Datasheet

t
THM_MIN
= 262 x C
THM
in second
V
CC
0V
3.25V
0.4V
V
Thermal
I
LED
(AVG)
Temperature
Thermal
foldback begin
100%
Approximately 0.5%
(Limited by t
THM_MIN
)
LM3464, LM3464A
SNVS652F –APRIL 2010–REVISED MAY 2013 www.ti.com
Figure 21. Average LED Current Reduces According to V
Thermal
The dimming frequency is defined by a sawtooth waveform that generated by charging and discharging the
capacitor C
THM
which connects across the Thermal_Cap pin and GND. The LM3464/64A charges the C
THM
up to
3.25V with 50uA constant current and discharge the C
THM
by pulling the Thermal_Cap pin to ground through a
125Ω (typ.) resistor until the pin voltage reaches 0.4V (V
CTHM-MIN
). When the voltages of the Thermal and DMIN
pins are both below 0.4V, the minimum dimming on time equals the discharge time of the C
THM
following the
equation:
(12)
Thus the minimum dimming duty cycle for thermal foldback that being restricted by the discharge time of C
THM
is
approximately 0.5%:
(13)
By comparing the voltage at the Thermal pin to the sawtooth voltage being generated at the Thermal_Cap pin of
the LM3464/64A, a PWM dimming signal for thermal foldback is generated as shown in Figure 22:
Figure 22. Signals Facilitating Thermal Foldback Control
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