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LT1111CS8-12 参数 Datasheet PDF下载

LT1111CS8-12图片预览
型号: LT1111CS8-12
PDF下载: 下载PDF文件 查看货源
内容描述: 微功率DC / DC转换器可调节和固定5V , 12V [Micropower DC/DC Converter Adjustable and Fixed 5V, 12V]
分类和应用: 转换器
文件页数/大小: 16 页 / 337 K
品牌: Linear [ Linear ]
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LT1111  
O U  
W
U
PPLICATI  
A
S I FOR ATIO  
Q1  
When the switch turns off, the SW2 pin falls rapidly and  
actually goes below ground. D1 turns on when SW2  
reaches 0.4V below ground. D1 MUST BE A SCHOTTKY  
DIODE. The voltage at SW2 must never be allowed to go  
below –0.5V. Asilicondiodesuchasthe1N4933willallow  
SW2togoto –0.8V, causingpotentiallydestructivepower  
dissipation inside the LT1111. Output voltage is deter-  
mined by:  
R1  
MJE210 OR  
ZETEX ZTX749  
0.3Ω  
L1  
V
30V  
MAX  
IN  
V
OUT  
R2  
220  
D1  
1N5821  
R3  
330  
V
I
L
SW1  
IN  
+
+
C2  
C1  
LT1111  
R4  
FB  
SW2  
GND  
R2  
R1  
R5  
R4  
V
= 1.25V  
(
1 +  
)
OUT  
V
= 1+  
1.25V  
(23)  
R5  
(
)
OUT  
LT1111 • TA08  
R3programsswitchcurrentlimit.Thisisespeciallyimpor-  
tant in applications where the input varies over a wide  
range.WithoutR3,theswitchstaysonforafixedtimeeach  
cycle. Under certain conditions the current in L1 can build  
up to excessive levels, exceeding the switch rating and/or  
saturating the inductor. The 100resistor programs the  
switch to turn off when the current reaches approximately  
700mA. When using the LT1111 in step-down mode,  
output voltage should be limited to 6.2V or less. Higher  
output voltages can be accommodated by inserting a  
1N5818 diode in series with the SW2 pin (anode con-  
nected to SW2).  
Figure 6. Q1 Permits Higher Current Switching.  
LT1111 Functions as Controller.  
Inverting Configurations  
The LT1111 can be configured as a positive-to-negative  
converter (Figure 7), or a negative-to-positive converter  
(Figure 8). In Figure 7, the arrangement is very similar to  
a step-down, except that the high side of the feedback is  
referredtoground.Thislevelshiftstheoutputnegative.As  
in the step-down mode, D1 must be a Schottky diode,  
and VOUT should be less than 6.2V. More negative out-  
put voltages can be accommodated as in the prior section.  
Higher Current Step-Down Operation  
V
IN  
R3  
Output current can be increased by using a discrete PNP  
pass transistor as shown in Figure 6. R1 serves as a  
current limit sense. When the voltage drop across R1  
equals a VBE, the switch turns off. For temperature com-  
pensation a Schottky diode can be inserted in series with  
theILIM pin.ThisalsolowersthemaximumdropacrossR1  
to VBE – VD, increasing efficiency. As shown, switch  
current is limited to 2A. Inductor value can be calculated  
based on formulas in the “Inductor Selection — Step-  
Down Converter” section with the following conservative  
I
V
IN  
SW1  
FB  
LIM  
+
C2  
LT1111  
GND  
L1  
SW2  
R1  
R2  
D1  
1N5818  
+
C1  
–V  
OUT  
LT1111 • F07  
Figure 7. Positive-to-Negative Converter  
expression for VSW  
= V + V 1.0V  
Q1SAT  
:
In Figure 8, the input is negative while the output is  
positive. In this configuration, the magnitude of the input  
voltage can be higher or lower than the output voltage. A  
levelshift, providedbythePNPtransistor, suppliesproper  
polarity feedback information to the regulator.  
V
(24)  
SW  
R1  
R2providesacurrentpathtoturnoffQ1. R3providesbase  
drivetoQ1. R4andR5setoutputvoltage. APMOSFETcan  
be used in place of Q1 when VIN is between 10V and 20V.  
11