Texas Instruments LM3445 Evaluation Board LM3445-230VFLBK/NOPB LM3445-230VFLBK/NOPB 데이터 시트
제품 코드
LM3445-230VFLBK/NOPB
1
2
3
4
5
6
7
8
9
10
GND
VCC
FLTR2
ASNS
FLTR1
ISNS
GATE
BLDR
COFF
DIM
LM3445MM
V+
D1
D2
R2
Q1
R5
C5
U1
C4
C3
R1
R11
SNVS570L – JANUARY 2009 – REVISED MAY 2013
Figure 17. LM3445 AC Line Sense Circuitry
D1 is typically a 15V zener diode which forces transistor Q1 to “stand-off” most of the rectified line voltage.
Having no capacitance on the source of Q1 allows the voltage on the BLDR pin to rise and fall with the rectified
line voltage as the line voltage drops below zener voltage D1 (see
Having no capacitance on the source of Q1 allows the voltage on the BLDR pin to rise and fall with the rectified
line voltage as the line voltage drops below zener voltage D1 (see
).
A diode-capacitor network (D2, C5) is used to maintain the voltage on the VCC pin while the voltage on the
BLDR pin goes low. This provides the supply voltage to operate the LM3445.
BLDR pin goes low. This provides the supply voltage to operate the LM3445.
Resistor R5 is used to bleed charge out of any stray capacitance on the BLDR node and may be used to provide
the necessary holding current for the dimmer when operating at light output currents.
the necessary holding current for the dimmer when operating at light output currents.
TRIAC HOLDING CURRENT RESISTOR
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. An external resistor (R5) needs to be
placed on the source of Q1 to GND to perform this function. Most existing 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 R5 will depend on:
•
require a small amount of holding current throughout the AC line cycle. An external resistor (R5) needs to be
placed on the source of Q1 to GND to perform this function. Most existing 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 R5 will depend on:
•
What type of triac the LM3445 will be used with
•
How many light fixtures are running off of the triac
With a single LM3445 circuit on a common triac dimmer, a holding current resistor between 3 k
Ω
and 5 k
Ω
will
be required. As the number of LM3445 circuits is added to a single dimmer, the holding resistor R5’s resistance
can be increased. A few triac dimmers will require a resistor as low as 1 k
can be increased. A few triac dimmers will require a resistor as low as 1 k
Ω
or lower for a single LM3445 circuit.
The trade-off will be performance vs efficiency. As the holding resistor R5 is increased, the overall efficiency per
LM3445 will also increase.
LM3445 will also increase.
Copyright © 2009–2013, Texas Instruments Incorporated
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