di

Q

= L

dt

-

C12

R3

Q2

-

D10

VLED

VBUCK

VL2

L2

VBUCK(MAX) = VAC-RMS(MAX) x

2

LM3444

SNVS682D – NOVEMBER 2010 – REVISED DECEMBER 2015

www.ti.com

Application Information (continued)

The maximum voltage seen by the Buck Converter is given by Equation 13.

(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 (VBUCK) 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 (VLED × off-time) remains constant. A constant volt-second product

makes it possible to keep the ripple through the inductor constant as the voltage at VBUCK varies.

Figure 15. LM3444 External Components of the Buck Converter

Use Equation 14 to calculate an ideal inductor.

(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.

VL(ON-TIME) = VBUCK - (VLED + VDS(Q2) + IL2 × 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,

VL(ON-TIME) = VBUCK - VLED

(16)

During the off-time, the voltage seen by the inductor is given by Equation 17.

VL(OFF-TIME) = VLED

(17)

16

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