LM3450

SNVS681D – NOVEMBER 2010 – REVISED MAY 2013

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DESIGN INFORMATION

HOW TO SELECT THE CORRECT DEVICE (LM3450 or LM3450A)

What application(s) are suitable for the LM3450, and when is the LM3450A appropriate? The difference centers

on the power dissipation in the passFET. The passFET stands off the high AC voltage from the LM3450/50A,

and provides a path for the hold current. The passFET operates in the linear region and dissipates power equal

to the product of the voltage across it and the current through it.

The LM3450 was designed to minimize the power dissipation in the passFET by applying holding current in a

sampled form. The standby sampling rate (when the dimmer is not moving) is infrequent, allowing minimal impact

on the thermal considerations of the passFET. Sampling of the dynamic hold is not desired for some applications

although. An example where the sampled hold current may cause undesirable effects is the single stage flyback

topology where the output of the flyback is directly connected to the LEDs. If the phase dimmer is allowed to

misfire or create erratic differences in the input voltage and current waveforms, this behavior will appear as a

"fluttering" of the LED light output at the sampling rate when the single stage topology is used. The light flutter is

most observable at low input currents (dimming). A small perturbation in the input voltage due to phase dimmer

misfire can create a visible difference in the output light. Using a secondary LED driver stage eliminates this

problem.

Cost sensitive applications may drive the design to a single stage solution, and the LM3450A was developed to

address this market. The LM3450A provides continuous dynamic hold current on every AC cycle preventing the

phase dimmer from misfire, and the sampling frequency from appearing at the output. Another design

consideration where continuous dynamic hold may be advantageous is the reduced stresses on input EMI R/C

snubber networks.

The designer must pay close attention to the power loss of the passFET when using the LM3450A. Designers

must consider worst case possibilities with any power conversion designs. Worst case operating conditions with

the LM3450A are usually found with the largest triac holding current requirements. Many phase dimmers require

25mA-40mA of holding current, and frequently designers are choosing 50mA as their minimum holding current

requirement. The passFET package for common LM3450/50A designs should be capable of dissipating between

1W and 1.5W. The pass-FET can always be increased in size and/or the hold current can be reduced. Power

calculations for the dynamic hold circuit as well as effective thermal protection are both described in the

DYNAMIC HOLD section.

The LM3450 and LM3450A is best differentiated in terms of the appropriate applications for each device. The

Table 1 can be used as a general guide for when to use each part.

Table 1. Device Selection Guide

Product

AC Input

Output Power

Device

Topology

POUT < 15W

LM3450

120V

POUT > 15W

LM3450A

High End Downlight

Two Stage Design

POUT < 25W

LM3450

230V

POUT > 25W

LM3450A

Low Cost Downlight

120V

POUT > 15W

or

LM3450A

Single Stage Design

230V

POUT > 25W

Large High End Bulb

26

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