=

I

MAX

-

HOLD

V

CC

:

+ 30

R

HLD

REG

MIN-

IN-

I

=

mV

200

SEN

R

1

356 mV

Turn-off

no bleeder

2

Turn-off

with bleeder

VAC

DIN

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.

Figure 34. Reverse Phase Waveforms

The dynamic hold will quickly bleed off the excess charge in an attempt to regulate the voltage across RSEN. 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:

(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:

(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 ISEN 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 ISEN 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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