參數(shù)資料
型號(hào)： LT1111
廠商： Linear Technology Corporation
英文描述： Micropower DC/DC Converter Adjustable and Fixed 5V, 12V
中文描述： 微功率DC / DC轉(zhuǎn)換可調(diào)和固定5V，12V的
文件頁(yè)數(shù)： 10/16頁(yè)
文件大?。?/td> 337K
代理商： LT1111
10
LT1111
U
S
A
O
PPLICATI
IU
U
Diode Selection
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
are unsuitablefor use in anyswitching 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
to output.
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.
At the end of the switch ON time the current in L1 is
1
:
I
V
L
t
PEAK
IN
ON
=
(
)
20
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
V
R
R
V
OUT
=
+
(
)
1
2
1
1 25
.
21
( )
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.
L1
LT1111 F04
GND
SW2
SW1
LIM
I
IN
V
D1
R3*
LT1111
+
V
OUT
R2
R1
C1
*OPTIONAL
V
IN
FB
Figure 4. Step-Up Mode Hookup.
Refer to Table 1 for Component Values.
Note 1:
This simple expression neglects the effect of switch and coil
resistance. This is taken into account in the “Inductor Selection” section.
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:
I
V
V
V
L
t
PEAK
IN
SW
OUT
ON
=
(
)
22
LT1111 F05
GND
SW2
SW1
LIM
I
IN
V
R3
100
FB
V
OUT
+
C2
+
C1
D1
1N5818
V
IN
R2
R1
L1
LT1111
Figure 5. Step-Down Mode Hookup
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