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参数资料
| 型号： | LT1111IS8#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 10/16页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK BOOST INV ADJ 8SOIC |
| 标准包装： | 100 |
| 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： | 管件 |
| 供应商设备封装： | 8-SOIC |
| 产品目录页面： | 1325 (CN2011-ZH PDF) |
LT1111
A PPLICATI
Diode Selection
S I FOR ATIO
At the end of the switch ON time the current in L1 is 1 :
Speed, forward drop, and leakage current are the three
main considerations in selecting a catch diode for LT1111
converters. General purpose rectifiers such as the 1N4001
I PEAK =
V I N
L
t ON
( 20 )
(
)
V OUT = ? 1 + ? 1 . 25 V
are unsuitable for use in any switching regulator applica-
tion. Although they are rated at 1A, the switching time of
a 1N4001 is in the 10 μ s to 50 μ s range. At best, efficiency
will be severely compromised when these diodes are
used; at worst, the circuit may not work at all. Most
LT1111 circuits will be well served by a 1N5818 Schottky
diode, or its surface mount equivalent, the MBRS130T3.
The combination of 500mV forward drop at 1A current,
fast turn ON and turn OFF time, and 4 μ A to 10 μ A leakage
current fit nicely with LT1111 requirements. At peak
switch currents of 100mA or less, a 1N4148 signal diode
may be used. This diode has leakage current in the 1nA to
5nA range at 25 ° C and lower cost than a 1N5818. (You can
also use them to get your circuit up and running, but
beware of destroying the diode at 1A switch currents.)
Step-Up (Boost Mode) Operation
A step-up DC/DC converter delivers an output voltage
higher than the input voltage. Step-up converters are not
short-circuit protected since there is a DC path from input
Immediately after switch turn-off, the SW1 voltage pin
starts to rise because current cannot instantaneously stop
flowing in L1. When the voltage reaches V OUT + V D , the
inductor current flows through D1 into C1, increasing
V OUT . This action is repeated as needed by the LT1111 to
keep V FB at the internal reference voltage of 1.25V. R1 and
R2 set the output voltage according to the formula
? R 2 ?
( 21 )
? R 1 ?
Step-Down (Buck Mode) Operation
A step-down DC/DC converter converts a higher voltage
to a lower voltage. The usual hookup for an LT1111 based
step-down converter is shown in Figure 5.
V IN
R3
100 ?
to output.
+
C2
I LIM
V IN
SW1
The usual step-up configuration for the LT1111 is shown
in Figure 4. The LT1111 first pulls SW1 low causing V IN –
V CESAT to appear across L1. A current then builds up in L1.
LT1111
GND
FB
SW2
L1
R2
V OUT
V IN
L1
D1
V OUT
D1
1N5818
+
C1
R1
R3*
I LIM
V IN
R2
Figure 5. Step-Down Mode Hookup
LT1111 ? F05
SW1
+
LT1111
FB
C1
When the switch turns on, SW2 pulls up to V IN – V SW . This
GND
SW2
R1
puts a voltage across L1 equal to V IN – V SW – V OUT ,
causing a current to build up in L1. At the end of the switch
ON time, the current in L1 is equal to:
*OPTIONAL
Figure 4. Step-Up Mode Hookup.
Refer to Table 1 for Component Values.
LT1111 ? F04
I PEAK =
V IN ? V SW ? V OUT
L
t ON
( 22 )
Note 1: This simple expression neglects the effect of switch and coil
resistance. This is taken into account in the “Inductor Selection” section.
1111fd
10
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