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
R
FLTR1
R
SNS
PWM
I-LIM
1.27V
ISNS
ASNS
GND
FLTR1
FLTR2
DIM
DIM DECODER
4.9V
Tri-State
50k
370k
C
FLTR2
C
FLTR1
1k
RAMP GEN.
5.9 kHz
3V
1V
1V
750 mV
125 ns
LEADING EDGE BLANKING
1V
Nearly a constant on-
time as the line varies
The PWM reference increases
as the line voltage increases.
As line voltage increases, the voltage across the
inductor increases, and the peak current increases.
LED Current
RAMP
t
V
FLTR2
SNOSB51C – SEPTEMBER 2011 – REVISED MAY 2013
By using the line voltage injection technique, the FLTR2 pin has the voltage wave shape shown in
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.
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
how (by injecting the
rectified voltage) the on-time is adjusted.
Figure 26. FLTR2 Waveform with No Dimmer
Figure 27. Typical Operation of Direct Line-Injection into FLTR2 Pin
TRIAC DIMMER HOLDING CURRENT
In order to emulate an incandescent light bulb (essentially a resistor) with any LED driver, the existing TRIAC will
require a small amount of holding current throughout the AC line cycle. As shown in
require a small amount of holding current throughout the AC line cycle. As shown in
, a simple circuit
consisting of R3, D1, Q1 and R4 can accomplish this. With R4 placed on the source of Q1, additional holding
current can be pulled from the TRIAC. Most TRIAC dimmers only require a few milliamps of current to hold them
on. A few “less expensive” TRIACs sold on the market will require a bit more current. The value of resistor R4
will depend on the type of TRIAC being used and how many light fixtures are running off the TRIAC.
current can be pulled from the TRIAC. Most TRIAC dimmers only require a few milliamps of current to hold them
on. A few “less expensive” TRIACs sold on the market will require a bit more current. The value of resistor R4
will depend on the type of TRIAC being used and how many light fixtures are running off the TRIAC.
With a single LM3448 circuit on a common TRIAC dimmer, a holding current resistor between 3k
Ω
and 5k
Ω
will
be required. As the number of LM3448 circuits added to a single dimmer increases, R4’s resistance can also be
increased. A few TRIAC dimmers will require a resistor as low as 1k
increased. A few TRIAC dimmers will require a resistor as low as 1k
Ω
or smaller for a single LM3448 circuit.
Therefore the trade-off will be dimming performance versus efficiency. As the holding resistor R4 is increased,
the overall system efficiency will also increase.
the overall system efficiency will also increase.
Copyright © 2011–2013, Texas Instruments Incorporated
17
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