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| 型号： | A6267KLPTR-T |
| 厂商： | Allegro Microsystems Inc |
| 文件页数： | 13/17页 |
| 文件大小： | 0K |
| 描述： | IC LED CTLR AUTO HI CURR 16TSSOP |
| 标准包装： | 8,000 |
| 恒定电流： | 是 |
| 拓扑： | 低端，PWM，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： | 6.5 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： | 带卷 (TR) |
| 工作温度： | -40°C ~ 150°C |
A6267
Automotive High Current LED Controller
Application Information
Component Selection
External component selection is critical to the successful appli-
cation of the LED driver. Although the inductor, the switching
MOSFET, and the output capacitor are the most critical elements,
the specification of the rectifying diode and sense resistors should
also be carefully considered.
The starting point for component selection is to define the maxi-
mum LED current, the voltage across the LEDs, and the input
operating voltage range. This then allows the average inductor
current under worst case conditions to be calculated. The induc-
tor value is then selected based on the acceptable inductor ripple
current. The amount of ripple current will then determine the
Inductor Selection Selecting the correct inductance is a
balance between choosing a value that is small enough to help
reduce size and cost, but high enough to ensure that the inductor
current ripple is kept to an acceptable level. A reasonable target
for the ripple current is 20% of the maximum average current.
The inductor current equations differ slightly depending on
whether the A6267 is configured as a boost or as a buck-boost
converter.
? In a boost converter configuration:
? The maximum average inductor current is approximately:
maximum inductor current under worst case conditions. From
this current the switch current sense resistor can be calculated.
I L(av) (max) = I LED (max) × V LED / V IN (min)
? The inductor current ripple is approximately:
(15)
LED Current Sense Resistor (R LS ) If the voltage at the
IREF pin, V IREF , is greater than 1V, or if IREF is tied to VREG,
then the value of the LED current sense resistor, R LS , can be
calculated from:
R LS = V IDL / I LED (max) (11)
where V IDL is the differential voltage across the LED current
sense amplifier and I LED (max) is the maximum LED current.
If V IREF is less than 1 V, then the value of the LED current sense
resistor can be calculated from:
I LRIP = V IN × ( V LED – V IN ) / ( f OSC × L × V LED ) (16)
? The inductor value is therefore:
L = V IN × ( V LED – V IN ) / ( f OSC × I LRIP × V LED ) (17)
? In a buck-boost configuration:
? The maximum average inductor current is approximately:
I L(av) (max) = I LED (max) × ( V IN (min) + V LED ) / V IN (min) (18)
? The inductor current ripple is approximately:
R LS = V IREF / (10 × I LED (max) ) (12)
The typical value for V IDL is 100 mV. Examples of various sense
resistor values are given in table 2.
I LRIP = V IN × V LED / ( f OSC × L × [ V IN + V LED ] )
? The inductor value is therefore:
(19)
In boost mode, the power loss in the current sense resistor is
worse at the lowest input voltage:
where:
L = V IN × V LED / ( f OSC × I LRIP × [ V IN + V LED ] ) (20)
P LOSS = ( V LED / V IN (min) ) × R LS × I 2LED (13)
In buck-boost mode, the power loss in the current sense resistor is
worse at the lowest input voltage:
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
P LOSS = ( [ V IN + V LED ] / V IN ) × R LS × I 2LED
(14)
f OSC is the oscillator frequency.
The resistors should be of a low inductance construction. Surface
mount chip resistors are usually the most suitable, however, axial
or radial leaded resistors can be used provided that the lead length
is kept to a minimum.
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:
Table 2. Sense Resistor Values
I L(PK) = I L(av) (max) + ( I RIP / 2)
(21)
I LED (max)
(mA)
350
700
1000
R LS
(m Ω )
286
143
100
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, I L(PK) . When selecting an
inductor from manufacturers datasheets, there are two current
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
13
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