Texas Instruments LM3445 Evaluation Boards LM3445-EDSNEV/NOPB LM3445-EDSNEV/NOPB Ficha De Dados
Códigos do produto
LM3445-EDSNEV/NOPB
V
DS(MAX)
= V
AC-RMS(MAX)
2
t
V
PEAK
V
+
V
DC
t
V
PEAK
V
+
V
DC
V
DC
t
V
PEAK
V
+
θ = 90°
t
θ = 45°
V
PEAK
V
+
V
+
t
V
PEAK
θ = 135°
SNVS570L – JANUARY 2009 – REVISED MAY 2013
Figure 31. AC Line with Various Firing Angles
Figure 32. V
BUCK
Waveforms 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.
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.
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:
across the switching MOSFET will equal:
(31)
The average current rating should be greater than:
I
DS-MAX
= I
LED(-AVE)
(D
MAX
)
(32)
26
Copyright © 2009–2013, Texas Instruments Incorporated
Product Folder Links: