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參數(shù)資料
| 型號： | LM3405AXMKX |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 1.6MHz, 1A Constant Current Buck LED Driver with Internal Compensation in Tiny SOT23 Package |
| 中文描述： | 2.8 A SWITCHING REGULATOR, 1900 kHz SWITCHING FREQ-MAX, PDSO6 |
| 封裝： | TSOT-23, 6 PIN |
| 文件頁數(shù)： | 10/20頁 |
| 文件大小： | 396K |
| 代理商： | LM3405AXMKX |
The catch diode (D1) forward voltage drop and the voltage
drop across the internal NMOS must be included to calculate
a more accurate duty cycle. Calculate D by using the following
formula:
V
SW
can be approximated by:
V
SW
= I
F
x R
DS(ON)
The diode forward drop (V
) can range from 0.3V to 0.7V
depending on the quality of the diode. The lower V
D1
is, the
higher the operating efficiency of the converter.
The inductor value determines the output ripple current (
Δ
i
L
,
as defined in
Figure 2
). Lower inductor values decrease the
size of the inductor, but increases the output ripple current.
An increase in the inductor value will decrease the output rip-
ple current. The ratio of ripple current to LED current is
optimized when it is set between 0.3 and 0.4 at 1A LED cur-
rent. This ratio r is defined as:
One must also ensure that the minimum current limit (1.2A)
is not exceeded, so the peak current in the inductor must be
calculated. The peak current (I
LPK
) in the inductor is calculated
as:
I
LPK
= I
F
+ Δ
i
L
/2
When the designed maximum output current is reduced, the
ratio r can be increased. At a current of 0.2A, r can be made
as high as 0.7. The ripple ratio can be increased at lighter
loads because the net ripple is actually quite low, and if r re-
mains constant the inductor value can be made quite large.
An equation empirically developed for the maximum ripple
ratio at any current below 2A is:
r = 0.387 x I
OUT-0.3667
Note that this is just a guideline.
The LM3405A operates at a high frequency allowing the use
of ceramic output capacitors without compromising transient
response. Ceramic capacitors allow higher inductor ripple
without significantly increasing LED current ripple. See the
output capacitor and feed-forward capacitor sections for more
details on LED current ripple.
Now that the ripple current or ripple ratio is determined, the
inductance is calculated by:
where f
SW
is the switching frequency and I
F
is the LED current.
When selecting an inductor, make sure that it is capable of
supporting the peak output current without saturating. Induc-
tor saturation will result in a sudden reduction in inductance
and prevent the regulator from operating correctly. Because
of the operating frequency of LM3405A, ferrite based induc-
tors are preferred to minimize core losses. This presents little
restriction since the variety of ferrite based inductors is huge.
Lastly, inductors with lower series resistance (DCR) will pro-
vide better operating efficiency. For recommended inductor
selection, refer to Circuit Examples and Recommended In-
ductance Range in
Table 1
.
TABLE 1. Recommended Inductance Range
Inductance Range and Inductor Current Ripple
6.8μH-15μH
Inductance
6.8μH
Δ
i
L
/ I
F
*
51%
10μH-22μH
Inductance
10μH
Δ
i
L
/ I
F
*
58%
15μH-27μH
Inductance
15μH
Δ
i
L
/ I
F
*
116%
I
F
1.0A
10μH
15μH
36%
24%
0.6A
15μH
39%
22μH
26%
0.2A
22μH
27μH
79%
65%
*Maximum over full range of V
IN
and V
OUT
.
INPUT CAPACITOR (C1)
An input capacitor is necessary to ensure that V
does not
drop excessively during switching transients. The primary
specifications of the input capacitor are capacitance, voltage
rating, RMS current rating, and ESL (Equivalent Series In-
ductance). The input voltage rating is specifically stated by
the capacitor manufacturer. Make sure to check any recom-
mended deratings and also verify if there is any significant
change in capacitance at the operating input voltage and the
operating temperature. The input capacitor maximum RMS
input current rating (I
RMS-IN
) must be greater than:
It can be shown from the above equation that maximum RMS
capacitor current occurs when D = 0.5. Always calculate the
RMS at the point where the duty cycle D, is closest to 0.5. The
ESL of an input capacitor is usually determined by the effec-
tive cross sectional area of the current path. A large leaded
capacitor will have high ESL and a 0805 ceramic chip capac-
itor will have very low ESL. At the operating frequency of the
LM3405A, certain capacitors may have an ESL so large that
the resulting inductive impedance (2
π
fL) will be higher than
that required to provide stable operation. It is strongly recom-
mended to use ceramic capacitors due to their low ESR and
low ESL. A 10μF multilayer ceramic capacitor (MLCC) is a
good choice for most applications. In cases where large ca-
pacitance is required, use surface mount capacitors such as
Tantalum capacitors and place at least a 1μF ceramic capac-
itor close to the V
pin. For MLCCs it is recommended to use
X7R or X5R dielectrics. Consult capacitor manufacturer
datasheet to see how rated capacitance varies over operating
conditions.
OUTPUT CAPACITOR (C2)
The output capacitor is selected based upon the desired re-
duction in LED current ripple. A 1μF ceramic capacitor results
in very low LED current ripple for most applications. Due to
the high switching frequency, the 1μF capacitor alone (without
feed-forward capacitor C4) can filter more than 90% of the
inductor current ripple for most applications where the sum of
LED dynamic resistance and R1 is larger than 1
. Since the
internal compensation is tailored for small output capacitance
with very low ESR, it is strongly recommended to use a ce-
ramic capacitor with capacitance less than 3.3μF.
www.national.com
10
L
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM3405AXMKX/NOPB | 功能描述:LED照明驅(qū)動器 RoHS:否 制造商:STMicroelectronics 輸入電壓:11.5 V to 23 V 工作頻率: 最大電源電流:1.7 mA 輸出電流: 最大工作溫度: 安裝風格:SMD/SMT 封裝 / 箱體:SO-16N |
| LM3405AXMY | 制造商:Texas Instruments 功能描述:LED DRVR 3.3V/5V/9V/12V/15V/18V 8-Pin MSOP T/R 制造商:Texas Instruments 功能描述:LED DRIVER, 1A, BUCK, 1.6MHZ, 8MSOP 制造商:Texas Instruments 功能描述:LED DRIVER, 1A, BUCK, 1.6MHZ, 8MSOP; Topology:Buck; No. of Outputs:1; Output Current:1A; Output Voltage:20V; Driver Case Style:MSOP; Input Voltage Min:3V; Input Voltage Max:22V; Dimming Control Type:PWM; Switching Frequency:1.6MHz; ;RoHS Compliant: Yes |
| LM3405AXMY/NOPB | 功能描述:LED照明驅(qū)動器 RoHS:否 制造商:STMicroelectronics 輸入電壓:11.5 V to 23 V 工作頻率: 最大電源電流:1.7 mA 輸出電流: 最大工作溫度: 安裝風格:SMD/SMT 封裝 / 箱體:SO-16N |
| LM3405AXMYEVAL | 功能描述:電源管理IC開發(fā)工具 LM3405AXMY EVAL BOARD RoHS:否 制造商:Maxim Integrated 產(chǎn)品:Evaluation Kits 類型:Battery Management 工具用于評估:MAX17710GB 輸入電壓: 輸出電壓:1.8 V |
| LM3405AXMYEVAL | 制造商:Texas Instruments 功能描述:LM3405AXMY EVAL BOARD |
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