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| 型号： | LT1111IS8#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 11/16页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK BOOST INV ADJ 8SOIC |
| 标准包装： | 2,500 |
| 类型： | 降压（降压），升压（升压），反相 |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 可调 |
| 输入电压： | 2 V ~ 30 V |
| 频率 - 开关： | 72kHz |
| 电流 - 输出： | 400mA |
| 同步整流器： | 无 |
| 工作温度： | -40°C ~ 105°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 8-SOIC（0.154"，3.90mm 宽） |
| 包装： | 带卷 (TR) |
| 供应商设备封装： | 8-SOIC |
LT1111
A PPLICATI
S I FOR ATIO
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
V IN
30V
MAX
R1
0.3 ?
Q1
MJE210 OR
ZETEX ZTX749
R2
L1
V OUT
DIODE. The voltage at SW2 must never be allowed to go
220
below –0.5V. A silicon diode such as the 1N4933 will allow
SW2 to go to –0.8V, causing potentially destructive power
dissipation inside the LT1111. Output voltage is deter-
mined by:
C2
+
V IN
LT1111
I L
SW1
FB
R3
330
R4
D1
1N5821
+
C1
= ? 1 +
( )
V OUT
?
?
R 2 ?
R 1 ? ?
(
1 . 25 V
)
( 23 )
GND
SW2
R5
R4
V OUT = 1.25V 1 + R5
LT1111 ? TA08
R3 programs switch current limit. This is especially impor-
tant in applications where the input varies over a wide
range. Without R3, the switch stays on for a fixed time each
cycle. Under certain conditions the current in L1 can build
up to excessive levels, exceeding the switch rating and/or
saturating the inductor. The 100 ? resistor 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
referred to ground. This level shifts the output negative. As
in the step-down mode, D1 must be a Schottky diode,
and ? V OUT ? 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
+
C2
R3
Output current can be increased by using a discrete PNP
pass transistor as shown in Figure 6. R1 serves as a
I LIM
V IN
SW1
current limit sense. When the voltage drop across R1
equals a V BE , the switch turns off. For temperature com-
pensation a Schottky diode can be inserted in series with
the I LIM pin. This also lowers the maximum drop across R1
to V BE – V D , increasing efficiency. As shown, switch
LT1111
GND
FB
SW2
L1
D1
1N5818
+
C1
R1
R2
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
expression for V SW :
Figure 7. Positive-to-Negative Converter
–V OUT
LT1111 ? F07
V SW = V R 1 + V Q 1 SAT ≈ 1 . 0 V
( 24 )
In Figure 8, the input is negative while the output is
positive. In this configuration, the magnitude of the input
R2 provides a current path to turn off Q1. R3 provides base
drive to Q1. R4 and R5 set output voltage. A PMOS FET can
be used in place of Q1 when V IN is between 10V and 20V.
voltage can be higher or lower than the output voltage. A
level shift, provided by the PNP transistor, supplies proper
polarity feedback information to the regulator.
1111fd
11
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相关代理商/技术参数 |
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