Datasheet

Thermal Foldback Control
The LM3464A evaluation board features an interface that en-
ables thermal foldback control by connecting a NTC thermal
sensor to the THM+ and THM- terminals. With the NTC sen-
sor attached to the chassis of the LEDs, the integrated ther-
mal foldback control circuit reduces the average LED current
and effectively reduces the LED temperature to prevent ther-
mal breakdown of the LEDs. The thermal foldback control
circuit reduces the LED currents by means of PWM dimming
which the dimming frequency is set by the capacitor, CTHM
following the equation shows below:
(10)
The default value of the CTHM on the LM3464A evaluation
board is 68nF, which set the thermal foldback dimming fre-
quency at 258Hz.
Thermal foldback control is activated when the voltage at the
Thermal pin, V
Thermal
is in between 3.25V and 0.4V as shown
in figure 5. Thermal foldback control begins when V
Thermal
is
below 3.25V. The LED current will be reduced to zero as
V
Thermal
falls below 0.4V. The average LED current varies ac-
cording to the Thermal pin voltage following the equation:
(11)
30127113
FIGURE 5. Changes of Average LED Current with Thermal Foldback Control
When the voltage at the DMIN pin is below 0.4V, the minimum
on time for thermal foldback control is restricted by the value
of C
THM
. As the voltage of the Thermal pin is set below 0.4V,
the on time for all output channels equals the discharge time
of the C
THM
following the equation:
(12)
Thus the minimum dimming duty cycle for thermal foldback is
calculated approximately equal to 0.5%:
(13)
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