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參數(shù)資料
| 型號： | LT1103CY |
| 英文描述： | Current-Mode SMPS Controller |
| 中文描述： | 電流模式開關電源控制器 |
| 文件頁數(shù)： | 9/16頁 |
| 文件大?。?/td> | 206K |
| 代理商： | LT1103CY |
9
LT1107
1107fa
U
S
A
The usual step-up configuration for the LT1107 is shown
in Figure 1. The LT1107 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
:
O
PPLICATI
U
U
I
V
L
t
PEAK
IN
ON
=
(
)
20
Figure 2. Step-Down Mode Hookup
D1
L1
R3
I
LIM
V
IN
FB
SW1
SW2
GND
C1
LT1107
1107 F01
V
IN
V
OUT
R2
R1
+
D1
1N5818
L1
R3
100
I
LIM
V
IN
FB
SW2
GND
C1
LT1107
1107 F02
V
IN
V
OUT
R2
R1
SW1
C2
+
+
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 LT1107 to
keep V
FB
at the internal reference voltage of 1.25V. R1 and
R2 set the output voltage according to the formula:
Step-Down (Buck Mode) Operation
A step-down DC/DC converter converts a higher voltage to
a lower voltage. The usual hookup for an LT1107 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:
Figure 1. Step-Up Mode Hookup
V
R
R
V
OUT
=
+
(
1
)
1
2
1 25
.
21
( )
I
V
V
V
L
t
PEAK
IN
SW
OUT
ON
=
( )
22
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 LT1107. Output voltage is deter-
mined by:
1
R3 programs switch current limit. This is especially im-
portant 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 pro-
grams the switch to turn off when the current reaches
approximately 700mA. When using the LT1107 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).
Inverting Configurations
The LT1107 can be configured as a positive-to-negative
converter (Figure 3), or a negative-to-positive converter
(Figure 4). In Figure 3, 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
R
R
V
OUT
=
+
(
)
1
2
1 25
.
23
(
)
Note 1:
This simple expression neglects the effects of switch and coil
resistance. This is taken into account in the “Inductor Selection” section.
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