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參數(shù)資料
| 型號： | LM3401 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Hysteretic PFET Controller for High Power LED Drive |
| 中文描述： | 遲滯PFET的控制器的高功率LED驅(qū)動 |
| 文件頁數(shù)： | 11/18頁 |
| 文件大小： | 324K |
| 代理商： | LM3401 |
at the input and provides the instantaneous current when the
PFET turns on. The important parameters for the input ca-
pacitor are the voltage rating and the RMS current rating.
Follow the manufacturer’s recommended voltage de-rating.
RMS current can be calculated with the equation below. The
highest RMS current will occur around 50% duty cycle.
A ceramic input capacitor must be placed close to the drain
of the PFET. This minimizes the trace inductance between
V
and the PFET, which is a source of switching noise. If the
input capacitor is not properly located, switching noise can
cause current limit and stability problems.
CATCH DIODE SELECTION
The catch diode provides the current path to the LED string
during the PFET off-time and must be rated higher than the
average current through the diode, which can be calculated
as shown:
I
DIODE
= I
LED
x (1-D)
The peak reverse voltage across the catch diode is approxi-
mately equal to the input voltage. Therefore, the diode’s peak
reverse voltage rating should be larger than the maximum in-
put voltage, plus some safety margin.
A Schottky diode is recommended because its low forward
voltage maximizes efficiency. For high temperature applica-
tions, diode leakage current may become significant and
require a higher reverse voltage rating or a low leakage diode
to achieve acceptable performance.
LED CURRENT ACCURACY
The total accuracy of average LED current is affected by sev-
eral factors, both internal and external to the LM3401. Total
static accuracy is the part-to-part variation and can be calcu-
lated from the equation below:
Where the worst case V
is ±6%, and R
is the sense
resistor accuracy. Because these factors are not correlated,
the RSS (root-sum-square) method of calculation is used.
The LED current will also show some variation with input volt-
age. This is primarily due to propagation delay and the dy-
namic resistance of the LED. In longer on-time operation, the
error due to dynamic resistance tends to dominate, while at
shorter on-time, the propagation delay will dominate. These
two effects counteract each other, resulting in typical regula-
tion curves similar to those shown in
Figure 9
. A larger induc-
tor will reduce the error due to propagation delay and will
result in better overall line regulation.
30021437
FIGURE 9. LED DC current line regulation LED Vf = 7.0V
For most applications, the average LED current will be the
highest at the maximum input voltage and lowest at a duty
cycle somewhat greater than 50%. The maximum LED cur-
rent variation can be estimated as:
Where V
is the input voltage corresponding to a duty
cycle of 60%. Since the actual input voltage where minimum
LED current occurs varies with the application, this is an ap-
proximation. As the duty cycle approaches 100%, the aver-
age LED current will approach I
. The average LED
current will be the highest at the point that 100% duty cycle is
reached. In the case that 100% duty cycle can occur, maxi-
mum LED current variation is calculated as:
PCB LAYOUT
PCB layout is very important in all switching regulator de-
signs. Poor layout can cause EMI problems, excess switching
noise, and improper device operation. The following key
points should be followed to ensure a quality layout.
Traces carrying large AC currents should be as wide and
short as possible to minimize trace inductance. These areas,
shown as darker regions in
Figure 10
, are:
- V
IN
between the input capacitor and PFET
- GND between the input capacitor and catch diode
- The switch node
As shown in
Figure 10
, place the input capacitor ground as
close as possible to the anode of the catch diode. The VIN
side of the input capacitor should be placed close to the top
of the PFET.
The CS node (the node connecting the catch diode cathode,
inductor, and PFET source) should be kept as small as pos-
sible. This node is one of the main sources for radiated EMI.
The SNS and HYS pins are sensitive to noise. Be sure to route
the SNS trace away from the inductor and the switch node,
which are sources of noise.
11
www.national.com
L
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相關(guān)代理商/技術(shù)參數(shù) |
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| LM340-12 | 制造商:GOL 功能描述:340 GOLDSTAR LOC#N10F8C |
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