LM3464 datasheet(8/14 Pages) NSC

8 / 14 page

30127114

FIGURE 6. Attaching NTC Thermistor to the LEDs

Assuming that thermal foldback control is required to begin at

70°C LED chassis temperature and reduce 55% average LED

current (45% dimming duty cycle) when the chassis temper-

ature reaches 125°C. Using the NTC thermister NXFT15W-

B473FA1B from MURATA, which has 4.704k

Ω resistance at

70°C (R

NTC(70°C)) and 1.436kΩ at 125°C (RNTC(125°C)).

Thermal at 125°C (45% dimming duty cycle):

(12)

(13)

(14)

In the above equation, V

Thermal is the voltage at the Thermal

pin and D

THMFB is the dimming duty cycle under thermal fold-

back control. When thermal foldback begins:

(15)

(16)

(17)

When the temperature goes up to 125°C

(18)

By combining the equations (17) and (18), the values of

RTHM1 and RTHM2 can be obtained:

(19)

The default values of RTHM1 and RTHM2 on the LM3464

evaluation board are 4.87k

Ω and 232Ω respectively.

Minimum Dimming Duty Cycle for

Thermal Foldback Control

The minimum dimming duty cycle for thermal foldback control

THMFB_MIN) can be limited by setting the voltage at the DMIN

pin of the LM3464. This limit will override the minimum dim-

ming level defined by RTHM1, RTHM2 and the NTC thermis-

tor. This function is especially useful for the applications that

require to maintain certain brightness level under high oper-

ation temperature. The minimum duty cycle limit is governed

by the following equation:

(20)

LM3464 Datasheet(PDF) 8 Page - National Semiconductor (TI)