Texas Instruments LM3448 Evaluation Board LM3448-EDSNEV/NOPB LM3448-EDSNEV/NOPB データシート
製品コード
LM3448-EDSNEV/NOPB
ON
i
L
t
V
'
v
t
di
L
d
R15
C11
R2
R7
V+
LM3448
11 FLTR2
R15
C11
R2
R7
V+
LM3448
11 FLTR2
C14
C11
LM3448
11 FLTR2
(a)
(b)
(c)
SNOSB51C – SEPTEMBER 2011 – REVISED MAY 2013
The valley-fill circuit can be optimized for power factor, voltage hold-up and overall application size and cost. The
LM3448 will operate with a single stage or a three stage valley-fill circuit as well. Resistor R8 functions as a
current limiting resistor during start-up and during the transition from series to parallel connection. Resistors R6
and R7 are 1M
LM3448 will operate with a single stage or a three stage valley-fill circuit as well. Resistor R8 functions as a
current limiting resistor during start-up and during the transition from series to parallel connection. Resistors R6
and R7 are 1M
Ω
bleeder resistors and may or may not be necessary for each application.
FLTR2 LINE-INJECTION
The technique of line-injection is another very effective means of improving power factor performance. When
using this method, the valley-fill circuit can be eliminated which results in a much simpler driver design. The trade
off will be an increase of 120Hz ripple on the LED current.
using this method, the valley-fill circuit can be eliminated which results in a much simpler driver design. The trade
off will be an increase of 120Hz ripple on the LED current.
Different FLTR2 circuits are shown in
(a) shows how to set up FLTR2 when a passive PFC
circuit (e.g. valley-fill) is already being used and no line-injection is utilized. If passive PFC is not being
implemented, then the “direct line-injection” of
implemented, then the “direct line-injection” of
(b) or “AC line-injection” of
(c) can be used.
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.
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.
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.
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
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,
shown in the following equations,
(2)
or,
(3)
Therefore a constant on-time (since inductor L is constant) can be obtained.
16
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