= D =

tON

tON + tOFF

= tON x fSW

VO

VIN

L2

R3

C11

Q2

Q3

D10

ICOLL

4

6

7

8

R4

PGND

ISNS

GATE

COFF

LM3444

VBUCK

C12

LM3444

SNVS682D – NOVEMBER 2010 – REVISED DECEMBER 2015

www.ti.com

Feature Description (continued)

Figure 12. LM3444 Buck Regulation Circuit

7.3.5 Overview Of Constant Off-Time Control

The conversion ratio of a buck converter is defined as given by Equation 3.

(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 varies 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.

12

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