LM3450 Datasheet, PDF (23/40 Pages) National Semiconductor (TI) – LED Driver with Active Power Factor Correction and Phase Dimming Decoder

LM3450
www.ti.com
SNVS681D – NOVEMBER 2010 – REVISED MAY 2013
The dynamic hold function is also necessary for reverse phase dimmers, but for a different reason. Reverse
phase dimmers do not use triacs, therefore they do not require a minimum “holding” current. Instead, they need
what is commonly called bleeder current. When a reverse phase dimmer turns off, the AC voltage is at a high
value. There is an RC time constant associated with discharging the total effective input capacitance (EMI
capacitors, PFC capacitor, damper capacitance). The decoder does not record the angle until the voltage
reaches the 356mV threshold. This can cause the decoded angle to be much larger than it actually is and
dependent on the RC time constant as shown in Figure 34.
Turn-off
with bleeder
Turn-off
no bleeder
356 mV
V_AC
D_IN
1
2
Figure 34. Reverse Phase Waveforms
The dynamic hold will quickly bleed off the excess charge in an attempt to regulate the voltage across R_SEN. This
will preserve the accuracy of the decoded phase angle.
During the conduction angle (θ), dynamic hold is enabled only during a sample period for the LM3450. However,
during the firing angle (delay time), dynamic hold is always enabled with the LM3450. This will ensure the
rectified line voltage does not begin to rise due to leakage currents through the phase dimmer. Again, with the
LM3450A the dynamic hold is continuously active during all conduction and firing angles.
The minimum regulated input current can be calculated:
I_{IN-MIN-REG =}
200 mV
R_SEN
(7)
The maximum possible additional holding current (which can occur when HOLD is still transitioning usually at the
rising edge of the triac firing) can be approximated:
I_HOLD-MAX
=
V_CC
R_HLD + 30:
(8)
It is recommended that the maximum hold current is set 10-15% higher than the minimum regulated input
current.
A minimum of 0.1µF capacitance should be placed between I_SEN and HOLD to limit the bandwidth of the dynamic
hold circuit to well below the switching frequency. However, if too large a capacitor is used, the bandwidth will be
too low to respond to line transients. A maximum of 0.47µF should ensure good performance.
Finally, a small Schottky diode should be placed from GND to I_SEN to absorb the large current spikes associated
with the triac firing edge. This diode should have a forward voltage above 200mV at the worst-case operating
temperature so that it won’t interfere with dynamic hold regulation.
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