LM3445MM datasheet(10/26 Pages) NSC

10 / 26 page

SENSING THE RECTIFIED TRIAC WAVEFORM

A bridge rectifier, BR1, converts the line (mains) voltage (5c)

into a series of half-sines as shown in 5b. Figure 5a shows a

typical voltage waveform after diode D3 (valley fill circuit, or

BUCK).

30060315

FIGURE 5. Voltage Waveforms After Bridge Rectifier

Without Triac Dimming

Figure 6c and 6b show typical triac dimmed voltage wave-

forms before and after the bridge rectifier. Figure 6a shows a

typical triac dimmed voltage waveform after diode D3 (valley

fill circuit, or V

BUCK).

30060316

FIGURE 6. Voltage Waveforms After Bridge Rectifier With

Triac Dimming

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.

30060317

FIGURE 7. 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 ca-

pacitance 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 the section on

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 capaci-

tance 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 dim-

mers 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 re-

quired. As the number of LM3445 circuits is added to a single

dimmer, the holding resistor R5’s resistance can be in-

creased. 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

LM3445MM データシート(PDF) 10 Page - National Semiconductor (TI)

LM3445MM データシート(HTML) 10 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

データシートダウンロード