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| 型号： | LM3421Q1MHX/NOPB |
| 厂商： | National Semiconductor |
| 文件页数： | 14/67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
The selection of the three resistors (R SNS , R CSH , and R HSP ) is not arbitrary. For matching and noise performance,
the suggested signal current I CSH is approximately 100 μA. This current does not flow in the LEDs and will not
affect either the off-state LED current or the regulated LED current. I CSH can be above or below this value, but
the high-side amplifier offset characteristics may be affected slightly. In addition, to minimize the effect of the
high-side amplifier voltage offset on LED current accuracy, the minimum V SNS is suggested to be 50 mV. Finally,
a resistor (R HSN = R HSP ) should be placed in series with the HSN pin to cancel out the effects of the input bias
current (~10 μA) of both inputs of the high-side sense amplifier.
The sense resistor (R SNS ) can be placed anywhere in the series string of LEDs as long as the voltage at the HSN
and HSP pins (V HSP and V HSN ) satisfies the following conditions.
V HSP < 76V
V HSN > 3.5V
(10)
Typically, for a buck-boost configuration, R SNS is placed at the bottom of the string (LED-) which allows for
greater flexibility of input and output voltage. However, if there is substantial input voltage ripple allowed, it can
help to place R SNS at the top of the string (LED+) which limits the output voltage of the string to:
V O = 76V - V IN
(11)
Note that he CSH pin can also be used as a low-side current sense input regulated to 1.24V. The high-side
sense amplifier is disabled if HSP and HSN are tied to AGND (or V HSN > V HSP ) .
ANALOG DIMMING
The CSH pin can be used to analog dim the LED current by adjusting the current sense voltage (V SNS ). There
are several different methods to adjust V SNS using the CSH pin:
1. External variable resistance : Adjust a potentiometer placed in series with R CSH to vary V SNS .
2. External variable current source: Source current (0 μA to I CSH ) into the CSH pin to adjust V SNS .
Variable Current Source
V REF
Q7
R MAX
Q8
V CC
LM3421/23
Q6
CSH
R ADJ
R BIAS
R CSH
R ADJ
Variable
Resistance
Figure 19. Analog Dimming Circuitry
In general, analog dimming applications require a lower switching frequency to minimize the effect of the leading
edge blanking circuit. As the LED current is reduced, the output voltage and the duty cycle decreases.
Eventually, the minimum on-time is reached. The lower the switching frequency, the wider the linear dimming
range. Figure 19 shows how both CSH methods are physically implemented.
Method 1 uses an external potentiometer in the CSH path which is a simple addition to the existing circuitry.
However, the LEDs cannot dim completely because there is always some resistance causing signal current to
flow. This method is also susceptible to noise coupling at the CSH pin since the potentiometer increases the size
of the signal current loop.
14
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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