VLED(MIN)

1 -

ID =

VBUCK(MAX)

x ILED(AVE)

t VAC-RMS(MAX) 2

VD

VDS(MAX) = VAC-RMS(MAX) 2

VBUCK(MIN) =

2

90

2

x SIN(135

o)

= 45V

VBUCK(MIN) =

2

VAC-RMS(MIN)

#stages

x SIN(T)

LM3444

SNVS682D – NOVEMBER 2010 – REVISED DECEMBER 2015

www.ti.com

Application Information (continued)

(28)

Example:

Line voltage = 90 VAC to 135 VAC

Valley-fill = two stage

(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:

(30)

The average current rating should be greater than what is given in Equation 31.

IDS-MAX = ILED(-AVE)(DMAX)

(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 VBUCK. For a

common 110 VAC ± 20% line, the reverse voltage could be as high as 190 V, as shown in Equation 32.

(32)

As shown in Equation 33, the current rating must be at least:

ID = (1 - DMIN) × ILED(AVE)

(33)

Or as shown in Equation 34:

(34)

8.2 Typical Application

The following design example illustrates the process of calculating external component values.

20

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