Texas Instruments LM3404 Evaluation Boards LM3404FSTDIMEV/NOPB LM3404FSTDIMEV/NOPB ユーザーズマニュアル
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LM3404FSTDIMEV/NOPB
L
MIN
=
V
IN
- V
O
'
i
L
x t
ON
R
ON
=
V
O
1.34 x 10
-10
x f
SW
GND
DIM
BOOT
SW
CS
RON
LM3404
VIN
D1
L1
C
B
R
SNS
C
F
R
ON
C
IN
V
IN
= 24V
I
F
= 700 mA
VCC
C
O
LED1
SNVS465F – OCTOBER 2006 – REVISED MAY 2013
Figure 30. Schematic for Design Example 1
R
ON
and t
ON
A moderate switching frequency is needed in this application to balance the requirements of magnetics size and
efficiency. R
efficiency. R
ON
is selected from the equation for switching frequency as follows:
(17)
R
ON
= 7.1 / (1.34 x 10
-10
x 4 x 10
5
) = 132.5 k
Ω
(18)
The closest 1% tolerance resistor is 133 k
Ω
. The switching frequency and on-time of the circuit can then be
found using the equations relating R
ON
and t
ON
to f
SW
:
f
SW
= 7.1 / (1.33 x 10
5
x 1.34 x 10
-10
) = 398 kHz
(19)
t
ON
= (1.34 x 10
-10
x 1.33 x 10
5
) / 24 = 743 ns
(20)
OUTPUT INDUCTOR
Since an output capacitor will be used to filter some of the AC ripple current, the inductor ripple current can be
set higher than the LED ripple current. A value of 40%
set higher than the LED ripple current. A value of 40%
P-P
is typical in many buck converters:
Δ
i
L
= 0.4 x 0.7 = 0.28A
(21)
With the target ripple current determined the inductance can be chosen:
(22)
L
MIN
= [(24 – 7.1) x 7.43 x 10
-7
] / (0.28) = 44.8 µH
(23)
The closest standard inductor value is 47 µH. The average current rating should be greater than 700 mA to
prevent overheating in the inductor. Separation between the LM3404 drivers and the LED arrays means that heat
from the inductor will not threaten the lifetime of the LEDs, but an overheated inductor could still cause the
LM3404 to enter thermal shutdown.
prevent overheating in the inductor. Separation between the LM3404 drivers and the LED arrays means that heat
from the inductor will not threaten the lifetime of the LEDs, but an overheated inductor could still cause the
LM3404 to enter thermal shutdown.
The inductance of the standard part chosen is ±20%. With this tolerance the typical, minimum, and maximum
inductor current ripples can be calculated:
inductor current ripples can be calculated:
Δ
i
L(TYP)
= [(24 - 7.1) x 7.43 x 10
-7
] / 47 x 10
-6
= 266 mA
P-P
(24)
Δ
i
L(MIN)
= [(24 - 7.1) x 7.43 x 10
-7
] / 56 x 10
-6
= 223 mA
P-P
(25)
Δ
i
L(MAX)
= [(24 - 7.1) x 7.43 x 10
-7
] / 38 x 10
-6
= 330 mA
P-P
(26)
24
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