LM3450 datasheet(11/28 Pages) NSC

11 / 28 page

30127422

FIGURE 1. Typical Flyback Application

Theory of Operation

The LM3450 is a single device with both power factor control

(PFC) and phase dimming decoder functions. This device is

designed to control isolated flyback converters and provide

active power factor correction. In addition to being a PFC, the

LM3450 can interpret a phase dimming (frequently called triac

dimming) input and provide a corresponding PWM output to

properly dim an LED load. This combination of features pro-

vides an excellent method to convert a standard AC mains

input to a dimmable LED output of 10-100W. It should be not-

ed that the LM3450 can control a boost converter in a similar

manner. However, this datasheet will focus mostly on the fly-

back topology due to the high demand for isolated LED driver

applications. Discussion of the LM3450 functionality will refer

to Figure 1 component designators.

The PFC control operates in critical conduction mode (CRM)

using zero crossing detection (ZCD) to terminate the off-time.

The PFC portion of this device includes an error amplifier,

multiplier, current sense circuit, zero crossing detector, and

gate driver. The internal error amplifier is used for feedback

of the output voltage in non-isolated designs. However, it can

be disabled for isolated designs where the error amplifier

needs to be on the secondary side.

The phase dimmer decoder detects the dimming angle of the

rectified AC line, decodes, filters and remaps it to a 500Hz

PWM output. The PWM output can then be sent directly, or

through optical isolation, to the dimming input of a second

stage LED driver. To ensure the decoder properly interprets

the dimming angle, dynamic hold is provided which prevents

the phase dimmer from misfiring. The input current is sensed

and when the current drops below a preset minimum, the

system adds more current.

Both the dynamic hold and the decoder are sampled syn-

chronously to reduce the overall efficiency drop due to the

additional hold current. When a decoding sample period oc-

curs, the dynamic hold is activated to ensure a proper angle

is decoded. Because of this sampling method, non-sampled

cycles will potentially cause the phase dimmer to misfire but

should not affect the output LED current regulation. Finally,

the dynamic filter and variable sampling rate provide fast,

smooth dimming transitions.

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