LM3444_15 Datasheet, PDF (20/27 Pages) Texas Instruments – AC-DC Offline LED Driver

LM3444
SNVS682D – NOVEMBER 2010 – REVISED DECEMBER 2015
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Application Information (continued)
V_{AC-RMS(MIN) 2 x SIN(T)}
V_BUCK(MIN) =
#stages
(28)
Example:
Line voltage = 90 V_AC to 135 V_AC
Valley-fill = two stage
V_BUCK(MIN) = ⁹⁰
2 x SIN(135^o)
2
= 45V
(29)
Depending on what type and value of capacitors are used, some derating should be used for voltage droop when
the capacitors are delivering power to the buck converter. With this derating, the lowest voltage the buck
converter sees is about 42.5 V in this example.
To determine how many LEDs can be driven, take the minimum voltage the buck converter sees (42.5 V) and
divide it by the worst-case forward voltage drop of a single LED.
Example: 42.5 V / 3.7 V = 11.5 LEDs (11 LEDs with margin)
8.1.9 Output Capacitor
A capacitor placed in parallel with the LED or array of LEDs can be used to reduce the LED current ripple while
keeping the same average current through both the inductor and the LED array. With a buck topology, the output
inductance (L2) can now be lowered, making the magnetics smaller and less expensive. With a well designed
converter, you can assume that all of the ripple is seen by the capacitor, and not the LEDs. One must ensure
that the capacitor you choose can handle the RMS current of the inductor. See the manufacturer data sheets to
ensure compliance. Usually an X5R or X7R capacitor from 1 µF and 10 µF of the proper voltage rating is
sufficient.
8.1.10 Switching MOSFET
The main switching MOSFET should be chosen with efficiency and robustness in mind. As shown in
Equation 30, the maximum voltage across the switching MOSFET equals:
V_DS(MAX) = V_{AC-RMS(MAX) 2}
(30)
The average current rating should be greater than what is given in Equation 31.
I_DS-MAX = I_LED(-AVE)(D_MAX)
(31)
8.1.11 Recirculating Diode
The LM3444 buck converter requires a recirculating diode D10 (see Figure 8) to carry the inductor current during
the MOSFET Q2 off-time. The most efficient choice for D10 is a diode with a low forward drop and near-zero
reverse recovery time that can withstand a reverse voltage of the maximum voltage seen at V_BUCK. For a
common 110 V_AC ± 20% line, the reverse voltage could be as high as 190 V, as shown in Equation 32.
V_D t V_AC-RMS(MAX) 2
(32)
As shown in Equation 33, the current rating must be at least:
I_D = (1 - D_MIN) × I_LED(AVE)
(33)
Or as shown in Equation 34:
I_D =
1 - ^VLED(MIN)
V_BUCK(MAX)
x I_LED(AVE)
(34)
8.2 Typical Application
The following design example illustrates the process of calculating external component values.
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