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LM3445MMX Datasheet(PDF) 11 Page - Texas Instruments |
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LM3445MMX Datasheet(HTML) 11 Page - Texas Instruments |
11 / 39 page 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. Copyright © 2009–2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM3445 |
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