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參數(shù)資料
| 型號(hào)： | LM3402HVMM |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 0.5A Constant Current Buck Regulator for Driving High Power LEDs |
| 中文描述： | 0.94 A SWITCHING REGULATOR, PDSO8 |
| 封裝： | PLASTIC, MSOP-8 |
| 文件頁數(shù)： | 16/22頁 |
| 文件大小： | 840K |
| 代理商： | LM3402HVMM |
Design Example 1: LM3402
(Continued)
I
L(PEAK)
= 0.35 + 0.258 / 2 = 479 mA
For this example the peak current rating of the inductor
should be greater than 479 mA. In the case of a short circuit
across the LED array, the LM3402 will continue to deliver
rated current through the short but will reduce the output
voltage to equal the CS pin voltage of 200 mV. Worst-case
peak current in this condition is equal to:
i
L(LED-SHORT)
= [(26.4 – 0.2) x 300 x 10
-9
] / 26.4 x 10
-6
= 298 mA
I
L(PEAK)
= 0.35 + 0.149 = 499 mA
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 735 mA. In order to prevent
inductor saturation during these short circuits the inductor’s
peak current rating must be above 735 mA. The device
selected is an off-the-shelf inductor rated 33 μH
±
20% with a
DCR of 96 m
and a peak current rating of 0.82A. The
physical dimensions of this inductor are 7.0 x 7.0 x 4.5 mm.
R
SNS
The current sensing resistor value can be determined by
re-arranging the expression for average LED current from
the LED Current Accuracy section:
R
SNS
= 0.74
, t
SNS
= 220 ns
Sub-1
resistors are available in both 1% and 5% tolerance.
A 1%, 0.75
resistor will give the best accuracy of the
average LED current. To determine the resistor size the
power dissipation can be calculated as:
P
= (I
)
2
x R
P
SNS
= 0.35
2
x 0.75 = 92 mW
Standard 0805 size resistors are rated to 125 mW and will be
suitable for this application.
To select the proper output capacitor the equation from Buck
Regulators with Output Capacitors is re-arranged to yield the
following:
The target tolerance for LED ripple current is
±
5% or 10%
= 35 mA
P-P
, and the LED datasheet gives a typical value for
r
of 1.0
at 350 mA. The required capacitor impedance to
reduce the worst-case inductor ripple current of 258 mA
P-P
is
therefore:
Z
C
= [0.035 / (0.258 - 0.035] x 1.0 = 0.157
A ceramic capacitor will be used and the required capaci-
tance is selected based on the impedance at 468 kHz:
C
O
= 1/(2 x
π
x 0.157 x 4.68 x 10
5
) = 2.18 μF
This calculation assumes that impedance due to the equiva-
lent series resistance (ESR) and equivalent series induc-
tance (ESL) of C
O
is negligible. The closest 10% tolerance
capacitor value is 2.2 μF. The capacitor used should be rated
to 10V or more and have an X7R dielectric. Several manu-
facturers produce ceramic capacitors with these specifica-
tions in the 0805 case size. A typical value for ESR of 1 m
can be read from the curve of impedance vs. frequency in
the product datasheet.
INPUT CAPACITOR
Following the calculations from the Input Capacitor section,
v
will be 1%
P-P
= 240 mV. The minimum required
capacitance is:
C
IN(MIN)
= (0.35 x 300 x 10
-9
) / 0.24 = 438 nF
In expectation that more capacitance will be needed to pre-
vent power supply interaction a 1.0 μF ceramic capacitor
rated to 50V with X7R dielectric in a 1206 case size will be
used. From the Design Considerations section, input rms
current is:
I
IN-RMS
= 0.35 x Sqrt(0.154 x 0.846) = 126 mA
Ripple current ratings for 1206 size ceramic capacitors are
typically higher than 1A, more than enough for this design.
RECIRCULATING DIODE
The first parameter for D1 which must be determined is the
reverse voltage rating. Schottky diodes are available at re-
verse ratings of 30V and 40V, often in the same package,
with the same forward current rating. To account for ringing a
40V Schottky will be used.
The next parameters to be determined are the forward cur-
rent rating and case size. In this example the low duty cycle
(D = 3.7 / 24 = 15%) requires the recirculating diode D1 to
carry the load current much longer than the internal power
MOSFET of the LM3402. The estimated average diode cur-
rent is:
I
D
= 0.35 x 0.85 = 298 mA
Schottky diodes are available at forward current ratings of
0.5A, however the current rating often assumes a 25C
ambient temperature and does not take into account the
application restrictions on temperature rise. A diode rated for
higher current may be needed to keep the temperature rise
below 40C.To determine the proper case size, the dissipa-
tion and temperature rise in D1 can be calculated as shown
in the Design Considerations section. V
D
for a small case
L
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM3402HVMM/NOPB | 功能描述:開關(guān)變換器、穩(wěn)壓器與控制器 RoHS:否 制造商:Texas Instruments 輸出電壓:1.2 V to 10 V 輸出電流:300 mA 輸出功率: 輸入電壓:3 V to 17 V 開關(guān)頻率:1 MHz 工作溫度范圍: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:WSON-8 封裝:Reel |
| LM3402HVMMX | 功能描述:開關(guān)變換器、穩(wěn)壓器與控制器 RoHS:否 制造商:Texas Instruments 輸出電壓:1.2 V to 10 V 輸出電流:300 mA 輸出功率: 輸入電壓:3 V to 17 V 開關(guān)頻率:1 MHz 工作溫度范圍: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:WSON-8 封裝:Reel |
| LM3402HVMMX/NOPB | 功能描述:開關(guān)變換器、穩(wěn)壓器與控制器 RoHS:否 制造商:Texas Instruments 輸出電壓:1.2 V to 10 V 輸出電流:300 mA 輸出功率: 輸入電壓:3 V to 17 V 開關(guān)頻率:1 MHz 工作溫度范圍: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:WSON-8 封裝:Reel |
| LM3402HVMR | 制造商:Texas Instruments 功能描述:Conv DC-DC Single Step Down 6V to 75V 8-Pin PSOP EP Rail 制造商:Texas Instruments 功能描述:Buck Reg for Hi Pwr LED Drvs, LM3402HVMR |
| LM3402HVMR/NOPB | 功能描述:開關(guān)變換器、穩(wěn)壓器與控制器 RoHS:否 制造商:Texas Instruments 輸出電壓:1.2 V to 10 V 輸出電流:300 mA 輸出功率: 輸入電壓:3 V to 17 V 開關(guān)頻率:1 MHz 工作溫度范圍: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:WSON-8 封裝:Reel |
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