LM3445 datasheet(12/46 Pages) TI2

12 / 46 page

GND

VCC

FLTR2

ASNS

FLTR1

ISNS

GATE

BLDR

COFF

DIM

LM3445MM

V+

R11

LM3445

SNVS570M – JANUARY 2009 – REVISED NOVEMBER 2015

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Feature Description (continued)

7.3.4 LM3445 Line Sensing Circuitry

An external series pass regulator (R2, D1, and Q1) translates the rectified line voltage to a level where it can be

sensed by the BLDR pin on the LM3445.

Figure 14. LM3445 AC Line Sense Circuitry

D1 is typically a 15-V 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.

7.3.5 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

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LM3445 Datasheet(PDF) 12 Page - Texas Instruments