,

I N

BU LK

L

LED

OUT LED(R IP)

P

C

2

R

V

I

³

f

p

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

Thermal Foldback Voltage, V

- V

TFB

(a) Circuit

(b) Relationship between V

and V

TSNS

REF

LM3447

www.ti.com

SNOSC65 A – APRIL 2012 – REVISED MAY 2012

where, To is the room temperature in Kelvin, and RNTC(To) is the NTC value at room temperature. A temperature

break-point ranging from 70°C (343K) to 90°C (363K) can be achieved by selecting an NTC resistance ranging

from 100k

Ω to 220kΩ and β-value of 3500K to 4500K.

Figure 30. Thermal Foldback

The precedence between the thermal foldback input, VTFB and the dimming input, VDIM, is decided by the

reference generator circuit. This allows dimming operation to be performed when thermal protection is engaged.

Dimming operation is allowed when the input power demanded by the decoder circuit, VDIM, is lower than the

maximum power limit set by the thermal protection circuit, VTFB. This feature provides optimal lamp utilization

under adverse operating conditions.

OUTPUT BULK CAPACITOR

The output bulk capacitor, CBULK, is required to store energy during the input voltage zero crossing interval and

limit twice the line frequency ripple component flowing through the LEDs. The value of output capacitor is given

by

(20)

where, RLED is the dynamic resistance of LED string, ILED(RIP) is the average to peak LED ripple current and fL is

line frequency. In typical applications, the solution size becomes a limiting factor and dictates the maximum

dimensions of the bulk capacitor. When selecting an electrolytic capacitor, manufacturer recommended de-rating

factors should be applied based on the worst case capacitor ripple current, output voltage and operating

temperature to achieve the desired operating lifetime.

It is essential to provide a minimum load at the output of the PFC to discharge the capacitor after the power is

switched off or during LED open circuit failures. A 20k

Ω resistor, RO, is recommended for best performance.

Copyright © 2012, Texas Instruments Incorporated

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