LM3445MM datasheet(18/26 Pages) NSC

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The off-time of the LM3445 can be programmed for switching

frequencies ranging from 30 kHz to over 1 MHz. A trade-off

between efficiency and solution size must be considered

when designing the LM3445 application.

The maximum switching frequency attainable is limited only

by the minimum on-time requirement (200 ns).

Worst case scenario for minimum on time is when V

BUCK is at

its maximum voltage (AC high line) and the LED string voltage

LED) is at its minimum value.

The maximum voltage seen by the Buck Converter is:

INDUCTOR SELECTION

The controlled off-time architecture of the LM3445 regulates

the average current through the inductor (L2), and therefore

the LED string current. The input voltage to the buck converter

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

LED x off-time) remains constant. A constant volt-second

product makes it possible to keep the ripple through the in-

ductor constant as the voltage at V

BUCK varies.

30060340

FIGURE 17. LM3445 External Components of the Buck

Converter

The equation for an ideal inductor is:

Given a fixed inductor value, L, this equation 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,

L(ON-TIME) = VBUCK - (VLED + VDS(Q2) + IL2 x R3)

Since the voltage across the MOSFET switch (Q2) is rela-

tively small, as is the voltage across sense resistor R3, we

can simplify this to approximately,

L(ON-TIME) = VBUCK - VLED

During the off-time, the voltage seen by the inductor is ap-

proximately:

L(OFF-TIME) = VLED

The value of V

L(OFF-TIME) will be relatively constant, because

the LED stack voltage will remain constant. If we rewrite the

equation for an inductor inserting what we know about the

circuit during the off-time, we get:

Re-arranging this gives:

From this we can see that the ripple current (

Δi) is proportional

to off-time (t

OFF) multiplied by a voltage which is dominated

by V

LED divided by a constant (L2).

These equations can be rearranged to calculate the desired

value for inductor L2.

Where:

Finally:

Refer to “Design Example” section of the datasheet to better

understand the design process.

LM3445MM Datasheet(PDF) 18 Page - National Semiconductor (TI)

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