L =

LM3434

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SNVS619B – MARCH 2010 – REVISED MAY 2013

INDUCTOR SELECTION

The most critical inductor parameters are inductance, current rating, and DC resistance. To calculate the

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Lower DC resistance inductors are larger. A good tradeoff point between the efficiency and the core size is

POWER FET SELECTION

efficiency with around 8m

the main switch FET. An additional important parameter for the synchronous FET is reverse recovery charge

DIM FET SELECTION

cycle. The output voltage during DIM will determine the switching frequency. A lower output voltage results in a

to force the switching frequency higher during the DIM cycle.

Placement of the Parallel Dimming FET

When using a FET in parallel with the LED for PWM dimming special consideration must be used for the location

of the FET. The ideal placement of the FET is directly next to the LED. Any distance between this FET and the

LED results in line inductance. Fast current changes through this inductance can induce large voltage spikes due

to v = Ldi/dt. These can be mitigated by either reducing the distance between the FET and the LED and/or

slowing the PWM edges, and therefore the dt, by using some gate resistance on the FET. In cases where the

dimming FET is not placed close to the LED and/or very fast switching edges are desired the induced voltages

can become great enough to damage the dimming FET and/or the LM3434 HS pin. This can also result in a

large spike of current into the LED when the FET is turned off. In these cases a snubber should be placed across

the dimming FET to protect the device(s).

BOOTSTRAP CAPACITORS

0.47µF is recommended between the HS and BST pins. A 0.1µF is recommended between BST2 and CGND.

SOFT-START CAPACITOR

The LM3434 integrates circuitry that, when used in conjunction with the SS pin, will slow the current ramp on

start-up. The SS pin is used to tailor the soft-start for a specific application. A capacitor value of 0.1µF on the SS

pin will yield a 12mS soft start time. For most applications soft start is not needed.

ENABLE OPERATION

The EN pin of the LM3434 is designed so that it may be controlled using a 1.6V or higher logic signal. If the

a 100k pull up resistor.

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