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| 型号： | LM3421Q1MHX/NOPB |
| 厂商： | National Semiconductor |
| 文件页数： | 12/67页 |
| 文件大小： | 0K |
| 描述： | IC LED DRVR HP CONS CURR 16TSSOP |
| 标准包装： | 2,500 |
| 系列： | PowerWise® |
| 恒定电流： | 是 |
| 拓扑： | PWM，SEPIC，降压（降压），升压（升压） |
| 输出数： | 1 |
| 内部驱动器： | 无 |
| 类型 - 主要： | 车载 |
| 类型 - 次要： | 高亮度 LED（HBLED） |
| 频率： | 2MHz |
| 电源电压： | 4.5 V ~ 75 V |
| 输出电压： | 3 V ~ 72 V |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 16-TSSOP（0.173"，4.40mm）裸露焊盘 |
| 供应商设备封装： | 16-TSSOP-EP |
| 包装： | 带卷 (TR) |
| 工作温度： | -40°C ~ 125°C |
| 其它名称： | LM3421Q1MHX |
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SNVS574E – JULY 2008 – REVISED MAY 2013
PREDICTIVE OFF-TIME (PRO) CONTROL
PRO control is used by the LM3421/23 to control I LED . It is a combination of average peak current control and a
one-shot off-timer that varies with input voltage. The LM3421/23 uses peak current control to regulate the
average LED current through an array of HBLEDs. This method of control uses a series resistor in the LED path
to sense LED current and can use either a series resistor in the MosFET path or the MosFET R DS-ON for both
cycle-by-cycle current limit and input voltage feed forward. D is indirectly controlled by changes in both t OFF and
t ON , which vary depending on the operating point.
Even though the off-time control is quasi-hysteretic, the input voltage proportionality in the off-timer creates an
essentially constant switching frequency over the entire operating range for boost and buck-boost topologies.
The buck topology can be designed to give constant ripple over either input voltage or output voltage, however
switching frequency is only constant at a specific operating point .
This type of control minimizes the control loop compensation necessary in many switching regulators, simplifying
the design process. The averaging mechanism in the peak detection control loop provides extremely accurate
LED current regulation over the entire operating range.
PRO control was designed to mitigate “current mode instability” (also called “sub-harmonic oscillation”) found in
standard peak current mode control when operating near or above 50% duty cycles. When using standard peak
current mode control with a fixed switching frequency, this condition is present, regardless of the topology.
However, using a constant off-time approach, current mode instability cannot occur, enabling easier design and
control.
Predictive off-time advantages:
?
?
There is no current mode instability at any duty cycle.
Higher duty cycles / voltage transformation ratios are possible, especially in the boost regulator.
The only disadvantage is that synchronization to an external reference frequency is generally not available.
SWITCHING FREQUENCY
An external resistor (R T ) connected between the RCT pin and the switch node (where D1, Q1, and L1 connect),
in combination with a capacitor (C T ) between the RCT and AGND pins, sets the off-time (t OFF ) as shown in
Figure 16 . For boost and buck-boost topologies, the V IN proportionality ensures a virtually constant switching
frequency (f SW ).
For a buck topology, R T and C T are also used to set t OFF , however the V IN proportionality will not ensure a
constant switching frequency. Instead, constant ripple operation can be achieved. Changing the connection of R T
in Figure 16 from V SW to V IN will provide a constant ripple over varying V IN . Adding a PNP transistor as shown in
The switching frequency is defined:
Buck (Constant Ripple vs. V IN )
f SW =
25 x ( V IN - V O )
R T x C T X V IN
(4)
Buck (Constant Ripple vs. V O )
25 x ( V IN x V O - V O )
R T x C T x V IN
f SW =
2
2
(5)
Boost and Buck-boost
f SW =
25
R T x C T
(6)
For all topologies, the C T capacitor is recommended to be 1 nF and should be located very close to the
LM3421/23.
12
Copyright ? 2008–2013, Texas Instruments Incorporated
Product Folder Links: LM3421 LM3421-Q1 LM3423 LM3423-Q1
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| LM3423BS2LYEV/NOPB | 功能描述:LED 照明开发工具 LM3423BS2LY EVAL BOARD RoHS:否 制造商:Fairchild Semiconductor 产品:Evaluation Kits 用于:FL7732 核心: 电源电压:120V 系列: 封装: |
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