t

t

t

V

PEAK

V

+

θ = 90°

t

θ = 45°

V

PEAK

V

+

V

+

t

V

PEAK

θ = 135°

LM3445

SNVS570L – JANUARY 2009 – REVISED MAY 2013

www.ti.com

Figure 31. AC Line with Various Firing Angles

To determine how many LEDs can be driven, take the minimum voltage the buck converter will see (42.5V) and

divide it by the worst case forward voltage drop of a single LED.

Example: 42.5V/3.7V = 11.5 LEDs (11 LEDs with margin)

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 will be seen by the capacitor, and not the LEDs. One must

ensure that the capacitor you choose can handle the RMS current of the inductor. Refer to manufacture’s

datasheets to ensure compliance. Usually an X5R or X7R capacitor between 1 µF and 10 µF of the proper

voltage rating will be sufficient.

SWITCHING MOSFET

The main switching MOSFET should be chosen with efficiency and robustness in mind. The maximum voltage

across the switching MOSFET will equal:

(31)

The average current rating should be greater than:

(32)

26

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