LM3450 Datasheet, PDF (10/40 Pages) National Semiconductor (TI) – LED Driver with Active Power Factor Correction and Phase Dimming Decoder

LM3450
SNVS681D – NOVEMBER 2010 – REVISED MAY 2013
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RETURN
AC
INPUT
C_PFC
C_REF
R_AD2
R_AD1
R_F2
C_F2
V_REF
V_ADJ
FLT2
BIAS
HOLD
ZCD
R_BS
R_HLD
Q_PS
R_ZCD
D_SW
C_OUT
+
FLYBACK
-^OUTPUT
R_AC2
C_F1
R_F1
PWM
TO OPTO
R_AC1
FEEDBACK
FROM OPTO
C_CMP
R_FB1
FLT1
V_CC
LM3450/50A
DIM
GATE
V_AC
COMP
CS
GND
FB
I_SEN
C_VCC
24V
Q_SW
C_HLD
C_SEN
R_CS
R_SEN
RETURN
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.
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