ZCD

GATE

LM3450/50A

GND

COMP

FB

HOLD

CS

DIM

FLT2

FLT1

PWM

TO OPTO

BIAS

+

-

RETURN

RETURN

AC

INPUT

24V

FLYBACK

OUTPUT

FEEDBACK

FROM OPTO

LM3450

SNVS681D – NOVEMBER 2010 – REVISED MAY 2013

www.ti.com

Figure 17. Typical Flyback Application

THEORY OF OPERATION

The LM3450/50A 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 LM3450/50A 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 provides an

excellent method to convert a standard AC mains input to a dimmable LED output of 10-100W. It should be

noted that the LM3450/50A can control a boost converter in a similar manner. However, this datasheet will focus

mostly on the flyback topology due to the high demand for isolated LED driver applications. Discussion of the

LM3450/50A functionality will refer to Figure 17 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 synchronously in the LM3450 to reduce the overall

efficiency drop due to the additional hold current. When a decoding sample period occurs, 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.

10

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