LM3445MM datasheet(17/26 Pages) NSC

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Design Guide

DETERMINING DUTY-CYCLE (D)

Duty cycle (D) approximately equals:

With efficiency considered:

For simplicity, choose efficiency between 75% and 85%.

CALCULATING OFF-TIME

The “Off-Time” of the LM3445 is set by the user and remains

fairly constant as long as the voltage of the LED stack remains

constant. Calculating the off-time is the first step in determin-

ing the switching frequency of the converter, which is integral

in determining some external component values.

PNP transistor Q3, resistor R4, and the LED string voltage

define a charging current into capacitor C11. A constant cur-

rent into a capacitor creates a linear charging characteristic.

Resistor R4, capacitor C11 and the current through resistor

R4 (i

COLL), which is approximately equal to VLED/R4, are all

fixed. Therefore, dv is fixed and linear, and dt (t

OFF) can now

be calculated.

Common equations for determining duty cycle and switching

frequency in any buck converter:

Therefore:

With efficiency of the buck converter in mind:

Substitute equations and rearrange:

Off-time, and switching frequency can now be calculated us-

ing the equations above.

SETTING THE SWITCHING FREQUENCY

Selecting the switching frequency for nominal operating con-

ditions is based on tradeoffs between efficiency (better at low

frequency) and solution size/cost (smaller at high frequency).

The input voltage to the buck converter (V

BUCK) changes with

both line variations and over the course of each half-cycle of

the input line voltage. The voltage across the LED string will,

however, remain constant, and therefore the off-time remains

constant.

The on-time, and therefore the switching frequency, will vary

as the V

BUCK voltage changes with line voltage. A good design

practice is to choose a desired nominal switching frequency

knowing that the switching frequency will decrease as the line

voltage drops and increase as the line voltage increases (see

figure 16).

30060310

FIGURE 16. Graphical Illustration of Switching

Frequency vs V

BUCK

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