D

tON

fSW =

, and

1 - D

tOFF

fSW =

1

tOFF + tON

fSW =

D =

tON

tON + tOFF

=

VLED

VBUCK

'¶ =

tOFF

tON + tOFF

tOFF = C11 x 1.276V x

R4

VLED

dv

i = C

dt

= D

1

K

VLED

VBUCK

u

= D =

tON

tON + tOFF

= tON x fSW

VLED

VBUCK

LM3444

SNVS682D – NOVEMBER 2010 – REVISED DECEMBER 2015

www.ti.com

8 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component

specification, and TI does not warrant its accuracy or completeness. TI’s customers are

responsible for determining suitability of components for their purposes. Customers should

validate and test their design implementation to confirm system functionality.

8.1 Application Information

8.1.1 Determining Duty-Cycle (D)

Equation 4 shows the duty-cycle (D).

(4)

Equation 5 shows the duty-cycle with efficiency considered.

(5)

For simplicity, choose efficiency from 75% to 85%.

8.1.2 Calculating Off-Time

The off-time of the LM3444 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 determining 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 current into a capacitor creates a linear charging characteristic.

(6)

Resistor R4, capacitor C11 and the current through resistor R4 (iCOLL), which is approximately equal to VLED/R4,

are all fixed. Therefore, dv is fixed and linear, and dt (tOFF) can now be calculated as shown in Equation 7.

(7)

Common equations for determining duty-cycle and switching frequency in any buck converter are shown in

Equation 8.

(8)

Therefore, Equation 9 shows:

(9)

14

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