LM3404EVAL/NOPB National Semiconductor, LM3404EVAL/NOPB Datasheet - Page 22

• Manufacturer Part Number: LM3404EVAL/NOPB
• Description: BOARD EVALUATION LM3404
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3404EVAL/NOPB

• Current - Output / Channel: 1A
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 4 V
• Features: Dimmable
• Voltage - Input: 6 ~ 42 V
• Utilized Ic / Part: LM3404
• Silicon Manufacturer: National
• Silicon Core Number: LM3404
• Kit Application Type: Power Management - Voltage Regulator
• Application Sub Type: Buck Regulator
• Kit Contents: Board, User Guide, LM3404 Datasheet
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *LM3404EVAL; LM3404EVAL

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Design Example 2: LM3404HV
The second example circuit will guide the user through com-
ponent selection for an outdoor general lighting application.
A regulated DC voltage input of 48V ±10% will power ten se-
R
A low switching frequency, 225 kHz, is needed in this appli-
cation, as high efficiency and low power dissipation take
precedence over the solution size. R
equation for switching frequency as follows:
The next highest 1% tolerance resistor is 1.18 MΩ. The
switching frequency and on-time of the circuit can then be
found using the equations relating R
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 30%
trade-off between the current ripple and the size of the induc-
tor:
With the target ripple current determined the inductance can
be chosen:
ON
and t
R
f
SW
ON
t
ON
= 35.2 / (1.18 x 10
= 35.2 / (1.34 x 10
ON
= (1.34 x 10
Δi
L
= 0.3 x 0.5 = 0.15A
-10
x 1.18 x 10
6
-10
x 1.34 x 10
x 2.25 x 10
ON
ON
6
) / 48 = 3.3 µs
and t
FIGURE 13. Schematic for Design Example 2
is selected from the
-10
5
) = 223 kHz
P-P
) = 1.16 MΩ
ON
makes a good
to f
SW
:
22
ries-connected LEDs at 500 mA ±10% with a ripple current of
50 mA
ule in thermal steady state is 35V, hence the average output
voltage will be 35.2V. A complete bill of materials can be found
in
The closest standard inductor value above 281 is 330 µH. The
average current rating should be greater than 0.5A to prevent
overheating in the inductor. In this example the LM3404HV
driver and the LED array share the same metal-core PCB,
meaning that heat from the inductor could threaten the lifetime
of the LEDs. For this reason the average current rating of the
inductor used should have a de-rating of about 50%, or 1A.
The inductance of the standard part chosen is ±20%. With this
tolerance the typical, minimum, and maximum inductor cur-
rent ripples can be calculated:
The peak inductor current is then estimated:
In the case of a short circuit across the LED array, the
LM3404HV will continue to deliver rated current through the
short but will reduce the output voltage to equal the CS pin
Table 2
L
P-P
MIN
Δi
Δi
Δi
L(MAX)
or less. The typical forward voltage of the LED mod-
L(TYP)
L(MIN)
at the end of this datasheet.
= [(48 – 35.2) x 3.3 x 10
I
L(PEAK)
= [(48 - 35.2) x 3.3 x 10
= [(48 - 35.2) x 3.3 x 10
= [(48 - 35.2) x 3.3 x 10
I
L(PEAK)
= 0.5 + 0.5 x 0.16 = 0.58A
= 128 mA
= 107 mA
= 160 mA
= I
L
+ 0.5 x Δi
P-P
P-P
P-P
-6
] / (0.15) = 281 µH
L(MAX)
-6
-6
-6
20205432
] / 330 x 10
] / 396 x 10
] / 264 x 10
-6
-6
-6