Texas Instruments LM3404 Evaluation Boards LM3404FSTDIMEV/NOPB LM3404FSTDIMEV/NOPB ユーザーズマニュアル
製品コード
LM3404FSTDIMEV/NOPB
'
i
F
x r
D
'
i
L
-
'
i
F
Z
C
=
SNVS465F – OCTOBER 2006 – REVISED MAY 2013
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.
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 current ripples can be calculated:
inductor current ripples can be calculated:
Δ
i
L(TYP)
= [(48 - 35.2) x 3.3 x 10
-6
] / 330 x 10
-6
= 128 mA
P-P
(57)
Δ
i
L(MIN)
= [(48 - 35.2) x 3.3 x 10
-6
] / 396 x 10
-6
= 107 mA
P-P
(58)
Δ
i
L(MAX)
= [(48 - 35.2) x 3.3 x 10
-6
] / 264 x 10
-6
= 160 mA
P-P
(59)
The peak inductor current is then estimated:
I
L(PEAK)
= I
L
+ 0.5 x
Δ
i
L(MAX)
(60)
I
L(PEAK)
= 0.5 + 0.5 x 0.16 = 0.58A
(61)
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 voltage of 200 mV. The inductor ripple current
and peak current in this condition would be equal to:
the short but will reduce the output voltage to equal the CS pin voltage of 200 mV. The inductor ripple current
and peak current in this condition would be equal to:
Δ
i
L(LED-SHORT)
= [(48 – 0.2) x 3.3 x 10
-6
] / 264 x 10
-6
= 0.598A
P-P
(62)
I
L(PEAK)
= 0.5 + 0.5 x 0.598 = 0.8A
(63)
In the case of a short at the switch node, the output, or from the CS pin to ground the short circuit current limit
will engage at a typical peak current of 1.5A. In order to prevent inductor saturation during these fault conditions
the inductor’s peak current rating must be above 1.5A. A 330 µH off-the shelf inductor rated to 1.9A (peak) and
1.0A (average) with a DCR of 0.56
will engage at a typical peak current of 1.5A. In order to prevent inductor saturation during these fault conditions
the inductor’s peak current rating must be above 1.5A. A 330 µH off-the shelf inductor rated to 1.9A (peak) and
1.0A (average) with a DCR of 0.56
Ω
will be used.
USING AN OUTPUT CAPACITOR
This application uses sub-1 kHz frequency PWM dimming, allowing the use of a small output capacitor to reduce
the size and cost of the output inductor. To select the proper output capacitor the equation from Buck Regulators
with Output Capacitors is re-arranged to yield the following:
the size and cost of the output inductor. To select the proper output capacitor the equation from Buck Regulators
with Output Capacitors is re-arranged to yield the following:
(64)
The target tolerance for LED ripple current is 50 mA
P-P
, and the typical value for r
D
is 10
Ω
with ten LEDs in
series. The required capacitor impedance to reduce the worst-case steady-state inductor ripple current of 160
mA
mA
P-P
is therefore:
Z
C
= [0.05 / (0.16 - 0.05] x 10 = 4.5
Ω
(65)
A ceramic capacitor will be used and the required capacitance is selected based on the impedance at 223 kHz:
C
O
= 1/(2 x
π
x 4.5 x 2.23 x 10
5
) = 0.16 µF
(66)
This calculation assumes that impedance due to the equivalent series resistance (ESR) and equivalent series
inductance (ESL) of C
inductance (ESL) of C
O
is negligible. The closest 10% tolerance capacitor value is 0.15 µF. The capacitor used
should be rated to 50V or more and have an X7R dielectric. Several manufacturers produce ceramic capacitors
with these specifications in the 0805 case size. ESR values are not typically provided for such low value
capacitors, however is can be assumed to be under 100 m
with these specifications in the 0805 case size. ESR values are not typically provided for such low value
capacitors, however is can be assumed to be under 100 m
Ω
, leaving plenty of margin to meet to LED ripple
current requirement. The low capacitance required allows the use of a 100V rated, 1206-size capacitor. The
rating of 100V ensures that the capacitance will not decrease significantly when the DC output voltage is applied
across the capacitor.
rating of 100V ensures that the capacitance will not decrease significantly when the DC output voltage is applied
across the capacitor.
28
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