LM3444_15 Datasheet, PDF (16/27 Pages) Texas Instruments – AC-DC Offline LED Driver

LM3444
SNVS682D – NOVEMBER 2010 – REVISED DECEMBER 2015
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Application Information (continued)
The maximum voltage seen by the Buck Converter is given by Equation 13.
V_BUCK(MAX) = V_AC-RMS(MAX) x 2
(13)
8.1.4 Inductor Selection
The controlled off-time architecture of the LM3444 regulates the average current through the inductor (L2), and
therefore the LED string current. The input voltage to the buck converter (V_BUCK) changes with line variations and
over the course of each half-cycle of the input line voltage. The voltage across the LED string is relatively
constant, and therefore the current through R4 is constant. This current sets the off-time of the converter and
therefore the output volt-second product (V_LED × off-time) remains constant. A constant volt-second product
makes it possible to keep the ripple through the inductor constant as the voltage at V_BUCK varies.
V_BUCK
C12
D10
V_LED
-
L2 V_L2
-
Q2
R3
Figure 15. LM3444 External Components of the Buck Converter
Use Equation 14 to calculate an ideal inductor.
di
Q = L _dt
(14)
Given a fixed inductor value, L, Equation 14 states that the change in the inductor current over time is
proportional to the voltage applied across the inductor.
During the on-time, the voltage applied across the inductor is given in Equation 15.
V_L(ON-TIME) = V_BUCK - (V_LED + V_DS(Q2) + I_L2 × R3)
(15)
Because the voltage across the MOSFET switch (Q2) is relatively small, as is the voltage across sense resistor
R3, we can approximately simplify this as shown in Equation 16,
V_L(ON-TIME) = V_BUCK - V_LED
(16)
During the off-time, the voltage seen by the inductor is given by Equation 17.
V_L(OFF-TIME) = V_LED
(17)
16
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