LM3444_15 Datasheet, PDF (11/27 Pages) Texas Instruments – AC-DC Offline LED Driver

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Feature Description (continued)
LM3444
SNVS682D – NOVEMBER 2010 – REVISED DECEMBER 2015
V+ _D3
V_BUCK
C7 ^{+ +}
V_BUCK
D9
-
D8
+
D4
V_BUCK
+
C9
-
C10
Figure 11. Two Stage Valley-Fill Circuit when AC Line is Low
A three stage valley-fill circuit performs exactly the same as two-stage valley-fill circuit, except now three
capacitors are charged in series when the line voltage decreases, as shown in Equation 2:
V_VF-CAP
=
V_AC-RMS
3
2
(2)
Diode D3 is reverse-biased and three capacitors are in parallel to each other.
The valley-fill circuit can be optimized for power factor, voltage hold-up, and overall application size and cost.
The LM3444 operates with a single-stage or a three-stage valley-fill circuit as well. Resistor R8 functions as a
current limiting resistor during start-up, and during the transition from series to parallel connection. Resistors R6
and R7 are 1-MΩ bleeder resistors, and may or may not be necessary for each application.
7.3.4 Buck Converter
The LM3444 is a buck controller that uses a proprietary constant off-time method to maintain constant current
through a string of LEDs. While transistor Q2 is on, current ramps up through the inductor and LED string. A
resistor R3 senses this current and this voltage is compared to the reference voltage at FILTER. When this
sensed voltage is equal to the reference voltage, transistor Q2 is turned off and diode D10 conducts the current
through the inductor and LEDs. Capacitor C12 eliminates most of the ripple current seen in the inductor. Resistor
R4, capacitor C11, and transistor Q3 provide a linear current ramp that sets the constant off-time for a given
output voltage.
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