Datasheet
356 mV
V
AC
D
IN
OPTO
0-3V
Analog
Input
V
CC
V
SB
PWM OUT
Rectified AC
R
F1
R
F2
C
F1
C
F2
R
PB
R
SB
R
AC2
R
AC1
+
-
DIM
V
AC
500k
Transition
LM3450/50A
ANGLE
DEMOD
SAMPLE / REMAP
D
IN
D
OUT
D
IN
D
OUT
FLT2
FLT1
V
ADJ
V
REF
3V
REF
500 Hz
LM3450
www.ti.com
SNVS681D –NOVEMBER 2010–REVISED MAY 2013
Figure 26. Dimming Decoder Circuit
PHASE DIMMING DECODER
The LM3450/50A uses the rectified AC line voltage to interpret the conduction angle. Figure 26 shows the
LM3450/50A decoder circuit with associated external circuitry. The rectified AC line voltage is scaled via a
resistor divider (R
AC1
, R
AC2
) and connected to the V
AC
pin. V
AC
is compared to a 356mV reference to generate a
twice line frequency PWM signal with corresponding duty cycle as shown in Figure 27.
Figure 27. Phase Angle Demodulation
For best results, R
AC1
and R
AC2
are suggested to be sized so that the V
AC
voltage crosses the 356mV threshold
when the rectified AC line is as follows:
• 120V systems: 25V to 45V
• 230V systems: 40V to 70V
The demodulated duty cycle is sampled and logarithmically remapped to a 300Hz PWM signal improving the
resolution of low dimming levels to the human eye. A minimum duty cycle limits the maximum achievable
contrast ratio to approximately 70:1. The remapped PWM signal is buffered and output at FLT1 with amplitude
equal to V
ADJ
as shown in Figure 28.
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