LM3445 datasheet(13/46 Pages) TI2

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LM3445

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SNVS570M – JANUARY 2009 – REVISED NOVEMBER 2015

Feature Description (continued)

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.

7.3.6 Angle Detect

The Angle Detect circuit uses a comparator with a fixed threshold voltage of 7.21 V to monitor the BLDR pin to

determine whether the TRIAC is on or off. The output of the comparator drives the ASNS buffer and also controls

the Bleeder circuit. A 4 µs delay line on the output is used to filter out noise that could be present on this signal.

The output of the Angle Detect circuit is limited to a 0 V to 4 V swing by the buffer and presented to the ASNS

pin. R1 and C3 comprise a low-pass filter with a bandwidth on the order of 1 Hz.

The Angle Detect circuit and its filter produce a DC level which corresponds to the duty cycle (relative on-time) of

the TRIAC dimmer. As a result, the LM3445 will work equally well with 50-Hz or 60-Hz line voltages.

7.3.7 Bleeder

While the BLDR pin is below the 7.21-V threshold, the bleeder MOSFET is on to place a small load (230

Ω) on

the series pass regulator. This additional load is necessary to complete the circuit through the TRIAC dimmer so

that the dimmer delay circuit can operate correctly. Above 7.21 V, the bleeder resistor is removed to increase

efficiency.

7.3.8 FLTR1 Pin

The FLTR1 pin has two functions. Normally, it is fed by ASNS through filter components R1 and C3 and drives

the dim decoder. However, if the FLTR1 pin is tied above 4.9 V (typical), for example, to VCC, the Ramp

Comparator is tri-stated, disabling the dim decoder. See Master/Slave Operation

7.3.9 Dim Decoder

The ramp generator produces a 5.85-kHz saw tooth wave with a minimum of 1 V and a maximum of 3 V. The

filtered ASNS signal enters pin FLTR1 where it is compared against the output of the Ramp Generator.

The output of the ramp comparator will have an on-time which is inversely proportional to the average voltage

level at pin FLTR1. However, since the FLTR1 signal can vary between 0 V and 4 V (the limits of the ASNS pin),

and the Ramp Generator signal only varies between 1 V and 3 V, the output of the ramp comparator will be on

continuously for VFLTR1 < 1 V and off continuously for VFLTR1 > 3 V. This allows a decoding range from 45° to

135° to provide a 0 to 100% dimming range.

The output of the ramp comparator drives both a common-source N-channel MOSFET through a Schmitt trigger

and the DIM pin (see Master/Slave Operation for further functions of the DIM pin). The MOSFET drain is pulled

up to 750 mV by a 50-k

Ω resistor.

Since the MOSFET inverts the output of the ramp comparator, the drain voltage of the MOSFET is proportional

to the duty cycle of the line voltage that comes through the TRIAC dimmer. The amplitude of the ramp generator

causes this proportionality to "hard limit" for duty cycles above 75% and below 25%.

The MOSFET drain signal next passes through an RC filter comprised of an internal 370-k

Ω resistor, and an

external capacitor on pin FLTR2. This forms a second low pass filter to further reduce the ripple in this signal,

which is used as a reference by the PWM comparator. This RC filter is generally set to 10 Hz.

The net effect is that the output of the dim decoder is a DC voltage whose amplitude varies from near 0 V to 750

mV as the duty cycle of the dimmer varies from 25% to 75%. This corresponds to conduction angles of 45° to

135°, respectively.

The output voltage of the Dim Decoder directly controls the peak current that will be delivered by Q2 during its

on-time. See Buck Converter for details.

As the TRIAC fires beyond 135°, the DIM decoder no longer controls the dimming. At this point the LEDs will dim

gradually for one of two reasons:

1. The voltage at VBUCK decreases and the buck converter runs out of headroom and causes LED current to

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