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LM3444

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SNVS682C – NOVEMBER 2010 – REVISED MAY 2013

Common equations for determining duty cycle and switching frequency in any buck converter:

(8)

Therefore:

(9)

With efficiency of the buck converter in mind:

(10)

Substitute equations and rearrange:

(11)

Off-time, and switching frequency can now be calculated using the equations above.

SETTING THE SWITCHING FREQUENCY

Selecting the switching frequency for nominal operating conditions is based on tradeoffs between efficiency

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

half-cycle of the input line voltage. The voltage across the LED string will, however, remain constant, and

therefore the off-time remains constant.

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 (Figure 17).

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