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參數(shù)資料
| 型號： | LM3402MR |
| 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, TSOP-8 |
| 文件頁數(shù)： | 15/24頁 |
| 文件大小： | 431K |
| 代理商： | LM3402MR |
INPUT CAPACITORS
Input capacitors at the VIN pin of the LM3402/02HV are se-
lected 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 cur-
rent as input voltage increases. This inverse proportionality of
input current to input voltage can cause oscillations (some-
times called ‘power supply interaction’) if the magnitude of the
negative input impedance is greater the the input filter
impedance. Minimum capacitance can be selected by com-
paring the input impedance to the converter’s negative resis-
tance; however this requires accurate 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 max-
imum voltage ripple which can be tolerated. This value,
Δ
v
IN
, is equal to the change in voltage across C
during the
converter on-time, when C
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 se-
lecting a ceramic capacitor, special attention must be paid to
the operating conditions of the application. Ceramic capaci-
tors can lose one-half or more of their capacitance at their
rated DC voltage bias and also lose capacitance with ex-
tremes in temperature. A DC voltage rating equal to twice the
expected maximum input voltage is recommended. In addi-
tion, 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 prac-
tice all switching converters have some ringing at the switch-
ing node due to the diode parasitic capacitance and the lead
inductance. D1 must also be rated to handle the average cur-
rent, 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 increas-
ing 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 multi-
plying 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 Schot-
tky. 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 esti-
mated 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
limit allows, with the emphasis on space savings over effi-
ciency. Efficiency cannot be ignored, however, as the con-
fined space with little air-flow requires a maximum tempera-
ture rise of 40°C in each circuit component. A complete bill of
materials can be found in Table 1 at the end of this datasheet.
15
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| 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 |
| LM3403 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Quad Operational Amplifiers |
| LM3403J | 制造商:National Semiconductor 功能描述:3403 NSC'94 S4A1D |
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