.

ADET

ADET

AC

-6

VAC( ANGLE)

V

V

R

=

=

I

66

10

´

VAC

LM3447

HOLD

RAC

CURRENT

MIRROR

10:1

42k

400mV

1V

VFLT2

+

±

BIAS

RBS

RHLD1

RHLD2

CHLD

QPASS

+

+

280mV

ANGLE

DECODING

CIRCUIT

VDIM

1.75V

+

FLT1

FLT2

ENHOLD

RFLT

CFLT

VTFB

VREF

REFERENCE

GENERATOR

-3

SN

P(PK,MAX)

275

10

R

=

2I

´

LM3447

SNOSC65 A – APRIL 2012 – REVISED MAY 2012

www.ti.com

and

(14)

For RSN It is recommended to use a film type SMD resistor, with power rating greater than PSN and with low ESL.

ANGLE DETECT CIRCUIT

Figure 27. Phase Angle Detection and HOLD Current Circuit

The LM3447 uses the input voltage, VREC, to detect the conduction phase angle. Figure 27 shows the LM3447

angle detect circuit, where the input voltage, VREC, is scaled by the current mirror circuits and re-generated

across an internal 42k

Ω resistor. This replica of the input voltage is compared with internal 280mV reference to

obtain the conduction information. The resulting PWM signal, with its on-time proportional to the conduction

period, is buffered and supplied through the FLT1 pin, as shown in Figure 28. To match the external phase

dimmer characteristics with the LM3447 decoding circuit and prevent EMI filter capacitors from interfering with

dimming operation, it is necessary to select an angle detection threshold, VADET(TH). This threshold can then be

programmed using the resistor, RAC, such that

(15)

For best results, set VADET(TH) as follows:

•

25V to 40V for 120V systems

•

50V to 80V for 230V systems

Resistor RAC should also limit the VAC current under worst case operating conditions. The value of RAC should

be optimized to meet both angle detect, VADET, and VAC current, IVAC constraints.

18

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