LM3444_15 Datasheet, PDF (15/27 Pages) Texas Instruments – AC-DC Offline LED Driver

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LM3444
SNVS682D – NOVEMBER 2010 – REVISED DECEMBER 2015
Application Information (continued)
With efficiency of the buck converter in mind, Equation 10 shows:
V_LED
V_BUCK ^{= K u D}
(10)
Substituting and rearranging the equations, Equation 11 shows:
¨©§¹
· ₁
¸¹ _K
u
V_LED
V_BUCK
f_SW =
t_OFF
(11)
Off-time and switching frequency can now be calculated using the previous equations.
8.1.3 Setting the Switching Frequency
Selecting the switching frequency for nominal operating conditions is based on tradeoffs between efficiency
(better at low frequency) and solution size and cost (smaller at high frequency).
The input voltage to the buck converter (V_BUCK) changes with both line variations and over the course of each
half-cycle of the input line voltage. The voltage across the LED string, however, remains constant, and therefore
the off-time remains constant.
The on-time, and therefore the switching frequency, varies as the V_BUCK voltage changes with line voltage. A
good design practice is to choose a desired nominal switching frequency knowing that the switching frequency
decreases as the line voltage drops, and increases as the line voltage increases (Figure 14).
1.50
Series
connected LEDs
1.25
3 LEDs
1.00
5 LEDs
0.75
7 LEDs
0.50
9 LEDs
0.25
0
50
100
150
200
V_BUCK (V)
Figure 14. Graphical Illustration of Switching Frequency vs V_BUCK
The off-time of the LM3444 can be programmed for switching frequencies ranging from 30 kHz to over 1 MHz. A
trade-off between efficiency and solution size must be considered when designing the LM3444 application.
The maximum switching frequency attainable is limited only by the minimum on-time requirement (200 ns).
Worst case scenario for minimum on time is when V_BUCK is at its maximum voltage (AC high line) and the LED
string voltage (V_LED) is at its minimum value, as shown in Equation 12.
1 ^VLED(MIN) 1
t_ON(MIN) = _K ^u _{VBUCK(MAX) fSW}
(12)
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