LM3450 Datasheet, PDF (26/40 Pages) National Semiconductor (TI) – LED Driver with Active Power Factor Correction and Phase Dimming Decoder

LM3450
SNVS681D – NOVEMBER 2010 – REVISED MAY 2013
DESIGN INFORMATION
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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.
Product
High End Downlight
Low Cost Downlight
or
Large High End Bulb
AC Input
120V
230V
120V
230V
Table 1. Device Selection Guide
Output Power
P_OUT < 15W
P_OUT > 15W
P_OUT < 25W
P_OUT > 25W
P_OUT > 15W
P_OUT > 25W
Device
LM3450
LM3450A
LM3450
LM3450A
LM3450A
Topology
Two Stage Design
Single Stage Design
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