dimming

PARALLEL MOSFET FOR HIGH-SPEED PWM DIMMING

For applications that require dimming at high frequency or with wide dimming duty cycle range neither the VINS

comparator or the DIM pin are capable of slewing the LED current from 0 to the target level fast enough. For

such applications the LED current slew rate can by increased by shorting the LED current with a N-MOSFET

placed in parallel to the LED or LED array, as shown in Figure 14. While the parallel FET is on the output current

flows through it, effectively reducing the output voltage to equal the CS pin voltage of 0.2V. This dimming method

maintains a continuous current through the inductor, and therefore eliminates the biggest delay in turning the

LED(s) or and off. The trade-off with parallel FET dimming is that more power is wasted while the FET is on,

although in most cases the power wasted is small compared to the power dissipated in the LEDs. Parallel FET

circuits should use no output capacitance or a bare minimum for noise filtering in order to minimize the slew rate

of output voltage. Dimming FET Q1 can be driven from a ground-referenced source because the source stays at

0.2V along with the CS pin.

Figure 14. Dimming with a Parallel FET

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