Datasheet

V
AUX
Q2
D9
R12
D11
D10
C4
V
CC
R3
D7
Q1
R8
V+
C7 C8
R22
D8
LM3448
6VCC
R17
Q2
D9
R12
D11
D10
C4
T1
SW
VLED+
VLED-
R1
V
CC
V
AUX
www.ti.com Design Guide
13.3 Bias Supplies and Capacitances
The bias supply circuits shown in Figure 33 and Figure 34 enables instant turn-on through Q1 while
providing an auxiliary winding for high efficiency steady state operation. The two bias paths are each
connected to VCC through a diode (D8, D9) to ensure the higher of the two is providing V
CC
current.
Figure 33. Bias Supply Circuits
Figure 34. Auxiliary Winding Bias Circuit
PassFET Bias Circuit
The passFET (Q1) is used in its linear region to stand-off the line voltage from the LM3448 regulator. Both
the V
CC
startup current and discharging of the EMI filter capacitance for proper phase angle detection are
handled by Q1. Therefore Q1 has to block the maximum peak input voltage and have both sufficient surge
and power handling capability with regards to its safe operating area (SOA). The design equations are,
(28)
21
SNOA554C–October 2011–Revised May 2013 AN-2090 LM3448 -120VAC, 6W Isolated Flyback LED Driver
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