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参数资料
| 型号： | LT3478IFE-1#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 14/24页 |
| 文件大小： | 0K |
| 描述： | IC LED DRVR HP CONS CURR 16TSSOP |
| 标准包装： | 95 |
| 恒定电流： | 是 |
| 拓扑： | PWM，降压（降压），升压（升压） |
| 输出数： | 1 |
| 内部驱动器： | 是 |
| 类型 - 主要： | 车载，背光 |
| 类型 - 次要： | 颜色，白色 LED |
| 频率： | 200kHz ~ 2.25MHz |
| 电源电压： | 2.8 V ~ 36 V |
| 输出电压： | 42V |
| 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： | 管件 |
| 工作温度： | -40°C ~ 125°C |
| 产品目录页面： | 1339 (CN2011-ZH PDF) |
LT3478/LT3478-1
APPLICATIO S I FOR ATIO
allowed level at higher temperatures, the CTRL2 voltage
curve may require a greater downward slope between
25°C and 50°C to compensate for that loss of slope at
higher temperatures.
Example: Calculate the resistor values required for generat-
ing CTRL1 and CTRL2 from V REF based on the following
requirements:
(a) I LED = 700mA at 25°C
(b) I LED derating curve breakpoint occurs at 25°C
(c) I LED derating curve has a slope of –200mA/25°C be-
tween 25°C and 50°C ambient temperature
Step1: Choose CTRL1 = 700mV for I LED = 700mA
CTRL1 = V REF /(1 + R2/R1)
R2 = R1 ? [(V REF /CTRL1) – 1]
For V REF = 1.24V and choosing R1 = 22.1k,
R2 = 22.1k [(1.24/0.7) – 1]
R2 = 17k (choose 16.9k)
CTRL1 = 1.24/(1 + (16.9/22.1))
CTRL1 = 703mV (I LED = 703mA)
Step 2: Choose resistor network option A (Figure 7) and
CTRL2 = CTRL1 for 25°C breakpoint
start with R4 = R2 = 16.9k, R NTC = 22k (closest value
available)
CTRL2 = 701mV (I LED = Min(CTRL1, CTRL2) ? 1A =
701mA)
Step 3: Calculate CTRL2 slope between 25°C and 50°C
CTRL2 (T) = 1.24/(1 + R4/R NTC (T))
at T = T O = 25°C, CTRL2 = 701mV
at T = 50°C, R NTC (T) = R NTC (T O ).e x , x = B [(1/(T + 273)
– 1/298)]
(B = B-constant; linear over the 25°C to 50°C temperature
range)
For R NTC B-constant = 3950 and T = 50°C
x = 3950 [(1/323) – 1/298] = –1.026
R NTC (50°C) = R NTC (25°C).e –1.026
R NTC (50°C) = 22k ? 0.358
R NTC (50°C) = 7.9k
CTRL2(50°C) = 1.24/(1 + 16.9/7.9) = 395mV
CTRL2 slope (25°C to 50°C) = [CTRL2(50°C)
– CTRL2(25°C)]/25°C
= (395 – 701)/25
= –306mV/25°C
I LED slope = –306mA/25°C
The required I LED slope is –200mA/25°C. To reduce the
slope of CTRL2 versus temperature it is easier to keep
the exact same NTC resistor value and B-constant (there
are limited choices) and simply adjust R4 and the type
of resistor network used for the CTRL2 pin. By changing
the resistor network to option C it is possible to place a
temperature independent resistor in series with R NTC to
reduce the effects of R NTC on the CTRL2 pin voltage over
temperature.
Step 4: Calculate the resistor value required for R Y in
resistor network option (c) (Figure 7) to provide an I LED
slope of –200mA/25°C between 25°C and 50°C ambient
temperature.
CTRL2 (25°C) = 0.7V = 1.24/(1 + (R4/(R NTC (25°C)+
R Y ))
R4 = 0.77 (R NTC (25°C) + R Y ) (a)
for –200mA/25°C slope ≥ CTRL2(50°C) = 0.7 – 0.2 =
0.5
CTRL2(50°C) = 0.5V = 1.24/(1 + (R4/(R NTC + R Y ))
R4 = 1.48 (R NTC (50°C) + R Y ) (b)
Equating (a) = (b) and knowing R NTC (25°C) = 22k and
R NTC (50°C) = 7.9k gives,
0.77 (22k + R Y ) = 1.48 (7.9k + R Y )
17k + 0.77 R Y = 11.7 k + 1.48 R Y
R Y = (17k – 11.7k)/(1.48 – 0.77)
R Y = 7.5k
34781f
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