Texas Instruments LM3448 Evaluation Board LM3448-EDSNEV/NOPB LM3448-EDSNEV/NOPB データシート
製品コード
LM3448-EDSNEV/NOPB
B U C K ( M IN )
9 0
2
s in 1 3 5
V
4 5 V
2
u
q
L E D ( M IN )
D
L E D ( A V E )
B U C K ( M A X )
V
I
1
I
V
§
·
¨
¸
u
¨
¸
©
¹
D
M IN
L E D ( A V E )
I
1
D
I
u
D
A C - R M S ( M A X )
V
V
2
t
SNOSB51C – SEPTEMBER 2011 – REVISED MAY 2013
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 1µF and 10µF 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 1µF and 10µF of the proper voltage rating will be
sufficient.
RE-CIRCULATING DIODE
The LM3448 Buck converter requires a re-circulating diode D10 to carry the inductor current during the off-time
of the internal SW FET. 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
of the internal SW FET. 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 110VAC ± 20% line, the reverse voltage could be as high as 190V.
(35)
The current rating must be at least,
(36)
or,
(37)
Another consideration when choosing a diode is to make sure that the diode’s reverse recovery time is much
greater than the leading edge blanking time for proper operation.
greater than the leading edge blanking time for proper operation.
Design Calculation Example
The following design example illustrates the process of actually calculating external component values for a
LM3448 non-isolated buck converter with valley-fill PFC according to the following specifications.
LM3448 non-isolated buck converter with valley-fill PFC according to the following specifications.
SPECIFICATIONS:
1. Input voltage range (90VAC – 135VAC)
2. Nominal input voltage = 115VAC
3. Number of LEDs in series = 7
4. Forward voltage drop of a single LED = 3.6V
5. LED stack voltage = (7 x 3.6V) = 25.2V
CHOSEN VALUES:
1. Target nominal switching frequency, f
SW
= 250kHz
2. I
LED(AVE)
= 400mA
3. P
OUT
= (25.2V) x (400mA) = 10.1W
4. Ripple current
Δ
i (usually 15% - 30% of I
LED(AVE)
) = (0.30 x 400mA) = 120mA
5. Valley fill stages = 2
6. Assumed minimum efficiency = 80%
CALCULATIONS:
1. Calculate minimum voltage V
BUCK
equals:
(38)
30
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