.

o

NT C(BK)

N TC (T )

BK

o

R

R

=

1

1

exp β

T

T

é

ù

æ

ö

-

ê

ú

ç

÷

ê

ú

è

ø

ë

û

NTC

TSNS(REF)

TSNS(TH)

7.88 kΩ

R

=

= 10.5 kΩ

V

V

-

1.2

1

0.8

0.6

0.4

0.2

0

0

0.3

0.6

0.9

1.2

1.5

1.8

FLT 2 Voltage, V

- V

FLT2

LM3447

SNOSC65 A – APRIL 2012 – REVISED MAY 2012

www.ti.com

The dimming command, VDIM is

•

held constant at 1V for VFLT2 ranging from 1.75V to 1.45V (conduction angle 180° to 150°)

•

linearly varied with gain of 0.877 for VFLT2 ranging from 1.45V to 280mV (conduction angle 150° to 30°)

•

saturated at 13mV for VFLT2 lower than 280mV (conduction angle less than 30°)

Figure 29. Relationship Between VFLT2 and VDIM

The relationship implemented by the angle decoding circuit is designed to map the non-linear power behavior of

external phase dimmer circuits and enhance Flyback PFC power stage compatibility.

Under normal operating conditions, the dimming command, VDIM is translated into a reference voltage, VREF,

where VREF = VDIM. As dimming progresses, the input power commanded by the feedforward loop is modulated

in accordance with VREF. This causes the output power and hence the LED current to vary based on the input

conduction angle. Using this feedforward control scheme and the internal angle decoding circuit of the LM3447, it

is possible to achieve monotonic, smooth and flicker free dimming with a dimming ratio of more than 50:1.

THERMAL FOLDBACK CIRCUIT

Thermal protection is necessary to prevent the LEDs and other power supply components from sustaining

damage when operated at elevated ambient temperatures. A thermal foldback circuit is incorporated into the

LM3447 to limit the maximum operating temperature of the LEDs by scaling the output power based on the

heatsink temperature. The LED temperature is sensed using an external NTC resistor, RNTC, connected between

the TSNS pin and GND, as shown in Figure 30(a). The thermal protection is engaged when the TSNS voltage

decreases below the thermal foldback threshold voltage, VTSNS(TH), of 1V. The power is scaled by adjusting the

reference voltage, VREF, based on the thermal foldback output voltage, VTFB, according to the relationship shown

in Figure 30(b). The resistor value, RNTC(BK), at which the device enters thermal protection is fixed by the internal

7.88k

Ω pull-up resistor and the TSNS reference voltage, VTSNS(REF) and is given by

(18)

The temperature break-point, TBK and rate-of-change (slope) are governed by the non-linear characteristics of

the NTC resistor, RNTC, given by its β-value. To achieve a break-point temperature, TBK, the NTC resistor, RNTC

should selected as

(19)

20

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