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参数资料
| 型号： | LT1108CS8-5#TR |
| 厂商： | Linear Technology |
| 文件页数： | 8/12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
In this mode the switch is arranged in common collector or
step-down mode. The switch drop can be modeled as a
0.75V source in series with a 0.65 ? resistor. When the
switch closes, current in the inductor builds according to
The usual step-up configuration for the LT1108 is shown in
Figure 1. The LT1108 first pulls SW1 low causing V IN –
V CESAT to appear across L1. A current then builds up in L1.
At the end of the switch-ON time the current in L1 is
?
1 – e L ?
t
I L () =
V L
R '
?
?
–R't ?
?
( 15 )
I PEAK =
V IN
L
t ON *
( 20 )
where: R' = 0.65 ? + DCR L
V L = V IN – 0.75V
As an example, suppose – 5V at 100mA is to be generated
from a 4.5V to 5.5V input. Recalling Equation (14),
V IN
R3
I LIM
V IN
SW1
L1
D1
R2
+
V OUT
P L = ( ? – 5V ? + 0.5V )( 100mA ) = 550mW.
(16)
LT1108
FB
C1
Energy required from the inductor is
GND
SW2
R1
= = 28 . 9 μ J
P L
f OSC
550mW
19 kHz
( 17 )
Figure 1. Step-Up Mode Hookup
LT1108 ? F01
Picking an inductor value of 220 μ H with 0.3 ? DCR results
in a peak switch current of
Immediately after switch turn-off, the SW1 voltage pin
starts to rise because current cannot instantaneously stop
( ) V ? 1 – e – 0 . 95 ? × 36 μ s ?
( 0 . 65 ? + 0 . 3 ? ) ? ?
4 . 5 V – 0 . 75
I PEAK = 220 μ H ?
= 568 mA
?
(18)
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 LT1108 to keep V FB
at the internal reference voltage of 1.245V. R1 and R2 set
the output voltage according to the formula
( )( )
220 μ H 0 . 568 A 2 = 35 . 5 μ J
E L =
(
)
V OUT = ? 1 +
?
R 1 ?
Substituting I PEAK into Equation (04) results in
1
2
( 19 )
? R 2 ?
? 1 . 245 V
STEP-DOWN (BUCK MODE) OPERATION
( 21 )
Since 35.5 μ J > 28.9 μ J, the 220 μ H inductor will work.
Finally, R LIM should be selected by looking at the Switch
Current vs R LIM curve. In this example, R LIM = 150 ? .
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
to output.
8
A step-down DC/DC converter converts a higher voltage to
a lower voltage. The usual hookup for an LT1108 based
step-down converter is shown in Figure 2.
When the switch turns on, SW2 pulls up to V IN – V SW . This
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
*Expression 20 neglects the effect of switch and coil resistance. This is taken into account in the
"Inductor Selection" section.
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