The LM3445 is an adaptive constant off-time AC/DC buck (step-down) constant current controller designed to be compatible with triac dimmers. The LM3445 provides a constant current for illuminating high power LEDs and includes a triac dim decoder. The dim decoder allows wide range LED dimming using standard triac dimmers. The high frequency capable architecture allows the use of small external passive components. The LM3445 includes a bleeder circuit to ensure proper triac operation by allowing current flow while the line voltage is low to enable proper firing of the triac. A passive PFC circuit ensures good power factor by drawing current directly from the line for most of the cycle, and provides a constant positive voltage to the buck regulator. Additional features include thermal shutdown, current limit and VCC under-voltage lockout.
■ Retro Fit Triac Dimming
■ Solid State Lighting
■ Industrial and Commercial Lighting
■ Residential Lighting
■ Triac dim decoder circuit for LED dimming
■ Application voltage range 80VAC – 270VAC
■ Capable of controlling LED currents greater than 1A
■ Adjustable switching frequency
■ Low quiescent current
■ Adaptive programmable off-time allows for constant ripple current
■ Thermal shutdown
■ No 120Hz flicker
■ Low profile 10 pin MSOP Package
■ Patent pending drive architecture
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 kΩ 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 kΩ 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.
The LM3445 is a buck controller that uses a proprietary constant off-time method to maintain constant current through a string of LEDs. While transistor Q2 is on, current ramps up through the inductor and LED string. A resistor R3 senses this current and this voltage is compared to the reference voltage at FLTR2. When this sensed voltage is equal to the reference voltage, transistor Q2 is turned off and diode D10 conducts the current through the inductor and LEDs. Capacitor C12 eliminates most of the ripple current seen in the inductor. Resistor R4, capacitor C11, and transistor Q3 provide a linear current ramp that sets the constant off-time for a given output
Multiple LM3445s can be configured so that large strings of LEDs can be controlled by a single triac dimmer. By doing so, smooth consistent dimming for multiple LED circuits is achieved. When the FLTR1 pin is tied above 4.9V (typical), preferably to VCC, the ramp comparator is tri-stated, disabling the dim decoder. This allows one or more LM3445 devices or PWM LED driver devices (slaves) to be controlled by a single LM3445 (master) by connecting their DIM pins together.
National Semiconductor offers an LM3445 demonstration PCB for customer evaluation through their website. The description and theory uses reference designators that follow our evaluation PCB. Each board contains a separate circuit for the Master and Slave function. Both the Master and Slave boards will need to be modified from their original stand alone function so that they can be coupled together. Only the Master LM3445 requires use of the Master/Slave circuit for any number of slaves.
Available also in Italian version: LM3445 - Driver per LED dimmabile a TRIAC senza fenomeni di Flicker
Read more: http://www.national.com/ds/LM/LM3445.pdf