LM3464 Datasheet, PDF (16/33 Pages) National Semiconductor (TI) – 4 Channel LED Driver Evaluation Board

LM3464, LM3464A
SNVS652F – APRIL 2010 – REVISED MAY 2013
V_Thermal
V_CC
3.25V
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I_LED (AVG)
100%
0.4V
0V
Thermal
foldback begin
Approximately 0.5%
(Limited by t_{THM_MIN})
Temperature
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:
t_{THM_MIN} = 262 x C_THM in second
(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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