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| 型号： | A6268KLPTR-T |
| 厂商： | Allegro Microsystems Inc |
| 文件页数： | 16/19页 |
| 文件大小： | 0K |
| 描述： | IC LED CTLR AUTOMOBILE 16TSSOP |
| 标准包装： | 1 |
| 恒定电流： | 是 |
| 拓扑： | 低端，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，背光 |
| 频率： | 100kHz ~ 700kHz |
| 电源电压： | 5 V ~ 50 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： | 标准包装 |
| 工作温度： | -40°C ~ 150°C |
| 其它名称： | 620-1473-6 |
A6268
? The inductor value is therefore:
Automotive High Current LED Controller
However, as the current rating increases, the diode capacitance
L = V IN × V LED / ( f OSC × I LRIP × [ V IN + V LED ] ) (23)
where:
V LED is the voltage across the LED string,
V IN is the supply voltage,
V IN (min) is the minimum supply voltage,
L is the inductor value, and
f OSC is the oscillator frequency.
With an internal oscillator frequency of 350 kHz, the value of the
inductor for most cases will be between 20 and 50 μ H.
The maximum inductor current can then be calculated as:
I L(PK) = I L(av) (max) + ( I RIP / 2) (24)
This defines the minimum peak switch current as set by the
switch current sense resistor.
The current rating for the inductor should be greater, by some
margin, than the peak value above . When selecting an inductor
from manufacturers datasheets, there are two current levels usu-
ally defined, the smallest value being the figure to work with:
? Saturation level, where the inductance value typically drops by
10%, or
? Temperature rise, where the part experiences a certain rise in
temperature at full rated current. This parameter can be defined
between a 20°C and 50°C rise in temperature. It is important to
understand how manufacturers define the maximum operating
temperature, because this can often incorporate the self-heating
temperature rise.
In most cases the limiting current is usually the saturation value.
To improve efficiency, the inductor should also have low winding
resistance, typically < 50 m Ω , and the core material will usually
be ferrite, with low losses at the oscillator frequency.
Recommended inductor manufacturers/series are:
? Coilcraft/ MSS1278T
? TDK/ SLF12575 type H
Diode The diode should have a low forward voltage, to reduce
conduction losses, and a low capacitance, to reduce switching
losses. Schottky diodes can provide both these features if care-
fully selected. The forward voltage drop is a natural advantage
for Schottky diodes and reduces as the current rating increases.
also increases so the optimum selection is usually the lowest cur-
rent rating above the required maximum, in this case I L(PK) .
Switch Current Sense Resistor (RSS) Neither the absolute
value of the switch current nor the accuracy of the measurement
is important, because the regulator will continuously adjust the
switch current, within a closed loop, to provide sufficient energy
for the output. For maximum accuracy the switch sense resistor
value should be chosen to maximize the differential signal seen
by the sense amplifier. The input limit of the sense amplifier,
V IDS , and the maximum switch current, I S (max), therefore define
the maximum value of the sense resistor as:
R SS = V IDS / I S (max) (25)
where I S (max) is the maximum switch current and should be set
above the maximum inductor current, I L(PK) .
This represents the maximum measurable value of the switch
(and inductor) current; however, the peak switch current will
always be less than this, set by the control circuit, depending on
the input voltage and the required load conditions. Because the
switch current control is within a closed loop, it is possible to
reduce the value of the sense resistor to reduce its power dissipa-
tion. However this will reduce the accuracy of the regulated LED
current.
In boost mode, the power loss in the switch sense resistor is
worse at the lowest input voltage:
P LOSS = R SS × I 2LED × ( V LED [ V LED – V IN (min)] / V 2 IN (min)) (26)
In buck-boost mode, the power loss in the switch sense resistor is
worse at the lowest input voltage:
P LOSS = R SS × I 2LED × ( V LED / V IN (min))( V LED + V IN (min)) (27)
The power rating of the sense resistor should exceed the above
rating at the maximum temperature.
External Switch MOSFET A logic-level N-channel MOSFET
is used as the switch for the DC-to-DC converter. In the boost
configuration the voltage at the drain of the MOSFET is equal
to the maximum voltage across the string of LEDs. In the
buck-boost configuration the output voltage is referenced to
the positive supply. This means that the voltage at the drain of
the MOSFET will reach a voltage equal to the sum of the LED
voltage and the supply voltage. Under load dump conditions,
up to 90 V may be present on this node. In this case the external
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
16
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