LM3444MM datasheet(11/30 Pages) TI

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= D =

tON

tON + tOFF

= tON x fSW

VIN

C11

D10

ICOLL

PGN

ISNS

GAT

COFF

LM3444

VBUCK

C12

LM3444

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SNVS682C – NOVEMBER 2010 – REVISED MAY 2013

Figure 15. LM3444 Buck Regulation Circuit

OVERVIEW OF CONSTANT OFF-TIME CONTROL

A buck converter’s conversion ratio is defined as:

(3)

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 VIN or VO changes. The output voltage is equal to the LED

string voltage (VLED), and should not change significantly for a given application. The input voltage or VBUCK 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 tON, MOSFET

switch Q2 is on causing the inductor current to increase. During the on-time, current flows from VBUCK, through

the LEDs, through L2, Q2, and finally through R3 to ground. At some point in time, the inductor current reaches a

maximum (IL2-PK) determined by the voltage sensed at R3 and the ISNS pin. This sensed voltage across R3 is

compared against the voltage of FILTER, at which point Q2 is turned off by the controller.

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LM3444MM Datasheet(PDF) 11 Page - Texas Instruments