Design Example
30127114
FIGURE 6. Attaching NTC Thermistor to the LEDs
A NTC thermistor is connected to the THM+ and THM- ter-
minals of the LM3464A evaluation board as shown in figure
6 to activate the thermal foldback control function.
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 1.436k
Ω
at 125°C (R
NTC(125°C)
).
V
Thermal
at 125°C (45% dimming duty cycle):
(14)
(15)
(16)
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:
(17)
(18)
(19)
When the temperature goes up to 125°C
(20)
By combining the equations (17) and (18), the values of
RTHM1 and RTHM2 can be obtained:
(21)
The default values of RTHM1 and RTHM2 on the LM3464A
evaluation board are 4.87k
Ω
and 232
Ω
respectively.
Minimum Dimming Duty Cycle for
Thermal Foldback Control
The minimum dimming duty cycle for thermal foldback control
(D
THMFB_MIN
) can be limited by setting the voltage at the DMIN
pin of the LM3464A. The minimum dimming duty cycle limit
overrides the minimum dimming level defined by RTHM1,
RTHM2 and the NTC thermistor. This function is especially
useful for the applications that require to maintain certain
brightness level under high operation temperature. The level
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