LM3448 Datasheet, PDF (16/43 Pages) Texas Instruments – Phase Dimmable Offline LED Driver with Integrated FET

LM3448
SNOSB51B – SEPTEMBER 2011 – REVISED NOVEMBER 2011
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Direct line-injection involves injecting a small portion (750mV to 1.00V) of rectified AC line voltage (i.e. V+) into
the FLTR2 pin. The result is that current shaping of the input current will yield power factor values greater than
0.94.
AC coupled line-injection goes one step further by adding a capacitor C14 between R15 and C11. This improves
LED line regulation but does so by trading out a small portion of the power factor improvement from the direct-
injection circuit. For example with AC coupled line-injection, LED current regulation of up to +/- 3% is possible for
an input voltage range of 105VAC to 135VAC when operating at a nominal 120VAC.
V+
V+
R2
R2
LM3448
11 FLTR2
C11
R7
R15
C11
LM3448
11 FLTR2
R7
C14
R15
C11
LM3448
11 FLTR2
(a)
(b)
(c)
Figure 25. (a) No line-injection, (b) Direct line-injection, (c) AC-coupled line injection
DIRECT LINE-INJECTION FOR FLYBACK TOPOLOGY
For flyback converters using the LM3448, direct-line injection can result in power factors greater than 0.95. Using
this technique, the LM3448 circuit is essentially turned into a constant power flyback converter operating in
discontinuous conduction mode (DCM). The LM3448 normally works as a constant off-time regulator, but by
injecting a 1.0V_PK rectified AC voltage into the FLTR2 pin, the on-time can be made to be constant. With a DCM
flyback converter the primary side current, i, needs to increase as the rectified input voltage, V+, increases as
shown in the following equations,
v L ^di
dt
(2)
or,
LV
t_ON 'i
(3)
Therefore a constant on-time (since inductor L is constant) can be obtained.
By using the line voltage injection technique, the FLTR2 pin has the voltage wave shape shown in Figure 26 on it
with no TRIAC dimmer in-line. Peak voltage at the FLTR2 pin should be kept below 1.25V otherwise current limit
will be tripped. Capacitor C11 is chosen small enough so as not to distort the AC signal but just add a little
filtering.
Although the on-time is probably never truly constant, it can be observed in Figure 27 how (by injecting the
rectified voltage) the on-time is adjusted.
V_FLTR2
t
Figure 26. FLTR2 Waveform with No Dimmer
16
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