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

LM3445

www.ti.com

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 ANGLE DETECT).

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.

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.

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:

•

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 kΩ 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

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.

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