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參數(shù)資料
| 型號(hào)： | LM3402MMX |
| 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(yè)數(shù)： | 14/22頁(yè) |
| 文件大小： | 840K |
| 代理商： | LM3402MMX |
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Design Considerations
(Continued)
INPUT CAPACITORS
Input capacitors at the VIN pin of the LM3402/02HV are
selected using requirements for minimum capacitance and
rms ripple current. The input capacitors supply pulses of
current approximately equal to I
while the power MOSFET
is on, and are charged up by the input voltage while the
power MOSFET is off. Switching converters such as the
LM3402/02HV have a negative input impedance due to the
decrease in input current as input voltage increases. This
inverse proportionality of input current to input voltage can
cause oscillations (sometimes called ‘power supply interac-
tion’) if the magnitude of the negative input impedance is
greater the the input filter impedance. Minimum capacitance
can be selected by comparing the input impedance to the
converter’s negative resistance; however this requires accu-
rate calculation of the input voltage source inductance and
resistance, quantities which can be difficult to determine. An
alternative method to select the minimum input capacitance,
C
, is to select the maximum voltage ripple which can
be tolerated. This value,
v
, is equal to the change in
voltage across C
during the converter on-time, when C
IN
supplies the load current. C
IN(MIN)
can be selected with the
following:
A good starting point for selection of C
is to use an input
voltage ripple of 5% to 10% of V
. A minimum input capaci-
tance of 2x the C
value is recommended for all
LM3402/02HV circuits. To determine the rms current rating,
the following formula can be used:
Ceramic capacitors are the best choice for the input to the
LM3402/02HV due to their high ripple current rating, low
ESR, low cost, and small size compared to other types.
When selecting a ceramic capacitor, special attention must
be paid to the operating conditions of the application. Ce-
ramic capacitors can lose one-half or more of their capaci-
tance at their rated DC voltage bias and also lose capaci-
tance with extremes in temperature. A DC voltage rating
equal to twice the expected maximum input voltage is rec-
ommended. In addition, the minimum quality dielectric which
is suitable for switching power supply inputs is X5R, while
X7R or better is preferred.
RECIRCULATING DIODE
The LM3402/02HV is a non-synchronous buck regulator that
requires a recirculating diode D1 (see the Typical Application
circuit) to carrying the inductor current during the MOSFET
off-time. The most efficient choice for D1 is a Schottky diode
due to low forward drop and near-zero reverse recovery
time. D1 must be rated to handle the maximum input voltage
plus any switching node ringing when the MOSFET is on. In
practice all switching converters have some ringing at the
switching node due to the diode parasitic capacitance and
the lead inductance. D1 must also be rated to handle the
average current, I
D
, calculated as:
I
D
= (1 – D) x I
F
This calculation should be done at the maximum expected
input voltage. The overall converter efficiency becomes
more dependent on the selection of D1 at low duty cycles,
where the recirculating diode carries the load current for an
increasing percentage of the time. This power dissipation
can be calculated by checking the typical diode forward
voltage, V
D
, from the I-V curve on the product datasheet and
then multiplying it by I
. Diode datasheets will also provide a
typical junction-to-ambient thermal resistance,
θ
, which
can be used to estimate the operating die temperature of the
Schottky. Multiplying the power dissipation (P
= I
x V
) by
θ
gives the temperature rise. The diode case size can then
be selected to maintain the Schottky diode temperature
below the operational maximum.
Design Example 1: LM3402
The first example circuit will guide the user through compo-
nent selection for an architectural accent lighting application.
A regulated DC voltage input of 24V
±
10% will power a
single 1W white LED at a forward current of 350 mA
±
5%.
The typical forward voltage of a 1W InGaN LED is 3.5V,
hence the estimated average output voltage will be 3.7V.
The objective of this application is to place the complete
current regulator and LED in the compact space formerly
occupied by an MR16 halogen light bulb. (The LED will be on
a separate metal-core PCB.) Switching frequency will be as
fast as the 300 ns t
ON
limit allows, with the emphasis on
space savings over efficiency. Efficiency cannot be ignored,
however, as the confined space with little air-flow requires a
maximum temperature rise of 40C in each circuit compo-
nent. A complete bill of materials can be found in Table 1 at
the end of this datasheet.
L
www.national.com
14
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM3402MMX/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 |
| LM3402MR | 制造商:Texas Instruments 功能描述:Conv DC-DC Single Step Down 6V to 42V 8-Pin PSOP EP Rail |
| LM3402MR/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 |
| LM3402MRX | 制造商:Texas Instruments 功能描述:Conv DC-DC Single Step Down 6V to 42V 8-Pin HSOP EP T/R |
| LM3402MRX/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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