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参数资料
| 型号： | LT1108CS8-5#TR |
| 厂商： | Linear Technology |
| 文件页数： | 9/12页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK BST INV 5V .4A 8SOIC |
| 标准包装： | 2,500 |
| 类型： | 降压（降压），升压（升压），反相 |
| 输出类型： | 固定 |
| 输出数： | 1 |
| 输出电压： | 5V |
| 输入电压： | 2 V ~ 30 V |
| 频率 - 开关： | 19kHz |
| 电流 - 输出： | 400mA |
| 同步整流器： | 无 |
| 工作温度： | 0°C ~ 70°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 |
LT1108
A PPLICATI
S I FOR ATIO
I PEAK =
V IN ? V SW ? V OUT
L
t ON
( 22 )
HIGHER CURRENT STEP-DOWN OPERATION
Output current can be increased by using a discrete PNP
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. A silicon diode such as the 1N4933 will allow
SW2 to go to –0.8V, causing potentially destructive power
dissipation inside the LT1108. Output voltage is deter-
pass transistor as shown in Figure 3. R1 serves as a
current limit sense. When the voltage drop across R1
equals 0.5V BE , the switch turns off. 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 expres-
sion for V SW :
mined by
V SW = V R 1 + V Q 1 SAT ≈ 1 . 0 V
( 24 )
V OUT = ? 1 +
(
R 1 ?
?
?
R 2 ?
? 1 . 245 V
)
( 23 )
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.
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
V IN
30V
MAX
R1
0.15 ?
Q1
ZETEX ZTX749
R2
100 ?
L1
V OUT
+
( )
V OUT = 1.245V 1 + R5
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 LT1108 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 connected
to SW2).
+
C2
V IN I L
SW1
LT1108
FB
GND SW2
R6
100 ?
R3
330 ?
R4
R5
D1
1N5821
C1
R4
LT1108 ? F03
V IN
R3
100 ?
Figure 3. Q1 Permits Higher Current Switching
The LT1108 Functions as Controller
+
C2
I LIM
V IN
LT1108
SW1
FB
SW2
L1
V OUT
INVERTING CONFIGURATIONS
The LT1108 can be configured as a positive-to-negative
converter (Figure 4), or a negative-to-positive converter
GND
D1
1N5818
+
C1
R2
R1
(Figure 5). In Figure 4, 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
Figure 2. Step-Down Mode Hookup
LT1108 ? F02
in the step-down mode, D1 must be a Schottky diode,
and ? V OUT ? should be less than 6.2V. More negative output
voltages can be accommodated as in the prior section.
In Figure 5, 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 level shift,
9
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|---|---|
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