LM3444_15 Datasheet, PDF (14/27 Pages) Texas Instruments – AC-DC Offline LED Driver

LM3444
SNVS682D – NOVEMBER 2010 – REVISED DECEMBER 2015
8 Application and Implementation
www.ti.com
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).
V_LED
t_ON
V_BUCK ^{= D =} _{tON + tOFF} ^{= tON x fSW}
(4)
Equation 5 shows the duty-cycle with efficiency considered.
1
K
u
V_LED
V_BUCK
=D
(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.
dv
i = C _dt
(6)
Resistor R4, capacitor C11 and the current through resistor R4 (i_COLL), which is approximately equal to V_LED/R4,
are all fixed. Therefore, dv is fixed and linear, and dt (t_OFF) can now be calculated as shown in Equation 7.
R4
t_OFF = C11 x 1.276V x _VLED
(7)
Common equations for determining duty-cycle and switching frequency in any buck converter are shown in
Equation 8.
1
f_SW = _{tOFF + tON}
D=
t_ON
t_ON + t_OFF
=
V_LED
V_BUCK
'¶
=
t_OFF
t_ON + t_OFF
(8)
Therefore, Equation 9 shows:
f_SW =
D,
t_ON
and
f_SW =
1-D
t_OFF
(9)
14
Submit Documentation Feedback
Product Folder Links: LM3444
Copyright © 2010–2015, Texas Instruments Incorporated