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| 型号： | LT1108CS8-5#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 11/12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
Figure 8 details current limit circuitry. Sense transistor Q1,
whose base and emitter are paralleled with power switch
V IN
5V
Q2, is ratioed such that approximately 0.5% of Q2’s
collector current flows in Q1’s collector. This current
passed through internal 80 ? resistor R1 and out through
the I LIM pin. The value of the external resistor connected
between I LIM and V IN sets the current limit. When sufficient
V BAT
R1
1.245V
REF
SET
–
+
LT1108
AO
47k
TO
PROCESSOR
switch current flows to develop a V BE across R1 + R LIM , Q3
V LB – 1.25V
35.1 μ A
turns on and injects current into the oscillator, turning off
the switch. Delay through this circuitry is approximately
2 μ s. The current trip point becomes less accurate for
switch-ON times less than 5 μ s. Resistor values program-
ming switch-ON time for 2 μ s or less will cause spurious
R2
GND
R3
R1 =
V LB = BATTERY TRIP POINT
response in the switch circuitry although the device will
still maintain output regulation.
R2 = 33k
R3 = 1.6M
Figure 9. Setting Low Battery Detector Trip Point
LT1108 ? F09
V IN
R LIM
(EXTERNAL)
I LIM
Table 1. Inductor Manufacturers
Q3
R1
80 ?
(INTERNAL)
MANUFACTURER
Coiltronics International
984 S.W. 13th Court
PART NUMBERS
OCTA-PAC TM
Series
OSCILLATOR
DRIVER
Q1
SW1
Q2
Pompano Beach, FL 33069
305-781-8900
Sumida Electric Co. USA
CD54
SW2
LT1108 ? F08
708-956-0666
CDR74
CDR105
Figure 8. LT1108 Current Limit Circuitry
Table 2. Capacitor Manufacturers
USING THE GAIN BLOCK
The gain block (GB) on the LT1108 can be used as an error
amplifier, low battery detector or linear post regulator. The
gain block itself is a very simple PNP input op amp with an
open collector NPN output. The negative input of the gain
block is tied internally to the 1.245V reference. The positive
input comes out on the SET pin.
Arrangement of the gain block as a low battery detector
is straightforward. Figure 9 shows hookup. R1 and R2
need only be low enough in value so that the bias current
of the SET input does not cause large errors. 33k for R2
is adequate. R3 can be added to introduce a small amount
of hysteresis. This will cause the gain block to “snap”
when the trip point is reached. Values in the 1M to 10M
range are optimal. The addition however, of R3 will
change the trip point.
MANUFACTURER
Sanyo Video Components
1201 Sanyo Avenue
San Diego, CA 92073
619-661-6322
Nichicon America Corporation
927 East State Parkway
Schaumberg, IL 60173
708-843-7500
AVX Corporation
Myrtle Beach, SC
803-946-0690
Table 3. Transistor Manufacturers
MANUFACTURER
Zetex Inc.
87 Modular Avenue
Commack, NY 11725
516-543-7100
PART NUMBERS
OS-CON Series
PL Series
TPS Series
PART NUMBERS
ZTX 749 (NPN)
ZTX 849 (NPN)
ZTX 949 (PNP)
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
11
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