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| 型号： | LT3505EDD#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 12/24页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK ADJ 1.2A 8DFN |
| 标准包装： | 121 |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 0.78 V ~ 33.8 V |
| 输入电压： | 3.6 V ~ 36 V |
| PWM 型： | 电流模式 |
| 频率 - 开关： | 200kHz ~ 3.01MHz |
| 电流 - 输出： | 1.2A |
| 同步整流器： | 无 |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 8-WFDFN 裸露焊盘 |
| 包装： | 管件 |
| 供应商设备封装： | 8-DFN-EP（3x3） |
| 产品目录页面： | 1331 (CN2011-ZH PDF) |
LT3505
APPLICATIONS INFORMATION
Component Selection for Input Voltage Frequency
Foldback Circuit
To determine the values of R4, R5, and D3 for a speci?c
application follow the procedure outlined in this section.
First select the value of R4 from Table 1.
Table 1. R T Pin Resistance
R T PIN RESISTANCE (k ? ) SWITCHING FREQUENCY (MHz)
357 0.20
237 0.30
165 0.40
124 0.50
100 0.60
84.5 0.69
71.5 0.80
and V SW is the voltage drop of the internal power switch
(~0.4V at maximum load), V IN(MAX) is the maximum input
voltage for the application (must be less than 36V), and
t ON(MIN) is the worst-case minimum on-time for the in-
tended application. The worst-case minimum on-time can
be determined from the graphs in the “Typical Performance
Characteristics” section of the datasheet. Next look up the
resistance that corresponds to f SW(MIN) in Table 1. This
resistance is R T(MAX) , the effective resistance from the R T
pin to ground at V IN(MAX) that programs the oscillator to
a switching frequency equal to f SW(MIN) .
Finally determine R5 from the following equation:
R5 = 2 ? (V IN(MAX) – V ZENER )/(1/R4 – 1/R T(MAX) )
61.9
54.9
48.7
44.2
40.2
37.4
34.0
31.6
29.4
27.4
25.5
23.7
22.6
21.0
0.91
1.00
1.11
1.21
1.31
1.39
1.50
1.60
1.70
1.80
1.90
2.02
2.10
2.22
where V ZENER is the zener diode breakdown voltage,
and V IN(MAX) is the maximum input voltage that will be
applied to the V IN pin. V IN(MAX) must not exceed 36V, the
maximum operating input voltage of the LT3505. The
equation to determine R5 assumes that R5 will com-
pensate a percentage of the current ?owing through R4
equal to R4/R T(MAX) . Be careful not to select a value of
R5 much less than that determined by the equation above
because it may become possible for R5 to compensate
100% of the current ?owing through R4 reducing the
frequency to 30kHz. In this state the part is not able to
start into large output current loads.
20.0 2.31
19.1 2.39
18.2 2.48
16.9 2.62
16.2 2.71
15.4 2.81
14.7 2.90
13.7 3.01
Second, determine the value of V IN(PS) from the equation
in the “Input Voltage Range” section of the data sheet.
Select the zener diode, D3, to have a breakdown voltage
(V ZENER ) below V IN(PS) . Next determine the desired foldback
frequency from the following equation:
f SW(MIN) = (V OUT + V D )/[t ON(MIN) ? (V IN(MAX) + V D – V SW )]
where V D is the forward drop of the catch diode (~0.4V),
Whenever the voltage at the FB pin is below 600mV, the
LT3505 folds back the switching frequency by reducing
the bias voltage at the R T pin. If the input voltage is higher
than the zener voltage, the reduced voltage at the R T pin
results in a larger voltage drop across R5, and a reduced
voltage drop across R4. The current carried by R5 may
be large enough to completely compensate the current
?owing through R4, reducing the frequency to 30kHz. In
this situation the input voltage will have to be reduced until
the input voltage is less than the zener voltage.
Note that when V IN is above V ZENER and the frequency is
reduced, the inductor ripple current will be higher and the
maximum load that the LT3505 can regulate will be lower.
See the Inductor Selection and Maximum Output Current
section of this data sheet for more information.
3505fc
12
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| LT3505EMS8E#PBF | 功能描述:IC REG BUCK ADJ 1.2A 8MSOP RoHS:是 类别:集成电路 (IC) >> PMIC - 稳压器 - DC DC 开关稳压器 系列:- 标准包装:250 系列:- 类型:降压（降压） 输出类型:固定 输出数:1 输出电压:1.2V 输入电压:2.05 V ~ 6 V PWM 型:电压模式 频率 - 开关:2MHz 电流 - 输出:500mA 同步整流器:是 工作温度:-40°C ~ 85°C 安装类型:表面贴装 封装/外壳:6-UFDFN 包装:带卷 (TR) 供应商设备封装:6-SON（1.45x1） 产品目录页面:1032 (CN2011-ZH PDF) 其它名称:296-25628-2 |
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