LM3445 datasheet(13/26 Pages) NSC

13 / 26 page

30060323

FIGURE 11. LM3445 Buck Regulation Circuit

OVERVIEW OF CONSTANT OFF-TIME CONTROL

A buck converter’s conversion ratio is defined as:

Constant off-time control architecture operates by simply

defining the off-time and allowing the on-time, and therefore

the switching frequency, to vary as either V

IN or VO changes.

The output voltage is equal to the LED string voltage (V

LED),

and should not change significantly for a given application.

The input voltage or V

BUCK in this analysis will vary as the

input line varies. The length of the on-time is determined by

the sensed inductor current through a resistor to a voltage

reference at a comparator. During the on-time, denoted by

ON, MOSFET switch Q2 is on causing the inductor current to

increase. During the on-time, current flows from V

BUCK,

through the LEDs, through L2, Q2, and finally through R3 to

ground. At some point in time, the inductor current reaches a

maximum (I

L2-PK) determined by the voltage sensed at R3 and

the ISNS pin. This sensed voltage across R3 is compared

against the voltage of dim decoder output, FLTR2, at which

point Q2 is turned off by the controller.

30060325

FIGURE 12. Inductor Current Waveform in CCM

During the off-period denoted by t

OFF, the current through L2

continues to flow through the LEDs via D10.

LM3445 Datasheet(PDF) 13 Page - National Semiconductor (TI)